The Marine Life Information Network

Global warming (extreme)

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

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

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

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

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

Evidence

Cerianthus lloydii adults are locally abundant in many localities on all coasts of the British Isles and in some areas are common on the shore. This species occurs on all western coasts of Europe from Greenland and Spitzbergen south to the Bay of Biscay. Larvae, but not adults, have been recorded from the Mediterranean (De Kluijver et al., 2024). OBIS (2024) lists records of Cerianthus lloydii from sea surface temperatures of -5 to 20°C, although the majority of records were from 10 to15°C. Similarly, the majority of records for other burrowing anemones (Mesacmaea mitchellii and Cereus pedunclatus) are also between 10 to 15°C. Cerianthus lloydii is predominantly distributed in northern regions, and is likely to move further northward as the climate changes and temperature shifts out of its preferred range. There is no further information available on the temperature tolerance of Cerianthus lloydii.

Cerianthids can occur across wide temperature range (8.36 to 11.51°C) and depth ranges from shallow waters to deep-sea environments (238 to 1,070 m). Hydrozoans have been recorded in depths ranging to 8,400 m, indicating adaptations to varied thermal niches throughout the water column (Davies et al., 2014; Stepanjants & Chernyshev, 2015). However, there is limited evidence for thermal thresholds and thermal ranges were available for the characterizing species recorded in this biotope and large knowledge gaps limit current understanding of the ecological feedback that may occur to cerianthids as a result of climate change driven temperature increase.

Sensitivity assessment. Under the middle and high emission and extreme scenarios seawater temperatures are expected to temperatures rise by 3 to 5°C to potential southern summer temperatures of 23 to 24°C and northern summer temperatures of 17 to 19°C. The distribution of the important characterizing species Cerianthus lloydii suggest that they are probably resistant of chronic change in temperature for a year, but exposure to a short-term acute increase in temperature may cause Cerianthus lloydii to withdraw into their burrows temporarily. But most records of Cerianthus lloydii occur north of the Bay of Biscay so there is the possibility that it may be driven north or the greater depth to avoid ongoing climate mediated temperature increases. Therefore, for all three scenarios (middle and high emission and extreme scenarios) resistance is assessed as ‘Medium’ as a precaution. Resilience is assessed as ‘Very Low’ due to the long-term nature of ocean warming. This biotope is assessed as ‘Medium’ sensitivity to ocean warming under all three scenarios. However, there is limited evidence on thermal tolerances of the characterizing species recorded in this biotope and confidence in the assessment is ‘Low’.

Global warming (high)

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

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

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

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

Evidence

Cerianthus lloydii adults are locally abundant in many localities on all coasts of the British Isles and in some areas are common on the shore. This species occurs on all western coasts of Europe from Greenland and Spitzbergen south to the Bay of Biscay. Larvae, but not adults, have been recorded from the Mediterranean (De Kluijver et al., 2024). OBIS (2024) lists records of Cerianthus lloydii from sea surface temperatures of -5 to 20°C, although the majority of records were from 10 to15°C. Similarly, the majority of records for other burrowing anemones (Mesacmaea mitchellii and Cereus pedunclatus) are also between 10 to 15°C. Cerianthus lloydii is predominantly distributed in northern regions, and is likely to move further northward as the climate changes and temperature shifts out of its preferred range. There is no further information available on the temperature tolerance of Cerianthus lloydii.

Cerianthids can occur across wide temperature range (8.36 to 11.51°C) and depth ranges from shallow waters to deep-sea environments (238 to 1,070 m). Hydrozoans have been recorded in depths ranging to 8,400 m, indicating adaptations to varied thermal niches throughout the water column (Davies et al., 2014; Stepanjants & Chernyshev, 2015). However, there is limited evidence for thermal thresholds and thermal ranges were available for the characterizing species recorded in this biotope and large knowledge gaps limit current understanding of the ecological feedback that may occur to cerianthids as a result of climate change driven temperature increase.

