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Alcyonium digitatum and faunal crust communities on vertical circalittoral bedrock

Distribution MapBIO Map Legend

Summary

UK and Ireland classification

Description

This biotope typically occurs on the vertical faces and overhangs of exposed to moderately exposed lower infralittoral and upper circalittoral bedrock subject to moderately strong to weak tidal streams. Due to the large numbers of the urchin Echinus esculentus often recorded, this biotope tends to have a grazed appearance, and the bedrock is often encrusted with pink coralline algae, encrusting bryozoans such as Parasmittina trispinosa and the calcareous tubeworm Spirobranchus triqueter. Dense aggregations of dead mans fingers Alcyonium digitatum may be present along with the cup coral Caryophyllia smithii. Other species present include the echinoderms Asterias rubens, Ophiothrix fragilis and Antedon bifida, the ascidians Clavelina lepadiformis, Ciona intestinalis and Ascidia mentula, the anthozoans Urticina felina, Cortynactis viridis, Metridium senile and Cylista elegans, the gastropod Calliostoma zizyphinum and the crustacean Cancer pagurus. Three regional variations of this biotope have been recorded. One variant found typically off the north-east coast of Scotland and around the Northern Isles, has a very impoverished appearance dominated by anthozoans. A second variant occurs along the west coast of Scotland, extending to Rockall in the west, and the Northern Isles in the north-east, and has a more fauna, characterized by hydroids, sponges, anthozoans and echinoderms. A third variant occurs along the north-east coast of England (Northumberland) up to the Northern Isles and is dominated by Alcyonium digitatum, brittlestars and Echinus esculentus. (Information from Connor et al., 2004; JNCC, 2015).

Depth range

5-10 m, 10-20 m, 20-30 m, 30-50 m

Additional information

-

Listed By

Sensitivity reviewHow is sensitivity assessed?

Sensitivity characteristics of the habitat and relevant characteristic species

The biotope occurs on vertical faces and overhangs and is characterized by a heavily grazed faunal crust of encrusting bryozoans such as Parasmittina trispinosa with dense aggregations of Alcyonium digitatum.       

Grazing by the sea urchin Echinus esculentus is considered significant in preserving the nature of this biotope and loss of this species is likely to significantly affect the biotope to the extent that reclassification would be necessary.  For this sensitivity assessment, Alcyonium digitatum and Echinus esculentus are the primary focus of research as the important characterizing species defining CR.MCR.EcCr.AdigVt, with species making up the faunal crust (such as the bryozoan Parasmittina trispinosa) considered where appropriate. Other species present in these biotopes are considered transient, mobile or ubiquitous and are therefore not considered significant to the assessment of the sensitivity of these biotopes.  However, information on the sensitivity of other characterizing species is included where appropriate. 

Resilience and recovery rates of habitat

Alcyonium digitatum is a colonial species of soft coral with a wide distribution in the North Atlantic, recorded from Portugal (41°N) to Northern Norway (70°N) as well as on the east coast of North America (Hartnoll, 1975). Colonies consist of stout “finger-like” projections (Hartnoll, 1975) that can reach up to 20 cm tall and can dominate circalittoral rock habitats (as in CR.HCR.FaT.CTub.Adig; Connor et al., 2004). Alcyonium digitatum colonies are likely to have a lifespan that exceeds 20 years as colonies have been followed for 28 years in marked plots (Lundälv, pers. comm., in Hartnoll, 1998). Colonies that are 10-15 cm in height have been aged between five and ten years old (Hartnoll, unpublished). The majority of colonies are unisexual, with the majority of individuals being female.  Sexual maturity is predicted, at the earliest when the colony reaches the second year of growth. However, the majority of colonies are not predicted to reach maturity until their third year when they attain biomass of approximately 20-30g (Hartnoll, 1975).

Alcyonium digitatum spawns from December and January. Gametes are released into the water and fertilization occurs externally. The embryos are neutrally buoyant and float freely for seven days. The embryos give rise to actively swimming lecithotrophic planulae which 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). In laboratory experiments, several larvae of Alcyonium digitatum failed to settle within 10 days, presumably finding the conditions unsuitable. These larvae were able to survive 35 weeks as non-feeding planulae. After 14 weeks some were still swimming and after 24 weeks the surface ciliation was still active although they rested on the bottom of the tanks. By the end of the experiment at 35 weeks, the larvae had shrunk to a diameter of 0.3 mm. This ability to survive for long periods in the plankton may favour the dispersal and eventual discovery of a site suitable for settlement (Hartnoll, 1975). 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. Young Alcyonium digitatum will consequently be able to take advantage of an abundant food resource in spring and be well developed before the appearance of other forms that may otherwise compete for the same substrata. In addition, because the planulae do not feed whilst in the pelagic zone, they do not suffer by being released at the time of minimum plankton density and they may also benefit from the scarcity of predatory zooplankton which would otherwise feed upon them (Hartnoll, 1975).

Echinus esculentus is a sea urchin found within the north-east Atlantic, recorded from Murmansk Coast, Russia to Portugal (Hansson, 1998). Echinus esculentus is estimated to have a lifespan of 8-16 years (Nichols, 1979; Gage, 1992) and reaches sexual maturity within 1-3 years (Tyler-Walters, 2008). 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 English Channel (Comely & Ansell, 1988) but decreases during spawning in spring and then increases again through summer and winter until the next spawning season. Spawning occurs just before the seasonal rise in temperature in temperate zones but is probably not triggered by rising temperature (Bishop, 1985). Echinus esculentus is a broadcast spawner, with a complex larval life history which includes a blastula, gastrula and a characteristic four-armed echinopluteus stage, which forms an important component of the zooplankton. MacBride (1914) observed planktonic larval development could take 45-60 days in captivity.

Recruitment is sporadic or variable depending on locality. For example, Millport populations showed annual recruitment, whereas few recruits were found in Plymouth populations during Nichols’ studies between 1980-1981 (Nichols, 1984). Bishop & Earll (1984) suggested that the population of Echinus esculentus at St Abbs had a high density and recruited regularly whereas the Skomer population was sparse, ageing and had probably not successfully recruited larvae in the previous six years (Bishop & Earll, 1984). Comely & Ansell (1988) noted that the largest number of Echinus esculentus occurred below the kelp forest.

