The Marine Life Information Network

Global warming (extreme)

Extreme emission scenario (by the end of this century 2081-2100) benchmark of:

• A 5°C rise in SST and NBT (coastal to the shelf seas),

• A 6°C rise in surface air temperature (in eulittoral and supralittoral habitats).

• A 1°C rise in Deep-sea habitats (>200 m) off the continental shelf, and

• A 5°C rise in surface air temperature in intertidal habitats exclusive to Scotland (Global warming pressure definitions).

Evidence

Caryophyllia smithii is a temperate cup coral that has a wide distribution from Norway to the Mediterranean and South Africa in the Atlantic, found in both shallow and deep water, with records of this species between temperatures of 5-30°C. Tranter et al. (1982) suggested Caryophyllia smithii reproduction was cued by seasonal increases in seawater temperature. Therefore, unseasonal increases in temperature may disrupt natural reproductive processes and negatively influence recruitment patterns given that gamete release is most likely triggered by seasonal temperature increases (Tranter et al., 1982).

Elevated seawater temperatures generally increase the metabolic rate of bryozoans and are expected to affect calcification (Smith, 2014; Moreno, 2020). Pentapora foliacea generally occurs in seawater temperatures between 10-20°C (www.obis.org). Multiple studies have shown Pentapora fascialis to display seasonal variation in the deposition of its calcareous skeleton, reflected by the difference in growth bands (Lombardi et al., 2008). Cocito et al. (2006) observed that colonies of Pentapora fascialis grew primarily during the colder months of the year, with slower growth recorded in the warmer months between May and September. Similarly, Lombardi et al. (2006) observed seasonal variation in the zooid size and growth banding, with longer zooid production during the colder months. Also, Lombardi et al. (2008) found the greatest weight percentage of calcite in the winter growth band of Pentapora. Therefore, the formation of Pentapora skeletons appears to be related to seawater temperatures.

Cocito & Sgorbini (2014) studied spatial and temporal patterns of colonial bryozoans in the Ligurian Sea for over nine years. The warming event (a temperature of 23.87 ± 1.4°C at 11 m and of 22.27 ± 1.2°C at 22 m ) in the eastern Ligurian Sea (NW Mediterranean) in 1999 caused rapid declines in the abundance of the bryozoan Pentapora between 11 and 22 m depth with an 86% reduction in colony cover (Cocito & Sgorbini, 2014). Recovery was gradual, with communities at 22 m deep recovering to pre-disturbance levels within four years, however, none of the larger colonies (>1,000 cm²) at 11 m deep survived after the first disturbance. 

Alcyonium digitatum is a boreal species of octocoral recorded along the Atlantic Coasts of Europe from Portugal to Norway and Iceland, and along the North West Atlantic coasts, at sea surface temperatures between 5 and 20°C but mostly between 10 and 15°C (www.obis.org).  Across this latitudinal gradient species are likely to experience a range of temperatures from approx. 5 and 18°C (Sea temperature, 2015). 

Alcyonium digitatum spawns during the winter months (Hartnoll, 1975). Gametes are fertilized while in the water column and the embryos give rise to actively swimming lecithotrophic planulae that may have an extended pelagic life before they eventually settle (usually within one or two further days) and metamorphose to polyps (Matthews, 1917; Hartnoll, 1975). However, laboratory experiments have observed settlement failure to occur in unsuitable conditions (Hartnoll, 1975). As spawning occurs when sea temperatures are low there is the probability that spawning and settlement could be correlated to climatic conditions, therefore global warming could impact the reproduction and recruitment of Alcyonium digitatum. However, the combination of spawning in winter and the long pelagic lifespan may allow a considerable length of time for the planulae to disperse, settle and metamorphose ahead of the spring plankton bloom (Hartnoll, 1975).

The duration, dispersal and survival of planktonic larvae are dependent on several factors including temperature (O’Connor et al., 2007). O’Connor et al. (2007) reported planktonic larval duration to increase with temperature, therefore cold-water species could see an increase in planktonic larval duration under global warming trends. Larval survival has been reported to decrease exponentially with time (planktonic larval duration) (O’Connor et al., 2007). Elevated temperatures may increase the occurrence of octocoral diseases caused by pathogens that act opportunistically to attack hosts that are under stressful conditions. For example, Cerrano et al. (2000) reported that ecosystems in the Mediterranean are rapidly declining from extensive attacks by microorganisms correlating to elevated seawater temperatures. 

The sponge Cliona celata is widespread from Norway, Iceland and the western Baltic Sea, to the Mediterranean. Elevated temperatures have been found to positively correlate with the growth of clionid sponges and provide opportunities for rapid territory expansion (Ruzler, 2002; Prezeslawski et al., 2008). Carver et al. (2010) observed the maximum growth rates of Cliona celata from June to August when seawater temperatures were >15°C. Duckworth & Peterson (2013) found high water temperatures of 31 °C to have limited negative effects on the growth, survival or boring rates of Cliona celata. 

The bryozoan Parasmittina trispinosa generally occurs in temperatures between 5 and 15°C, however, there are records of this species between 25 and 30°C (www.obis.org). Therefore, Parasmittina trispinosa is considered unlikely to be affected by long-term changes in temperature, as acclimation to elevated temperatures appears possible. Menon (1972), for example, reported that the upper lethal temperature and median lethal temperature of Electra pilosa varied significantly with acclimation temperature, e.g. 24 hr upper lethal temperature was ca 25°C in colonies acclimated to 5°C but ca 29°C when acclimated to 22 °C (Menon, 1972).  An acute temperature change may affect growth, feeding and hence reproduction in bryozoans. The bryozoan Porella compressa is primarily recorded between temperatures of 0-15°C (www.obis.org). Therefore, Porella compressa is likely to be influenced by the elevation of seawater temperatures.  

Echinus esculentus is a sea urchin distributed across the N.E. Atlantic from Iceland, north to Finmark, Norway and south to Portugal. Echinus esculentus is common on most coasts of the British Isles but absent from most of the east coast of England, the eastern English Channel and some parts of north Wales. Echinus esculentus has been recorded most commonly at sea temperatures of 5-15°C (www.obis.org).

Echinus esculentus occurred at temperatures between 0 and 18°C in Limfjord, Denmark (Ursin 1960). Temperature, photoperiod and food availability are considered to be factors that control the reproduction of echinoids (Kelly, 2001). Bishop (1985) noted that gametogenesis proceeded at temperatures between 11 - 19 °C although continued exposure to 19°C destroyed synchronicity of gametogenesis between individuals. Embryos and larvae developed abnormally after up to 24hr at 15°C (Tyler & Young 1998) but normally at the other temperatures tested (4, 7 and 11 °C). Tyler & Young (1998) concluded that embryos and larvae were more tolerant of depth and temperature than adults. Bishop (1985) suggested that Echinus esculentus cannot tolerate high temperatures for prolonged periods due to increased respiration rate and resultant metabolic stress. Therefore, Echinus esculentus is likely to be intolerant of chronic long term temperature change but would probably be more intolerant of sudden or short term acute change (e.g. 5 °C for one week) in temperature. 

Sensitivity assessment.  Under the middle, high and extreme emission scenarios seawater temperatures are expected to rise by 3-5°C, with potential southern summer temperatures of 22-24°C. The distribution of Alcyonium digitatum suggests that the species may be impacted by ocean warming, as populations of this species have been reported where seawater temperatures range up to 20°C (www.obis.org). Caryophyllia smithii and Parasmittina trispinosa are found in geographical locations with higher temperatures than in the UK, therefore these species are unlikely to be affected by an increase in seawater temperature, as these species are likely to adjust to gradual ocean warming. Elevated temperatures have shown to be advantageous to clionid sponges, with increased growth rates and rapid territorial expansion. The sea urchin Echinus esculentus cannot tolerate high temperatures for prolonged periods and loss of grazing may change the community. While no evidence on the thermal tolerance of Pentapora foliacea and Porella compressa was found, evidence on the impacts on bryozoan species including Pentapora foliacea (Cocito & Sgorbini, 2014) and Porella compressa could be adversely affected by increased seawater temperatures. Therefore, resistance is assessed as ‘Low’ and resilience as ‘Very low’ so that sensitivity is, therefore, assessed as 'High' at levels predicted for the end of this century.

Global warming (high)

High emission scenario (by the end of this century 2081-2100) benchmark of:

• A 4°C rise in SST, NBT (coastal to the shelf seas) and surface air temperature (in eulittoral and supralittoral habitats).

• A 1°C rise in Deep-sea habitats (>200 m) off the continental shelf, and

• A 3°C rise in surface air temperature in intertidal habitats exclusive to Scotland. 

Evidence

Caryophyllia smithii is a temperate cup coral that has a wide distribution from Norway to the Mediterranean and South Africa in the Atlantic, found in both shallow and deep water, with records of this species between temperatures of 5-30°C. Tranter et al. (1982) suggested Caryophyllia smithii reproduction was cued by seasonal increases in seawater temperature. Therefore, unseasonal increases in temperature may disrupt natural reproductive processes and negatively influence recruitment patterns given that gamete release is most likely triggered by seasonal temperature increases (Tranter et al., 1982).

Elevated seawater temperatures generally increase the metabolic rate of bryozoans and are expected to affect calcification (Smith, 2014; Moreno, 2020). Pentapora foliacea generally occurs in seawater temperatures between 10-20°C (www.obis.org). Multiple studies have shown Pentapora fascialis to display seasonal variation in the deposition of its calcareous skeleton, reflected by the difference in growth bands (Lombardi et al., 2008). Cocito et al. (2006) observed that colonies of Pentapora fascialis grew primarily during the colder months of the year, with slower growth recorded in the warmer months between May and September. Similarly, Lombardi et al. (2006) observed seasonal variation in the zooid size and growth banding, with longer zooid production during the colder months. Also, Lombardi et al. (2008) found the greatest weight percentage of calcite in the winter growth band of Pentapora. Therefore, the formation of Pentapora skeletons appears to be related to seawater temperatures.

Cocito & Sgorbini (2014) studied spatial and temporal patterns of colonial bryozoans in the Ligurian Sea for over nine years. The warming event (a temperature of 23.87 ± 1.4°C at 11 m and of 22.27 ± 1.2°C at 22 m ) in the eastern Ligurian Sea (NW Mediterranean) in 1999 caused rapid declines in the abundance of the bryozoan Pentapora between 11 and 22 m depth with an 86% reduction in colony cover (Cocito & Sgorbini, 2014). Recovery was gradual, with communities at 22 m deep recovering to pre-disturbance levels within four years, however, none of the larger colonies (>1,000 cm²) at 11 m deep survived after the first disturbance. 

Alcyonium digitatum is a boreal species of octocoral recorded along the Atlantic Coasts of Europe from Portugal to Norway and Iceland, and along the North West Atlantic coasts, at sea surface temperatures between 5 and 20°C but mostly between 10 and 15°C (www.obis.org).  Across this latitudinal gradient species are likely to experience a range of temperatures from approx. 5 and 18°C (Sea temperature, 2015). 

Alcyonium digitatum spawns during the winter months (Hartnoll, 1975). Gametes are fertilized while in the water column and the embryos give rise to actively swimming lecithotrophic planulae that may have an extended pelagic life before they eventually settle (usually within one or two further days) and metamorphose to polyps (Matthews, 1917; Hartnoll, 1975). However, laboratory experiments have observed settlement failure to occur in unsuitable conditions (Hartnoll, 1975). As spawning occurs when sea temperatures are low there is the probability that spawning and settlement could be correlated to climatic conditions, therefore global warming could impact the reproduction and recruitment of Alcyonium digitatum. However, the combination of spawning in winter and the long pelagic lifespan may allow a considerable length of time for the planulae to disperse, settle and metamorphose ahead of the spring plankton bloom (Hartnoll, 1975).

The duration, dispersal and survival of planktonic larvae are dependent on several factors including temperature (O’Connor et al., 2007). O’Connor et al. (2007) reported planktonic larval duration to increase with temperature, therefore cold-water species could see an increase in planktonic larval duration under global warming trends. Larval survival has been reported to decrease exponentially with time (planktonic larval duration) (O’Connor et al., 2007). Elevated temperatures may increase the occurrence of octocoral diseases caused by pathogens that act opportunistically to attack hosts that are under stressful conditions. For example, Cerrano et al. (2000) reported that ecosystems in the Mediterranean are rapidly declining from extensive attacks by microorganisms correlating to elevated seawater temperatures. 

The sponge Cliona celata is widespread from Norway, Iceland and the western Baltic Sea, to the Mediterranean. Elevated temperatures have been found to positively correlate with the growth of clionid sponges and provide opportunities for rapid territory expansion (Ruzler, 2002; Prezeslawski et al., 2008). Carver et al. (2010) observed the maximum growth rates of Cliona celata from June to August when seawater temperatures were >15°C. Duckworth & Peterson (2013) found high water temperatures of 31 °C to have limited negative effects on the growth, survival or boring rates of Cliona celata. 

The bryozoan Parasmittina trispinosa generally occurs in temperatures between 5 and 15°C, however, there are records of this species between 25 and 30°C (www.obis.org). Therefore, Parasmittina trispinosa is considered unlikely to be affected by long-term changes in temperature, as acclimation to elevated temperatures appears possible. Menon (1972), for example, reported that the upper lethal temperature and median lethal temperature of Electra pilosa varied significantly with acclimation temperature, e.g. 24 hr upper lethal temperature was ca 25°C in colonies acclimated to 5°C but ca 29°C when acclimated to 22 °C (Menon, 1972).  An acute temperature change may affect growth, feeding and hence reproduction in bryozoans. The bryozoan Porella compressa is primarily recorded between temperatures of 0-15°C (www.obis.org). Therefore, Porella compressa is likely to be influenced by the elevation of seawater temperatures.  

Echinus esculentus is a sea urchin distributed across the N.E. Atlantic from Iceland, north to Finmark, Norway and south to Portugal. Echinus esculentus is common on most coasts of the British Isles but absent from most of the east coast of England, the eastern English Channel and some parts of north Wales. Echinus esculentus has been recorded most commonly at sea temperatures of 5-15°C (www.obis.org).

Echinus esculentus occurred at temperatures between 0 and 18°C in Limfjord, Denmark (Ursin 1960). Temperature, photoperiod and food availability are considered to be factors that control the reproduction of echinoids (Kelly, 2001). Bishop (1985) noted that gametogenesis proceeded at temperatures between 11 - 19 °C although continued exposure to 19°C destroyed synchronicity of gametogenesis between individuals. Embryos and larvae developed abnormally after up to 24hr at 15°C (Tyler & Young 1998) but normally at the other temperatures tested (4, 7 and 11 °C). Tyler & Young (1998) concluded that embryos and larvae were more tolerant of depth and temperature than adults. Bishop (1985) suggested that Echinus esculentus cannot tolerate high temperatures for prolonged periods due to increased respiration rate and resultant metabolic stress. Therefore, Echinus esculentus is likely to be intolerant of chronic long term temperature change but would probably be more intolerant of sudden or short term acute change (e.g. 5 °C for one week) in temperature. 

Sensitivity assessment.  Under the middle, high and extreme emission scenarios seawater temperatures are expected to rise by 3-5°C, with potential southern summer temperatures of 22-24°C. The distribution of Alcyonium digitatum suggests that the species may be impacted by ocean warming, as populations of this species have been reported where seawater temperatures range up to 20°C (www.obis.org). Caryophyllia smithii and Parasmittina trispinosa are found in geographical locations with higher temperatures than in the UK, therefore these species are unlikely to be affected by an increase in seawater temperature, as these species are likely to adjust to gradual ocean warming. Elevated temperatures have shown to be advantageous to clionid sponges, with increased growth rates and rapid territorial expansion. The sea urchin Echinus esculentus cannot tolerate high temperatures for prolonged periods and loss of grazing may change the community. While no evidence on the thermal tolerance of Pentapora foliacea and Porella compressa was found, evidence on the impacts on bryozoan species including Pentapora foliacea (Cocito & Sgorbini, 2014) and Porella compressa could be adversely affected by increased seawater temperatures. Therefore, resistance is assessed as ‘Low’ and resilience as ‘Very low’ so that sensitivity is, therefore, assessed as 'High' at levels predicted for the end of this century.

Global warming (middle)

Middle emission scenario (by the end of this century 2081-2100) benchmark of:

• A 3°C rise in SST, NBT (coastal to the shelf seas) and surface air temperature (in eulittoral and supralittoral habitats).

• A 1°C rise in Deep-sea habitats (>200 m) off the continental shelf.

• A 2°C rise in surface air temperature in intertidal habitats exclusive to Scotland. 

Evidence

Caryophyllia smithii is a temperate cup coral that has a wide distribution from Norway to the Mediterranean and South Africa in the Atlantic, found in both shallow and deep water, with records of this species between temperatures of 5-30°C. Tranter et al. (1982) suggested Caryophyllia smithii reproduction was cued by seasonal increases in seawater temperature. Therefore, unseasonal increases in temperature may disrupt natural reproductive processes and negatively influence recruitment patterns given that gamete release is most likely triggered by seasonal temperature increases (Tranter et al., 1982).

Elevated seawater temperatures generally increase the metabolic rate of bryozoans and are expected to affect calcification (Smith, 2014; Moreno, 2020). Pentapora foliacea generally occurs in seawater temperatures between 10-20°C (www.obis.org). Multiple studies have shown Pentapora fascialis to display seasonal variation in the deposition of its calcareous skeleton, reflected by the difference in growth bands (Lombardi et al., 2008). Cocito et al. (2006) observed that colonies of Pentapora fascialis grew primarily during the colder months of the year, with slower growth recorded in the warmer months between May and September. Similarly, Lombardi et al. (2006) observed seasonal variation in the zooid size and growth banding, with longer zooid production during the colder months. Also, Lombardi et al. (2008) found the greatest weight percentage of calcite in the winter growth band of Pentapora. Therefore, the formation of Pentapora skeletons appears to be related to seawater temperatures.

Cocito & Sgorbini (2014) studied spatial and temporal patterns of colonial bryozoans in the Ligurian Sea for over nine years. The warming event (a temperature of 23.87 ± 1.4°C at 11 m and of 22.27 ± 1.2°C at 22 m ) in the eastern Ligurian Sea (NW Mediterranean) in 1999 caused rapid declines in the abundance of the bryozoan Pentapora between 11 and 22 m depth with an 86% reduction in colony cover (Cocito & Sgorbini, 2014). Recovery was gradual, with communities at 22 m deep recovering to pre-disturbance levels within four years, however, none of the larger colonies (>1,000 cm²) at 11 m deep survived after the first disturbance. 

Alcyonium digitatum is a boreal species of octocoral recorded along the Atlantic Coasts of Europe from Portugal to Norway and Iceland, and along the North West Atlantic coasts, at sea surface temperatures between 5 and 20°C but mostly between 10 and 15°C (www.obis.org).  Across this latitudinal gradient species are likely to experience a range of temperatures from approx. 5 and 18°C (Sea temperature, 2015). 

Alcyonium digitatum spawns during the winter months (Hartnoll, 1975). Gametes are fertilized while in the water column and the embryos give rise to actively swimming lecithotrophic planulae that may have an extended pelagic life before they eventually settle (usually within one or two further days) and metamorphose to polyps (Matthews, 1917; Hartnoll, 1975). However, laboratory experiments have observed settlement failure to occur in unsuitable conditions (Hartnoll, 1975). As spawning occurs when sea temperatures are low there is the probability that spawning and settlement could be correlated to climatic conditions, therefore global warming could impact the reproduction and recruitment of Alcyonium digitatum. However, the combination of spawning in winter and the long pelagic lifespan may allow a considerable length of time for the planulae to disperse, settle and metamorphose ahead of the spring plankton bloom (Hartnoll, 1975).

The duration, dispersal and survival of planktonic larvae are dependent on several factors including temperature (O’Connor et al., 2007). O’Connor et al. (2007) reported planktonic larval duration to increase with temperature, therefore cold-water species could see an increase in planktonic larval duration under global warming trends. Larval survival has been reported to decrease exponentially with time (planktonic larval duration) (O’Connor et al., 2007). Elevated temperatures may increase the occurrence of octocoral diseases caused by pathogens that act opportunistically to attack hosts that are under stressful conditions. For example, Cerrano et al. (2000) reported that ecosystems in the Mediterranean are rapidly declining from extensive attacks by microorganisms correlating to elevated seawater temperatures. 

The sponge Cliona celata is widespread from Norway, Iceland and the western Baltic Sea, to the Mediterranean. Elevated temperatures have been found to positively correlate with the growth of clionid sponges and provide opportunities for rapid territory expansion (Ruzler, 2002; Prezeslawski et al., 2008). Carver et al. (2010) observed the maximum growth rates of Cliona celata from June to August when seawater temperatures were >15°C. Duckworth & Peterson (2013) found high water temperatures of 31 °C to have limited negative effects on the growth, survival or boring rates of Cliona celata. 

The bryozoan Parasmittina trispinosa generally occurs in temperatures between 5 and 15°C, however, there are records of this species between 25 and 30°C (www.obis.org). Therefore, Parasmittina trispinosa is considered unlikely to be affected by long-term changes in temperature, as acclimation to elevated temperatures appears possible. Menon (1972), for example, reported that the upper lethal temperature and median lethal temperature of Electra pilosa varied significantly with acclimation temperature, e.g. 24 hr upper lethal temperature was ca 25°C in colonies acclimated to 5°C but ca 29°C when acclimated to 22 °C (Menon, 1972).  An acute temperature change may affect growth, feeding and hence reproduction in bryozoans. The bryozoan Porella compressa is primarily recorded between temperatures of 0-15°C (www.obis.org). Therefore, Porella compressa is likely to be influenced by the elevation of seawater temperatures.  

Echinus esculentus is a sea urchin distributed across the N.E. Atlantic from Iceland, north to Finmark, Norway and south to Portugal. Echinus esculentus is common on most coasts of the British Isles but absent from most of the east coast of England, the eastern English Channel and some parts of north Wales. Echinus esculentus has been recorded most commonly at sea temperatures of 5-15°C (www.obis.org).

Echinus esculentus occurred at temperatures between 0 and 18°C in Limfjord, Denmark (Ursin 1960). Temperature, photoperiod and food availability are considered to be factors that control the reproduction of echinoids (Kelly, 2001). Bishop (1985) noted that gametogenesis proceeded at temperatures between 11 - 19 °C although continued exposure to 19°C destroyed synchronicity of gametogenesis between individuals. Embryos and larvae developed abnormally after up to 24hr at 15°C (Tyler & Young 1998) but normally at the other temperatures tested (4, 7 and 11 °C). Tyler & Young (1998) concluded that embryos and larvae were more tolerant of depth and temperature than adults. Bishop (1985) suggested that Echinus esculentus cannot tolerate high temperatures for prolonged periods due to increased respiration rate and resultant metabolic stress. Therefore, Echinus esculentus is likely to be intolerant of chronic long term temperature change but would probably be more intolerant of sudden or short term acute change (e.g. 5 °C for one week) in temperature. 