Sensitivity assessment. Under the middle and high emission and extreme scenarios seawater temperatures are expected to temperatures rise by 3 to 5°C to potential southern summer temperatures of 23 to 24°C and northern summer temperatures of 17 to 19°C. The distribution of the important characterizing species Cerianthus lloydii suggest that they are probably resistant of chronic change in temperature for a year, but exposure to a short-term acute increase in temperature may cause Cerianthus lloydii to withdraw into their burrows temporarily. But most records of Cerianthus lloydii occur north of the Bay of Biscay so there is the possibility that it may be driven north or the greater depth to avoid ongoing climate mediated temperature increases. Therefore, for all three scenarios (middle and high emission and extreme scenarios) resistance is assessed as ‘Medium’ as a precaution. Resilience is assessed as ‘Very Low’ due to the long-term nature of ocean warming. This biotope is assessed as ‘Medium’ sensitivity to ocean warming under all three scenarios. However, there is limited evidence on thermal tolerances of the characterizing species recorded in this biotope and confidence in the assessment is ‘Low’.

Global warming (middle)

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

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

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

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

Evidence

Cerianthus lloydii adults are locally abundant in many localities on all coasts of the British Isles and in some areas are common on the shore. This species occurs on all western coasts of Europe from Greenland and Spitzbergen south to the Bay of Biscay. Larvae, but not adults, have been recorded from the Mediterranean (De Kluijver et al., 2024). OBIS (2024) lists records of Cerianthus lloydii from sea surface temperatures of -5 to 20°C, although the majority of records were from 10 to15°C. Similarly, the majority of records for other burrowing anemones (Mesacmaea mitchellii and Cereus pedunclatus) are also between 10 to 15°C. Cerianthus lloydii is predominantly distributed in northern regions, and is likely to move further northward as the climate changes and temperature shifts out of its preferred range. There is no further information available on the temperature tolerance of Cerianthus lloydii.

Cerianthids can occur across wide temperature range (8.36 to 11.51°C) and depth ranges from shallow waters to deep-sea environments (238 to 1,070 m). Hydrozoans have been recorded in depths ranging to 8,400 m, indicating adaptations to varied thermal niches throughout the water column (Davies et al., 2014; Stepanjants & Chernyshev, 2015). However, there is limited evidence for thermal thresholds and thermal ranges were available for the characterizing species recorded in this biotope and large knowledge gaps limit current understanding of the ecological feedback that may occur to cerianthids as a result of climate change driven temperature increase.

Sensitivity assessment. Under the middle and high emission and extreme scenarios seawater temperatures are expected to temperatures rise by 3 to 5°C to potential southern summer temperatures of 23 to 24°C and northern summer temperatures of 17 to 19°C. The distribution of the important characterizing species Cerianthus lloydii suggest that they are probably resistant of chronic change in temperature for a year, but exposure to a short-term acute increase in temperature may cause Cerianthus lloydii to withdraw into their burrows temporarily. But most records of Cerianthus lloydii occur north of the Bay of Biscay so there is the possibility that it may be driven north or the greater depth to avoid ongoing climate mediated temperature increases. Therefore, for all three scenarios (middle and high emission and extreme scenarios) resistance is assessed as ‘Medium’ as a precaution. Resilience is assessed as ‘Very Low’ due to the long-term nature of ocean warming. This biotope is assessed as ‘Medium’ sensitivity to ocean warming under all three scenarios. However, there is limited evidence on thermal tolerances of the characterizing species recorded in this biotope and confidence in the assessment is ‘Low’.

Marine heatwaves (high)

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

Evidence

Cerianthus lloydii adults are locally abundant in many localities on all coasts of the British Isles and in some areas are common on the shore. This species occurs on all western coasts of Europe from Greenland and Spitzbergen south to the Bay of Biscay. Larvae, but not adults, have been recorded from the Mediterranean (De Kluijver et al., 2024). OBIS (2024) lists records of Cerianthus lloydii from sea surface temperatures of -5 to 20°C, although the majority of records were from 10 to 15°C. Similarly, the majority of records for other burrowing anemones (Mesacmaea mitchellii and Cereus pedunclatus) are also between 10 to 15°C. There is no further evidence available on the upper thermal limit of Cerianthus lloydii.

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 occur 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. As a precaution, this biotope has been assessed as having ‘Medium’ sensitivity to marine heatwaves. The recovery of Cerianthus lloydii populations is likely to be ‘Low’, hence, recovery may be interrupted by the occurrence of a further heatwave before the habitat can recover.  Therefore, resilience has been assessed as ‘Very Low’, leading to a sensitivity assessment of ‘Medium’ for this biotope, but with ‘Low’ confidence.