Echinus esculentus is a mobile species and could therefore migrate and re-populate an area quickly if removed. For example, Lewis & Nichols (1979a) found that adults were able to colonize an artificial reef in small numbers within three months and the population steadily grew over the following year.  If completely removed from a site and local populations are naturally sparse, then recruitment may be dependent on larval supply, which can be highly variable. As suggested by Bishop & Earll (1984), the Skomer, Wales Echinus esculentus population had most likely not successfully recruited for six years, which would suggest the mature population would be highly sensitive to removal and may not return for several years. The Prestige oil tanker spilt 63 000t of fuel 130 nautical miles off Galicia, Spain in November 2002. High wave action and strong weather systems increased the mixing of the oil to “some” depth within the water column, causing sensitive faunal communities to be affected. The biological community of Guéthary, France was monitored preceding and for nine years following the oil spill. Following the oil spill, taxonomic richness decreased significantly from 57 recorded species to 41, which included the loss of Echinus esculentus from the site. Two to three years after the oil spill, taxonomic richness had increased to pre-spill levels and Echinus esculentus had returned (Castège et al., 2014).

Coralline crusts, Parasmittina trispinosa and Caryophyllia smithii are also important within the CR.MCR.EcCr.AdigVt biotope. Studies by Edyvean & Ford (1984a; 1986; 1987) of populations of coralline crusts, namely Lithophyllum incrustans, suggest that reproduction may be sexual or asexual (on average early in the third year), and spores are released throughout the year with seasonal variation as fewer spores were produced in the summer. The authors also found that spore survival was extremely low and young mortality was high, but individuals after the age of ten appear relatively long-lived (up to 30 years). Some repair of damaged encrusting coralline occurs through vegetative growth, so recolonization by propagules may also be an important mechanism for rapid recovery (Chamberlain, 1996; Airoldi, 2000).

There is limited information on the life history traits of Parasmittina trispinosa. Eggleston (1972a) noted in the Isle of Man, a peak in reproductive and vegetative growth was not well marked and that the number of embryos present was fairly constant throughout the year, which indicated that Parasmittina trispinosa could potentially reproduce annually within the UK. Caryophyllia smithii is a small (max 3 cm across) solitary coral common within tide swept sites of the UK (Wood, 2005). Fowler & Laffoley (1993) suggested that Caryophyllia smithii was a slow-growing species, which suggested that inter-specific spatial competition with colonial faunal or algae species were important factors in determining local abundance (Bell & Turner, 2000). Caryophyllia smithii reproduces sexually with gamete release most likely triggered by seasonal temperature increases (typically from January-April) (Tranter et al., 1982). The pelagic stage of the larvae may last up to ten weeks, which provides this species with a good dispersal capability (Tranter et al., 1982). 

Resilience assessment. Echinus esculentus can reportedly reach sexual maturity within 1-2 years (Tyler-Walters, 2008), however, as highlighted by Bishop & Earll (1984) and Castège et al. (2014) recovery may take 2-6 years (possibly more if local recruitment is poor).  Alcyonium digitatum can recruit onto bare surfaces within two years, however, may take up to five years to become a dominant component of the community (Whomersley & Picken, 2003; Hiscock et al., 2010). The faunal crust is heavily grazed by Echinus esculentus and, together with the evidence presented, is likely to be quite resilient.   If the community declined significantly (resistance of ‘None’ or ‘Low’) resilience would be assessed as ‘Medium’ (recovery in 2-10 years). However, where resistance was assessed as ‘Medium’ or ‘High’ then resilience would be assessed as ‘High’.

Climate Change Pressures

Use / to open/close text displayedResistanceResilienceSensitivity
Low Very Low High
Q: Medium
A: Medium
C: Medium
Q: High
A: High
C: High
Q: Medium
A: Medium
C: Medium

Alcyonium digitatum is a boreal species of octocoral recorded along the Atlantic Coasts of Europe from Portugal to Norway and Iceland, and along the northwest 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. 

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 primarily between sea temperatures of 5 and 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 24 hr 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. 

Elevated seawater temperatures generally increase the metabolic rate of bryozoans and are expected to affect calcification (Smith, 2014; Moreno, 2020). The bryozoan Parasmittina trispinosa has been recorded from Britain, north to western Norway, the Faroe Isles, northwestern Atlantic coasts, the Californian coast and the Gulf of Mexico. 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.

Coralline crusts, including Lithophyllum incrustans are found further south than the UK and are considered to tolerate increased temperatures, although they may be more sensitive to drying rather than higher temperatures. Edyvean & Ford (1984b) suggest that populations of Lithophyllum incrustans are affected by temperature changes and salinity and that temperature and salinity ‘shocks’ induce spawning but no information on thresholds was provided (Edyvean & Ford, 1984b). Populations of Lithophyllum incrustans were less stable in tide pools with a smaller volume of water, which were more exposed to temperature and salinity changes due to lower buffering capacity. Sexual plants (or the spores that give rise to them) were suggested to be more susceptible than asexual plants to extremes of local environmental variables (temperature, salinity etc.) as they occur with greater frequency at sites where temperature and salinity were more stable (Edyvean & Ford, 1984b). Lithophyllum incrustans is close to the northern edge of its range and is likely to tolerate increased temperatures.

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 and 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).

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. While no evidence on the impacts of ocean warming on the characterizing species Alcyonium digitatum was found, biogeographic distribution is often a good predictor of temperature tolerance (Jeffree & Jeffree, 1994). The distribution of Alcyonium digitatum suggests that the species is likely to be impacted by ocean warming, as populations of this species only occur where seawater temperatures range up to 20°C (www.obis.org).  Parasmittina trispinosa, Caryophyllia smithii and the coralline crusts are found in geographical locations with higher temperatures than in the UK so that these species are unlikely to be affected by an increase in seawater temperature under these scenarios. However, the sea urchin Echinus esculentus cannot tolerate high temperatures for prolonged periods. A reduction in grazing due to loss of Echinus esculentus may result in loss of the biotope if it is replaced by a more abundant and diverse faunal turf, eg. FaT.CTub.Adig.  As CR.MCR.EcCr.AdigVt is a grazed biotope and the main characterizing species Alcyonium digitatum is unknown to tolerate high temperatures 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.