Sensitivity assessment.  Under the middle, high and extreme emission scenarios seawater temperatures are expected to rise by 3-5°C, with potential southern summer temperatures of 22-24°C. The distribution of Alcyonium digitatum suggests that the species may be impacted by ocean warming, as populations of this species have been reported where seawater temperatures range up to 20°C (www.obis.org). Caryophyllia smithii and Parasmittina trispinosa are found in geographical locations with higher temperatures than in the UK, therefore these species are unlikely to be affected by an increase in seawater temperature, as these species are likely to adjust to gradual ocean warming. Elevated temperatures have shown to be advantageous to clionid sponges, with increased growth rates and rapid territorial expansion. The sea urchin Echinus esculentus cannot tolerate high temperatures for prolonged periods and loss of grazing may change the community. While no evidence on the thermal tolerance of Pentapora foliacea and Porella compressa was found, evidence on the impacts on bryozoan species including Pentapora foliacea (Cocito & Sgorbini, 2014) and Porella compressa could be adversely affected by increased seawater temperatures. Therefore, resistance is assessed as ‘Low’ and resilience as ‘Very low’ so that sensitivity is, therefore, assessed as 'High' at levels predicted for the end of this century.

Marine heatwaves (high)

High emission scenario benchmark: A marine heatwave occurring every two years, with a mean duration of 120 days, and a maximum intensity of 3.5°C (Marine heatwave pressure definitions).

Evidence

Marine heatwaves are extreme weather events defined as periods of extreme sea surface temperature that persists for days to months (Frölicher et al., 2018). Marine heatwaves are predicted to occur more frequently, last for longer and at increased intensity by the end of this century under both middle and high emission scenarios (Frölicher et al., 2018).

Caryophyllia smithii is a temperate cup coral that has a wide distribution and is found in both shallow and deep waters, with records of this species between temperatures of 5-30°C. Tranter et al. (1982) suggested Caryophyllia smithii reproduction was cued by seasonal increases in seawater temperature. Therefore, unseasonal increases in temperature may disrupt natural reproductive processes and negatively influence recruitment patterns given that gamete release is most likely triggered by seasonal temperature increases (Tranter et al., 1982). Therefore, marine heatwaves could impact the reproduction and recruitment of Caryophyllia smithii. 

Elevated seawater temperatures generally increase the metabolic rate of bryozoans and are expected to affect calcification (Smith, 2014; Leveroni, 2020). Pentapora foliacea generally occurs in seawater temperatures between 10-20°C (www.obis.org). Cocito & Sgorbini (2014) studied spatial and temporal patterns of colonial bryozoans in the Ligurian Sea for over nine years. The warming event (a temperature of 23.87 ± 1.4°C at 11 m and of 22.27 ± 1.2°C at 22 m ) in the eastern Ligurian Sea (NW Mediterranean) in 1999 caused rapid declines in the abundance of the bryozoan Pentapora between 11 and 22 m depth with an 86% reduction in colony cover (Cocito & Sgorbini, 2014). Recovery was gradual, with communities at 22 m deep recovering to pre-disturbance levels within four years, however, none of the larger colonies (>1,000 cm²) at 11 m deep survived after the first disturbance. 

The sponge Cliona celata is widespread from Norway, Iceland and the western Baltic Sea, to the Mediterranean. Elevated temperatures have been found to positively correlate with the growth of clionid sponges and provide opportunities for rapid territory expansion (Ruzler, 2002; Prezeslawski et al., 2008). Carver et al, (2010) observed the maximum growth rates of Cliona celata from June to August when seawater temperatures were >15°C. Duckworth & Peterson (2013) found high water temperatures of 31 °C to have limited negative effects on the growth, survival or boring rates of Cliona celata. Therefore, marine heatwaves are unlikely to impact the survival of Cliona celata. 

Echinus esculentus cannot tolerate high temperatures for prolonged periods due to increased respiration rate resulting in metabolic stress (Bishop 1985). Bishop (1985) observed gametogenesis to occur between 11-19°C however, continued exposure to 19°C disrupted gametogenesis. In addition, embryos and larvae developed abnormally after 24hr exposure to 15°C (Bishop, 1985). Therefore, marine heatwaves have the potential to impact the reproduction, recruitment and survival of Echinus esculentus.  No evidence on the impacts of marine heatwaves on Alcyonium digitatum was found, this species appears to be restricted to colder waters. With Alcyonium digitatum occurring in seawater temperatures between 5- 20°C(www.obis.org).  Therefore, Alcyonium digitatum is likely to be impacted by heatwaves under both emission scenarios. 

No evidence on the impacts of marine heatwaves was found for Parasmittina trispinosa was found. Though Parasmittina trispinosa generally occurs in temperatures between 5 and 15°C, there are also records of this species between 25 and 30°C (www.obis.org). Therefore, Parasmittina trispinosa is considered unlikely to be affected by long-term changes in temperature in the UK, as Parasmittina trispinosa is likely to acclimate to temperatures with time. However, the occurrence of marine heatwaves could cause mass mortality to populations that have not been acclimated to warmer temperatures. The bryozoan Porella compressa is primarily recorded between temperatures of 0-15°C (www.obis.org). Therefore, Porella compressa is likely to be influenced by heatwave events. 

Sensitivity assessment. Under the middle emission scenario, if heatwaves occurred every three years, with a maximum intensity of 2°C for 80 days by the end of this century, this could lead to summer sea temperatures reaching up to 24°C in southern England. Under the high emission scenario, if heatwaves occurred every two years by the end of this century, reaching a maximum intensity of 3.5°C for 120 days, this could lead to the heatwave lasting the entire summer with temperatures reaching up to 26.5°C. There is no experimental evidence of the impact of marine heatwaves on Alcyonium digitatum, however, the species is unknown to tolerate seawater temperatures >20°C. Caryophyllia smithii occurs in seawater temperatures higher than marine heatwave predictions, however, reproduction is cued by changes in temperature therefore marine heatwaves could disrupt natural reproductive processes. Elevated temperatures have shown to be advantageous to clionid sponges, with increased growth rates and rapid territory expansion, therefore Cliona celata is unlikely to be impacted by heatwaves. However, the sea urchin Echinus esculentus cannot tolerate high temperatures for prolonged periods and loss of grazing may alter the community.  Bryozoans are extremely sensitive to elevated temperatures; therefore, marine heatwaves could affect the calcification and survival of the characteristic bryozoan species Pentapora foliacea. Therefore, resistance is assessed as ‘Low’ and resilience as ‘Very low' so that sensitivity is, therefore, assessed as  'High' at levels predicted for the end of this century.

Marine heatwaves (middle)

Middle emission scenario benchmark:  A marine heatwave occurring every three years, with a mean duration of 80 days, with a maximum intensity of 2°C. 

Evidence

Marine heatwaves are extreme weather events defined as periods of extreme sea surface temperature that persists for days to months (Frölicher et al., 2018). Marine heatwaves are predicted to occur more frequently, last for longer and at increased intensity by the end of this century under both middle and high emission scenarios (Frölicher et al., 2018).

Caryophyllia smithii is a temperate cup coral that has a wide distribution and is found in both shallow and deep waters, with records of this species between temperatures of 5-30°C. Tranter et al. (1982) suggested Caryophyllia smithii reproduction was cued by seasonal increases in seawater temperature. Therefore, unseasonal increases in temperature may disrupt natural reproductive processes and negatively influence recruitment patterns given that gamete release is most likely triggered by seasonal temperature increases (Tranter et al., 1982). Therefore, marine heatwaves could impact the reproduction and recruitment of Caryophyllia smithii. 

Elevated seawater temperatures generally increase the metabolic rate of bryozoans and are expected to affect calcification (Smith, 2014; Leveroni, 2020). Pentapora foliacea generally occurs in seawater temperatures between 10-20°C (www.obis.org). Cocito & Sgorbini (2014) studied spatial and temporal patterns of colonial bryozoans in the Ligurian Sea for over nine years. The warming event (a temperature of 23.87 ± 1.4°C at 11 m and of 22.27 ± 1.2°C at 22 m ) in the eastern Ligurian Sea (NW Mediterranean) in 1999 caused rapid declines in the abundance of the bryozoan Pentapora between 11 and 22 m depth with an 86% reduction in colony cover (Cocito & Sgorbini, 2014). Recovery was gradual, with communities at 22 m deep recovering to pre-disturbance levels within four years, however, none of the larger colonies (>1,000 cm²) at 11 m deep survived after the first disturbance. 

The sponge Cliona celata is widespread from Norway, Iceland and the western Baltic Sea, to the Mediterranean. Elevated temperatures have been found to positively correlate with the growth of clionid sponges and provide opportunities for rapid territory expansion (Ruzler, 2002; Prezeslawski et al., 2008). Carver et al, (2010) observed the maximum growth rates of Cliona celata from June to August when seawater temperatures were >15°C. Duckworth & Peterson (2013) found high water temperatures of 31 °C to have limited negative effects on the growth, survival or boring rates of Cliona celata. Therefore, marine heatwaves are unlikely to impact the survival of Cliona celata. 

Echinus esculentus cannot tolerate high temperatures for prolonged periods due to increased respiration rate resulting in metabolic stress (Bishop 1985). Bishop (1985) observed gametogenesis to occur between 11-19°C however, continued exposure to 19°C disrupted gametogenesis. In addition, embryos and larvae developed abnormally after 24hr exposure to 15°C (Bishop, 1985). Therefore, marine heatwaves have the potential to impact the reproduction, recruitment and survival of Echinus esculentus.  No evidence on the impacts of marine heatwaves on Alcyonium digitatum was found, this species appears to be restricted to colder waters. With Alcyonium digitatum occurring in seawater temperatures between 5- 20°C(www.obis.org).  Therefore, Alcyonium digitatum is likely to be impacted by heatwaves under both emission scenarios. 

No evidence on the impacts of marine heatwaves was found for Parasmittina trispinosa was found. Though Parasmittina trispinosa generally occurs in temperatures between 5 and 15°C, there are also records of this species between 25 and 30°C (www.obis.org). Therefore, Parasmittina trispinosa is considered unlikely to be affected by long-term changes in temperature in the UK, as Parasmittina trispinosa is likely to acclimate to temperatures with time. However, the occurrence of marine heatwaves could cause mass mortality to populations that have not been acclimated to warmer temperatures. The bryozoan Porella compressa is primarily recorded between temperatures of 0-15°C (www.obis.org). Therefore, Porella compressa is likely to be influenced by heatwave events. 

Sensitivity assessment. Under the middle emission scenario, if heatwaves occurred every three years, with a maximum intensity of 2°C for 80 days by the end of this century, this could lead to summer sea temperatures reaching up to 24°C in southern England. Under the high emission scenario, if heatwaves occurred every two years by the end of this century, reaching a maximum intensity of 3.5°C for 120 days, this could lead to the heatwave lasting the entire summer with temperatures reaching up to 26.5°C. There is no experimental evidence of the impact of marine heatwaves on Alcyonium digitatum, however, the species is unknown to tolerate seawater temperatures >20°C. Caryophyllia smithii occurs in seawater temperatures higher than marine heatwave predictions, however, reproduction is cued by changes in temperature therefore marine heatwaves could disrupt natural reproductive processes. Elevated temperatures have shown to be advantageous to clionid sponges, with increased growth rates and rapid territory expansion, therefore Cliona celata is unlikely to be impacted by heatwaves. However, the sea urchin Echinus esculentus cannot tolerate high temperatures for prolonged periods and loss of grazing may alter the community.  Bryozoans are extremely sensitive to elevated temperatures; therefore, marine heatwaves could affect the calcification and survival of the characteristic bryozoan species Pentapora foliacea. Therefore, resistance is assessed as ‘Low’ and resilience as ‘Very low' so that sensitivity is, therefore, assessed as  'High' at levels predicted for the end of this century.

Ocean acidification (high)

High emission scenario benchmark: a further decrease in pH of 0.35 (annual mean) and corresponding 120% increase in H+ ions, seasonal aragonite saturation of 20% of UK coastal waters and North Sea bottom waters, and the aragonite saturation horizon in the NE Atlantic, off the continental shelf, occurring at a depth of 400 m by the end of this century 2081-2100 (Ocean acidification pressure definitions).

Evidence

Increasing levels of CO₂ in the atmosphere have led to the average pH of sea surface waters dropping from 8.25 in the 1700s to 8.14 in the 1990s (Jacobson, 2005). In general, it is thought that calcifying invertebrates will be more sensitive to ocean acidification than non-calcifying invertebrates, which appear to have a more mixed response (Hofmann et al., 2010). It must be noted that many species show variation in their response to pCO₂ independent of their taxonomic group or habitat preferences (Widdicombe & Spicer, 2008; Kroeker et al., 2013).

Ocean acidification has negative impacts on numerous species of coral; however, laboratory evidence has shown that the temperate cup coral Caryophyllia smithii might have some resistance to ocean acidification. Rodolfo-Metalpa et al. (2015) exposed Caryophyllia smithii samples to elevated CO₂ conditions expected for the end of this century for several months. All of the corals survived the treatment and no significant differences in respiration or gross and net calcification rates were observed under high seawater pCO₂. 

Bryozoans are invertebrate calcifiers, therefore they are potentially highly sensitive to ocean acidification (Smith, 2009). The decrease in water pH from global climate change could cause corrosion, changes in mineralogy and decrease the survival of Bryozoans (Smith, 2014). No evidence on the impacts of ocean acidification on the characterizing bryozoan species Parasmittina trispinosa, Pentapora foliacea and Porella compressa were found. However, Swezey et al. (2017) observed that populations of bryozoans raised under high CO₂ conditions grew faster, invested less in reproduction and produced lighter skeletons when compared to genetically identical clones raised under current surface atmospheric CO₂values. In addition, the bryozoans under high CO₂ altered Mg/Ca ratio of skeletal calcite, which could be a protective mechanism against acidification (Swezey et al., 2017). 

Lombardi et al. (2011) investigated the impacts of ocean acidification on the growth, organic tissue and protein profile of bryozoan Myriapora truncata along a gradient of different pH levels in a natural volcanic CO₂ vent site. At sites with normal pH levels (mean pH 8.10),  Myriapora truncata produced new and complete zooids. However, at the intermediate (pH 7.83) and low pH (pH 7.32) sites neither partial nor complete zooids were produced. At the intermediate pH sites, Myriapora truncata increased its skeleton thickness suggesting a protective defence against dissolution, but at the low pH sites, there was a decrease in skeletal weights and corrosion of skeletal structures. Additionally, at intermediate and low pH sites Myriapora truncata upregulated protein production to potentially overcome the low pH conditions, however, the upregulation came at a cost, and fitness was reduced resulting in mortality particularly in the lower pH sites. 

No evidence of the impacts of ocean acidification on Alcyonium digitatum was found. However, studies on the impacts of ocean acidification on octocorals reported various responses (Conci et al., 2021). Gomez et al. (2014) found a significant negative correlation between calcification and CO₂ concentrations for Eunicea flexuosa at a pH range of 8.1–7.1. But another study on Eunicea flexuosa observed no significant differences in branch extension and sclerite structure at pH 7.75 and suggested that Eunicea flexuosa had a degree of resilience to ocean acidification (Enochs et al., 2016). Similarly, ocean acidification did not significantly impact the octocorals Ovabunda macrospiculata, Heteroxenia fuscescens and Sarcophyton sp. with no effects on polyp weight and protein concentration, nor any significant differences in chlorophyll abundance or density of zooxanthellae at pH 7.6 and 7.3 when compared to controls at pH 8.2. The findings suggested that the octocoral’s tissue may provide a protective role against acidification (Gabay et al., 2013; Gabay et al., 2014).

The planktonic larval stage is often thought to be the most sensitive stage to ocean acidification in benthic organisms (Kurihara, 2008, Chan et al., 2015).  The embryos of Alcyonium digitatum are neutrally buoyant and float freely for several days before they give rise to actively swimming lecithotrophic planulae which may have an extended pelagic life before they eventually settle (usually within one or two additional days) and metamorphose to polyps (Matthews, 1917; Hartnoll, 1975; Budd, 2008). In laboratory experiments, larvae of Alcyonium digitatum failed to settle within ten days, presumably finding the conditions unsuitable (Hartnoll, 1975), however, the water conditions were not recorded. 

The sponge Cliona celata is a boring species that have been observed to have reduced survival when exposed to lowered pH levels. Duckworth & Peterson (2013) observed that the lowered pH of 7.8 reduced the survival of the sponges with 20% mortality in the lowered pH treatment, while all sponges in the control survived. Additionally, the reduced pH conditions caused Cliona celata to bore more holes and bore two times the amount of shell weight than at pH 8.1.  

Dupont et al. (2010) analysed the literature and suggested that echinoderms were generally robust to ocean acidification, although different life stages and species were affected differently. Limited evidence on the impacts of ocean acidification on Echinus esculentus was found. However, near future CO2-driven ocean acidification (-0.4 units for the end of this century) had negative impacts on survival and developmental dynamics of Echinus esculentus (Dupont and Thorndyke, personal communication, 2009).  Evidence on the reproduction or early life stages of Echinus esculentus was not found, however, studies have found a variety of responses to ocean acidification depending on the species of sea urchin. Dworjanyn & Byrne (2018) found acidification to decrease the gonad index of Tripneustes gratilla, with almost no gonads in urchins at pH 7.6 regardless of temperature. Clark et al, (2009) observed the effects of lowered pH on larvae from tropical (Tripneustes gratilla), temperate (Pseudechinus huttoni, Evechinus chloroticus), and a polar species (Sterechinus neumayeri) of sea urchin. The results indicated that the survival of larvae may not be directly affected by the pH levels predicted for 2100, but the low pH may cause reduced growth and calcification, which could compromise survival. Lee et al. (2019) observed metabolic rates of Strongylocentrotus purpuratus larvae to increase with decreasing pH and reach a threshold between pH 7.0 and pH 7.3 where metabolic rates decreased again. Therefore, ocean acidification could have detrimental effects on the survival, reproduction and recruitment of Echinus esculentus. 

Nevertheless, Suckling et al. (2014) emphasized that studies that presented stressors in a shock-type exposure (as above) may reflect stress response outcomes rather than the results of gradual change in the climate. Cross generation echinoderm studies observed a variety of responses to the progeny produced by adults that have been exposed to low pH. The evidence indicated that the effect on progeny depended on the level of acidification and the conditioning duration of the parents (Byrne et al., 2019). Suckling et al. (2014) found that when Psammechinus miliaris larvae were raised from parents pre-exposed to low pH conditions (pH 7.7 compared to control pH of 7.98), settlement rates were similar to control larvae, and the test (i.e. the urchin shell) diameter was larger, which suggested that this species can acclimate and possibly adapt to low pH conditions. Similarly, Clark et al. (2019) observed that gene expression profiles associated with transgenerational plasticity contributed to Psammechinus miliaris larval resilience when the adults were conditioned to low pH.

From observations at natural vent sites, Connell et al. (2018) observed that increased CO₂ enrichment reduced the abundance and feeding rates of primary grazers (the urchin Evechinus chloroticus), allowing turf algae to increase in abundance. Therefore, ocean acidification could cause changes to community structure. 

Sensitivity Assessment. While limited evidence of the impacts of ocean acidification on Echinus esculentus was found, the effects of ocean acidification on other echinoderms species have shown species dependent sensitivity. No evidence of the effect of ocean acidification on Alcyonium digitatum was found, although the effects of ocean acidification on other species of octocoral have indicated some resilience to low pH. Though Alcyonium digitatum larvae settlement has been reported to be sensitive to environmental conditions, however, the conditions were not verified. Bryozoans appear to be highly sensitive to ocean acidification, with impacts on health, survival and reproduction.  Laboratory evidence has shown that Caryophyllia smithii has some resistance to ocean acidification, therefore the evidence suggests that Caryophyllia smithii may be able to tolerate future pH predictions, however, impacts on reproduction and recruitment were not reviewed. At present, there are no studies to determine whether Echinus esculentus can adapt or acclimate to future pH conditions, but on the evidence available, Echinus esculentus could be lost from this biotope under the predicted emissions scenarios. Loss of this important grazer may change the abundance and diversity of the faunal turf, and result in a different biotope. Similarly, the characteristic bryozoans, especially Pentapora foliacea may be lost. Therefore, resistance is assessed as ‘Low’ under both the middle and high emission scenarios (0.15 and 0.35 pH unit decrease, respectively). Resilience is assessed as ‘Very low’ because of the long-term nature of ocean acidification so that sensitivity is assessed as ‘High’  albeit with ‘Low’ confidence.

Ocean acidification (middle)

Middle emission scenario benchmark: a further decrease in pH of 0.15 (annual mean) and a corresponding 35% increase in H+ ions with no coastal aragonite undersaturation and the aragonite saturation horizon in the NE Atlantic, off the continental shelf, at a depth of 800 m by the end of this century, 2081-2100. 

Evidence

Increasing levels of CO₂ in the atmosphere have led to the average pH of sea surface waters dropping from 8.25 in the 1700s to 8.14 in the 1990s (Jacobson, 2005). In general, it is thought that calcifying invertebrates will be more sensitive to ocean acidification than non-calcifying invertebrates, which appear to have a more mixed response (Hofmann et al., 2010). It must be noted that many species show variation in their response to pCO₂ independent of their taxonomic group or habitat preferences (Widdicombe & Spicer, 2008; Kroeker et al., 2013).

Ocean acidification has negative impacts on numerous species of coral; however, laboratory evidence has shown that the temperate cup coral Caryophyllia smithii might have some resistance to ocean acidification. Rodolfo-Metalpa et al. (2015) exposed Caryophyllia smithii samples to elevated CO₂ conditions expected for the end of this century for several months. All of the corals survived the treatment and no significant differences in respiration or gross and net calcification rates were observed under high seawater pCO₂. 

Bryozoans are invertebrate calcifiers, therefore they are potentially highly sensitive to ocean acidification (Smith, 2009). The decrease in water pH from global climate change could cause corrosion, changes in mineralogy and decrease the survival of Bryozoans (Smith, 2014). No evidence on the impacts of ocean acidification on the characterizing bryozoan species Parasmittina trispinosa, Pentapora foliacea and Porella compressa were found. However, Swezey et al. (2017) observed that populations of bryozoans raised under high CO₂ conditions grew faster, invested less in reproduction and produced lighter skeletons when compared to genetically identical clones raised under current surface atmospheric CO₂values. In addition, the bryozoans under high CO₂ altered Mg/Ca ratio of skeletal calcite, which could be a protective mechanism against acidification (Swezey et al., 2017). 