Marine heatwaves (middle)

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

Evidence

Cerianthus lloydii adults are locally abundant in many localities on all coasts of the British Isles and in some areas are common on the shore. This species occurs on all western coasts of Europe from Greenland and Spitzbergen south to the Bay of Biscay. Larvae, but not adults, have been recorded from the Mediterranean (De Kluijver et al., 2024). OBIS (2024) lists records of Cerianthus lloydii from sea surface temperatures of -5 to 20°C, although the majority of records were from 10 to 15°C. Similarly, the majority of records for other burrowing anemones (Mesacmaea mitchellii and Cereus pedunclatus) are also between 10 to 15°C. There is no further evidence available on the upper thermal limit of Cerianthus lloydii.

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 occur 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. As a precaution, this biotope has been assessed as having ‘Medium’ sensitivity to marine heatwaves. The recovery of Cerianthus lloydii populations is likely to be ‘Low’, hence, recovery may be interrupted by the occurrence of a further heatwave before the habitat can recover.  Therefore, resilience has been assessed as ‘Very Low’, leading to a sensitivity assessment of ‘Medium’ for this biotope, but with ‘Low’ confidence.

Ocean acidification (high)

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

Evidence

Increasing levels of CO₂ in the atmosphere have led to the average pH of sea surface waters dropping from 8.25 in the 1700s to 8.14 in the 1990s (Jacobson, 2005). Decreased pH, and therefore shoaling of the calcium carbonate saturation horizon, may adversely impact UK sessile benthic communities (Danovaro et al., 2017). However, cerianthid anemones are not calcifying organisms, and therefore, may not be impacted by inhibited calcification due to pH decline. Furthermore, analyses that focused on taxa present in the North Atlantic indicated that deep-sea communities containing cerianthids may not be adversely impacted by ocean acidification (Johnson et al., 2018). Knowledge gaps and data paucity were highlighted as key limitations in the study, therefore, conclusive assessments of pH decline impacts to such communities could not be made (Johnson et al., 2018). There is no direct evidence regarding the effects of ocean acidification on Cerinthus lloydii or other burrowing anemones.

Ocean acidification has been documented to reduce calcification rates in some hydrocorals (de Barros Marangoni et al., 2017). In addition, hydrozoan metabolites required for cell protection from osmotic and thermal stress (betaine) have been found to be inhibited under extreme acidification (pH 7.7 to 7.75) as a result of climate change (Boco et al., 2019). In contrast, studies utilising over 40 years of Continuous Plankton Recorder (CPR) data from the North Sea have indicated a significant correlative relationship between increased hydrozoan abundance and a temporal pH decline of 0.2 (Attrill et al., 2007; Fabry et al., 2008). Furthermore, some hydrozoans have been indicated to have high tolerances to wide pH variability, exhibiting complete mortality at pH 4 and 10, and with high survival rates (>50%) within the pH range of 5 to 8.5 (Gutierre, 2012). However, tolerance ranges are likely to be species specific, therefore, it is possible that these findings cannot be inferred beyond the study.

Sensitivity Assessment. Cerianthids are not calcifying organisms, therefore, they are not considered to be impacted by the shoaling of the calcium carbonate saturation horizon due to ocean acidification (Danovaro et al., 2017). Ocean acidification may limit calcification in some hydrocorals and has also been attributed to inhibiting metabolic function in hydrozoan metabolites required for cell protection against osmotic and thermal stress. (Boco et al., 2019; de Barros Marangoni et al., 2017). However, these impacts may be species-specific. Conversely, some hydrozoans have been found to be highly tolerant to wide pH ranges and CPR data have indicated a significant correlative relationship between increased hydrozoan abundance and increased acidification in the North Sea (Attrill et al., 2007; Fabry et al., 2008; Gutierre, 2012).

Therefore, based on the evidence available, under both the middle and high emission scenarios resistance is assessed as ‘Medium’ to represent the possible impact on hydroids. Resilience is assessed as ‘Very Low’ as it is an ongoing long-term pressure, and sensitivity as ‘Medium’ at the benchmark level but with ‘Low’ confidence.