Low Very Low High
Q: Medium
A: Medium
C: Medium
Q: High
A: High
C: High
Q: Medium
A: Medium
C: Medium

Alcyonium digitatum is a boreal species of octocoral recorded along the Atlantic Coasts of Europe from Portugal to Norway and Iceland, and along the northwest 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. 

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 primarily between sea temperatures of 5 and 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 24 hr 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. 

Elevated seawater temperatures generally increase the metabolic rate of bryozoans and are expected to affect calcification (Smith, 2014; Moreno, 2020). The bryozoan Parasmittina trispinosa has been recorded from Britain, north to western Norway, the Faroe Isles, northwestern Atlantic coasts, the Californian coast and the Gulf of Mexico. 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.

Coralline crusts, including Lithophyllum incrustans are found further south than the UK and are considered to tolerate increased temperatures, although they may be more sensitive to drying rather than higher temperatures. Edyvean & Ford (1984b) suggest that populations of Lithophyllum incrustans are affected by temperature changes and salinity and that temperature and salinity ‘shocks’ induce spawning but no information on thresholds was provided (Edyvean & Ford, 1984b). Populations of Lithophyllum incrustans were less stable in tide pools with a smaller volume of water, which were more exposed to temperature and salinity changes due to lower buffering capacity. Sexual plants (or the spores that give rise to them) were suggested to be more susceptible than asexual plants to extremes of local environmental variables (temperature, salinity etc.) as they occur with greater frequency at sites where temperature and salinity were more stable (Edyvean & Ford, 1984b). Lithophyllum incrustans is close to the northern edge of its range and is likely to tolerate increased temperatures.

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 and 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).

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. While no evidence on the impacts of ocean warming on the characterizing species Alcyonium digitatum was found, biogeographic distribution is often a good predictor of temperature tolerance (Jeffree & Jeffree, 1994). The distribution of Alcyonium digitatum suggests that the species is likely to be impacted by ocean warming, as populations of this species only occur where seawater temperatures range up to 20°C (www.obis.org).  Parasmittina trispinosa, Caryophyllia smithii and the coralline crusts are found in geographical locations with higher temperatures than in the UK so that these species are unlikely to be affected by an increase in seawater temperature under these scenarios. However, the sea urchin Echinus esculentus cannot tolerate high temperatures for prolonged periods. A reduction in grazing due to loss of Echinus esculentus may result in loss of the biotope if it is replaced by a more abundant and diverse faunal turf, eg. FaT.CTub.Adig.  As CR.MCR.EcCr.AdigVt is a grazed biotope and the main characterizing species Alcyonium digitatum is unknown to tolerate high temperatures 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.

Low Very Low High
Q: Medium
A: Medium
C: Medium
Q: High
A: High
C: High
Q: Medium
A: Medium
C: Medium

Alcyonium digitatum is a boreal species of octocoral recorded along the Atlantic Coasts of Europe from Portugal to Norway and Iceland, and along the northwest 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. 

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 primarily between sea temperatures of 5 and 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 24 hr 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. 

Elevated seawater temperatures generally increase the metabolic rate of bryozoans and are expected to affect calcification (Smith, 2014; Moreno, 2020). The bryozoan Parasmittina trispinosa has been recorded from Britain, north to western Norway, the Faroe Isles, northwestern Atlantic coasts, the Californian coast and the Gulf of Mexico. 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.

Coralline crusts, including Lithophyllum incrustans are found further south than the UK and are considered to tolerate increased temperatures, although they may be more sensitive to drying rather than higher temperatures. Edyvean & Ford (1984b) suggest that populations of Lithophyllum incrustans are affected by temperature changes and salinity and that temperature and salinity ‘shocks’ induce spawning but no information on thresholds was provided (Edyvean & Ford, 1984b). Populations of Lithophyllum incrustans were less stable in tide pools with a smaller volume of water, which were more exposed to temperature and salinity changes due to lower buffering capacity. Sexual plants (or the spores that give rise to them) were suggested to be more susceptible than asexual plants to extremes of local environmental variables (temperature, salinity etc.) as they occur with greater frequency at sites where temperature and salinity were more stable (Edyvean & Ford, 1984b). Lithophyllum incrustans is close to the northern edge of its range and is likely to tolerate increased temperatures.

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 and 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).

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. While no evidence on the impacts of ocean warming on the characterizing species Alcyonium digitatum was found, biogeographic distribution is often a good predictor of temperature tolerance (Jeffree & Jeffree, 1994). The distribution of Alcyonium digitatum suggests that the species is likely to be impacted by ocean warming, as populations of this species only occur where seawater temperatures range up to 20°C (www.obis.org).  Parasmittina trispinosa, Caryophyllia smithii and the coralline crusts are found in geographical locations with higher temperatures than in the UK so that these species are unlikely to be affected by an increase in seawater temperature under these scenarios. However, the sea urchin Echinus esculentus cannot tolerate high temperatures for prolonged periods. A reduction in grazing due to loss of Echinus esculentus may result in loss of the biotope if it is replaced by a more abundant and diverse faunal turf, eg. FaT.CTub.Adig.  As CR.MCR.EcCr.AdigVt is a grazed biotope and the main characterizing species Alcyonium digitatum is unknown to tolerate high temperatures 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.

Low Low High
Q: Medium
A: Medium
C: Medium
Q: High
A: High
C: High
Q: Medium
A: Medium
C: Medium

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).

No studies on the impacts of marine heatwaves on Alcyonium digitatum were found. However, this species appears to be restricted to colder waters and occurs in seawater temperatures between 5 and 20°C (www.obis.org).  Therefore, Alcyonium digitatum is likely to be impacted by heatwaves under both scenarios. 