Lombardi et al. (2011) investigated the impacts of ocean acidification on the growth, organic tissue and protein profile of bryozoan Myriapora truncata along a gradient of different pH levels in a natural volcanic CO₂ vent site. At sites with normal pH levels (mean pH 8.10),  Myriapora truncata produced new and complete zooids. However, at the intermediate (pH 7.83) and low pH (pH 7.32) sites neither partial nor complete zooids were produced. At the intermediate pH sites, Myriapora truncata increased its skeleton thickness suggesting a protective defence against dissolution, but at the low pH sites, there was a decrease in skeletal weights and corrosion of skeletal structures. Additionally, at intermediate and low pH sites Myriapora truncata upregulated protein production to potentially overcome the low pH conditions, however, the upregulation came at a cost, and fitness was reduced resulting in mortality particularly in the lower pH sites. 

No evidence of the impacts of ocean acidification on Alcyonium digitatum was found. However, studies on the impacts of ocean acidification on octocorals reported various responses (Conci et al., 2021). Gomez et al. (2014) found a significant negative correlation between calcification and CO₂ concentrations for Eunicea flexuosa at a pH range of 8.1–7.1. But another study on Eunicea flexuosa observed no significant differences in branch extension and sclerite structure at pH 7.75 and suggested that Eunicea flexuosa had a degree of resilience to ocean acidification (Enochs et al., 2016). Similarly, ocean acidification did not significantly impact the octocorals Ovabunda macrospiculata, Heteroxenia fuscescens and Sarcophyton sp. with no effects on polyp weight and protein concentration, nor any significant differences in chlorophyll abundance or density of zooxanthellae at pH 7.6 and 7.3 when compared to controls at pH 8.2. The findings suggested that the octocoral’s tissue may provide a protective role against acidification (Gabay et al., 2013; Gabay et al., 2014).

The planktonic larval stage is often thought to be the most sensitive stage to ocean acidification in benthic organisms (Kurihara, 2008, Chan et al., 2015).  The embryos of Alcyonium digitatum are neutrally buoyant and float freely for several days before they give rise to actively swimming lecithotrophic planulae which may have an extended pelagic life before they eventually settle (usually within one or two additional days) and metamorphose to polyps (Matthews, 1917; Hartnoll, 1975; Budd, 2008). In laboratory experiments, larvae of Alcyonium digitatum failed to settle within ten days, presumably finding the conditions unsuitable (Hartnoll, 1975), however, the water conditions were not recorded. 

The sponge Cliona celata is a boring species that have been observed to have reduced survival when exposed to lowered pH levels. Duckworth & Peterson (2013) observed that the lowered pH of 7.8 reduced the survival of the sponges with 20% mortality in the lowered pH treatment, while all sponges in the control survived. Additionally, the reduced pH conditions caused Cliona celata to bore more holes and bore two times the amount of shell weight than at pH 8.1.  

Dupont et al. (2010) analysed the literature and suggested that echinoderms were generally robust to ocean acidification, although different life stages and species were affected differently. Limited evidence on the impacts of ocean acidification on Echinus esculentus was found. However, near future CO2-driven ocean acidification (-0.4 units for the end of this century) had negative impacts on survival and developmental dynamics of Echinus esculentus (Dupont and Thorndyke, personal communication, 2009).  Evidence on the reproduction or early life stages of Echinus esculentus was not found, however, studies have found a variety of responses to ocean acidification depending on the species of sea urchin. Dworjanyn & Byrne (2018) found acidification to decrease the gonad index of Tripneustes gratilla, with almost no gonads in urchins at pH 7.6 regardless of temperature. Clark et al, (2009) observed the effects of lowered pH on larvae from tropical (Tripneustes gratilla), temperate (Pseudechinus huttoni, Evechinus chloroticus), and a polar species (Sterechinus neumayeri) of sea urchin. The results indicated that the survival of larvae may not be directly affected by the pH levels predicted for 2100, but the low pH may cause reduced growth and calcification, which could compromise survival. Lee et al. (2019) observed metabolic rates of Strongylocentrotus purpuratus larvae to increase with decreasing pH and reach a threshold between pH 7.0 and pH 7.3 where metabolic rates decreased again. Therefore, ocean acidification could have detrimental effects on the survival, reproduction and recruitment of Echinus esculentus. 

Nevertheless, Suckling et al. (2014) emphasized that studies that presented stressors in a shock-type exposure (as above) may reflect stress response outcomes rather than the results of gradual change in the climate. Cross generation echinoderm studies observed a variety of responses to the progeny produced by adults that have been exposed to low pH. The evidence indicated that the effect on progeny depended on the level of acidification and the conditioning duration of the parents (Byrne et al., 2019). Suckling et al. (2014) found that when Psammechinus miliaris larvae were raised from parents pre-exposed to low pH conditions (pH 7.7 compared to control pH of 7.98), settlement rates were similar to control larvae, and the test (i.e. the urchin shell) diameter was larger, which suggested that this species can acclimate and possibly adapt to low pH conditions. Similarly, Clark et al. (2019) observed that gene expression profiles associated with transgenerational plasticity contributed to Psammechinus miliaris larval resilience when the adults were conditioned to low pH.

From observations at natural vent sites, Connell et al. (2018) observed that increased CO₂ enrichment reduced the abundance and feeding rates of primary grazers (the urchin Evechinus chloroticus), allowing turf algae to increase in abundance. Therefore, ocean acidification could cause changes to community structure. 

Sensitivity Assessment. While limited evidence of the impacts of ocean acidification on Echinus esculentus was found, the effects of ocean acidification on other echinoderms species have shown species dependent sensitivity. No evidence of the effect of ocean acidification on Alcyonium digitatum was found, although the effects of ocean acidification on other species of octocoral have indicated some resilience to low pH. Though Alcyonium digitatum larvae settlement has been reported to be sensitive to environmental conditions, however, the conditions were not verified. Bryozoans appear to be highly sensitive to ocean acidification, with impacts on health, survival and reproduction.  Laboratory evidence has shown that Caryophyllia smithii has some resistance to ocean acidification, therefore the evidence suggests that Caryophyllia smithii may be able to tolerate future pH predictions, however, impacts on reproduction and recruitment were not reviewed. At present, there are no studies to determine whether Echinus esculentus can adapt or acclimate to future pH conditions, but on the evidence available, Echinus esculentus could be lost from this biotope under the predicted emissions scenarios. Loss of this important grazer may change the abundance and diversity of the faunal turf, and result in a different biotope. Similarly, the characteristic bryozoans, especially Pentapora foliacea may be lost. Therefore, resistance is assessed as ‘Low’ under both the middle and high emission scenarios (0.15 and 0.35 pH unit decrease, respectively). Resilience is assessed as ‘Very low’ because of the long-term nature of ocean acidification so that sensitivity is assessed as ‘High’  albeit with ‘Low’ confidence.

Sea level rise (extreme)

Extreme scenario benchmark: a 107 cm rise in average UK sea-level by the end of this century (2018-2100) (Sea-level rise pressure definitions).

Evidence

Sea-level rise is occurring through a combination of thermal expansion and ice melt.  Sea levels have risen 1-3 mm/yr. in the last century (Cazenave & Nerem, 2004, Church et al., 2004, Church & White, 2006). Evidence appears to suggest that impacts of sea-level rise on exposure or tidal energy will be non-linear and site-specific (Pickering et al., 2012, Li et al., 2016).  This biotope occurs on mixed sediment, in moderately exposed to sheltered areas, subject to strong to weak tidal streams and, therefore, should be reasonably robust to any changes which occur. Furthermore, this biotope occurs at depths of 10-50 m around the UK, and sea-level rises predicted for the end of this century should have limited impacts on this biotope.

Sensitivity assessment. As this biotope CR.MCR.EcCr.CarSp.PenPcom can occur from 10-50 m depth, in a range of different energy environments, it is assumed that a sea-level rise of 50 cm, 70 cm or 107 cm (middle and high emission, and extreme scenarios) would have limited effect. Therefore, resistance is assessed as ‘High’ under all three scenarios, so that resilience is ‘High’ and sensitivity assessed as ‘Not sensitive’. 

Sea level rise (high)

High emission scenario benchmark: a 70 cm rise in average UK sea-level by the end of this century (2018-2100). 

Evidence

Sea-level rise is occurring through a combination of thermal expansion and ice melt.  Sea levels have risen 1-3 mm/yr. in the last century (Cazenave & Nerem, 2004, Church et al., 2004, Church & White, 2006). Evidence appears to suggest that impacts of sea-level rise on exposure or tidal energy will be non-linear and site-specific (Pickering et al., 2012, Li et al., 2016).  This biotope occurs on mixed sediment, in moderately exposed to sheltered areas, subject to strong to weak tidal streams and, therefore, should be reasonably robust to any changes which occur. Furthermore, this biotope occurs at depths of 10-50 m around the UK, and sea-level rises predicted for the end of this century should have limited impacts on this biotope.

Sensitivity assessment. As this biotope CR.MCR.EcCr.CarSp.PenPcom can occur from 10-50 m depth, in a range of different energy environments, it is assumed that a sea-level rise of 50 cm, 70 cm or 107 cm (middle and high emission, and extreme scenarios) would have limited effect. Therefore, resistance is assessed as ‘High’ under all three scenarios, so that resilience is ‘High’ and sensitivity assessed as ‘Not sensitive’. 

Sea level rise (middle)

Middle emission scenario benchmark: a 50 cm rise in average UK sea-level by the end of this century (2081-2100).

Evidence

Sea-level rise is occurring through a combination of thermal expansion and ice melt.  Sea levels have risen 1-3 mm/yr. in the last century (Cazenave & Nerem, 2004, Church et al., 2004, Church & White, 2006). Evidence appears to suggest that impacts of sea-level rise on exposure or tidal energy will be non-linear and site-specific (Pickering et al., 2012, Li et al., 2016).  This biotope occurs on mixed sediment, in moderately exposed to sheltered areas, subject to strong to weak tidal streams and, therefore, should be reasonably robust to any changes which occur. Furthermore, this biotope occurs at depths of 10-50 m around the UK, and sea-level rises predicted for the end of this century should have limited impacts on this biotope.

Sensitivity assessment. As this biotope CR.MCR.EcCr.CarSp.PenPcom can occur from 10-50 m depth, in a range of different energy environments, it is assumed that a sea-level rise of 50 cm, 70 cm or 107 cm (middle and high emission, and extreme scenarios) would have limited effect. Therefore, resistance is assessed as ‘High’ under all three scenarios, so that resilience is ‘High’ and sensitivity assessed as ‘Not sensitive’. 

Temperature increase (local)

Benchmark. A 5°C increase in temperature for one month, or 2°C for one year (Temperature change pressure definition).

Evidence

CR.MCR.EcCr.CarSp.PenPcom is distributed across the west coasts of Scotland, Ireland, and England. Sea surface temperature across this distribution ranges from northern to southern Sea Surface Temperature (SST) of 8 to 16°C in summer and 6 to 13°C in winter (Beszczynska-Möller & Dye, 2013). 

Alcyonium digitatum is described as a northern species by Hiscock et al. (2004) but is distributed from Northern Norway (70°N) to Portugal (41°N) in the North East Atlantic (Hartnoll, 1975; Budd, 2008; OBIS, 2026). Temperature at the seafloor is an important predictor of Alcyonium digitatum, yet there are no published studies that explore the thermal minima or maxima for the species, so the only current evidence that gives insight into the thermal niches of this species is the sea temperatures at the locations where they are observed (Jenkins & Stevens, 2022). Currently, Alcyonium digitatum is commonly found in inshore and offshore areas of northwest and northern France, the Channel Islands, most of the British Isles, including the Shetland Islands, parts of the southern North Sea, parts of southern Norway, and along the Atlantic slope (Jenkins & Stevens, 2022). Alcyonium digitatum can be found alongside Eunicella verrucosa, and in Britain and Ireland, Eunicella verrucosa has been observed in waters where 9.2°C was the lowest average seafloor temperature, the median temperature was 10.5°C, and the highest temperature was 11.4°C (Jenkins & Stevens, 2022). While trying to understand the impact of climate change on Alcyonium digitatum distribution, Jenkins & Stevens (2022) noted that there will likely be a shift north as more suitable habitat becomes available in higher latitudes. However, suitability predictions in the southern portion of their study area decreased.

Caryophyllia smithii is found across the British Isles (NBN, 2015; Coolen et al., 2015) and has been recorded in Greece (Koukouras, 2010). In the Mediterranean, Caryophyllia smithii has been recorded in seawater between 13 and 14°C (Rodolfo‐Metalpa et al., 2015). It is therefore unlikely to be significantly affected by an increase at the benchmark level. However, Tranter et al. (1982) suggested Caryophyllia smithii reproduction was cued by seasonal increases in seawater temperature. Therefore, unseasonal increases in temperature may disrupt natural reproductive processes and negatively influence recruitment patterns. Holt (pers. comm.) also suggested that long-term increases in temperature due to climate change may allow the parasitic barnacle Adna anglica to extend its range northwards and overlap the range of this biotope. Adna anglica is a southern species, limited to the southwest of Britain, where it parasitizes Caryophillia and has probably contributed to the decrease in abundance of Leptopsammia (Holt pers. comm.). It may impact the abundance of Caryophyllia if climate change allows it to extend its range northwards (Holt pers. comm.).

The bryozoan Porella compressa is regarded as a Boreal-Arctic species (Hayward & Ryland 1999), although it is distributed from the Faroe Islands to southwest of the Iberian Peninsula (Reverter-Gil et al. 2014 cited in Souto & Reverter-Gil, 2019), where it is found between 16 and 300 m deep (Souto & Reverter-Gil, 2019). In Franz Josef Land (northwest Barents Sea), Porella compressa is subjected to a constant influx of cold Arctic water. The lowest air temperatures are recorded in February and March, with temperatures as low as −50°C, while the warmest months are July and August, with temperatures reaching up to 13°C; water temperature in the region ranges from −1.9°C in the winter to 4.5°C in exceptional cases in the summer (Dzhenyuk, 2014 cited in Evseeva & Dvoretsky, 2024). During their observations in Franz Josef Land, Evseeva & Dvoretsky (2024) noted that between the three sites observed, the cumulative contribution of Porella compressa to the groups was between 25.32 and 36.39%. Evseeva & Dvoretsky (2024) state that the primary environmental drivers of bryozoan communities are depth and temperature (negative association) and percentage of stones and shells in the substratum (positive association). Denisenko & Grebmeier (2015), cited by Evseeva & Dvoretsky (2024), suggested that both boreal and Arctic species of bryozoans can survive in areas where near-bottom water temperatures fluctuate greatly and may warm above 4°C, but with limited reproductive capabilities. They found a positive correlation between bryozoan species richness and temperature in the Chukchi Sea, where the proportion of boreal species was two times higher (15.7%), and the proportion of Arctic species was two times lower (17.2%) than in Franz Josef Land.

Cocito & Sgorbini (2014) studied spatial and temporal patterns of colonial bryozoans in the Ligurian Sea over nine years. High temperature events caused mass mortality among a number of species. The decline in Pentapora fascialis colony cover between 11 and 22 m depth followed the unusually warm summer in 1999 (temperature at 11 m of 23.87 ± 1.4 °C), with an 86% reduction in live colony portion and the larger colonies were most affected. Gradual recovery took place, with deeper communities recovering to pre-disturbance levels within four years. Pagès-Escolà et al. (2020) studied the restoration of the bryozoan Pentapora foliacea and observed that the highest growth values for this species occurred during the cold months of the year (October to March), whereas in the warmer season, the growth rates decreased. In addition, necrosis was highest during the warmer months, illustrating the vulnerability of Pentapora foliacea to high temperatures, with high necrotic proportions occurring in the largest colonies.

There is little information available about the tolerance of the characterizing sponges in this biotope. However, Kazanidis et al. (2019) reported how water temperature and salinity explained a significant amount of variation in sponge density, and in the North Atlantic, the highest density values were found where temperature and salinity ranged from 6.52 to 8.98°C and 34.91 to 35.13 psu, respectively. In Lough Hyne, a small (approx. 0.5 km²) lough on the southwest coast of Ireland, sponge communities are observed in waters where temperatures range from 7 to 9°C in winter and 14 to 18°C in summer (Micaroni et al., 2025). Long-term temperature increases may cause extension of the British Isles sponge populations, and decreases in temperature may result in population shrinkage. Goodwin et al. (2013) noted increases in the abundance of Axinella damicornis and Axinella dissimilis in Northern Ireland over a 20-year period and suggested the increase was due to sea temperature warming (relating to a 0.3 to 0.5°C increase in Northern Irish sea surface temperature between 1850 and 2007).

The sponge Pachymatisma johnstonia is one of the most common and well-known sponges throughout the North East Atlantic coasts, recorded from the Orkneys to Spain and is ubiquitous across the western and southern coasts of Britain, and it is mainly found in the littoral and sublittoral zones (Ackers et al., 1992; Schiavo et al., 2024).

The sponge Cliona celata is found throughout the Atlantic between 1 and 200 m deep and has been recorded from Sweden to the Mediterranean in Europe, across the southeastern USA in North America, and off Argentina in South America (Ackers et al., 1992; Stubler et al., 2024; Novarin et al., 2025). Cliona celata have been recorded in waters with a temperature range of 23.19 to 35.11°C off southeastern USA (Stubler et al., 2024), and in a similar boring sponge, Pione truitti, growth was documented to decrease as temperatures dropped below 20°C (Pomponi & Meritt, 1985 cited in Stubler et al., 2024). One way in which sponges respond to increases in temperature is with higher respiration rates, and Cliona celata has been recorded as having a significant change in respiration rate as a stress response (Wood et al., 2025). However, Cliona celata have been reported to be resistant to temperatures up to 4 to 5°C higher than ambient temperatures (Bosch-Belmar et al., 2024). Duckworth & Peters (2013) demonstrated that an increase in water temperature to 26 and 31°C had no detectable effect on the boring activity of Cliona celata. Whereas cold winter temperatures (<5°C) can cause boring activity to cease, the incurrent papillae to withdraw and the excurrent papillae constrict (Fell et al., 1984; Carver et al., 2010), which indicate that cold temperatures are more limiting to Cliona celata than hot temperatures. Piscitelli et al. (2011) observed an annual peak in reproductive activity in April to May from individuals in the Mediterranean, and suggested this was a result of a sharp seasonal increase in water temperature, which suggests variable temperatures could affect larval recruitment processes, but not otherwise negatively affect Cliona celata.

The effect of temperature on the sponge Haliclona viscosa was tested on samples from Moorea, French Polynesia, between the 22^nd of September and the 15^th of November 2023. Evans (2024) collected Haliclona sp. at a depth of 1.1 m in waters where average ocean temperature was 28.4°C, after which samples were placed into temperature tanks where the average water bath temperatures were 26.9°C, 28.9°C, and 31.6°C. Although the Haliclona sp. displayed cytotoxic properties (damage, death, or disruption to living cells) in totality (becoming discoloured and malodorous at 31.6°C), temperature did not have a statistically significant effect on toxicity (not primarily affecting the regulation of secondary metabolites present in the extracts, which would have, in turn, killed brine shrimp at three statistically distinct rates) and failed to cause upregulation, as hypothesized (Evans, 2024). Despite a surprising finding, Evans (2024) concluded that warming water temperatures do not seem to damage Haliclona sponges’ ability to produce chemical defences and did not ultimately cause differences between the antimicrobial or cytotoxic properties of different treatments. Marra (2019) also studied the biological mechanisms underlying the bioactivity detected in several Irish and Mediterranean sponge species currently placed into the genus Haliclona. As part of the study, specimens of Haliclona sp. were collected for lab analysis from multiple areas with differing temperatures, including: the coasts of Ireland (14.8 to 15.6°C in August and September 2014 and 2015), the UK (10°C in May 2014), and France (14.7°C in April 2015).

Berman et al. (2013) monitored sponge communities off Skomer Island, UK, over four years, with all characterizing sponges for this biotope assessed. Seawater temperature, turbidity, photosynthetically active radiation, and wind speed were all recorded during the study. They concluded that, despite changes in species composition, primarily driven by the non-characterizing Hymeraphia stellifera and Halicnemia patera, no significant difference in sponge density was recorded in all sites studied. Morphological changes most strongly correlated with a mixture of water visibility and temperature. Research by Webster et al. (2008, 2011), Webster & Taylor (2012) and Preston & Burton (2015) suggested that many sponges rely on a holobiont of many synergistic microbes. Webster et al. (2011) described a much higher thermal tolerance of sponge larval holobiont when compared with adult sponges. 

Bishop (1985) suggested that Echinus esculentus cannot tolerate high temperatures for prolonged periods due to increased respiration rate and resultant metabolic stress. Ursin (1960) reported that Echinus esculentus occurred at temperatures between 0 and 18°C in Limfjord, Denmark. Bishop (1985) noted that gametogenesis occurred at 11 to 19°C, however, continued exposure to 19°C disrupted gametogenesis. Embryos and larvae developed abnormally after 24 hr exposure to 15°C but normally at 4, 7 and 11°C (Tyler & Young 1998). In addition, maximum spawning occurs in spring, although individuals may spawn over a protracted period throughout the year. Gonad weight is at its maximum in February/March in the English Channel (Comely & Ansell, 1988; Hamed et al., 2024) but decreases during spawning in spring and then increases again through summer and winter until the next spawning season.

Sensitivity assessment

The characterizing species are widely distributed across the British Isles. Morphological changes were observed in sponge communities, with temperature as a factor, but the characterizing sponges assessed display a level of tolerance to increasing temperature (Bosch-Belmar et al., 2024; Evans, 2024). An increase in sea surface temperature of 2°C for a period of one year, combined with high temperatures, may approach the upper temperature threshold of Alcyonium digitatum and Echinus esculentus, and may, therefore, cause minor declines in abundance (Bishop, 1985; Jenkins & Stevens, 2022). Biotopes in the North of the UK are unlikely to be affected at the benchmark level. Therefore, resistance has been assessed as ‘Medium’, and resilience has been assessed as ‘High’. Sensitivity has been assessed as ‘Low’.

Temperature decrease (local)

Benchmark. A 5°C decrease in temperature for one month, or 2°C for one year (Temperature change pressure definition).

Evidence

CR.MCR.EcCr.CarSp.PenPcom is distributed across the west coasts of Scotland, Ireland, and England. Sea surface temperature across this distribution ranges from northern to southern Sea Surface Temperature (SST) of 8 to 16°C in summer and 6 to 13°C in winter (Beszczynska-Möller & Dye, 2013). 