Ocean acidification (middle)

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

Evidence

Increasing levels of CO₂ in the atmosphere have led to the average pH of sea surface waters dropping from 8.25 in the 1700s to 8.14 in the 1990s (Jacobson, 2005). Decreased pH, and therefore shoaling of the calcium carbonate saturation horizon, may adversely impact UK sessile benthic communities (Danovaro et al., 2017). However, cerianthid anemones are not calcifying organisms, and therefore, may not be impacted by inhibited calcification due to pH decline. Furthermore, analyses that focused on taxa present in the North Atlantic indicated that deep-sea communities containing cerianthids may not be adversely impacted by ocean acidification (Johnson et al., 2018). Knowledge gaps and data paucity were highlighted as key limitations in the study, therefore, conclusive assessments of pH decline impacts to such communities could not be made (Johnson et al., 2018). There is no direct evidence regarding the effects of ocean acidification on Cerinthus lloydii or other burrowing anemones.

Ocean acidification has been documented to reduce calcification rates in some hydrocorals (de Barros Marangoni et al., 2017). In addition, hydrozoan metabolites required for cell protection from osmotic and thermal stress (betaine) have been found to be inhibited under extreme acidification (pH 7.7 to 7.75) as a result of climate change (Boco et al., 2019). In contrast, studies utilising over 40 years of Continuous Plankton Recorder (CPR) data from the North Sea have indicated a significant correlative relationship between increased hydrozoan abundance and a temporal pH decline of 0.2 (Attrill et al., 2007; Fabry et al., 2008). Furthermore, some hydrozoans have been indicated to have high tolerances to wide pH variability, exhibiting complete mortality at pH 4 and 10, and with high survival rates (>50%) within the pH range of 5 to 8.5 (Gutierre, 2012). However, tolerance ranges are likely to be species specific, therefore, it is possible that these findings cannot be inferred beyond the study.

Sensitivity Assessment. Cerianthids are not calcifying organisms, therefore, they are not considered to be impacted by the shoaling of the calcium carbonate saturation horizon due to ocean acidification (Danovaro et al., 2017). Ocean acidification may limit calcification in some hydrocorals and has also been attributed to inhibiting metabolic function in hydrozoan metabolites required for cell protection against osmotic and thermal stress. (Boco et al., 2019; de Barros Marangoni et al., 2017). However, these impacts may be species-specific. Conversely, some hydrozoans have been found to be highly tolerant to wide pH ranges and CPR data have indicated a significant correlative relationship between increased hydrozoan abundance and increased acidification in the North Sea (Attrill et al., 2007; Fabry et al., 2008; Gutierre, 2012).

Therefore, based on the evidence available, under both the middle and high emission scenarios resistance is assessed as ‘Medium’ to represent the possible impact on hydroids. Resilience is assessed as ‘Very Low’ as it is an ongoing long-term pressure, and sensitivity as ‘Medium’ at the benchmark level but with ‘Low’ confidence.

Sea level rise (extreme)

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

Evidence

Sea-level rise is occurring through a combination of thermal expansion and ice melt. Sea levels have risen 1-3 mm yr^-1 in the last century (Cazenave & Nerem, 2004, Church et al., 2004, Church & White, 2006). The most recent projections on sea-level rise suggest a rise of 50 cm under the middle emission scenario, 70 cm under the high emission scenario, and 107 cm under the extreme scenario.

This biotope occurs in areas sheltered from wave action and subject to weak or negligible tidal streams (Hughes, 1998a). Understanding of how sea-level rise will affect tidal energy, and the tide-swept nature of a habitat, is fraught with uncertainty, although evidence appears to suggest that any alterations will be non-linear (Pickering et al., 2012, Li et al., 2016). Modelling potential outcomes of sea-level rise on the tidal and residual currents in the Bohai Sea, China showed effects were site dependent, with energy either increasing or decreasing (Li et al., 2016). Similarly, Pickering et al. (2012) found a similar pattern around the UK for tidal amplitude. The effects of sea-level rise and increased wave action may be increased further due to storms and storms surges. IPCC (2019) note that the frequency of extreme sea-level events (e.g. due to storms) are predicted to increase as sea-level rises, however there is no consensus on the future storm and, hence, wave climate around UK coasts (Mossman et al., 2015; Lowe et al., 2018; Palmer et al., 2018).

Sensitivity assessment. This habitat occurs from 5 to 30 m, although Cerianthus lloydii has been recorded at depths ranging from 0 to 900 m (OBIS, 2024). Therefore, an increase in sea-level rise is unlikely to have a significant impact on this biotope. Therefore, resistance to sea-level rise has been assessed as ‘High’ for the middle (50 cm), and high (70 cm) emission scenario, and for the extreme scenario (107 cm). As no recovery is deemed necessary, resilience has been assessed as ‘High’, and therefore this species has been classified as ‘Not sensitive’ to sea-level rise at each of the benchmarks.