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 and 19°C however, continued exposure to 19°C disrupted gametogenesis. In addition, embryos and larvae developed abnormally after 24 hr exposure to 15°C (Bishop, 1985). Therefore, marine heatwaves have the potential to impact the reproduction, recruitment and survival of Echinus esculentus. 

Elevated seawater temperatures generally increase the metabolic rate of bryozoans and is expected to affect calcification (Smith, 2014; Leveroni, 2020). However, although Parasmittina trispinosa generally occurs in temperatures between 5 and 15°C, 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 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.

Cocito & Sgorbini (2014) studied spatial and temporal patterns of colonial bryozoans in the Ligurian Sea over nine years, during those years marine heatwave events caused mass mortality among a number of species. A 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. 

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 and 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. 

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 the characteristic species of this biotope, however as Alcyonium digitatum is not known to tolerant seawater temperatures >20°C and the sea urchin Echinus esculentus cannot tolerate high temperatures, resistance is assessed as “Low”, under the middle and high emissions scenario, and resilience is assessed as “Low”, so the biotope is assessed as “High” sensitivity.

Low Low High
Q: Medium
A: Medium
C: Medium
Q: High
A: High
C: High
Q: Medium
A: Medium
C: Medium

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).

No studies on the impacts of marine heatwaves on Alcyonium digitatum were found. However, this species appears to be restricted to colder waters and occurs in seawater temperatures between 5 and 20°C (www.obis.org).  Therefore, Alcyonium digitatum is likely to be impacted by heatwaves under both scenarios. 

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 and 19°C however, continued exposure to 19°C disrupted gametogenesis. In addition, embryos and larvae developed abnormally after 24 hr exposure to 15°C (Bishop, 1985). Therefore, marine heatwaves have the potential to impact the reproduction, recruitment and survival of Echinus esculentus. 

Elevated seawater temperatures generally increase the metabolic rate of bryozoans and is expected to affect calcification (Smith, 2014; Leveroni, 2020). However, although Parasmittina trispinosa generally occurs in temperatures between 5 and 15°C, 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 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.

Cocito & Sgorbini (2014) studied spatial and temporal patterns of colonial bryozoans in the Ligurian Sea over nine years, during those years marine heatwave events caused mass mortality among a number of species. A 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. 

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 and 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. 

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 the characteristic species of this biotope, however as Alcyonium digitatum is not known to tolerant seawater temperatures >20°C and the sea urchin Echinus esculentus cannot tolerate high temperatures, resistance is assessed as “Low”, under the middle and high emissions scenario, and resilience is assessed as “Low”, so the biotope is assessed as “High” sensitivity.

Medium Very Low Medium
Q: Medium
A: Medium
C: Medium
Q: High
A: High
C: High
Q: Medium
A: Medium
C: Medium

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 pCOindependent of their taxonomic group or habitat preferences (Widdicombe & Spicer, 2008; Kroeker et al., 2013).

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. 

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 huttoniEvechinus 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. 

NeverthelessSuckling 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 (urchins, Evechinus chloroticus), allowing turf algae to increase in abundance. Therefore, ocean acidification could cause changes to community structure. 

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 was found. However, Swezey et al. (2017) observed that populations of bryozoans raised under high CO2 (1254 μatm; pH 7.60) conditions grew faster, invested less in reproduction and produced lighter skeletons when compared to genetically identical clones raised under current surface atmospheric CO2 values (400 μatm; pH 8.04). In addition, the bryozoans under high CO2 altered the 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. 

Brodie et al. (2014) reported that Corallina species were more resilient to ocean acidification than other calcified algae species, although competition from flesh algal species that benefit from high CO2 may indirectly cause the loss of calcified species from biotopes. Similarly, observations have indicated Corallinales to be adversely affected at locations where CO2 gradients occur naturally, with evidence of Corallinales being outcompeted by heterokont algae at Mediterranean CO2 seeps (Martin and Hall-Spencer, 2017).  

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 acidificationRodolfo-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

Sensitivity Assessment. While no evidence of the effect of ocean acidification on Alcyonium digitatum was found, the effects of ocean acidification on other species of octocoral show some resilience to low pH. Alcyonium digitatum larvae settlement has been reported to be sensitive to environmental conditions, but the conditions were not stated. Ocean acidification studies have shown negative impacts on the health and reproduction of sea urchins. In addition, bryozoans appear to be highly sensitive to ocean acidification, with impacts on health, survival and reproduction.  Unfortunately, at present, there are no studies to determine whether Alcyonium digitatum, Echinus esculentus and Parasmittina trispinosa can adapt or acclimate to future pH conditions but, on the evidence available, Parasmittina trispinosa and Echinus esculentus could be lost from this biotope. Therefore, as CR.MCR.EcCr.AdigVt is a grazed biotopeunder both the middle and high emission scenarios (0.15 and 0.35 pH unit decrease, respectively) the biotope is assessed as having a resistance level of 'low', and a resilience level of ‘Very low’ because of the long-term nature of ocean acidification.  Therefore, sensitivity is assessed as ‘High’ under both scenarios, albeit with ‘Low’ confidence.

Medium Very Low Medium
Q: Medium
A: Medium
C: Medium
Q: High
A: High
C: High
Q: Medium
A: Medium
C: Medium

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 pCOindependent of their taxonomic group or habitat preferences (Widdicombe & Spicer, 2008; Kroeker et al., 2013).

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. 

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 huttoniEvechinus 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. 

NeverthelessSuckling 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 (urchins, Evechinus chloroticus), allowing turf algae to increase in abundance. Therefore, ocean acidification could cause changes to community structure. 

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 was found. However, Swezey et al. (2017) observed that populations of bryozoans raised under high CO2 (1254 μatm; pH 7.60) conditions grew faster, invested less in reproduction and produced lighter skeletons when compared to genetically identical clones raised under current surface atmospheric CO2 values (400 μatm; pH 8.04). In addition, the bryozoans under high CO2 altered the 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. 