Alcyonium digitatum is described as a northern species by Hiscock et al. (2004) but is distributed from Northern Norway (70°N) to Portugal (41°N) in the North East Atlantic (Hartnoll, 1975; Budd, 2008; OBIS, 2026). Across this latitudinal gradient, both species are likely to experience a range of temperatures from approximately 5 to 18°C. Alcyonium digitatum was also reported to be apparently unaffected by the severe winter of 1962 to 1963, when air temperature reached -5.8°C (Crisp, 1964). Temperature at the seafloor is an important predictor of Alcyonium digitatum, yet there are no published studies that explore the thermal minima or maxima for the species, so the only current evidence that gives insight into the thermal niches of this species is the sea temperatures at the locations where they are observed (Jenkins & Stevens, 2022). Currently, Alcyonium digitatum is commonly found in inshore and offshore areas of northwest and northern France, the Channel Islands, most of the British Isles, including the Shetland Islands, parts of the southern North Sea, parts of southern Norway, and along the Atlantic slope (Jenkins & Stevens, 2022). Alcyonium digitatum can be found alongside Eunicella verrucosa, and in Britain and Ireland, Eunicella verrucosa has been observed in waters where 9.2°C was the lowest average seafloor temperature, the median temperature was 10.5°C, and the highest temperature was 11.4°C (Jenkins & Stevens, 2022).

Caryophyllia smithii is a southern species (Fish & Fish, 1996) with a northern range limit in the Shetland Isles (NBN, 2015). It is therefore likely to be close to its northerly range limit and therefore likely to be negatively affected by a decrease in temperature at the benchmark level in northern examples of the biotope.

The bryozoan Porella compressa is regarded as a Boreal-Arctic species (Hayward & Ryland 1999), although it is distributed from the Faroe Islands to southwest of the Iberian Peninsula (Reverter-Gil et al. 2014 cited in Souto & Reverter-Gil, 2019), where it is found between 16 and 300 m deep (Souto & Reverter-Gil, 2019). In Franz Josef Land (northwest Barents Sea), Porella compressa is subjected to a constant influx of cold Arctic water. The lowest air temperatures are recorded in February and March, with temperatures as low as −50°C, while the warmest months are July and August, with temperatures reaching up to 13°C; water temperature in the region ranges from −1.9°C in the winter to 4.5°C in exceptional cases in the summer (Dzhenyuk, 2014 cited in Evseeva & Dvoretsky, 2024). During their observations in Franz Josef Land, Evseeva & Dvoretsky (2024) noted that between the three sites observed, the cumulative contribution of Porella compressa to the groups was between 25.32 and 36.39%. Evseeva & Dvoretsky (2024) state that the primary environmental drivers of bryozoan communities are depth and temperature (negative association) and percentage of stones and shells in the substratum (positive association). Denisenko & Grebmeier (2015), cited by Evseeva & Dvoretsky (2024), suggested that both boreal and Arctic species of bryozoans can survive in areas where near-bottom water temperatures fluctuate greatly and may warm above 4°C, but with limited reproductive capabilities. They found a positive correlation between bryozoan species richness and temperature in the Chukchi Sea, where the proportion of boreal species was two times higher (15.7%), and the proportion of Arctic species was two times lower (17.2%) than in Franz Josef Land.

Pentapora foliacea is found as far north as the Minch off western Scotland (Lombardi et al., 2010). Patzold et al. (1987) recorded the formation of growth bands in Pentapora foliacea during times of reduced reproduction, which appeared during periods of colder water temperatures. Once established, colonies are most likely able to withstand occasional lower or higher than normal temperatures, but long-term decreases in temperature may cause the distribution range to shrink. Pagès-Escolà et al. (2020) studied the restoration of the bryozoan Pentapora foliacea and observed that the highest growth values for this species were observed during the cold months of the year (October to March), whereas in the warmer season, the growth rates decreased.

There is little information available about the tolerance of the characterizing sponges in this biotope. However, Kazanidis et al. (2019) reported how water temperature and salinity explained a significant amount of variation in sponge density, and in the North Atlantic, the highest density values were found where temperature and salinity ranged from 6.52 to 8.98°C and 34.91 to 35.13 psu, respectively. In Lough Hyne, a small (approx. 0.5 km²) lough on the southwest coast of Ireland, sponge communities are observed in waters where temperatures range from 7 to 9°C in winter and 14 to 18 °C in summer (Micaroni et al., 2025). Long-term temperature increases may cause extension of the British Isles sponge populations, and decreases in temperature may result in population shrinkage. Goodwin et al. (2013) noted increases in the abundance of Axinella damicornis and Axinella dissimilis in Northern Ireland over a 20-year period and suggested the increase was due to sea temperature warming (relating to a 0.3 to 0.5°C increase in Northern Irish sea surface temperature between 1850 and 2007). 

The sponge Pachymatisma johnstonia is one of the most common and well-known sponges throughout the North East Atlantic coasts, recorded from the Orkneys to Spain and is ubiquitous across the western and southern coasts of Britain, and it is mainly found in the littoral and sublittoral zones (Ackers et al., 1992; Schiavo et al., 2024).

The sponge Cliona celata is found throughout the Atlantic between 1 and 200 m deep and has been recorded from Sweden to the Mediterranean in Europe, across the southeastern USA in North America, and off Argentina in South America (Ackers et al., 1992; Stubler et al., 2024; Novarin et al., 2025). Cliona celata have been recorded in waters with a temperature range of 23.19 to 35.11°C off southeastern USA (Stubler et al., 2024), and in a similar boring sponge, Pione truitti, growth was documented to decrease as temperatures dropped below 20°C (Pomponi & Meritt, 1985 cited in Stubler et al., 2024). Duckworth & Peters (2013) demonstrated that an increase in water temperature to 26 and 31°C had no detectable effect on the boring activity of Cliona celata. Whereas cold winter temperatures (<5°C) can cause boring activity to cease, the incurrent papillae to withdraw and the excurrent papillae constrict (Fell et al., 1984; Carver et al., 2010), which indicate that cold temperatures are more limiting to Cliona celata than hot temperatures.

Marra (2019) studied the biological mechanisms underlying the bioactivity detected in several Irish and Mediterranean sponge species currently placed into the genus Haliclona. As part of the study, specimens of Haliclona sp. were collected for lab analysis from multiple areas with differing temperatures, including: the coasts of Ireland (14.8 to 15.6°C in August and September 2014 and 2015), the UK (10°C in May 2014), and France (14.7°C in April 2015).

Berman et al. (2013) monitored sponge communities off Skomer Island, UK, over four years, with all characterizing sponges for this biotope assessed. Seawater temperature, turbidity, photosynthetically active radiation, and wind speed were all recorded during the study. They concluded that, despite changes in species composition, primarily driven by the non-characterizing Hymeraphia stellifera and Halicnemia patera, no significant difference in sponge density was recorded in all sites studied. Morphological changes most strongly correlated with a mixture of water visibility and temperature. Research by Webster et al. (2008, 2011), Webster & Taylor (2012) and Preston & Burton (2015) suggested that many sponges rely on a holobiont of many synergistic microbes. Webster et al. (2011) described a much higher thermal tolerance of sponge larval holobiont when compared with adult sponges. 

Ursin (1960) reported that Echinus esculentus occurred at temperatures between 0 and 18°C in Limfjord, Denmark. Bishop (1985) noted that gametogenesis occurred at 11 to 19°C, however, continued exposure to 19°C disrupted gametogenesis. Embryos and larvae developed abnormally after 24 hr exposure to 15°C but normally at 4, 7 and 11°C (Tyler & Young 1998). Echinus esculentus has been recorded from the Murmansk Coast, Russia. Due to the high latitude at which Echinus esculentus can occur, it is unlikely to be affected by a decrease in temperature at the pressure benchmark. In addition, maximum spawning occurs in spring, although individuals may spawn over a protracted period throughout the year. Gonad weight is at its maximum in February/March in the English Channel (Comely & Ansell, 1988; Hamed et al., 2024) but decreases during spawning in spring and then increases again through summer and winter until the next spawning season.

Sensitivity assessment

Alcyonium digitatum, Porella compressa, and Echinus esculentus have northern/boreal distributions and are unlikely to be affected at the benchmark level. Long-term decreases in temperature may cause population shrinkage of the British Isles distributions of sponges (Berman et al., 2013). Apparent shrinkage of individual Axinella dissimilis (negative growth rate) was observed in Lundy in some years and was attributed to particularly cold winters, notably between 1985 and 1986 (Hiscock, 1993). However, limited information on the characterizing sponges was found, but they are widely distributed across the British Isles. Pentapora foliacea and Caryophyllia smithii could experience reduced growth rates in colder temperatures, but are likely to be less affected by short-term decreases in temperature. In addition, the depth of the biotope probably protects it from short-term acute decreases in temperature. The important characterizing Caryophyllia smithii is close to its northern distribution limit within the British Isles, and a decrease at the benchmark level may result in some mortality in northern examples of the biotope. Therefore, resistance is ‘Medium’, resilience is ‘High’, and sensitivity is ‘Low’.

Salinity increase (local)

Benchmark. An increase in one MNCR salinity category above the usual range of the biotope or habitat (Salinity regime change pressure definition).

Evidence

Alcyonium digitatum’s distribution and the depth at which it occurs also suggest it would not likely experience regular salinity fluctuations and therefore would not tolerate significant increases in salinity.

Ryland (1970) stated that, with a few exceptions, the Gymnolaemata bryozoans were fairly stenohaline and restricted to full salinity (30 to 35 ppt), noting that reduced salinities result in an impoverished bryozoan fauna. For example, in Franz Josef Land (northwest Barents Sea), Porella compressa is subjected to a constant influx of cold Arctic water where salinity ranges from <33 to >35 psu (Evseeva & Dvoretsky, 2024).

Kazanidis et al. (2019) reported how water temperature and salinity explained a significant amount of variation in sponge density, and in the North Atlantic, the highest density values were found where temperature and salinity ranged from 6.52 to 8.98°C and 34.91 to 35.13 psu, respectively. 

Cliona celata is tolerant of higher salinities and is found in more polyhaline (e.g. >20 psu) parts of waterbodies, such as the Chesapeake Bay, USA, where it was differentially distributed along the Bay’s salinity gradient, peaking in the mid-salinity sites (spanning from mesohaline to low polyhaline, approx. 5 to 22 psu) (Anchondo et al., 2024). However, when Choptank in Chesapeake Bay experienced a high-salinity event pre-2017, clionid populations apparently thrived but then subsequently decreased over the next two years until 2019 due to a freshet (the flood of a river from heavy rain or melted snow) (Anchondo et al., 2024). Marin (1998) describes the presence of Dysidea fragilis in a hypersaline coastal lagoon (at 42 to47 g/l; 42 to 47 ppt) in La Mar Menor, Spain. 

Echinoderms are generally stenohaline and possess no osmoregulatory organ (Boolootian, 1966; Russell, 2013). Therefore, an increase in salinity may cause Echinus esculentus mortality.

Sensitivity assessment

CR.MCR.EcCr.CarSp.PenPcom is restricted to full salinity (Connor et al., 2004; JNCC, 2022), it therefore seems likely that an increase in salinity from ‘full’ to hypersaline (>40‰) at the benchmark level may cause a decline in the abundance of characterizing species. Furthermore, a reduction in Echinus esculentus may cause an increase in faunal turf and red algae growth, which would change the character of the biotope. Therefore, resistance has been assessed as ‘Low’, resilience as ‘Medium’ and sensitivity as ‘Medium’. Due to the lack of information regarding salinity effects on Alcyonium digitatum, Caryophyllia smithii, Pentapora foliacea, and Echinus esculentus, confidence in this assessment has been assessed as Low.

Salinity decrease (local)

Benchmark. A decrease in one MNCR salinity category above the usual range of the biotope or habitat (Salinity regime change pressure definition detail).

Evidence

Alcyonium digitatum inhabits situations such as the entrances to sea lochs (Budd, 2008) or the entrances to estuaries (Braber & Borghouts, 1977) where salinity may vary occasionally. However, its distribution and the depth at which it occurs suggest that Alcyonium digitatum would not likely often experience salinity fluctuations and therefore is unlikely to survive significant reductions in salinity (Budd, 2008).

Ryland (1970) stated that, with a few exceptions, the Gymnolaemata bryozoans were fairly stenohaline and restricted to full salinity (30 to 35 ppt), noting that reduced salinities result in an impoverished bryozoan fauna. For example, in Franz Josef Land (northwest Barents Sea), Porella compressa is subjected to a constant influx of cold Arctic water where salinity ranges from <33 to >35 psu (Evseeva & Dvoretsky, 2024).

Kazanidis et al. (2019) reported how water temperature and salinity explained a significant amount of variation in sponge density, and in the North Atlantic, the highest density values were found where temperature and salinity ranged from 6.52 to 8.98°C and 34.91 to 35.13 psu, respectively. 

Castric-Fey & Chassé (1991) conducted a factorial analysis of the subtidal rocky ecology near Brest, France and rated the distribution of species from estuarine to offshore conditions. Dysidea fragilis and Raspailia ramosa were rated as indifferent to this range. Cliona celata and Pachymatisma johnstonia had a slight preference for more estuarine conditions. Mean salinity difference between the two farthest zones was low (35.1 and 33.8‰, respectively), but with a greater range being experienced in the Inner Rade (±2.4‰ compared with ±0.1‰). It should be noted that the range of salinities identified in this study does not reach the lower benchmark level, and at least some of the characterizing sponges are likely to be affected at the benchmark level. 

Carver et al. (2010) suggested Cliona celata can function efficiently in salinities as low as 20‰ and can withstand exposure to 15‰ for brief periods, but prevailing salinities less than 10 to 15‰ are likely to be lethal (Hartman 1958, Hopkins 1962). Cliona celata is found in more polyhaline (e.g. >20 psu) parts of waterbodies, such as the Chesapeake Bay, USA, where it was differentially distributed along the Bay’s salinity gradient, peaking in the mid-salinity sites (spanning from mesohaline to low polyhaline, approx. 5 to 22 psu) (Anchondo et al., 2024). However, when Choptank in Chesapeake Bay experienced a high-salinity event pre-2017, clionid populations apparently thrived but then subsequently decreased over the next two years until 2019 due to a freshet (the flood of a river from heavy rain or melted snow) (Anchondo et al., 2024).

Echinoderms are generally unable to tolerate low salinity (stenohaline) and possess no osmoregulatory organ (Boolootian, 1966; Russell, 2013). At low salinity, urchins gain weight, and the epidermis loses its pigment as patches are destroyed; prolonged exposure is fatal. However, within Echinus esculentus, there is some evidence to suggest intracellular regulation of osmotic pressure due to increased amino acid concentrations. Barrett et al. (2024) studied the effect of low salinity on Echinus esculentus. Experiments were conducted on populations of Echinus esculentus originally collected from Loch Linnhe, Scotland, and were exposed to low salinity over the short term (11‰, 16, 21, 26 and 31‰ for 24 h) and long term (21, 26 and 31‰ for 25 days). Over the short term, oxygen consumption, activity coefficient and coelomic fluid osmolality were directly correlated with reduced salinity, with 100% survival at ≥21‰ and 0% at ≤16‰ (Barrett et al., 2024). Over the long term at 21‰ (25 days), oxygen consumption was significantly higher, feeding was significantly reduced, and activity coefficient values were significantly lower than at control salinity (31‰) (Barrett et al., 2024). Furthermore, beneficial functional resistance (righting ability and metabolic capacity) to acute low salinity was observed at 26‰. Barrett et al. (2024) concluded that Echinus esculentus demonstrated phenotypic plasticity that enabled acclimation to reduced salinity around 26‰. However, 21‰ represented a lower acclimation threshold, potentially limiting its distribution in coastal areas prone to high freshwater input. In addition, Echinus esculentus is recovered within several variable and reduced salinity biotopes, e.g. IR.LIR.KVS. (Connor et al., 2004; JNCC, 2022).

Sensitivity review

CR.MCR.EcCr.CarSp.PenPcom is recorded exclusively in full marine conditions (30 to 40 ‰) (Connor et al., 2004; JNCC, 2022). The lack of records within “Reduced” salinity (18 to 30‰) suggests the community would not persist/be recognisable if salinity was reduced.  Records from the MNCR suggest Alcyonium digitatum, Caryophyllia smithii, and Echinus esculentus can occur in reduced salinity habitats. However, the evidence suggests that these species would decrease in abundance. Some of the characterizing sponges and bryozoans are likely to be adversely affected by a reduction in salinity. Therefore, resistance has been assessed as ‘Low’, resilience as ‘Medium’, and sensitivity assessed as ‘Medium’.

Water flow (tidal current) changes (local)

Benchmark. A change in peak mean spring bed flow velocity of between 0.1 m/s and 0.2 m/s for more than one year (Water flow pressure definition). 

Evidence

Alcyonium digitatum, Caryophyllia smithii, sponges, bryozoans and hydroids are suspension feeders, relying on water currents to supply food (Hiscock, 1983; O’Reilly et al., 2022). These taxa therefore thrive in conditions of vigorous water flow, e.g. around Orkney and St Abbs, Scotland, where Alcyonium digitatum-dominated biotopes may experience tidal currents of 3 and 4 knots (approximately 1.5 m/sec) during spring tides (De Kluijver, 1993; Coolen et al., 2015). 

In flume experiments, Hiscock (1983) noted that the tentacles of Caryophyllia smithii were displaced by currents over ca 0.5 m/s but withdrawn at 0.75 m/s and took several hours to re-emerge after cessation of strong flow. Hiscock (1983) noted that Caryophyllia smithii was most abundant in semi-exposed and sheltered habitats. Other cup corals (Balanophyllia regia, Hoplangia durotrix and Leptopsammia pruvoti) were recorded from weak and very weak tidal streams in cave and overhang biotopes (Connor et al., 2004). 

The life cycle of Caryophyllia smithii includes a larval planktotrophic stage with a duration of 8 to 10 weeks, and during this time, the released larvae float freely in the water column and are transported in the direction of net water movement, which is driven by tidal currents and wind. These residual currents in the North Sea, UK, range between 0.02 and 0.08 cm/s (Coolen et al., 2015). Caryophyllia smithii is described as favouring sites with a high tidal flow (Bell & Turner, 2000; Wood, 2005; Coolen et al., 2015). Caryophyllia smithii has been recorded in biotopes from negligible to strong water flow (0 to 6 knots; 0 to >3 m/s) (Connor et al., 2004). Rodolfo‐Metalpa et al. (2015) noted that Caryophyllia smithii was recorded in waters with a tidal current of 24 cm/s (±15) off the coast of Italy. This biotope consists mainly of species firmly attached to the substratum, which would be unlikely to be displaced by an increase in the strength of tidal streams at the benchmark level.

Water flow has been shown to be important for the development of bryozoan communities and the provision of suitable hard substrata for colonization (Eggleston, 1972b; Ryland, 1976). In addition, areas subject to the high mass transport of water, such as the Menai Strait and tidal rapids, generally support large numbers of bryozoan species (Moore, 1977). Although active suspension feeders, their feeding currents are probably fairly localised, and they are dependent on water flow to bring adequate food supplies within reach (McKinney, 1986). For example, in Franz Josef Land (northwest Barents Sea), Porella compressa is subjected to a constant influx of cold Arctic water where the maximum current velocity is 75 cm/s (Evseeva & Dvoretsky, 2024). A substantial decrease in water flow will probably result in impaired growth due to a reduction in food availability, and an increased risk of siltation (Tyler-Walters, 2005c).

Riisgard et al. (1993) discussed the low energy cost of filtration for sponges and concluded that passive current-induced filtration may be insignificant for sponges. Pumping and filtering occur in choanocyte cells, which generate water currents in sponges using flagella (de Vos et al., 1991). The sponges Pachymatisma johnstonia and Dysidea fragilis have been recorded in biotopes from very weak to very strong water flow (0 to >3 m/s). Some studies have also stated how massive and encrusting sponge morphotypes are more abundant at high-flow areas in the sublittoral zone, due to a high basal area-to-volume ratio, which decreases removal from the substrata (Kazanidis et al., 2019). In northern Norway, large-growth sponges, Phakellia ventilabrum and Axinella infundibuliformis, were primarily observed at sites with a relatively slower horizontal current velocity (0.02 to 0.03 m/s) (Dunlop et al., 2020). 

In Lough Hyne, a small (approx. 0.5 km²) lough on the southwest coast of Ireland, sponge communities experience currents reaching >300 cm/s (>3 m/s), and greater sponge growth/recovery was observed at sites with greater water movement, possibly due to sponges using ambient currents to reduce the high energy costs associated with their filtration activity (Micaroni et al., 2025). Furthermore, the dispersal ability of sponge larvae is generally low, which can slow the recolonization rates in areas, and it is possible that very slow current speeds could reduce the larvae supply to sites (Micaroni et al., 2025). However, this biotope occurs in wave-exposed conditions, and although ameliorated by depth, wave action might be a more important source of water movement than tidal streams. Bell (2002) documented Cliona celata regeneration, following artificial damage, at two sites within Lough Hyne. The study demonstrated that 80% of Cliona celata were fully regenerated at the site which received high water flow (2 m/sec) within 100 days, whereas no Cliona celata were found in slower water velocities (0.1 m/sec) had fully regenerated. The author suggested an increase in water flow may increase food availability and therefore be of benefit to Cliona celata, whereas slow water flow (0.1 m/sec) may limit food supply and therefore slow regeneration if damaged.

Echinus esculentus is recorded in kelp beds on the west coast of Scotland in currents of about 0.5 m/sec. Outside the beds, specimens were occasionally seen being rolled by the current (Comely & Ansell, 1988), which may have been up to 1.4 m/sec. Urchins are removed from the stipe of kelps by wave and current action. Echinus esculentus are also displaced by storm action. After disturbance, Echinus esculentus migrates up the shore, an adaptation to being washed to deeper water by wave action (Lewis & Nichols, 1979). Therefore, increased water flow may remove the population from the affected area, probably to deeper water, although individuals would probably not be killed in the process and could recolonize the area quickly.

Sensitivity assessment

CR.MCR.EcCr.CarSp.PenPcom is recorded from moderately strong to negligible tidal currents (<1.5m/sec) (Connor et al., 2004; JNCC, 2022). The characteristic sponges are recorded in biotopes with both stronger and weaker tidal flow and are, therefore, unlikely to be affected by a change in water flow at the benchmark level. Echinus esculentus may become dislodged, but are unlikely to be killed and may recolonize quickly. Bryozoan communities rely on the movement of water for feeding, and a severe reduction in water flow over an extended period could cause mortality. The abundance of the characterizing species may be affected by a large-scale increase/decrease in water flow (e.g. >1m/sec). However, a change in tidal velocity of 0.1 to 0.2 m/s is not likely to have a significant effect. Therefore, resistance has been assessed as ‘High’, resilience as ‘High’, and sensitivity as ‘Not sensitive’.

Emergence regime changes

Benchmark.  1) A change in the time covered or not covered by the sea for a period of ≥1 year, or 2) an increase in relative sea level or decrease in high water level for ≥1 year. (Emergence regime change pressure definition).

Evidence

Changes in emergence are ‘Not relevant’ to this biotope as it is restricted to fully subtidal/circalittoral conditions - the pressure benchmark is relevant only to littoral and shallow sublittoral fringe biotopes.

Wave exposure changes (local)

Benchmark. A change in near shore significant wave height of >3% but <5% for more than one year (Wave action pressure definition). 

Evidence

Alcyonium digitatum, Caryophyllia smithii, bryozoans, hydroids and sponges are suspension feeders relying on water currents to supply food. These taxa, therefore, thrive in conditions of vigorous water flow. Jenkins & Stevens (2022) noted how seabed slope, temperature at the seafloor, and wave orbital velocity were important predictors of distribution in Alcyonium digitatum, and that, specifically, wave orbital velocity is more important than tidal velocity for bringing in fresh nutrients and oxygen, both for polyps to feed on and for exporting waste products.

Caryophyllia smithii has been recorded in very sheltered to extremely exposed biotopes (Connor et al., 2004; JNCC, 2015). Bell (2002) reported that Caryophyllia smithii near Lough Hyne (Ireland), exposed to strong wave action on open coasts, were relatively small, possibly due to juvenile morphological variability, as Caryophyllia smithii found deeper and in sediment were thinner and taller.

Pentapora foliacea is recorded as occurring in biotopes experiencing moderate to extreme wave exposure (Connor et al., 2004). However, extreme wave action (storms) has been noted to cause widespread destruction of colonies (Cocito et al., 1998a). Hiscock (2003) suggested that ‘prolonged Easterly gales in 1985’ might account for the loss of Axinella dissimilis specimens at Lundy. Nevertheless, following severe gales in 2013/2014, the abundance of sea fans, Pentapora fascialis, and associated branching sponges on stable rocky habitats appeared much as always (Hiscock, pers. comm.).

In northern Norway, large-growth sponges, such as Phakellia ventilabrum and Axinella infundibuliformis, have been observed to dominate sponge communities on wave-exposed circalittoral rock habitat (Dunlop et al., 2020). Similarly, in the Faroe-Shetland Channel, sponge abundance was noted to be highest in the region of internal wave activity at the seabed (Eerkes-Medrano et al., 2020). High sponge diversity and aggregations are likely found in areas of high-wave activity, such as near shelf breaks, due to the wave activity providing an abundant and stable food supply to them, with diversity and densities of sponges decreasing away from such areas (Santín et al., 2018). In contrast, Roberts et al. (2006) studied deep sponge reef communities (18 to 20 m) in sheltered and exposed locations in Australia. They reported greater diversity and cover (>40% cover) of sponges in wave-sheltered areas compared with a sparser and more temporal cover in exposed sites (25% cover).

Echinus esculentus occurred in kelp beds on the west coast of Scotland in currents of about 0.5 m/sec. Outside the beds, specimens were occasionally seen being rolled by the current (Comely & Ansell, 1988), which may have been up to 1.4 m/sec. Urchins are removed from the stipe of kelps by wave and current action. Echinus esculentus are also displaced by storm action. After disturbance, Echinus esculentus migrates up the shore, an adaptation to being washed to deeper water by wave action (Lewis & Nichols, 1979). Keith Hiscock (pers. comm.) reported that Echinus esculentus occurred in significant numbers as shallow as 15m below low water at the extremely wave-exposed site of Rockall, Scotland. Therefore, localised increases in wave height may remove the population from the affected area, probably to deeper water, although individuals would probably not be killed in the process and could recolonize the area quickly.

Sensitivity assessment

CR.MCR.EcCr.CarSp.PenPcom is recorded from the upper and vertical faces of extremely wave-exposed to moderately wave-exposed, moderately strong to weakly tide-swept sites, on circalittoral bedrock or boulders (Connor et al., 2004; JNCC, 2022). Wave action is a fundamental environmental variable controlling the biological community of sublittoral biotopes. A large and significant change in wave height may fundamentally alter the character of CR.MCR.EcCr.CarSp.PenPcom.

An increase in one wave exposure category is unlikely to be significant, since the biotope can occur in extremely exposed conditions. However, a reduction in wave exposure may transition moderately wave-exposed examples of the biotope to sheltered conditions and more sheltered conditions overall. This reduction may result in an increase in grazing pressure, especially from Echinus, resulting in a transition to the severely grazed CR.MCR.EcCr.FaAlCr biotope. Most of the characteristic species are long-lived, so unlikely to be lost during one year (the benchmark duration), but opportunistic species may be able to increase in abundance while the faunal turf species reduce in abundance due to grazing. Hence, resistance is assessed as Medium to reflect some changes in the biotope. Resilience is probably ‘High’, so sensitivity is assessed as ‘Low’. A prolonged decrease in wave exposure (greater than one year) would result in loss of the biotope.

Transition elements & organo-metal contamination

Benchmark. Exposure of marine species or habitat to one or more relevant Transitional metal or organometal (e.g. TBT) contaminants via uncontrolled releases or incidental spills (Transitional metals and organometals pressure definition).

Evidence

This pressure is Not assessed, but evidence is presented where available.

Little is known about the effects of heavy metals on echinoderms. Bryan (1984) reported that early work had shown that echinoderm larvae were sensitive to heavy metals contamination, for example, Migliaccio et al. (2014) reported that exposure of Paracentrotus lividis larvae to increased levels of cadmium and manganese caused abnormal larval development and skeletal malformations. Kinne (1984) reported developmental disturbances in Echinus esculentus exposed to waters containing 25 µg / l of copper (Cu).

No information was found on the direct biological effects of heavy metal contamination on Alcyonium digitatum. Possible sub-lethal effects of exposure to heavy metals, may result in a change in morphology, growth rate or disruption of the reproductive cycle. The vulnerability of this species to concentrations of pollutants may also depend on variations in other factors, e.g. temperature and salinity conditions outside the normal range.

Bryozoans are common members of the fouling community, and amongst those organisms most resistant to antifouling measures, such as copper-containing anti-fouling paints (Soule & Soule, 1979; Holt et al., 1995). Bryozoans were shown to bioaccumulate heavy metals to a certain extent (Holt et al., 1995). For example, Bowerbankia gracialis and Nolella pusilla accumulated Cd, exhibiting sublethal effects (reduced sexual reproduction and inhibited resting spore formation) between 10-100 µg Cd /l and fatality above 500 µg Cd/l (Kayser, 1990).

Hydrocarbon & PAH contamination

Benchmark. Exposure of marine species or habitat to one or more relevant hydrocarbon or polyaromatic hydrocarbon (PAH) contaminants via uncontrolled releases or incidental spills (Hydrocarbon & PAH pressure definition).

Evidence

This pressure is Not assessed, but evidence is presented where available.

Echinus esculentus was reported absent after the oil spill, however, it returned after 2-5 years. Large numbers of dead Echinus esculentus were found between 5.5 and 14.5 m in the vicinity of Sennen Cove, presumably due to a combination of wave exposure and heavy spraying of dispersants following the Torrey Canyon oil spill (Smith, 1968). Smith (1968) also demonstrated that 0.5 -1ppm of the detergent BP1002 resulted in developmental abnormalities in its echinopluteus larvae. Echinus esculentus populations in the vicinity of an oil terminal in La Coruna Bay, Spain, showed developmental abnormalities in the skeleton. The tissues contained high levels of aliphatic hydrocarbons, naphthalenes, pesticides and heavy metals (Zn, Hg, Cd, Pb, and Cu) (Gommez & Miguez-Rodriguez, 1999).

Oil pollution is mainly a surface phenomenon, so its impact upon circalittoral turf communities is likely to be limited. However, as in the case of the Prestige oil spill off the coast of France, high swell and winds can cause oil pollutants to mix with the seawater and potentially negatively affect sub-littoral habitats (Castège et al., 2014). Smith (1968) reported dead colonies of Alcyonium digitatum at a depth of 16m in the locality of Sennen Cove, Cornwall, which was likely a result of toxic detergents sprayed along the shoreline to disperse oil from the Torrey Canyon tanker spill (Budd, 2008). Little information on the effects of hydrocarbons on bryozoans could be found. Ryland & De Putron (1998) did not detect adverse effects of oil contamination on the bryozoan Alcyonidium spp. in Milford Haven or St. Catherine's Island, south Pembrokeshire, although it did alter the breeding period. Echinus esculentus is subtidal and unlikely to be directly exposed to oil spills. However, as with the ‘Prestige’ oil spill, rough seas can cause mixing with the oil and the seawater, and therefore sub-tidal habitats can be affected by the oil spill. Castège et al. (2014) recorded the recovery of rocky shore communities following the Prestige oil spill, which impacted the French Atlantic coast. Rough weather at the time of the spill increased mixing between the oil and seawater, causing sub-tidal communities/habitats to be affected.

Synthetic compound contamination

Benchmark. Exposure of marine species or habitat to one or more synthetic compound contaminants via uncontrolled releases or incidental spills (Synthetic compound contamination pressure definition).

Evidence

This pressure is Not assessed, but evidence is presented where available.

Smith (1968) reported dead colonies of Alcyonium digitatum at a depth of 16 m in the locality of Sennen Cove, Cornwall, resulting from the offshore spread and toxic effect of detergents (a mixture of a surfactant and an organic solvent). Possible sub-lethal effects of exposure to synthetic chemicals may result in a change in morphology, growth rate or disruption of the reproductive cycle. The vulnerability of this species to concentrations of pollutants may also depend on variations in other factors, e.g. temperature and salinity conditions outside the normal range (Budd, 2008).

Hoare & Hiscock (1974) suggested that polyzoa (bryozoa) were amongst the most intolerant species to acidified halogenated effluents in Amlwch Bay, Anglesey and reported that Flustra foliacea did not occur less than 165m from the effluent source. The evidence therefore suggests that Parasmittina trispinosa would be sensitive to synthetic compounds.

Radionuclide contamination

Benchmark. An increase in 10µGy/h above background levels (Radionuclides contamination pressure definition).

Evidence

'No evidence' was found.

Introduction of other substances

Benchmark. Exposure of marine species or habitat to one or more relevant "other" substances (solid, liquid or gas) contaminants via uncontrolled releases or incidental spills (Introduction of other substances pressure definition). 

Evidence

This pressure is Not assessed.

De-oxygenation

Benchmark. Exposure to dissolved oxygen concentration of less than or equal to 2 mg/l for one week (a change from WFD poor status to bad status) (deoxygenation pressure definition).

Evidence

In general, respiration in most marine invertebrates does not appear to be significantly affected until extremely low concentrations are reached. For many benthic invertebrates, this concentration is about 2 ml/l, or even less (Herreid, 1980; Rosenberg et al., 1991; Diaz & Rosenberg, 1995).

Alcyonium digitatum mainly inhabits environments in which the oxygen concentration usually exceeds 5 ml/l, and respiration is aerobic (Budd, 2008). In August 1978, a dense bloom of a dinoflagellate, Gyrodinium aureolum, occurred surrounding Geer Reef in Penzance Bay, Cornwall and persisted until September that year. Observations by local divers indicated a decrease in underwater visibility (<1 m) from below 8 m Below Sea Level. It was also noted that many of the faunal species appeared to be affected, e.g. no live Echinus esculentus were observed, whereas on surveys prior to August they were abundant; Alcyonium sp. and bryozoans were also in an impoverished state. Caryophyllia smithii were also in a contracted state, apparently dead. During follow-up surveys conducted in early September, Alcyonium sp. was noted to be much healthier and feeding. It was suggested that the decay of Gyrodinium aureolum either reduced oxygen levels or physically clogged faunal feeding mechanisms. Adjacent reefs were also surveyed during the same time period, and the effects of the Gyrodinium aureolum bloom were less apparent. It was suggested that higher water agitation in shallow water on reefs more exposed to wave action were less affected by the phytoplankton bloom (Griffiths, 1979).

Little information on the effects of oxygenation on bryozoans was found. Sagasti et al. (2000) reported that epifauna communities, including dominant species such as the bryozoans were unaffected by periods of moderate hypoxia (ca 0.35 to 1.4 ml/l; ca 0.5 to 2 mg/l) and short periods of hypoxia (<0.35 ml/l; ca <0.5 mg/l) in the York River, Chesapeake Bay, although bryozoans were more abundant in the area with generally higher oxygen. However, estuarine species are likely to be better adapted to periodic changes in oxygenation. 

Demosponges maintained under laboratory conditions can tolerate hypoxic conditions for brief periods. Gunda & Janapala (2009) investigated the effects of variable dissolved oxygen (DO) levels on the survival of the marine sponge, Haliclona pigmentifera. Under hypoxic conditions (1.5 to 2.0 ppm O₂; 1.5 to 2 mg/l), Haliclona pigmentifera with intact ectodermal layers and subtle oscula survived for 42 ± 3 days. Sponges with prominent oscula, foreign material, and damaged pinacoderm exhibited poor survival (of 1 to 9 days) under similar conditions. Complete mortality of the sponges occurred within two days under anoxic conditions (<0.3 ppm O₂; <0.3 mg/l). Schiaparelli et al. (2007) described the decline of the cup-coral Leptopsammia pruvoti by 85% following an anoxic event caused by decomposing mucilage, due to an unexpected bloom of the brown alga Acinetospora crinite because of a summer heatwave in 2003 in the Mediterranean.

Bell et al. (2024) studied the stability of shallow water sponges at Lough Hyne, Ireland, and concluded that changes to the deeper subtidal sponge assemblages were possibly driven by local processes associated with deeper water, potentially related to the seasonal oxythermocline that forms within Lough Hyne. This low-oxygen layer is thought to have a strong influence on the ecology and biology of organisms in the deeper areas of the lough, with a marked decline in the biodiversity of sponges and other organisms below approximately 25 m (Bell et al., 2024). However, explicit testing of Lough Hyne sponges’ oxygen tolerance found sponges to be resilient to short-term oxygen stress, with the focus now being on the presence of hydrogen sulphide as the main driver of change (Bell et al., 2024). Bell (2002) also reported that an oxycline at Lough Hyne (<5% surface concentration; ca 0.5 mg/l) limited vertical colonization by Caryophillia smithii.

Micaroni et al. (2022) examined oxygenation tolerance in Cliona celata and Suberites carnosus from Lough Hyne and Polymastia crocea and Suberites australiensis from New Zealand. The sponges were exposed to a total of five hypoxic treatments, with increasing severity (3.3, 1.6, 0.5, 0.4 and 0.13 mg O²/l) over seven days. They concluded that sponges were very tolerant of hypoxia, as all sponges survived the experiments, except Polymastia crocea, which experienced significant mortality at the lowest concentration of 0.13 mg O²/l (LT50 = 286 hours). Respiration rate in the sponges was not significantly affected down to 0.4 mg O²/l, except in Suberites carnosus. However, the species exhibited a range of physiological responses to hypoxia. Micaroni et al. (2022) concluded that sponges showed generally high tolerance to hypoxia compared with other sessile species.

Hiscock & Hoare (1975) reported an oxycline (between 1.5 and 78% oxygen saturation between 0 and 25 m) forming in the summer months (June to September) in a quarry lake (Abereiddy, Pembrokeshire) from close to full oxygen saturation at the surface to <5% saturation (ca 0.5 mg/l O²) below ca 10 m. No sponges, including Tethya aurantia, Hymeniacidon pereleve, Polymastia boletiformis, nor the cup coral Caryophyllia smithii, the hydroid Kirchenpaueria pinnata, or the sea squirt Ascidia mentula were recorded at depths below 10 to 11 m. Other example species observed included the tubeworms Protula tubular (14 m deep in late summer – 4% oxygen saturation) and Apomatus similis (16 m deep in early autumn – 3% oxygen saturation), and the bamboo worm Micromaldane ornithochaeta (20 m deep in spring – 1.5% oxygen saturation) (Hiscock & Hoare, 1975).

Sensitivity assessment

CR.MCR.EcCr.CarSp.PenPcom is recorded from moderately strong to negligible tidal streams (<0.5 m/sec) and in extremely to moderately wave-exposed sites (Connor et al., 2004; JNCC, 2022). Therefore, water movement (through wave action and/or tidal flow) could potentially cause mixing with surrounding oxygenated water and may therefore decrease the effects of de-oxygenation rapidly. The evidence suggests that sponges are very resistant to hypoxia for short periods (Gunda & Janapala, 2009; Micaroni et al., 2022; Bell et al., 2024) but may be excluded or die during prolonged hypoxic episodes due to seasonal oxythermoclines. Caryophyllia smithii was excluded below <5% saturation (ca 0.5 mg/l O²) in Lough Hyne (Bell, 2002) and Abereiddy (Hiscock & Hoare, 1975), but probably due to the seasonal oxythermocline rather than a short-term hypoxia.

However, Echinus esculenta, Caryophyllia smithii, Alcyonium sp. and bryozoans were adversely affected or killed by short-term (ca one month) hypoxia (un-quantified) due to an algal bloom (Griffiths et al., 1979). Therefore, resistance has been assessed as ‘Low’ as a precaution, albeit with ‘’Low’ confidence in the absence of quantified evidence. Hence, resilience is assessed as ‘Medium’, and sensitivity as ‘Medium’.

Nutrient enrichment

Benchmark. Increased levels of the elements nitrogen, phosphorus, silicon, and iron in the marine environment compared to background concentrations (Nutrient enrichment pressure definition).

Evidence

Alcyonium digitatum, Caryophyllia smithii, and sponges are suspension feeders. Nutrient enrichment of coastal waters that enhances the population of phytoplankton may be beneficial to Alcyonium digitatum and Caryophyllia smithii in terms of an increased food supply, but the effects are uncertain (Hartnoll, 1998). The survival of Alcyonium digitatum and Caryophyllia smithii may be influenced indirectly. High primary productivity in the water column combined with high summer temperatures and the development of thermal stratification (which prevents mixing of the water column) can lead to hypoxia of the bottom waters, which faunal species are likely to be highly intolerant of (see de-oxygenation pressure).

Hartikainen et al. (2009) reported that increased nutrient concentrations resulted in freshwater bryozoans achieving higher biomass. For example, O’Dea & Okamura (2000) found that the annual growth of Flustra foliacea in western Europe substantially increased since 1970. They suggested that this could be due to eutrophication in coastal regions due to organic pollution, leading to increased phytoplankton biomass (see Allen et al., 1998). 

Kazanidis et al. (2019) studied the nitrogen assimilation rates in the deep-sea sponge Spongosorites coralliophaga and cluster anemone Parazoanthus anguicomus. They found that Spongosorites coralliophaga preferentially assimilated particulate organic nitrogen over particulate organic carbon. This was not the case for Parazoanthus anguicomus. They suggested that the metabolic flexibility of Spongosorites coralliophaga played an important role in its survival under the food-limited conditions in the deep sea. Therefore, it seems that Spongosorites coralliophaga can cope with high levels of nitrogen. Kazanidis et al. (2019) reported that this species has been observed in concentrations of ammonium from 30, 100, and 200 μM (from depths of 2 to 15 m).

Gochfeld et al. (2012) studied the effect of nutrient enrichment (≤0.05 to 0.07 μM for nitrate and ≤0.5 μM for phosphate) as a potential stressor in the sponge Aplysina caulifornis and its bacterial symbionts and found that nutrient enrichment had no effects on sponge or symbiont physiology when compared to the control. This study does contradict findings in Gochfeld et al. (2007), in which Aplysina spp. sponges were virtually absent from a site of anthropogenic stress in Bocas del Toro, Panama, which experienced high rainfall and terrestrial runoff. The author suggested that whilst this site did include elevated nutrient concentrations, other pressures and stresses could be contributing.

Wood et al. (2025) exposed North East Atlantic sponges (Cliona celata, Stelligera stuposa, Axinella dissimilis, and Suberites carnosus) from Lough Hyne to nitrogenous fertiliser (4 to 6 mg/l NO₃-N (286 to 426 μM NO₃), and 16 to 19 mg/l NO₃-N (1143 to 1357 μM NO₃) for 13 days and SW Pacific Ocean sponges (Crella incrustans, Suberites australiensis, and Tethya bergquistae) for 10 days at 1 mg/l NO₃-N (71 μM NO₃), and 5 to 7 mg/l NO₃-N (357 to 500 μM NO₃) and found that sponges showed high survival rates (>95%) and only one species, Cliona celata, showed evidence of health effects. Cliona celata exhibited both a significant change in respiration rates, coupled with visible changes in surface colouration, but only at the highest fertilizer concentration. Only two Suberites carnosus exposed to 16 to 19 mg/l NO₃-N concentrations showed necrosis after 13 days of exposure. Another two specimens died in the controls.

Lough Hyne has experienced major shifts in intertidal and subtidal communities in the last two decades, and in particular, a large decline in the abundance of subtidal sponges at some sites in the lough (Micaroni et al., 2021 cited in Micaroni et al., 2025). Excess nitrogen was proposed as a possible cause of these changes in the subtidal sponges (Micaroni et al., 2021 cited in Micaroni et al., 2025), including a decline in Cliona celata.

In contrast, Stubler et al. (2024) evaluated the impacts of nitrate and phosphate addition (ranging from 7.6+/-3.4 µmol/l nitrate and 1.7+/-0.6 µmol/l phosphate in aquaria to 2.9+/-0.9 µmol/l nitrate and 0.95+/-0.2 µmol/l phosphate in field experiments) on the bioerosion of Cliona celata inhabiting carbonate substrata in the subtropical southeastern USA. Overall, there were no differences in loss of calcium carbonate substratum among treatments in any of the experiments, though very high rates of bioerosion (up to 0.11 g CaCO₃/day) were observed in the field experiments. Cliona celata is known to thrive under nutrient-replete conditions and presents high adaptive plasticity to environmental variables, and it is likely that the sponges were generally unimpacted by the nutrient treatments (Stubler et al., 2024). However, previous studies have shown a positive correlative relationship between organic nutrient loading and abundance of clionid boring sponges in the Caribbean and Mediterranean (Rose and Risk, 1985; Ward-Paige et al., 2005; Chaves-Fonnegra et al., 2007; and Muricy, 1991 cited in Stubler et al., 2024). Rose & Risk (1985) described an increase in the abundance of Cliona delitrix in an organically polluted section of the Grand fringing reef affected by the discharge of untreated faecal sewage. Ward-Paige et al. (2005) described that the greatest size and biomass of clionids corresponded with the highest nitrogen, ammonia and δ¹⁵N levels. Dissolved organic matter has been demonstrated to be an important component of nutrition in tropical encrusting and boring sponges, and a higher amount of organic nutrients may directly benefit boring sponges (Stubler et al., 2024).