Brodie et al. (2014) reported that Corallina species were more resilient to ocean acidification than other calcified algae species, although competition from flesh algal species that benefit from high CO2 may indirectly cause the loss of calcified species from biotopes. Similarly, observations have indicated Corallinales to be adversely affected at locations where CO2 gradients occur naturally, with evidence of Corallinales being outcompeted by heterokont algae at Mediterranean CO2 seeps (Martin and Hall-Spencer, 2017).  

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 acidificationRodolfo-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

Sensitivity Assessment. While no evidence of the effect of ocean acidification on Alcyonium digitatum was found, the effects of ocean acidification on other species of octocoral show some resilience to low pH. Alcyonium digitatum larvae settlement has been reported to be sensitive to environmental conditions, but the conditions were not stated. Ocean acidification studies have shown negative impacts on the health and reproduction of sea urchins. In addition, bryozoans appear to be highly sensitive to ocean acidification, with impacts on health, survival and reproduction.  Unfortunately, at present, there are no studies to determine whether Alcyonium digitatum, Echinus esculentus and Parasmittina trispinosa can adapt or acclimate to future pH conditions but, on the evidence available, Parasmittina trispinosa and Echinus esculentus could be lost from this biotope. Therefore, as CR.MCR.EcCr.AdigVt is a grazed biotopeunder both the middle and high emission scenarios (0.15 and 0.35 pH unit decrease, respectively) the biotope is assessed as having a resistance level of 'low', and a resilience level of ‘Very low’ because of the long-term nature of ocean acidification.  Therefore, sensitivity is assessed as ‘High’ under both scenarios, albeit with ‘Low’ confidence.

High High Not sensitive
Q: Low
A: NR
C: NR
Q: High
A: High
C: High
Q: Low
A: Low
C: Low

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 5-50 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.AdigVt can occur from 5-50 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, high and extreme emission 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’. 

High High Not sensitive
Q: Low
A: NR
C: NR
Q: High
A: High
C: High
Q: Low
A: Low
C: Low

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 5-50 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.AdigVt can occur from 5-50 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, high and extreme emission 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’. 

High High Not sensitive
Q: Low
A: NR
C: NR
Q: High
A: High
C: High
Q: Low
A: Low
C: Low

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 5-50 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.AdigVt can occur from 5-50 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, high and extreme emission 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’. 

Hydrological Pressures

Use / to open/close text displayedResistanceResilienceSensitivity
Medium High Low
Q: Low
A: NR
C: NR
Q: High
A: High
C: High
Q: Low
A: Low
C: Low

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) (Hartnoll, 1975; Budd, 2008) and is commonly found across the British Isles (Fish  & Fish, 1992).  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 Echinus esculentus occurred at temperatures between 0-18°C in Limfjord, Denmark. Bishop (1985) noted that gametogenesis occurred at 11-19°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).  Parasmittina trispinosa is commonly found across the whole of the British Isles (NBN, 2015) and is distributed from the Northern coast of Norway to the Mediterranean (Hayward & Ryland, 1990). 

Cocito & Sgorbini (2014) studied spatial and temporal patterns of colonial bryozoans in the Ligurian Sea over 9 years.  High temperature events were recorded, the first causing mass mortality among a number of species.  Decline in Pentapora fascialis colony cover between 11 and 22m depth following unusually warm summer in 1999 (temperature at 11m of 23.87 ± 1.4 °C and resulted in a 86% reduction in live colony portion) with larger colonies most affected.  Gradual recovery took place, with deeper communities recovering to pre-disturbance levels within 4 years.

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 Echinus esculentus occurred at temperatures between 0-18°C in Limfjord, Denmark. Bishop (1985) noted that gametogenesis occurred at 11-19°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).  High temperature (up to 31 °C) had little effect on the growth, survival and boring rate of the sponge Cliona celata (Duckworth & Bradley, 2012).  

Sensitivity assessment. Whilst Alcyonium digitatum and Parasmittina trispinosa are likely to tolerate an increase in temperature at the benchmark level, evidence suggests that Echinus esculentus may be affected. Resistance has been assessed as ‘Medium’, resilience has been assessed as ‘High’ and sensitivity has been assessed as ‘Low’.

High High Not sensitive
Q: Medium
A: Medium
C: Medium
Q: High
A: High
C: High
Q: Medium
A: Medium
C: Medium

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) (Hartnoll, 1975; Budd, 2008).   Alcyonium digitatum was also reported to be apparently unaffected by the severe winter of 1962-1963 where air temperature reached -5.8°C (Crisp, 1964a). Parasmittina trispinosa is commonly found across the whole of the British Isles (NBN, 2015) and is distributed from the northern coast of Norway to the Mediterranean (Hayward& Ryland, 1990).

Ursin (1960) reported Echinus esculentus occurred at temperatures between 0-18°C in Limfjord, Denmark. Bishop (1985) noted that gametogenesis occurred at 11-19°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.

Sensitivity assessment. None of the characterizing species are at their southern distribution limit and are unlikely to suffer mortality by a decrease in temperature at the benchmark level.  Resistance is therefore recorded as ‘High’, resilience as ‘High’ and the biotope is ‘Not Sensitive’ at the benchmark level.

Low Medium Medium
Q: Low
A: NR
C: NR
Q: Medium
A: Medium
C: Medium
Q: Low
A: Low
C: Low

Echinoderms are generally stenohaline and possess no osmoregulatory organ (Boolootian, 1966) and lack the ability to osmo- and ion-regulate (Stickle & Diehl, 1987). The inability of echinoderms to osmoregulate extracellularly causes body fluid volume to decrease when individuals experience higher external salinity.  Protracted hypersalinity is likely to result in the decline of echinoderm populations. Echinoderm larvae have a narrow range of salinity tolerance and will develop abnormally and die if exposed to increased salinity (Tyler-Walters, 2008).   Alcyonium digitatum distribution and the depth at which it occurs also suggest it would not likely experience regular salinity fluctuations and therefore not resists significant increases in salinity.  CR.MCR.EcCr.AdigVt occurs in full salinity (Connor et al., 2004), it is therefore possible that an increase in salinity may cause a decline in the abundance of Alcyonium digitatum, Echinus esculentus and the faunal crust.