Cliona celata is considered a hardy sponge, tolerant of environmental stressors such as high nutrient loads, low salinity, and large temperature variation (Duckworth & Peters, 2013). Carballo et al. (1994) suggested Cliona celata was a good indicator species of pollution and noted its abundance at polluted sites that receive sewage discharge within Algeciras Bay, Spain.

Echavarri-Erasun et al. (2007) described the effects of deep-water sewage discharge on the relative abundance of rocky reef communities. Species typical of hard substrata (including Caryophyllia smithii and bryozoans) increased in total richness and abundance near the outfall.

It was suggested by Comely & Ansell (1988) that Echinus esculentus could absorb dissolved organic material for the purposes of nutrition. Nutrient enrichment may encourage the growth of ephemeral and epiphytic algae and therefore increase sea-urchin food availability. Lawrence (1975) reported that sea urchins had persisted over 13 years on barren grounds near sewage outfalls, presumably feeding on dissolved organic material, detritus, plankton, and microalgae, although individuals died at an early age.

Sensitivity assessment

Most of the evidence suggests that sponges, including Cliona celata, are resistant to nutrient or organic enrichment. In experiments, Cliona celata demonstrated health impacts but only at high nutrient levels of 16 to 19 mg/l (1,143 to 1,357 μM NO₃) (Wood et al., 2025). However, high nutrient levels are significantly higher (100-fold) than those tested by Stubler et al. (2024) and two-fold higher than the nutrient levels defined as ‘Poor’ under the Water Framework Directive (2015). However, the evidence on the important characteristic species is limited, but the presence of Caryophyllia and Echinus in the vicinity of sewage outfalls suggests resistance. However, the evidence is probably ‘insufficient’ to form the basis of an assessment for the whole community.

Organic enrichment

Benchmark. A deposit of 100 gC/m²/yr (Organic enrichment pressure definition).

Evidence

Alcyonium digitatum, Caryophyllia smithii and Spirobranchus triqueter are suspension feeders of phytoplankton and zooplankton. Organic enrichment of coastal waters that enhances the population of phytoplankton may be beneficial to Alcyonium digitatum, Caryophyllia smithii and Spirobranchus triqueter in terms of an increased food supply, but the effects are uncertain (Hartnoll, 1998). The survival of Alcyonium digitatum, Caryophyllia smithii and Spirobranchus triqueter may be influenced indirectly. High primary productivity in the water column combined with high summer temperature and the development of thermal stratification (which prevents mixing of the water column) can lead to hypoxia of the bottom waters, which faunal species are likely to be highly intolerant of (see de-oxygenation pressure).

Cliona celata is considered a hardy sponge, tolerant of environmental stressors such as high nutrient loads, low salinity, and large temperature variation (Duckworth & Peters, 2013). Carballo et al. (1994) suggested Cliona celata was a good indicator species of pollution, and noted its abundance at polluted sites which receive sewage discharge within Algeciras Bay, Spain.

Johnston & Roberts (2009) conducted a meta-analysis, which reviewed 216 papers to assess how a variety of contaminants (including sewage and nutrient loading) affected 6 marine habitats (including subtidal reefs). A 30-50% reduction in species diversity and richness was identified in all habitats exposed to the contaminant types.

It was suggested by Comely & Ansell (1988) that Echinus esculentus could absorb dissolved organic material for the purposes of nutrition. Organic enrichment may encourage the growth of ephemeral and epiphytic algae and therefore increase sea-urchin food availability. Lawrence (1975) reported that sea urchins had persisted over 13 years on barren grounds near sewage outfalls, presumably feeding on dissolved organic material, detritus, plankton and microalgae, although individuals died at an early age.

Sensitivity assessment

Organic enrichment is not likely to directly negatively affect the characterizing species within this biotope, however, chronic organic enrichment may cause secondary effects such as hypoxia (refer to de-oxygenation pressure). Resistance has been assessed as ‘Low’, and resilience as ‘Medium’. Sensitivity is ‘Medium’.

Physical loss (to land or freshwater habitat)

Benchmark. A permanent loss of existing saline habitat within the site (Physical loss pressure definition). 

Evidence

All marine habitats and benthic species are considered to have a resistance of ‘None’ to this pressure and to be unable to recover from a permanent loss of habitat (resilience is ‘Very low’). Sensitivity within the direct spatial footprint of this pressure is therefore ‘High’. Although no specific evidence is described, confidence in this assessment is ‘High’ due to the incontrovertible nature of this pressure.

Physical change (to another seabed type)

Benchmark. Permanent change from sedimentary or soft rock substrata to hard rock or artificial substrata, or vice versa (Physical change in subtratum type pressure definition).

Evidence

The characterizing species (except Echinus esculentus) require a hard substratum to attach to, such as rock, steel, and other coralligenous or hard formations (Langhamer, 2016; Chava & Mokievsky, 2022; Jenkins & Stevens, 2022).

Alcyonium digitatum prefers hard substrata but is capable of settling on other substrata, including shells, cobble, and other (unstable) coarse substrata (Jenkins & Stevens, 2022). High terrain ruggedness index (TRI) values are often associated with hard substrata, which may explain the positive relationship between the density of records and TRI. Areas with a high TRI would indicate a more complex seabed with local topographic highs, which coral species have been found to prefer (Langton, Stirling & Boulcott, 2023). A study by Becker et al. (2020) focusing on the German North Sea (Helgoländer Steingrund between 9 and 18 m deep) noted how Alcyonium digitatum had a significant positive correlation with hard ground greater than 22%. Becker et al. (2020) also stated that Flustra foliacea dominated the surface structure of bottom waters (meaning coverage of nearly 100%), being significantly positively correlated with hard ground (25%) more than and shell <2 mm more than 0.05 weight% (Becker et al., 2020); in the same region, Michaelis et al. (2019) noted how Flustra foliacea largely colonized boulder-sized stones. In addition, the probability of occurrence of Echinus esculentus was highest in areas with hard ground greater than 19%, and shell >2 mm, more than 0.04 weight% (Becker et al., 2020). While observing an undersea pipeline in the North Sea, Rouse et al. (2019) reported that Alcyonium digitatum was present in the highest densities on pipelines located on mud, while Echinus esculentus were more common on pipelines in sand. 

In northern Norway, large-growth sponges, such as Phakellia ventilabrum and Axinella infundibuliformis, were primarily observed at sites with hard substratum, including sandy-gravels and cobbles, and dominate sponge communities on wave-exposed circalittoral rock habitat (Dunlop et al. 2020). This biotope is also characteristic of circalittoral rock (JNCC, 2022). Kazanidis et al. (2019) found that the type of substratum explained a significant amount of variation in sponge density, with the highest densities found in cobble with boulders.

Sensitivity assessment

If rock were replaced with sediment, this would represent a fundamental change to the physical character of the biotope, and the species would be unlikely to recover. The biotope would be lost. Resistance to the pressure is considered ‘None’, and resilience is ‘Very low’. Sensitivity has been assessed as ‘High’.

Physical change (to another sediment type)

Benchmark. Permanent change in one Folk class (based on UK SeaMap simplified classification) (Physical change in sediment type pressure definition). 

Evidence

‘Not relevant’ to biotopes occurring on bedrock.

Habitat structure changes - removal of substratum (extraction)

Benchmark. The extraction of substratum to 30 cm (where substratum includes sediments and soft rock but excludes hard bedrock) (Removal of substratum pressure definition). 

Evidence

The species characterizing this biotope are epifauna or epiflora occurring on rock and would be sensitive to the removal of the habitat. However, extraction of rock substratum is considered unlikely, and this pressure is considered to be ‘Not relevant’ to hard substratum habitats.

Abrasion / disturbance of the surface of the substratum or seabed

Benchmark. Damage to surface features (e.g. species and physical structures within the habitat) (Surface abrasion/disturbance pressure definition).

Evidence

In the subtidal, abrasion is most likely to be a result of bottom or pot fishing gear, cable laying, etc., which may cause localised mobility of the substrata and mortality of the resident community. The effect would be situation-dependent; however, if bottom fishing gear were towed over a site, it may mobilise a high proportion of the rock substrata and cause high mortality in the resident community.

The characterizing species are likely to be affected by physical disturbances. Physical disturbance by fishing gear has been shown to adversely affect sessile benthic, slow-moving, and emergent epifaunal communities, with hydroid and bryozoan matrices reported to be greatly reduced in fished areas and increase when fishing activity is removed (Jennings & Kaiser, 1998; Sheehan et al., 2017; Kaiser et al., 2018; Long et al., 2021; Langton, Stirling & Boulcott, 2023). Also, heavy mobile gears could also result in the movement of boulders (Bullimore, 1985; Jennings & Kaiser, 1998). All characterizing sponge species for this biotope are sessile or slow-moving epifauna, being either branching or cup-like. The species that create biogenic habitats, such as sponges and corals, often form complex ecological associations and tend to be long-lived, slow-growing, and fragile, sensitive to disturbance and vulnerable to damage (Kaiser et al. 2018; Graves et al., 2023). Fishing disturbance is one of the largest pressures for these characterizing species (Kaiser et al. 2018; Kazanidis et al., 2019; Graves et al., 2023). Less ‘aggressive’ fishing practices can still be damaging, such as some large Pentapora foliacea individuals being badly smashed by potting (Eno et al., 2001). However, they are also vulnerable to other anthropogenic physical disturbances, such as those from oil and gas exploration, deep-sea mining, and recreational SCUBA diving (Vad et al., 2018; Betti et al., 2019; Graves et al., 2023).

Picton & Goodwin (2007) noted that an area of boulders with a rich fauna of sponges and hydroids on the east coast of Rathlin Island, Northern Ireland, was significantly altered since the 1980s. Scallop dredging had begun in 1989, and boulders were observed to have been turned and the gravel harrowed. In addition, many of the boulders had disappeared, and rare hydroid communities were greatly reduced (Picton & Goodwin, 2007). Prior records indicated the presence of large sponges, mainly Axinella infundibuliformis (Picton & Goodwin, 2007). Hiscock (2014) identified Axinella dissimilis as being very susceptible to towed fishing gear. Hinz et al. (2011) studied the effects of scallop dredging in Lyme Bay, UK, and found that the presence of the erect sponge Axinella dissimilis was significantly higher at non-fished sites (33% occurrence) compared to fished sites (15% occurrence). Similarly, these results were observed by Kazanidis et al. (2019), who found that variation in sponge density was strongly explained by fisheries pressure, with higher densities of sponges found in the areas with lowest values of demersal landings, while densities were significantly lower in the areas with higher demersal landings.

Van Dolah et al. (1987) studied the effects on sponges and corals of one trawl event over a low-relief hard bottom habitat off Georgia, US. The densities of individuals taller than 10 cm of three species of sponges in the trawl path and in the adjacent control area were assessed by divers and were compared before, immediately after and 12 months after trawling. Of the total number of sponges remaining in the trawled area, 32% were damaged. Most of the affected sponges were the barrel sponges Cliona spp., whereas Haliclona oculata and Ircina campana were not significantly affected. Twelve months after trawling, the abundance of sponges had increased to pre-trawl densities or greater. Tilmant (1979) found that, following a shrimp trawl in Florida, US, over 50% of sponges, including Neopetrosia, Spheciospongia, Spongia and Hippiospongia, were torn loose from the bottom. The highest damage incidence occurred to the finger sponge Neopetrosia longleyi. Size did not appear to be important in determining whether a sponge was affected by the trawl. Recovery was ongoing, but not complete, 11 months after the trawl, although no specific data was provided.

Freese et al. (1999) studied the effects of trawling on seafloor habitats and associated invertebrates in the Gulf of Alaska. They found that a transect following a single trawling event showed a significant reduction in ‘vase’ sponges (67% expressed damage) and ‘morel’ sponges (total damage could not be quantified as their brittle nature meant that these sponges were completely torn apart and scattered). The ‘finger’ sponges, the smallest and least damaged of the sponges assessed (14%), were damaged by being knocked over. Freese (2001) studied deep cold-water sponges in Alaska a year after a trawl event; 46.8% of sponges exhibited damage, with 32.1% having been torn loose. None of the damaged sponges displayed signs of regrowth or recovery. This was in stark contrast to early work by Freese (1999) on warm shallow sponge communities. Impacts of trawling activity in Alaska were more persistent due to the slower growth/regeneration rates of deep, cold-water sponges. Given the slow growth rates and long lifespans of the rich, diverse fauna, it was considered likely to take many years for deep sponge communities to recover if adversely affected by physical damage (Freese, 2001).

Boulcott & Howell (2011) conducted experimental Newhaven scallop dredging over a circalittoral rock habitat in the Sound of Jura, Scotland and recorded the damage to the resident community. The results indicated that the sponge Pachymatisma johnstoni was highly damaged by the experimental trawl. However, only 13% of photographic samples showed visible damage to Alcyonium digitatum. Where Alcyonium digitatum damage was evident, it tended to be small colonies that were ripped off the rock. The authors highlight that physical damage to faunal turfs (erect bryozoans and hydroids) was difficult to quantify in the study. However, the faunal turf communities did not show large signs of damage and were only damaged by the scallop dredge teeth, which was often limited in extent (approximately 2 cm wide tracts). The authors indicated that species such as Alcyonium digitatum and faunal turf communities were not as vulnerable to damage through trawling as sedimentary fauna, and whilst damage to circalittoral rock fauna did occur, it was of an incremental nature, with loss of species such as Alcyonium digitatum and faunal turf communities increasing with repeated trawls.

Alcyonium digitatum has been documented as locally depleted (reductions in colony numbers and size) in some areas due to benthic trawling, such as Lyme Bay, before the trawling ban (Holland, Jenkins & Stevens, 2017). Magorrian & Service (1998) reported that trawling for queen scallops resulted in the removal of emergent epifauna and damage to horse mussel beds in Strangford Lough. They suggested that the emergent epifauna, such as Alcyonium digitatum, were more intolerant than the horse mussels themselves and reflected early signs of damage (Service & Magorrian, 1997; Magorrian & Service, 1998; Service, 1998). Veale et al. (2000) reported that the abundance, biomass, and production of epifaunal assemblages, including Alcyonium digitatum, decreased with increasing fishing effort. However, the fact that Alcyonium digitatum is more abundant on high fishing effort grounds suggests that this seemingly fragile species is more resistant to abrasive disturbance than might be assumed (Bradshaw et al., 2000), presumably owing to the ability for the replacement of senescent cells and regeneration of damaged tissue, in addition to the early larval colonization of available substrata.

Kaiser et al. (2018) specifically studied the recovery of sessile epifauna following the exclusion of towed mobile fishing gear in Lyme Bay, UK. Their estimates suggest that no recovery occurred within the timescale of the study, and that some biogenic habitats (particularly sponges and soft corals) could require up to, or more than, 20 to 30 years before signs of recolonization and recovery may occur. The maximum recovery time modelled was 51 years for yellow branched sponges, while species such as Eunicella verrucosa and Pentapora foliacea increased in abundance but had not fully recovered, with their projected recovery time being 17 to 20 years (Kaiser et al., 2018). Therefore, recovery rates of biota depend on life-history factors and habitat-specific requirements, with the longer-lived species that require specific habitats and have low dispersal potential taking longer to recover (Kaiser et al., 2018). A 15-year review of the Lyme Bay trawling ban by Renn et al. (2024) highlighted definitive evidence of recovery, in terms of increased species richness, with key sessile taxa (Pentapora foliacea and Phallusia mammillata) showing signs of early recovery between 2008 and 2013. In terms of exploited species, between 2008 and 2019, fish experienced a 430% increase in taxon richness and a 370% increase in total abundance inside the Marine Protected Area (MPA), but invertebrates (crab, lobster, cuttlefish, and whelk) exhibited no signs of recovery (Renn et al., 2024). Renn et al. (2024) concluded that the evidence of recovery recorded in Lyme Bay broadly aligned with the wider literature by detecting early stages of recovery within the first few years of MPA establishment. However, full recovery is thought to occur over decadal timescales, and measuring full recovery rates in-situ remains a priority for future research in Lyme Bay.

Species with fragile tests, such as Echinus esculentus, were reported to suffer badly due to scallop or queen scallop dredging (Bradshaw et al., 2000; Hall-Spencer & Moore, 2000). Kaiser et al. (2000) reported that Echinus esculentus were less abundant in areas subject to high trawling disturbance in the Irish Sea. Jenkins et al. (2001) conducted experimental scallop trawling in the North Irish Sea and recorded the damage caused to several conspicuous megafauna species, both when caught as by-catch and when left on the seabed. The authors predicted 16.4% of Echinus esculentus were crushed/dead, 29.3% would have >50% spine loss/minor cracks, 1.1% would have <50% spine loss, and the remaining 53.3% would be in good condition. Sea urchins can rapidly regenerate spines; e.g. Psammechinus miliaris were found to re-grow all spines within a period of two months (Hobson, 1930).

The trawling examples mentioned above were conducted on sedimentary habitats, and thus the evidence is not directly relevant to the rock-based biotopes

Sensitivity assessment

Given the sessile, emergent nature of the biotopes characterizing epifauna, damage and mortality following a physical disturbance are likely to be significant, however, some studies have brought into question the extent of damage to the faunal turf. The physiology of the bryozoans affords some protection in the event of abrasion events, and recovery is likely to be rapid if stolons remain undamaged. Mortality amongst the characterizing species Alcyonium digitatum and Echinus esculentus is likely to be ‘Medium’ (<25% loss), but this is dependent on the abrasion activity, and heavier gears may well cause more damage. However, based on the damage to sponges, resistance has been assessed as ‘Medium’, and resilience has been assessed as ‘High’. Sensitivity has been assessed as ‘Low’

Please note Boulcott & Howell (2011) did not mention the abrasion caused by fully loaded collection bags on the Newhaven dredges. A fully loaded Newhaven dredge may cause higher damage to the community, as indicated in their study.

Penetration or disturbance of the substratum subsurface

Benchmark. Damage to sub-surface features (e.g. species and physical structures within the habitat) (Sub-surface penetration pressure definition).

Evidence

The species characterizing this biotope group are epifauna or epiflora occurring on rock which is resistant to subsurface penetration. The assessment for abrasion at the surface only is therefore considered to equally represent sensitivity to this pressure. This pressure is considered to be ‘Not Relevant’ to hard rock biotopes.

Changes in suspended solids (water clarity)

Benchmark. A change in one rank on the WFD (Water Framework Directive) scale, e.g. from clear to intermediate for one year (Suspended sediment pressure definition).

Evidence

Alcyonium digitatum, Caryophyllia smithii and sponges are not thought to be highly susceptible to changes in water clarity because they are suspension-feeding organisms and are not directly dependent on sunlight for nutrition. Alcyonium digitatum has been shown to be tolerant of high levels of suspended sediment. Hill et al. (1997) demonstrated that Alcyonium digitatum sloughed off settled particles with a large amount of mucous. Alcyonium digitatum is also known to inhabit the entrances to sea lochs (Budd, 2008) or the entrances to estuaries (Braber & Borghouts, 1977) where water clarity is likely to be highly variable.

Bell & Turner (2000) studied populations of Caryophyllia smithii at three sites of differing sedimentation regimes in Lough Hyne, Ireland. Calyx size was largest at the site of least sedimentation and smallest at the site of most sedimentation. In contrast, the height of individuals was greatest at the site of most sedimentation and smallest at the site of least sedimentation. The height of individuals correlated with the level of surrounding sediment. High density was correlated with high sedimentation and depth (Bell & Turner, 2000).

Bryozoans are suspension feeders that may be adversely affected by increases in suspended sediment, due to clogging of their feeding apparatus. However, Tyler-Walters & Ballerstedt (2007) reported Flustra foliacea as tolerant to increased suspended sediment based on its occurrence in areas of high suspended sediment, e.g. abundant in turbid, fast-flowing waters of the Menai Straits (Moore 1977). Also, communities dominated by Flustra foliacea were described on tide-swept seabed, exposed to high levels of suspended sediment and sediment scour in the English Channel (Holme & Wilson, 1985). Flustra foliacea is also characteristic of sediment-scoured, silty rock communities CR.HCR.XFa.FluCoAs and CR.MCR.EcCr.UrtScr (Connor et al., 2004), and is also found at greater depths, where organic particulates (detritus) are probably more important.

Schönberg (2015) reviewed and observed the interactions between sediments and marine sponges in Australia and described the lack of research on Porifera. Bell et al. (2015) reviewed the effects and interactions of sponges with sediment in suspension and after deposition. Whilst many sponges are disadvantaged by sedimentation (as would be expected, being sessile filter feeders) (Gerrodette & Flechsig, 1979), many examples exist of sponges adapting to sediment presence (Bell et al., 2015; Schönberg, 2015), and many encrusting sponges appear to be able to survive in highly sedimented conditions, and, in fact, many species prefer such habitats (Bell & Barnes, 2001; Bell & Smith, 2004). Adaptations included sediment incorporation, sediment encrusting, structural modification (such as reduction in numbers of oscula, or repositioning of inhalant and exhalant openings), soft sediment anchoring using spicules, modification of spicules to shield the body from sediment, backwashing, mucus production, morphology (e.g. upright forms intercept less settling sediment) and living, at least partially, embedded within the sediment (Bell et al., 2015; Schönberg, 2015). 

Among the sponges, Schönberg (2015) found that Axinellids frequently formed external crusts and sediment interaction was observed in 5.8 ± 4.8% of observations, but required rock substrata under the sediment for attachment. Ackers et al. (1992) describe Axinella dissimilis as preferring clean oceanic water but tolerating silt. Sanchez et al. (2009) described finding communities composed primarily of Phakellia ventilabrum and Dendrophyllia cornigera in circalittoral rocky habitats in the Cantabrian Sea, northern Spain. Phakellia ventilabrum showed greater tolerance to sedimentation pressures than the coral. The authors concluded that Phakellia ventilabrum preferred a mixed rock–sand habitat where deposition processes predominate, and hence sedimentation, together with hard substrata where it settles (Sanchez et al., 2009). Axinella dissimilis is mainly found on upward-facing clean or silty rock, and whilst it tends to prefer clean oceanic water, it is tolerant of silt (Ackers et al., 1992).

Castric-Fey & Chassé (1991) conducted a factorial analysis of the subtidal rocky ecology near Brest, France, and rated the distribution of species in varying turbidity (corroborated by the depth at which laminarians disappeared). Cliona celata and Stelligera rigida were classed as indifferent to turbidity, Tethya citrina, Pachymatisma johnstonia and Polymastia boletiformis (as Polymastia robusta) had a slight preference for clearer water, while Dysidea fragilis, Polymastia mamillaris, and Raspailia ramosa had a strong preference for turbid water.