Sensitivity assessment. It is likely that Echinus esculentus is stenohaline and hypersaline conditions would probably affect the species.  Resistance has been assessed as ‘Low’, resilience as ‘Medium’, and sensitivity has been assessed as ‘Medium’.  Due to the lack of information regarding salinity effects on the characterizing species, confidence in this assessment has been assessed as low.

Low Medium Medium
Q: Low
A: NR
C: NR
Q: Medium
A: Medium
C: Medium
Q: Low
A: Low
C: Low

Alcyonium digitatum is found at the entrances to sea lochs (Budd, 2008) and estuaries (Braber & Borghouts, 1977) where salinity may vary occasionally. Furthermore, Alcyonium digitatum is found within a number of variable salinity biotopes, e.g. MCR.BYH.Flu.Hocu (Connor et al., 2004).  However, its distribution and the depth, at which it occurs suggest that Alcyonium digitatum would not likely often experience salinity fluctuations and is, therefore, unlikely to survive significant reductions in salinity (Budd, 2008).

Echinoderms are generally unable to tolerate low salinity (stenohaline) and possess no osmoregulatory organ (Boolootian, 1966). 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. Echinus esculentus is found within a number of variable and reduced salinity biotopes, e.g. IR.LIR.KVS.SlatPsaVS (Connor et al., 2004).

Sensitivity assessment. CR.MCR.EcCr.AdigVt is recorded exclusively in full marine conditions (30-35 ppt) (Connor et al., 2004). Records from the MNCR suggest Alcyonium digitatum and Echinus esculentus can occur in reduced salinity habitats, however the evidence suggests that these species would decrease in abundance.  In addition, a reduction in salinity may result in a reduction in species richness of the biotope anTherefore, resistance has been assessed as ‘Low’, Resilience as ‘Medium’. Sensitivity has been assessed as ‘Medium’.

High High Not sensitive
Q: Medium
A: Medium
C: Medium
Q: High
A: High
C: High
Q: Medium
A: Medium
C: Medium

The biotope (EcCr.AdigVT) occurs in extremely to moderately wave exposed conditions in areas of moderately strong to negligible water flow at 5-50m depth (Connor et al., 2004).  The biotope is structured by grazing, especially by Echinus esculentus.  The biotope probably occurs at a critical range of water movement that allows the Echinus population to remain in high enough abundance to structure the biotope.  Deep examples probably depend on water flow or extreme wave action, while shallow examples depend on wave action or water flow.

Alcyonium digitatum and the bryozoans are suspension feeders relying on water currents to supply food. These taxa, therefore, thrive in conditions of vigorous water flow e.g. around Orkney and St Abbs, Scotland, where the community may experience tidal currents of 3 and 4 knots (1.5 and 2 m/s) during spring tides (De Kluijver, 1993).

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. 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, 1979a). Therefore, increased water flow may remove the population from the affected area, probably to deeper water, however individuals would probably not be killed in the process and could recolonize the area quickly.

Sensitivity assessment. This biotope occurs negligible water flow, so a reduction in water flow would therefore not affect the biotope.  All characterizing species are likely to be tolerant of an increase at the benchmark level (0.1-0.2 m/s), being present in biotopes with stronger water flow. Resistance is therefore ‘High’, resilience is ‘High’ and the biotope is ‘Not sensitive’ at the benchmark level.

Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

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.

High High Not sensitive
Q: Medium
A: Medium
C: Medium
Q: High
A: High
C: High
Q: Medium
A: Medium
C: Medium

The biotope (EcCr.AdigVT) occurs in extremely to moderately wave exposed conditions in areas of moderately strong to negligible water flow at 5-50 m depth (Connor et al., 2004).  The biotope is structured by grazing, especially by Echinus esculentus.  The biotope probably occurs at a critical range of water movement that allows the Echinus population to remain in high enough abundance to structure the biotope.  Deep examples probably depend on water flow or extreme wave action, while shallow examples depend on wave action or water flow.

Alcyonium digitatum are suspension feeders relying on water currents to supply food. These taxa therefore thrive in conditions of vigorous water flow. As a circalittoral biotope (recorded from 5 - 50 m), the depth at which these biotopes occur may therefore also reduce the direct physical effects of a localised change in wave height; wave attenuation is directly related to water depth (Hiscock, 1983).

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, 1979a). Keith Hiscock (pers. comm.) reported Echinus esculentus occurred in significant numbers as shallow as 15 m below low water at the extremely wave exposed site of Rockall, Scotland.

Sensitivity assessment. Whilst storm events may have an impact on the biotope, a change at the benchmark level is not likely to have a significant effect on the characterizing species. Resistance has been assessed as ‘High’, resilience has been assessed as ‘High’ and the biotope is assessed as ‘Not sensitive’ at the benchmark level.

Chemical Pressures

Use / to open/close text displayedResistanceResilienceSensitivity
Not Assessed (NA) Not assessed (NA) Not assessed (NA)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

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 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 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).

Not Assessed (NA) Not assessed (NA) Not assessed (NA)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

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

Echinus esculentus was reported absent after the oil spill however 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.

Not Assessed (NA) Not assessed (NA) Not assessed (NA)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

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 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.

No evidence (NEv) Not relevant (NR) No evidence (NEv)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

'No evidence' was found.

Not Assessed (NA) Not assessed (NA) Not assessed (NA)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

This pressure is Not assessed.

Low Medium Medium
Q: High
A: Medium
C: Medium
Q: High
A: High
C: High
Q: High
A: Medium
C: Medium

Mass mortality of species including Echinus esculentus was observed due to a stratified hypoxic event below 8 m caused by a phytotplankton bloom ( Griffiths et al., 1979).  Hiscock & Hoare (1975) reported an oxycline forming in the summer months (Jun-Sep) in a quarry lake (Abereiddy, Pembrokeshire) from close to full oxygen saturation at the surface to <5% saturation below ca 10 m.  During these summer events, no echinoderms were recorded at depths below 10 - 11 m.  At the time of writing there was insufficient evidence on which to assess this pressure. There is anecdotal evidence to suggest that Alcyonium digitatum is sensitive to hypoxic events. However, because the degree of de-oxygenation wasn’t quantified the evidence cannot be compared to the pressure benchmark.  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. 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 were abundant, Alcyonium sp. and bryozoans were also in an impoverished state. During follow up surveys conducted in early September Alcyonium sp. were noted to be much healthier and feeding. It was suggested the decay of Gyrodinium aureolum either reduced oxygen levels or physically clogged faunal feeding mechanisms. Adjacent reefs where 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 effected by the phytoplankton bloom (Griffiths et al., 1979).   CR.MCR.EcCr.AdigVt is recorded from very weak to moderately strong tidal streams (negligible to 1.5 m/sec) (Connor et al., 2004). Whilst mixing with surrounding oxygenated water is likely to occur in examples of this biotope that experience moderate water movement (Dennis, 1979), de-oxygenation is likely to become a chronic factor in examples where there is negligible water movement.

Sensitivity assessment. The characterizing species are likely to suffer significant mortality in a hypoxic event at the benchmark level, especially in examples of the biotope that occur with negligible water flow in which it may take longer for the oxygen levels to recover.  Whilst the majority of assessed species are sessile, Echinus esculentus is mobile and may escape the hypoxic event (depending on extent and conditions). Resistance is assessed as ‘Low’, Resilience as ‘Medium’ (assuming recovery to normal oxygen conditions) and sensitivity as ‘Medium’.

Not relevant (NR) Not relevant (NR) Not sensitive
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

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.  Alcyonium digitatum is a suspension feeders of phytoplankton and zooplankton. Nutrient enrichment of coastal waters that enhances the population of phytoplankton may be beneficial to these species in terms of an increased food supply but the effects are uncertain (Hartnoll, 1998). 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 (see de-oxygenation).  Nutrient enrichment could also lead to algal blooms.

This biotope is considered to be 'Not sensitive' at the pressure benchmark that assumes compliance with good status as defined by the WFD.

No evidence (NEv) Not relevant (NR) No evidence (NEv)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

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. Alcyonium digitatum is a suspension feeders of phytoplankton and zooplankton. Organic enrichment of coastal waters that enhances the population of phytoplankton may be beneficial to these species in terms of an increased food supply but the effects are uncertain (Hartnoll, 1998). 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, (see de-oxygenation)

Sensitivity assessment.  ‘No evidence’ of the effects of organic enrichment in circalittoral faunal crusts was found.

Physical Pressures

Use / to open/close text displayedResistanceResilienceSensitivity
None Very Low High
Q: High
A: High
C: High
Q: High
A: High
C: High
Q: High
A: High
C: High

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.

None Very Low High
Q: High
A: High
C: High
Q: High
A: High
C: High
Q: High
A: High
C: High

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.

Sensitivity assessment. Resistance to the pressure is considered ‘None’, and resilience ‘Very low’. Sensitivity has been assessed as ‘High’.

Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

‘Not relevant’ to biotopes occurring on bedrock.

Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

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.

Medium High Low
Q: Medium
A: Medium
C: Low
Q: High
A: High
C: High
Q: Medium
A: Medium
C: Low

Alcyonium digitatum, Echinus esculentus and Parasmittina trispinosa are sessile or slow moving species that might be expected to suffer from the effects of abrasion. 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. 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 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.  Species with fragile tests, such as Echinus esculentus were reported to suffer badly as a result of scallop or queen scallop dredging (Bradshaw et al., 2000; Hall-Spencer & Moore, 2000a). 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.  The authors used simultaneous assessment of both bycatch and organisms left on the seabed to estimate capture efficiency for both target and non-target organisms. This found 16.4% of Echinus esculentus were crushed or dead, 29.3% had >50% spine loss/minor cracks, 1.1% had  <50% spine loss and the remaining 53.3% were in good condition. Sea urchins can rapidly regenerate spines, e.g. Psammechinus miliaris were found to re-grow all spines within a period of 2 months (Hobson, 1930).  The trawling examples mentioned above were conducted on sedimentary habitats and thus the evidence is not directly relevant to rock based, however it does indicate the likely effects of abrasion on Echinus esculentus.

Sensitivity assessment. Whilst abrasion pressures tend to heavily impact sessile or slow moving marine species, the evidence suggests that mortality amongst the characterizing species is ‘Medium’ (<25% loss) for the characterizing  Alcyonium digitatum and Echinus esculentus.  It should be noted that this is dependent on the abrasion activity and heavier gears may well cause more damage.  Based on the evidence for the characterizing species, resistance is ‘Medium’, resilience as ‘High’ and sensitivity as ‘Low’. Please note, Boulcott & Howell (2011) did not mention the abrasion caused by fully loaded collection bags on the new haven dredges. A fully loaded Newhaven dredge may cause higher damage to community than indicated in their study.

Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

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.

High High Not sensitive
Q: Medium
A: Medium
C: Medium
Q: High
A: High
C: High
Q: Medium
A: Medium
C: Medium

Alcyonium digitatum and Parasmittina trispinosa are not thought to be highly susceptible to changes in water clarity due to the fact 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.  Also, Moore (1977a) suggested that Echinus esculentus was unaffected by turbid conditions. Echinus esculentus is an important grazer in CR.MCR.EcCr.AdigVt but also feeds on detritus or dissolved organic material (Lawrence, 1975, Comely & Ansell, 1988). 

Sensitivity assessment. The biotope is faunally dominated and circalittoral and is therefore not dependent on light, so a change in suspended sediment is unlikely to affect the characterizing species and Resistance is therefore assessed as ‘High’, Resilience as ‘High’ and the biotope is ‘Not sensitive’.