Bell et al. (2015) noted that upright forms intercepted a smaller amount of settling sediment than encrusting forms. For example, Bell & Barnes (2002; cited in Bell et al., 2015) reported considerable variation in the branching characteristics of Raspaillia ramosa and Stelligera stuposa across a sediment gradient in Lough Hyne, Northern Ireland, although the patterns were due to the interaction between sedimentation and water flow. Raspailia ramosa and Stelligera stuposa have a reduced maximum size in areas of high sedimentation (Bell et al., 2002). Storr (1976) observed the sponge Sphecispongia vesparium backwashing to eject sediment and noted that other sponges (such as Condrilla nucula) use secretions to remove settled material. Tjensvoll et al. (2013) found that Geodia barretti physiologically shuts down when exposed to sediment concentrations of 100 mg /l that caused an 86% reduction in respiration. Rapid recovery to initial respiration levels directly after the exposure indicated that Geodia barretti can cope with a single short exposure to elevated sediment concentrations. However, it should be noted that a laboratory study on the impact of elevated sedimentation rates on deep-water sponges found that a sediment load of 30 mg sed./l resulted in significantly higher sponge mortality compared with sponges exposed to 5 and 10 mg sed./l, although no additional information was provided (Hoffman & Tore Rapp, pers comm. cited in Lancaster et al., 2014).

Pineda et al. (2017a) examined the effect of suspended sediments in three species of sponge from New Zealand: two phototrophic (due to symbiotic algae) (Cliona orientalis and Carteriospongia foliascens) and one heterotrophic (Ianthella basta) under laboratory conditions. All sponges exhibited a short-term response to suspended sediment, e.g. closed oscula, mucus production, and tissue regression. Most survived low to medium turbidity (≤33 mg/l) for up to 28 days, but at high turbidity (≤76 mg/l), Cliona orientalis and Carteriospongia foliascens experienced 20-90% mortality, and Ianthella basta showed tissue regression. Pineda et al. (2017a) suggested that suspended sediment combined with low light due to turbidity increased mortality in the phototrophic species, but noted that there was considerable interspecies variation in their response. In addition, Kazanidis et al. (2018) noted that cold-water sponges exclusively feed on dissolved organic matter particles smaller than 10 μm, and that suspended Particulate Organic Matter (POM) concentration decreased, along with its quality, with depth. For example, in surface waters 0 to 150 m water depth, the average concentration of suspended POM is about four times higher than between 150 and 4,000 m water depth.

Moore (1977) suggested that Echinus esculentus was unaffected by turbid conditions. Echinus esculentus is an important grazer of red macro-algae within CR.MCR.EcCr. Increased turbidity and resultant reduced light penetration is likely to negatively affect algal growth. However, Echinus esculentus can feed on alternative prey, detritus or dissolved organic material (Lawrence, 1975; Comely & Ansell, 1988).

Sensitivity assessment

Most of the characteristic species are probably adapted to the silty conditions experienced in this biotope (JNCC, 2022) and resistant to suspended sediment. Therefore, resistance has been assessed as ‘High’, and resilience as ‘High’. Sensitivity has been assessed as ‘Not Sensitive’.

Smothering and siltation rate changes (light)

Benchmark. ‘Light’ deposition of up to 5 cm of fine material added to the seabed in a single discrete event (Smothering pressure definition).

Evidence

Alcyonium digitatum, Caryophyllia smithii, and sponges are sessile and thus would be unable to avoid the deposition of a smothering layer of sediment. Some Alcyonium digitatum colonies can attain a height of up to 20 cm (Edwards, 2008), so they would still be able to feed in the event of sediment deposition. Smothering by 5 cm of sediment is likely to prevent feeding, and hence growth and reproduction, as well as respiration in the bryozoans. In addition, associated sediment abrasion may remove the bryozoan colonies. A layer of sediment will probably also interfere with larval settlement (Tyler-Walters, 2005c).

Caryophyllia smithii is a small (approx. <3 cm in height from the seabed) species and would therefore likely be inundated in a “light” sedimentation event. Coolen et al. (2015) noted how a low abundance of Caryophyllia smithii was typically observed at locations with low tidal current strength and high sedimentation. For example, in the Skomer Island, UK, Marine Conservation Zone, higher numbers of Caryophyllia smithii were observed on vertical walls, likely due to less surface sediment accumulating there (Lock et al., 2025). However, Bell & Turner (2000) reported that Caryophyllia smithii was abundant at sites of “moderate” sedimentation (7 mm ± 0.5 mm) in Lough Hyne. It is therefore likely that Caryophyllia smithii would be resistant to periodic sedimentation. If 5 cm of sediment were removed rapidly, via tidal currents, Caryophyllia smithii would likely remain within the biotope. Lock et al. (2006) partly attributed fluctuations in Caryophyllia smithii abundance at Skomer Island to surface sediment cover. Bell (2002) reported that juvenile Caryophyllia smithii are morphologically variable and initially undergo rapid growth with tall and thin forms in deeper, sheltered, relatively sedimented conditions near Lough Hyne, Ireland. It was concluded that this was to escape the thin layer of sediment present. Coolen et al. (2015) noted that a low abundance of Caryophyllia smithii was typically observed at locations with low tidal current strength and high sedimentation. Hiscock (1983) reported that Caryophyllia smithii was able to survive for five days smothered by fine sediment (mud) (Hiscock, 1983).

Colonies of Pentapora fascialis can reach a height of 30 cm in the British Isles (Hayward & Ryland, 1979). Partial mortality due to siltation has been recorded in the Mediterranean (Cocito et al., 1998a), although recovery was observed in all but one colony (which fragmented into two smaller colonies).

Schönberg (2015) reviewed and observed the interactions between sediments and marine sponges in Australia and described the lack of research on Porifera. Bell et al. (2015) reviewed the effects and interactions of sponges with sediment in suspension and after deposition. Despite sediment being considered to have a negative impact on suspension feeders (Gerrodette & Flechsig, 1979), many encrusting sponges appear to be able to survive in highly sedimented conditions, and, in fact, many species prefer such habitats (Bell & Barnes, 2001; Bell & Smith, 2004). While studying coralligenous assemblages of the Apulian continental shelf in the Mediterranean Sea, Piazzi et al. (2019) found that sedimentation was higher on deep outcrops and suggested that it was the main driver of differences between shallow and deep assemblages. Furthermore, in these areas of high sedimentation, an abundance of stress-tolerant organisms was observed, such as the anemone Parazoanthus axinellae (Piazzi et al., 2019).

Cliona celata can either have a boring life form (it bores into rock) or a massive form (grows on top of rock). With the boring form, only inhalant and exhalent papillae are visible just above the rock surface (Wood, 2007). The massive form has raised, rounded ridges with large specimens growing up to 1 m across and 50 cm high (Snowden, 2007). Haliclona viscosa is described as a cushion-forming species, which forms a thick crust which may reach a diameter of 30 to 40 cm and a height of 1.5 to 5cm (Topsent, 1888). Pachymatisma johnstonia can reach up to 15 cm in diameter and a height of up to 10 cm (Neish, 2007).

Pineda et al. (2017b) exposed three phototrophic (due to symbiotic algae) and two heterotrophic sponges from New Zealand to repeated deposition events and sediment cover over 80 to 100% of sponge surface to a depth of ca 0.5 mm for up to 30 days in laboratory conditions. All five species survived with minimal physiological effects. However, Wulff (2006) described mortality in three sponge groups following four weeks of complete burial under sediment; 16% of Amphimedon biomass died compared with 40% and 47% in Iotrochota and Aplysina, respectively. In Lough Hyne, a small (approx. 0.5 km²) lough on the southwest coast of Ireland, sponge communities experience varying levels of sedimentation, from negligible levels (3 ± 0.2 mm) to higher rates (18 to 34 g /m/ day), and sponges have been continually recorded in these areas under these levels of sedimentation (Micaroni et al., 2025). The complete disappearance of the ‘sponges associated’ with the sea squirt Ascidiella aspersa in the Black Sea near the Kerch Strait was attributed to siltation (Terent'ev, 2008 cited in Tillin & Tyler-Walters, 2014). It should also be noted that some of the characterizing sponges are likely to be buried by 5 cm of sediment deposition.

Bell et al. (2015) noted that upright forms intercepted a smaller amount of settling sediment than encrusting forms. For example, Bell & Barnes (2002; cited in Bell et al., 2015) reported considerable variation in the branching characteristics of Raspaillia ramosa and Stelligera stuposa across a sediment gradient in Lough Hyne, Northern Ireland, although the patterns were due to the interaction between sedimentation and water flow. Raspailia ramosa and Stelligera stuposa have a reduced maximum size in areas of high sedimentation (Bell et al., 2002). 

Hiscock & Jones (2004) reported that Axinella dissimilis (as Axinella polypoides) grew up to a height of ca. 30 cm. Axinella sponges are commonly found alongside Eunicella verrucosa and can therefore tolerate a similar/high level of smothering (Canessa et al., 2022). Ackers et al. (1992) described Axinella dissimilis as preferring clean oceanic water but tolerating silt. No evidence of smothering of axinellids was found.

Echinus esculentus are mobile, large globular urchins which can reach a diameter of 17 cm (Tyler-Walters, 2008). Comely & Ansell (1988) recorded large Echinus esculentus from kelp beds on the west coast of Scotland in which the substratum was seasonally covered with "high levels" of silt. This suggests that Echinus esculentus is unlikely to be killed by smothering. However, smaller specimens and juveniles may be less resistant. A layer of sediment may interfere with larval settlement. If retained within the host biotope for extended periods, a layer of 5 cm of sediment may negatively affect successive recruitment events.

Sensitivity assessment

CR.MCR.EcCr.CarSp.PenPcom is recorded from moderately strong to negligible tidal currents (<1.5m/sec) in extremely exposed to moderately wave-exposed conditions (Connor et al., 2004; JNCC, 2022). Therefore, water movement (through wave action and/or tidal flow) would be expected to clear 5 cm of deposited sediment within a few tidal cycles. Smothering by 5 cm would cover the majority of Caryophyllia smithii and the smallest examples of the other characterizing species and could result in limited mortality. Caryophyllia smithii has been reported as quite tolerant of temporary burial, and the biotope occurs in moderate energy conditions, and the sediment would likely be removed rapidly. Therefore, resistance has been assessed as ‘High’, and resilience as ‘High’. Sensitivity has therefore been assessed as ‘Not Sensitive’.

Smothering and siltation rate changes (heavy)

Benchmark. ‘Heavy’ deposition of up to 30 cm of fine material added to the seabed in a single discrete event (Smothering pressure definition).

Evidence

Alcyonium digitatum, Caryophyllia smithii, and sponges are sessile and thus would be unable to avoid the deposition of a smothering layer of sediment. Some Alcyonium digitatum colonies can attain a height of up to 20 cm (Edwards, 2008), so they would still be able to feed in the event of sediment deposition. Smothering by 30 cm of sediment is likely to prevent feeding, and hence growth and reproduction, as well as respiration in the bryozoans. In addition, associated sediment abrasion may remove the bryozoan colonies. A layer of sediment will probably also interfere with larval settlement (Tyler-Walters, 2005c).

Caryophyllia smithii is a small (approx. <3 cm in height from the seabed) species and would therefore likely be inundated in a “light” sedimentation event. Coolen et al. (2015) noted how a low abundance of Caryophyllia smithii was typically observed at locations with low tidal current strength and high sedimentation. For example, in the Skomer Island, UK, Marine Conservation Zone, higher numbers of Caryophyllia smithii were observed on vertical walls, likely due to less surface sediment accumulating there (Lock et al., 2025). However, Bell & Turner (2000) reported that Caryophyllia smithii was abundant at sites of “moderate” sedimentation (7 mm ± 0.5 mm) in Lough Hyne. It is therefore likely that Caryophyllia smithii would be resistant to periodic sedimentation. If 30 cm of sediment were removed rapidly, via tidal currents, Caryophyllia smithii would likely remain within the biotope. Lock et al. (2006) partly attributed fluctuations in Caryophyllia smithii abundance at Skomer Island to surface sediment cover. Bell (2002) reported that juvenile Caryophyllia smithii are morphologically variable and initially undergo rapid growth with tall and thin forms in deeper, sheltered, relatively sedimented conditions near Lough Hyne, Ireland. It was concluded that this was to escape the thin layer of sediment present. Coolen et al. (2015) noted that a low abundance of Caryophyllia smithii was typically observed at locations with low tidal current strength and high sedimentation. Hiscock (1983) reported that Caryophyllia smithii was able to survive for five days smothered by fine sediment (mud) (Hiscock, 1983).  

Colonies of Pentapora fascialis can reach a height of 30 cm in the British Isles (Hayward & Ryland, 1979). Partial mortality due to siltation has been recorded in the Mediterranean (Cocito et al., 1998a), although recovery was observed in all but one colony (which fragmented into two smaller colonies).

Schönberg (2015) reviewed and observed the interactions between sediments and marine sponges in Australia and described the lack of research on Porifera. Bell et al. (2015) reviewed the effects and interactions of sponges with sediment in suspension and after deposition. Despite sediment being considered to have a negative impact on suspension feeders (Gerrodette & Flechsig, 1979), many encrusting sponges appear to be able to survive in highly sedimented conditions, and, in fact, many species prefer such habitats (Bell & Barnes, 2001; Bell & Smith, 2004). While studying coralligenous assemblages of the Apulian continental shelf in the Mediterranean Sea, Piazzi et al. (2019) found that sedimentation was higher on deep outcrops and suggested that it was the main driver of differences between shallow and deep assemblages. Furthermore, in these areas of high sedimentation, an abundance of stress-tolerant organisms was observed, such as the anemone Parazoanthus axinellae (Piazzi et al., 2019).

Cliona celata can either have a boring life form (it bores into rock) or a massive form (grows on top of rock). With the boring form, only inhalant and exhalant papillae are visible just above the rock surface (Wood, 2007). The massive form has raised, rounded ridges with large specimens growing up to 1 m across and 50 cm high (Snowden, 2007). Haliclona viscosa is described as a cushion-forming species, which forms a thick crust which may reach a diameter of 30 to 40 cm and a height of 1.5 to 5cm (Topsent, 1888). Pachymatisma johnstonia can reach up to 15 cm in diameter and a height of up to 10 cm (Neish, 2007).

Pineda et al. (2017b) exposed three phototrophic (due to symbiotic algae) and two heterotrophic sponges from New Zealand to repeated deposition events and sediment cover over 80 to 100% of sponge surface to a depth of ca 0.5 mm for up to 30 days in laboratory conditions. All five species survived with minimal physiological effects. However, Wulff (2006) described mortality in three sponge groups following four weeks of complete burial under sediment; 16% of Amphimedon biomass died compared with 40% and 47% in Iotrochota and Aplysina, respectively. In Lough Hyne, a small (approx. 0.5 km²) lough on the southwest coast of Ireland, sponge communities experience varying levels of sedimentation, from negligible levels (3 ± 0.2 mm) to higher rates (18 to 34 g /m/ day), and sponges have been continually recorded in these areas under these levels of sedimentation (Micaroni et al., 2025). The complete disappearance of the ‘sponges associated’ with the sea squirt Ascidiella aspersa in the Black Sea near the Kerch Strait was attributed to siltation (Terent'ev, 2008 cited in Tillin & Tyler-Walters, 2014). It should also be noted that most of the characterizing sponges are likely to be buried by 30 cm of sediment deposition.

Bell et al. (2015) noted that upright forms intercepted a smaller amount of settling sediment than encrusting forms. For example, Bell & Barnes (2002; cited in Bell et al., 2015) reported considerable variation in the branching characteristics of Raspaillia ramosa and Stelligera stuposa across a sediment gradient in Lough Hyne, Northern Ireland, although the patterns were due to the interaction between sedimentation and water flow. Raspailia ramosa and Stelligera stuposa have a reduced maximum size in areas of high sedimentation (Bell et al., 2002). 

Hiscock & Jones (2004) reported that Axinella dissimilis (as Axinella polypoides) grew up to a height of ca 30 cm. Axinella sponges are commonly found alongside Eunicella verrucosa and can therefore tolerate a similar/high level of smothering (Canessa et al., 2022). Ackers et al. (1992) described Axinella dissimilis as preferring clean oceanic water but tolerating silt. No evidence of smothering of axinellids was found.

Echinus esculentus are mobile, large globular urchins which can reach a diameter of 17 cm (Tyler-Walters, 2008). Comely & Ansell (1988) recorded large Echinus esculentus from kelp beds on the west coast of Scotland in which the substratum was seasonally covered with "high levels" of silt. This suggests that Echinus esculentus is unlikely to be killed by smothering, however, smaller specimens and juveniles may be less resistant. A layer of sediment may interfere with larval settlement. If retained within the host biotope for extended periods, a layer of 30 cm of sediment may negatively affect successive recruitment events.

Sensitivity assessment

CR.MCR.EcCr.CarSp.PenPcom is recorded from moderately strong to negligible tidal currents (<1.5m/sec) in extremely exposed to moderately wave-exposed conditions (Connor et al., 2004; JNCC, 2022). Smothering by 30 cm of sediment could cause mortality amongst most characterizing species of this biotope if it remained on the surface of the seabed. In addition, only upward-facing areas within the biotope are likely to be affected, while vertical faces would be unaffected. Hence, the damage to the resident community would depend on the time taken for the deposited sediment to be removed. The biotope occurs in moderate energy environments, and it is likely that the sediment would be removed rapidly. Therefore, resistance has been assessed as ‘Medium’, and resilience as ‘High’. Sensitivity has therefore been assessed as ‘Low’.

Litter

Benchmark. The introduction of man-made objects able to cause physical harm (surface, water column, seafloor or strandline) (Litter pressure definition). 

Evidence

All characterizing species for this biotope are sessile epifauna or slow-moving, being either encrusting, branching or cup-like, or having a fragile test. Physical disturbance by fishing gear has been shown to adversely affect sessile benthic and emergent epifaunal communities, with hydroid and bryozoan matrices reported to be greatly reduced in fished areas and increased when fishing activity is removed (Jennings & Kaiser, 1998; Sheehan et al., 2017; Kaiser et al., 2018). Both Sheehan et al. (2017) and Giusti et al. (2019) highlight how, in addition to the direct damage from fishing, ghost fishing may also be responsible for some coral mortality, Eunicella verrucosa in this case, either through direct damage or making them more vulnerable to removal from their anchorage to the sea floor, particularly during storms.

There are no records of ghost fishing affecting the characterizing species for this biotope. However, epifaunal communities are vulnerable to damage from fishing gear and are likely vulnerable to being dislodged or damaged through lost fishing gear, and possibly certain types of marine litter. At present, there is 'Insufficient evidence' to complete a sensitivity assessment on the effect of litter on this biotope.

Electromagnetic changes

Benchmark. A local electric field of 1 V/m or a local magnetic field of 10 µT (Electromagnetic pressure definition).

Evidence

Evidence on the effect of electromagnetic fields (EMFs) on benthic organisms is still severely lacking. Some studies have investigated the effect of anthropogenically induced EMFs on benthic invertebrates at intensities ranging between 2 nT and 40 mT, which is often much higher than in-situ measurements from subsea cables. While some report changes to behaviour, physiology, reproduction, development, immunology, cytotoxicity and orientation, others demonstrate no effect from exposure to the EMF (Albert et al., 2020; Hutchison et al., 2020), depending on the study species and duration and intensity of exposure. There have been no studies investigating the effect of EMFs at the population or community level for benthic organisms. 

One study exists on the effect of EMFs on Echinus esculentus. Chapman et al. (2023) simulated an EMF of 500 μT, as modelled for an export cable over a rocky shore, where the industry standard cable burial would not be possible; however, no significant differences were found in either behavioural or physiological responses in Echinus esculentus. A similar study was performed on the reef-forming annelid, Ficopomatus enigmaticus (Oliva et al., 2023). Sperm cells from this species were exposed to 0.5 and 1.0 mT of a static magnetic field. After only three hours of exposure, sperm fertilization rate was reduced, and significant increases in DNA damage and mitochondrial activity, indicative of a stress response, were reported. However, there is ‘Insufficient evidence’ on which to base an assessment of the likely sensitivity of this biotope to EMFs.

Underwater noise changes

Benchmark. MSFD indicator levels (SEL or peak SPL) exceeded for 20% of days in a calendar year. Further detail

Evidence

Echinus esculentus, Alcyonium digitatum and Parasmittina trispinosa have no hearing perception, but vibrations may cause an impact, however, there is ‘No evidence’ to support an assessment.   

Introduction of light or shading

Benchmark. A change in incident light via anthropogenic means (Introduced light or shade pressure definition).

Evidence

Although no evidence was found for the effect of light on the characterizing species, it is well understood that light influences the spawning of tropical corals, along with other environmental cues such as solar insolation, day length, and temperature (Davies et al., 2023; Egger et al., 2025). However, for temperate and intermediate-water species, some of these cues may be absent or differ significantly. Many cold-water corals live beyond the reach of moonlight, but the species in this biotope, Caryophyllia smithii, can be found within the first 200 m of depth where light, both natural and artificial, would reach (Holland, Jenkins & Stevens, 2017; Egger et al., 2025). In addition, shading of light or the introduction of light within the first 50 m could influence marine organisms, such as triggering early coral spawning or affecting the opening and reproduction rhythm of bivalves (Charifi et al., 2023; Davies et al., 2023; Smyth et al.,2021). Below 200 m, it is unlikely that these species would be impacted, as the light level that reaches beyond this point is very low and unsuitable for photosynthesis.

Jones et al. (2012) compiled a report on the monitoring of sponges around Skomer Island and found that many sponges, particularly encrusting species, preferred vertical or shaded bedrock to open, light surfaces, presumably due to a lack of competition from algae.

Sensitivity assessment

Whilst no evidence could be found for the effect of light on the characterizing species of these biotopes, it is unlikely that these species would be impacted. As a circalittoral biotope, a decrease in light (due to shading) is unlikely to be important as growth ceases for several red algae (such as Chondrus crispus) below ca. 1.0 μmol /m²/s (ca 50 Lux). However, an increase in light could be significant if it duplicated sunlight.

Although there is some evidence that the basiepithelial nerve plexus below the entire outer skin of echinoderms is sensitive to light (Hill, 2008), there is no evidence to suggest the impact of artificial light on echinoderms.

CR.MCR.EcCr.CarSp.PenPcom is a circalittoral biotope and is thus by definition a naturally shaded environment with low light levels. Increased shading (e.g. by construction of a pontoon, pier, etc.) could be beneficial to the characterizing species within these biotopes. However, given the rapid expansion of the evidence base and the continuing lack of data at the level of individual biotopes, resistance and resilience cannot be robustly assessed. Sensitivity is therefore recorded as ‘Insufficient evidence’.

Barrier to species movement

Benchmark. A permanent or temporary barrier to species movement over ≥50% of water body width or a 10% change in tidal excursion (Barrier to species movement pressure definition).

Evidence

Barriers and changes in tidal excursion are 'Not relevant' to biotopes restricted to open waters.

Death or injury by collision

Benchmark. Injury or mortality from collisions of biota with both static or moving structures due to 0.1% of tidal volume on an average tide, passing through an artificial structure (Death for collision pressure definition).

Evidence

'Not relevant' to seabed habitats. NB. Collision by grounding vessels is addressed under ‘surface abrasion’.

Visual disturbance

Benchmark. The daily duration of transient visual cues exceeds 10% of the period of site occupancy by the feature (Visual disturbance pressure definition). 

Evidence

'Not relevant'.

Genetic modification & translocation of indigenous species

Benchmark. Translocation of indigenous species or the introduction of genetically modified or genetically different populations of indigenous species may result in changes in the genetic structure of local populations, hybridization, or a change in community structure (Translocation pressure definition).

Evidence

Echinus esculentus was identified by Kelly & Pantazis (2001) as a species suitable for culture for the urchin roe industry. However, at present, no evidence could be found to suggest that significant Echinus esculentus mariculture was present in the UK. If industrially cultivated, it is feasible that Echinus esculentus individuals could be translocated. ‘No evidence’ of cultivation or translocation of other characterizing species was found.

Introduction of microbial pathogens

Benchmark. The introduction of relevant microbial pathogens or metazoan disease vectors to an area where they are currently not present (e.g. Martelia refringens and Bonamia, Avian influenza virus, viral Haemorrhagic Septicaemia virus) (pathogen or disease pressure definition).

Evidence

Echinus esculentus is susceptible to 'Bald-sea-urchin disease', which causes lesions, loss of spines, tube feet, pedicellariae, destruction of the upper layer of skeletal tissue and death. It is thought to be caused by the bacteria Vibrio anguillarum and Aeromonas salmonicida. Bald sea-urchin disease was recorded from Echinus esculentus on the Brittany Coast. Although associated with mass mortalities of Strongylocentrotus franciscanus in California and Paracentrotus lividus in the French Mediterranean, it is not known if the disease induces mass mortality (Bower, 1996).

Alcyonium digitatum acts as the host for the endoparasitic species Enalcyonium forbesi and Enalcyonium rubicundum (Stock, 1988). Parasitisation may reduce the viability of a colony but not to the extent of causing mortality.  No further evidence was found to substantiate this suggestion. Specimens of Cliona spp. exhibited blackened damage since 2013 in Skomer. Preliminary results have shown that clean, fouled and blackened Cliona all have very different bacterial communities. The blackened Cliona are effectively dead and have a bacterial community similar to marine sediments. The fouled Cliona have a very distinct bacterial community that may suggest a specific pathogen caused the effect (Burton, pers comm; Preston & Burton, 2015).  Stebbing (1971b) reported that encrusting epizoites reduced the growth rate of Flustra foliacea by ca 50%. The bryozoan Bugula flabellata produces stolons that grow in and through the zooids of Flustra foliacea, causing "irreversible degeneration of the enclosed polypide" (Stebbing, 1971b). No evidence of Parasmittina trispinosa disease could be found.

Sensitivity assessment

Whilst evidence of disease in the characterizing species could be found, ‘No evidence’ of mass-mortality through disease could be found.

Removal of target species

Benchmark. Removal of species targeted by fishery, shellfishery or harvesting at a commercial or recreational scale (targeted removal pressure definition).

Evidence

Despite historic extraction as a curio (Jangoux, 1980; Nichols, 1984), Echinus esculentus is not thought to be currently targeted. 'No evidence' for the targeted removal of Alcyonium digitatum or bryozoans could be found.

Removal of non-target species

Benchmark. Removal of features or incidental non-targeted catch (by-catch) through targeted fishery, shellfishery or harvesting at a commercial or recreational scale (non-targeted removed pressure definition).

Evidence

The sensitivity assessment for this pressure considers any biological/ecological effects resulting from the removal of non-target species on this biotope. Alcyonium digitatum goes through an annual cycle, from February to July, all Alcyonium digitatum colonies are feeding, from July to November, an increasing number of colonies stop feeding. During this period, a large number of polyps can retract, and a variety of filamentous algae, hydroids and amphipods can colonize the surface of colonies epiphytically. From December to February, the epiphytic community is, however, sloughed off (Hartnoll, 1975). If Alcyonium digitatum were removed, the epiphytic species would likely colonize rock surfaces and are therefore not dependent on Alcyonium digitatum.

While recovery of the characterizing species should be possible within 2-10 years following non-targeted removal (e.g. from static or mobile gears), loss of Echinus esculentus from the biotope, subsequent loss of grazing pressure, would result in increasing competition from algae and increased competition for space, which could lead to a change in biotope classification e.g. to XFa biotopes with a more.  Alcyonium digitatum and faunal turf communities (which include bryozoans such as Parasmittina trispinosa) are probably resistant to abrasion through bottom fishing (see abrasion pressure).

Sensitivity assessment

A decrease in Caryophyllia smithii or Alcyonium digitatum would result in a decline in the biotope richness.  However, removal of Echinus esculentus could result in restructuring of the biotope.  If all are lost, reclassification would be necessary.  Resistance has therefore been assessed as ‘None’, resilience as’ Medium’, and sensitivity as ‘Medium’.

The American slipper limpet, Crepidula fornicata

Evidence

Crepidula fornicata larvae require hard substrata for settlement. It prefers muddy, gravelly, shell-rich substrata that include gravel, the shells of other Crepidula, or other species, e.g., oysters and mussels. It is highly gregarious and seeks out adult shells for settlement, forming characteristic ‘stacks’ of adults. But it also recorded from rock, artificial substrata, and Sabellaria alveolata reefs (Blanchard, 1997, 2009; Bohn et al., 2012, 2013a, 2013b, 2015; De Montaudouin et al., 2018; Hinz et al., 2011; Helmer et al., 2019; Powell-Jennings & Calloway, 2018; Preston et al., 2020; Tillin et al., 2020). Close examination of the literature (2023) shows that evidence of its colonization and density on bedrock in the infralittoral or circalittoral was lacking. Tillin et al. (2020) suggested that Crepidula could colonize circalittoral rock due to its presence on tide-swept rough grounds at 60 metres in the English Channel (Hinz et al., 2011). However, Hinz et al. (2011) reported that Crepidula fornicata only dominated one assemblage (with an average of 181 individuals per trawl) on a gravel substratum with boulders. Bohn et al. (2015) noted that Crepidula occurred at low density or was absent in areas dominated by boulders. Bohn et al. (2013a, 2013b, 2015) and Preston et al. (2020) showed that while Crepidula could settle on slate panels or ‘stone’, it preferred shell, especially that of conspecifics. In addition, no evidence was found of the effect of Crepidula populations on faunal turf-dominated habitats. It was only recorded at low density (0.1-0.9/m²) in one faunal turf biotope (CR.MCR.CFaVS.CuSpH.As) (JNCC, 2015). Faunal turfs are dominated by suspension feeders, so larval predation is probably high, which may prevent colonization by Crepidula. Also, faunal turf species actively compete for space, and many are fast-growing and opportunistic, so they may out-compete Crepidula for space even if it gained a foothold in the community. 

Sensitivity assessment

The wave-exposed circalittoral rock characterizing this biotope is likely to be unsuitable for the colonization by Crepidula fornicata. Crepidula has been recorded from areas of strong tidal streams (Hinz et al., 2011), and has been recorded from the lower intertidal to ca 160 m in depth, but it is most common in the shallow subtidal above 50 m (Blanchard, 1997; Thieltges et al., 2003; Bohn et al., 2012, 2015; Hinz et al., 2011; OBIS, 2023; Tillin et al., 2020). Therefore, colonization of Crepidula would be limited to low densities in deeper examples of the biotope. However, no evidence was found of the effect of Crepidula populations on faunal turf-dominated habitats or circalittoral rock habitats. At present, there is 'Insufficient evidence' to suggest that the circalittoral rock biotopes are sensitive to colonization by Crepidula fornicata; further evidence is required.

The carpet sea squirt, Didemnum vexillum

Evidence

The carpet sea squirt Didemnum vexillum (syn. Didemnum vestitum; Didemnum vestum) is a colonial ascidian with rapidly expanding populations that have invaded most temperate coastal regions around the world (Kleeman, 2009; Stefaniak et al., 2012; Tillin et al., 2020). It is an ‘ecosystem engineer’ that can change or modify invaded habitats and alter biodiversity (Griffith et al., 2009; Mercer et al., 2009). Didemnum vexillum has colonized and established populations in the northeast Pacific, Canadian and USA coast; New Zealand; France, Spain, and the Wadden Sea, Netherlands; the Mediterranean Sea and Adriatic Sea (Bullard et al., 2007; Coutts & Forrest, 2007; Dijkstra et al., 2007; Valentine et al., 2007a; Valentine et al., 2007b; Lambert, 2009; Hitchin, 2012; Tagliapietra et al., 2012; Gittenberger et al., 2015; Vercaemer et al., 2015; McKenzie et al., 2017; Cinar & Ozgul, 2023; Holt, 2024). In the UK, Didemnum vexillum has colonized Holyhead marina and Milford Haven, Wales; the west coast of Scotland (marinas around Largs, Clyde, Loch Creran and Loch Fyne), South Devon (Plymouth, Yealm, and Dartmouth estuaries), the Solent, northern Kent, Essex, and Suffolk coasts (Griffith et al., 2009; Lambert, 2009; Hitchin, 2012; Minchin & Nunn, 2013; Bishop et al., 2015; McKenzie et al., 2017; Tillin et al., 2020, Holt, 2024; NBN, 2024).

Although a widespread invader, Didemnum vexillum has a limited ability for natural dispersal since the pelagic larvae remain in the water column for a short time (up to 36 hours). Therefore, it has a short dispersal phase that can allow the species to build localized populations (Herborg et al., 2009; Vercaemer et al., 2015; Holt, 2024). However, Bullard et al. (2007) suggested that Didemnum vexillum can form new colonies asexually by fragmentation. Colonies can produce long tendrils from an encrusting colony, which can fragment, disperse and settle, attaching to suitable hard substrata elsewhere (Bullard et al., 2007; Lambert, 2009; Stefaniak & Whitlatch, 2014). A fragmented colony can spread naturally for up to three weeks, transported by ocean currents, attached to floating seaweed, seagrass or other floating biota, or as free-floating spherical colonies (Bullard et al., 2007; Lengyel et al., 2009; Stefaniak & Whitlatch, 2014; Holt, 2024). Fragments can reattach to suitable substrata within six hours of contact. Fragments have the potential to disperse around 20 km before reattachment (Lengyel et al., 2009). Valentine et al. (2007a) reported that colonies of Didemnum vexillum enlarged by 6 to 11 times by asexual budding after 15 days and enlarged 11 to 19 times after 30 days. Valentine et al. (2007a) concluded fragments could successfully grow, survive, and help to spread Didemnum vexillum.

While natural fragmentation of tendrils is thought to allow Didemnum vexillum to invade longer distances and increase its dispersal potential, Stefaniak & Whitlatch (2014) found that only one tendril out of 80 reattached to the flat, bare substrata used in their study, because tendrils required an extensive (at least eight-hour) period of contact to reattach. Stefaniak & Whitlatch (2014) suggested that once fragmented from a colony, the success of tendril reattachment was limited, and reattachment was not a major contributor to the invasive success of Didemnum vexillum. However, Stefaniak & Whitlatch (2014) also found that larvae-packed tendril fragments may increase natural dispersal distance, reproduction, and invasive success of Didemnum vexillum, and increase the distance larvae can travel. Not all colonies produce tendrils at all locations.

Human-mediated transport via aquaculture facilities, boat hulls, commercial fishing vessels, and ballast water is probably the most important vector that has aided the long-distance dispersal of Didemnum vexillum and explains its prevalence in harbours and marinas (Bullard et al., 2007; Dijkstra et al., 2007; Griffith et al., 2009; Herborg et al., 2009). Fragmentation of colonies during transport or human disturbance (such as trawling or dredging) could indirectly disperse the species and enable it to find suitable conditions for establishment (Herborg et al., 2009). For example, in oyster farms in British Columbia, large fragments of Didemnum sp. come off oyster strings when they are pulled out of water, and other fragments can be pulled off oysters and mussels and thrown back into the water, which is likely to aid dispersal of the invasive species (Bullard et al., 2007). Dijkstra et al. (2007) hypothesised that Didemnum sp. was introduced to the Gulf of Maine with oyster aquaculture in the Damariscotta River and transported via Pacific oysters.

Didemnum vexillum was likely introduced into the UK from northern Europe or Ireland via poorly maintained or not antifouled vessels, movement of contaminated shellfish stock and aquaculture equipment, or via marine industries such as oil, gas, renewables, and dredging (Holt, 2024). Recent evidence from genetic material suggests that human-mediated dispersal between marinas and shellfish culture sites is the most likely pathway for connectivity of Didemnum vexillum populations throughout Ireland and Britain (Prentice et al., 2021; Holt, 2024). Didemnum vexillum can disperse away from artificial substrata, invading and colonizing natural substrata in surrounding areas (Tillin et al., 2020). Holt (2024) noted that Didemnum vexillum had not spread as far as feared in the UK since it was first recorded. The current evidence of Didemnum vexillum’s ability to spread on natural habitats in this area is sparse and often conflicting, complicated by genetics, its apparent variable habitat preferences and tolerances and its variable ability to adapt to ‘new’ conditions (Holt 2024).

Didemnum vexillum has a seasonal growth cycle that is influenced by temperature (Valentine et al., 2007a). In warmer months (June and July), colonies may be large and well-developed encrusting mats. Populations experience more rapid growth from July to September, sometimes continuing into December. Colonies begin to decline in health and ‘die-off’ when temperatures drop below 5°C during winter months from around October to April (Gittenberger, 2007; Valentine et al., 2007a; Herborg et al., 2009). Cold-water months cause colonies to regress and reduce in size, yet they often regenerate as temperatures warm (Griffith et al., 2009; Kleeman, 2009; Mercer et al., 2009), although some populations may not survive winter at all (Dijkstra et al., 2007). The early growth phase, from May to July, is initiated by smaller colonies developing from remnants of colonies that survived the cold water (Valentine et al., 2007a). The seasonal growth cycle is also likely influenced by location. For example, the Didemnum sp. growth cycle for colonies in Sandwich tide pool (temperature range from -1 °C to 24 °C, with daily fluctuations) probably does not occur in deep offshore subtidal habitats in Georges Bank (annual temperature range from 4 °C to 15°C, and daily fluctuations are minimal) (Valentine et al., 2007a). Larval release and recruitment typically occur between 14 and 20°C and slow or cease below 9 to 11°C as summer ends (Griffith et al., 2009; McKenzie et al., 2017). In New Zealand, recruitment occurs from November to July, where the highest average temperatures were recorded in February (18 to 22°C), and the lowest average temperatures were recorded in July (9 to 10°C) (Fletcher et al., 2013a). In this New Zealand study, higher water temperatures were associated with a higher level of recruitment (Fletcher et al., 2013a).

Didemnum vexillum requires suitable hard substrata for successful settlement and the establishment of colonies. It can grow quickly and establish large colonies of dense encrusting mats on a variety of hard substrata (Valentine et al., 2007a; Griffith et al., 2009; Lambert, 2009; Groner et al., 2011; Cinar & Ozgul, 2023). Gittenberger (2007) stated that invasive Didemnum sp. was a threat to native ecosystems because of its ability to overgrow virtually all hard substrata present. Suitable hard substrata can include rocky substrata such as bedrock, gravel, pebble, cobble, or boulders or artificial substrata such as a variety of maritime structures, such as pontoons, docks, wood and metal pilings, chains, ropes and moorings, plastic and ship hulls and at aquaculture facilities (Valentine et al., 2007a&b; Bullard et al., 2007; Griffith et al., 2009; Lambert, 2009; Tagliapietra et al., 2012; Tillin et al., 2020). Didemnum vexillum has been reported colonizing these types of hard substrata in the USA, Canada, northern Kent, and the Solent (Bullard et al., 2007; Valentine et al., 2007a; Valentine et al., 2007b; Hitchin, 2012; Vercaemer et al., 2015; Tillin et al., 2020).

Didemnum vexillum has the ability to rapidly overgrow and displace on other sessile organisms such as other colonial ascidians (Ciona intestinalis, Styela clava, Ascidiella aspera, Botrylloides violaceus, Botryllus schlosseri, Diplosoma listerianium and Aplidium spp.), bryozoan, hydroids, sponges (Clione celata and Halichrondria sp.), anemone (Diadumene cincta), calcareous tube worms, eelgrass (Zostera marina), kelp (Laminaria spp. and Agarum sp.), green algae (Codium fragile subsp. fragile), red algae (Plocamium, Chondrus crispus and bush weed Agardhiella subulata), brown algae (Ascophyllum nodosum, Sargassum, Halidrys, Fucus evanescens and Fucus serratus), calcareous algae (Corallina officinalis), mussels (Mytilus galloprovincialis, Perna canaliculus  and Mytilus edulis), barnacles, oysters (Magallana gigas, Ostrea edulis and Crassostrea virginica), sea scallops (Placopecten magellanicus), or dead shells (Dijkstra et al., 2007; Gittenberger, 2007; Valentine et al., 2007a; Valentine et al., 2007b; Griffith et al., 2009; Carman & Grunden, 2010; Dijkstra & Nolan, 2011; Groner et al., 2011; Hitchin, 2012; Tagliapietra et al., 2012; Minchin & Nunn, 2013; Gittenberger et al., 2015; Long & Groholz, 2015; Vercaemer et al., 2015).

In contrast, Didemnum vexillum has a preference for sheltered conditions, as established colonies observed in Georges Bank and Long Island Sound were exposed to moderately strong tidal currents (1 to 2 knots; ca 0.5 to 1 m/s recorded at both sites) that may mobilise sediment (Valentine et al., 2007b; Mercer et al., 2009; Tillin et al., 2020). However, Valentine et al. (2007b) describe the substratum as immobile, presumably consolidated, gravel, cobbles, and pebbles. Kleeman (2009) stated that the presence of a consistent mild wave action or ‘swash zone’ appears to favour Didemnum sp. establishment in the intertidal. Although some evidence suggests that waves and currents can facilitate the fragmentation and spread of Didemnum vexillum (McKenzie et al., 2017), the tidal current velocities at some sites where Didemnum vexillum has been reported (for example, New England, where current velocities reach up to around 3 m/s) is lower than the current velocity required for the dislodgement of Didemnum vexillum fragments (around 7.6 m/s) (Reinhardt et al., 2012). This suggests that not all tidal currents are likely to dislodge Didemnum vexillum fragments. When on boat hulls, the species can experience higher current velocities, which are enough to cause dislodgement (Reinhardt et al., 2012).

Sensitivity assessment

Didemnum vexillum has been recorded in the sublittoral to depths of 81 m in Georges Bank and 30 m in Long Island, USA (Bullard et al., 2007; Valentine et al., 2007b; Mercer et al., 2009). This biotope, and child biotopes, occurs on bedrock, which could provide a suitable hard substratum for colonization by Didemnum sp. Didemnum vexillum is reported to prefer sheltered conditions but has also been recorded in moderately strong currents (Valentine et al., 2007b; Mercer et al., 2009; Tillin et al., 2020) and is predicted to survive stronger currents, as the current velocity which will dislodge Didemnum vexillum is around 7.6 m/s (Reinhardt et al., 2012). This biotope, and child biotopes, experiences very weak to moderately strong water flow (1 to 3 m/s) and moderately exposed to extremely exposed wave exposure. However, the effect of wave action reduces with depth, so it is possible that only the most wave-exposed examples of the biotope could be unsuitable for Didemnum. Didemnum vexillum regresses as temperatures decline in winter, so shallow examples may be able to recover their condition in winter (Gittenberger, 2007; Valentine et al., 2007a; Herborg et al., 2009). However, deeper examples may not experience enough temperature change to trigger the decline in Didemnum vexillum (Valentine et al., 2007a). The EcCR.CarSp biotopes are characterized by grazing, primarily due to Echinus esculentus, a general forager and grazer of faunal and floral turfs. Switzer et al. (2011) reported that green (Strongylocentrotus droebachiensis) and red (Strongylocentrotus franciscanus) sea urchins consumed this tunicate, but concluded that Strongylocentrotus droebachiensis was an ineffective biological control for the tunicate in oyster culture. No evidence was found to suggest that Echinus could not consume Didemnum, but the sea squirt’s ability to produce acidic secretions suggests it could be unpalatable. If Didemnum sp. could gain a 'foothold', it might overgrow, smother or cause mortality of epifauna. Holt (2024) noted that Didemnum vexillum had not spread as far as feared in the UK since it was first recorded. Therefore, a resistance of 'Medium' (some, <25% mortality) is suggested as a precaution in case Didemnum vexillum could colonize the biotope, but with 'Low' confidence due to the lack of direct evidence. Resilience is assessed as 'Very low' as recovery would require the physical removal of Didemnum sp., so sensitivity is assessed as 'Medium'. 

The Pacific oyster, Magallana gigas

Evidence

Most of the evidence indicates that circalittoral rock and other habitats that occur at depths more than 10 m are unlikely to be suitable for Magallana gigas because it is considered an intertidal and shallow subtidal species rarely recorded below extreme low water (Herbert et al., 2012, 2016; Tillin et al., 2020). Therefore, this biotope is probably 'Not sensitive to this INIS.

Wireweed, Sargassum muticum

Evidence

The depth and sedimentation probably exclude macroalgae from this biotope. Hence, it is unlikely to be colonized by Sargassum. Therefore, this biotope is probably 'Not sensitive to this INIS.

Wakame, Undaria pinnatifida

Evidence

The depth and sedimentation probably exclude macroalgae from this biotope. Hence, it is unlikely to be colonized by Undaria. Therefore, this biotope is probably 'Not sensitive’ to this INIS.

Other INIS

Evidence

This biotope is classified as circalittoral, and therefore no algal species have been considered. Several invasive bryozoans are of concern, including Bugula neritina (Porter et al., 2017), Schizoporella japonica (Ryland et al., 2014) and Tricellaria inopinata (Dyrynda et al., 2000; Cook et al., 2013b), however, evidence of potential effects is sparse. Limited evidence exists for Bugula neritina in the UK; however, it was found in Northern Ireland for the first time in 2006, then seen again in both 2008 and 2013 in abundance and attached to buoys and kelp stipes throughout Carrickfergus Marina (Porter et al., 2017). Tricellaria inopinata has been reported to colonize the byssal threads of the mussel Mytilus galloprovincialis, Hymeniacidon perleve and the ascidian Styela plicata (Dyrynda et al., 2000). Tricellaria inopinata dominated the fouling community in the Lagoon of Venice, within seven years of being introduced (Ambrogi, 2000). At present, there is 'Insufficient evidence' to suggest that the circalittoral biotopes are sensitive to colonization by algal or other invasive species; further evidence is required.