High High Not sensitive
Q: Low
A: NR
C: NR
Q: High
A: High
C: High
Q: Low
A: Low
C: Low

CR.MCR.EcCr.AdigVt occurs on vertical faces and overhangs which would afford the characterizing species protection in the event of sediment deposition.  Alcyonium digitatum is sessile and thus would be unable to avoid the deposition of a smothering layer of sediment, however, colonies can attain a height of up to 20 cm (; Budd, 2008; Edwards, 2008), so would still be able to feed in the event of sediment deposition. Parasmittina trispinosa is an encrusting species and would thus likely be smothered, and depending on sediment retention, could block larval settlement. Echinus esculentus are mobile, large globular urchins which can reach a diameter of 17 cm (Tyler-Walters, 2000).  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 the sediment may negatively affect successive recruitment events, however this is unlikely given the typically vertical nature of the biotope.

Sensitivity assessment. CR.MCR.EcCr.AdigVt occurs on vertical faces and overhangs and sedimentation would be unlikely, with removal likely to be rapid.  Areas at the base of the rock could be affected, but overall, resistance is assessed as ‘High’, resilience as ‘High’ and the biotope is ‘Not sensitive’ at the benchmark level.

Medium High Low
Q: Low
A: NR
C: NR
Q: High
A: High
C: High
Q: Low
A: Low
C: Low

CR.MCR.EcCr.AdigVt typically occurs on vertical faces and overhangs which would afford the characterizing species some protection in the event of sediment deposition. 

Alcyonium digitatum is sessile and thus would be unable to avoid the deposition of a smothering layer of sediment.  However Alcyonium digitatum colonies can attain a height of up to 20 cm (; Budd, 2008; Edwards, 2008), so would still be able to feed in the event of sediment deposition. However, Parasmittina trispinosa is an encrusting species and would thus likely be smothered, and depending on sediment retention, could block larval settlement. Echinus esculentus are mobile, large globular urchins which can reach a diameter of 17 cm (Tyler-Walters, 2000).  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 could interfere with larval settlement.  If sediment is retained within the host biotope for extended periods a layer sediment may negatively affect successive recruitment events, however given that the biotope tends to occur on vertical faces and overhangs, this is unlikely.

Sensitivity assessment. CR.MCR.EcCr.AdigVt occurs on vertical faces and overhangs and sedimentation would be unlikely, with removal likely to be rapid.  Smothering at the base of rocks could result in burial of the encrusting bryozoans and would affect Alcyonium digitatum which grows to 20 cm tall (Budd, 2008; Edwards, 2008). A cautious assessment of ‘Medium’ resistance is applied. Resilience is ‘High’ and sensitivity is ‘Low’.

Not Assessed (NA) Not assessed (NA) Not assessed (NA)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

Not assessed.

No evidence (NEv) Not relevant (NR) No evidence (NEv)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

'No evidence' was found.

Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

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.   

High High Not sensitive
Q: Low
A: NR
C: NR
Q: High
A: High
C: High
Q: Low
A: Low
C: Low

There is some evidence that the basiepithelial nerve plexus below the entire outer skins of echinoderms is sensitive to light (Hill, 2008). There is no evidence to suggest that algal species would benefit if exposed to anthropogenic light sources. CR.MCR.EcCr.AdigVt is a circalittoral biotope and therefore defined as occurring at low light levels due to depth. Increased shading (e.g. by construction of a pontoon, pier etc) could be beneficial to the characterizing species within these biotopes and

Sensitivity assessment. Resistance is assessed as 'High', with 'High' resilience and the biotope is 'Not Sensitive'.

Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

‘Not relevant’ as barriers and changes in tidal excursion are not relevant to biotopes restricted to open waters.

Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

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

Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

Not relevant

Biological Pressures

Use / to open/close text displayedResistanceResilienceSensitivity
Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

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 Alcyonium digitatum and Parasmittina trispinosa was found.

No evidence (NEv) Not relevant (NR) No evidence (NEv)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

Styela clava was first recorded in the UK at Plymouth in 1952 (Eno et al., 1997). Where Styela clava and Ciona intestinalis co-occur they may compete for space and food (Jackson, 2008).

Didemnum vexillum is an invasive colonial sea squirt native to Asia which was first recorded in the UK in Darthaven Marina, Dartmouth in 2005. Didemnum vexillum can form extensive mats over the substrata it colonizes; binding boulders, cobbles and altering the host habitat (Griffith et al., 2009). Didemnum vexillum can also grow over and smother the resident biological community. Recent surveys within Holyhead Marina, North Wales have found Didemnum vexillum growing on and smothering native tunicate communities, including Ciona intestinalis (Griffith et al., 2009). Due to the rapid-re-colonization of Didemnum vexillum eradication attempts have to date failed. 

Presently, Didemnum vexillum is isolated to several sheltered locations in the UK (NBN, 2015). However Didemnum vexillum has successfully colonized the offshore location of the Georges Bank, USA (Lengyel et al., 2009) which is more exposed than the locations which Didemnum vexillum have colonized in the UK. It is therefore possible that Didemnum vexillum could colonize more exposed locations within the UK and could therefore pose a threat to these biotopes.  A number of invasive bryozoans are of concern including Schizoporella japonica (Ryland et al., 2014) and Tricellaria inopinata (Dyrynda et al., 2000; Cook et al., 2013b).

However, there is ‘No evidence’ regarding known invasive species colonizing this biotope.  Due to the constant risk of new invasive species, the literature for this pressure should be revisited.

No evidence (NEv) Not relevant (NR) No evidence (NEv)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

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.

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

No evidence (NEv) Not relevant (NR) No evidence (NEv)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

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.

None Medium Medium
Q: Low
A: NR
C: NR
Q: Medium
A: Medium
C: Medium
Q: Low
A: Low
C: Low

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

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Citation

This review can be cited as:

Readman, J.A.J. & Williams, E., 2021. Alcyonium digitatum and faunal crust communities on vertical circalittoral bedrock. In Tyler-Walters H. and Hiscock K. Marine Life Information Network: Biology and Sensitivity Key Information Reviews, [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 10-12-2022]. Available from: https://www.marlin.ac.uk/habitat/detail/1097

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Last Updated: 30/03/2021