Sea level rise (high)

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

Evidence

Sea-level rise is occurring through a combination of thermal expansion and ice melt. Sea levels have risen 1-3 mm yr^-1 in the last century (Cazenave & Nerem, 2004, Church et al., 2004, Church & White, 2006). The most recent projections on sea-level rise suggest a rise of 50 cm under the middle emission scenario, 70 cm under the high emission scenario, and 107 cm under the extreme scenario.

This biotope occurs in areas sheltered from wave action and subject to weak or negligible tidal streams (Hughes, 1998a). Understanding of how sea-level rise will affect tidal energy, and the tide-swept nature of a habitat, is fraught with uncertainty, although evidence appears to suggest that any alterations will be non-linear (Pickering et al., 2012, Li et al., 2016). Modelling potential outcomes of sea-level rise on the tidal and residual currents in the Bohai Sea, China showed effects were site dependent, with energy either increasing or decreasing (Li et al., 2016). Similarly, Pickering et al. (2012) found a similar pattern around the UK for tidal amplitude. The effects of sea-level rise and increased wave action may be increased further due to storms and storms surges. IPCC (2019) note that the frequency of extreme sea-level events (e.g. due to storms) are predicted to increase as sea-level rises, however there is no consensus on the future storm and, hence, wave climate around UK coasts (Mossman et al., 2015; Lowe et al., 2018; Palmer et al., 2018).

Sensitivity assessment. This habitat occurs from 5 to 30 m, although Cerianthus lloydii has been recorded at depths ranging from 0 to 900 m (OBIS, 2024). Therefore, an increase in sea-level rise is unlikely to have a significant impact on this biotope. Therefore, resistance to sea-level rise has been assessed as ‘High’ for the middle (50 cm), and high (70 cm) emission scenario, and for the extreme scenario (107 cm). As no recovery is deemed necessary, resilience has been assessed as ‘High’, and therefore this species has been classified as ‘Not sensitive’ to sea-level rise at each of the benchmarks.

Sea level rise (middle)

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

Evidence

Sea-level rise is occurring through a combination of thermal expansion and ice melt. Sea levels have risen 1-3 mm yr^-1 in the last century (Cazenave & Nerem, 2004, Church et al., 2004, Church & White, 2006). The most recent projections on sea-level rise suggest a rise of 50 cm under the middle emission scenario, 70 cm under the high emission scenario, and 107 cm under the extreme scenario.

This biotope occurs in areas sheltered from wave action and subject to weak or negligible tidal streams (Hughes, 1998a). Understanding of how sea-level rise will affect tidal energy, and the tide-swept nature of a habitat, is fraught with uncertainty, although evidence appears to suggest that any alterations will be non-linear (Pickering et al., 2012, Li et al., 2016). Modelling potential outcomes of sea-level rise on the tidal and residual currents in the Bohai Sea, China showed effects were site dependent, with energy either increasing or decreasing (Li et al., 2016). Similarly, Pickering et al. (2012) found a similar pattern around the UK for tidal amplitude. The effects of sea-level rise and increased wave action may be increased further due to storms and storms surges. IPCC (2019) note that the frequency of extreme sea-level events (e.g. due to storms) are predicted to increase as sea-level rises, however there is no consensus on the future storm and, hence, wave climate around UK coasts (Mossman et al., 2015; Lowe et al., 2018; Palmer et al., 2018).

Sensitivity assessment. This habitat occurs from 5 to 30 m, although Cerianthus lloydii has been recorded at depths ranging from 0 to 900 m (OBIS, 2024). Therefore, an increase in sea-level rise is unlikely to have a significant impact on this biotope. Therefore, resistance to sea-level rise has been assessed as ‘High’ for the middle (50 cm), and high (70 cm) emission scenario, and for the extreme scenario (107 cm). As no recovery is deemed necessary, resilience has been assessed as ‘High’, and therefore this species has been classified as ‘Not sensitive’ to sea-level rise at each of the benchmarks.

Temperature increase (local)

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

Evidence

Synarachnactis lloydii adults are locally abundant in many localities on all coasts of the British Isles and, in some areas, are common on the shore. This species occurs on all western coasts of Europe from Greenland and Spitzbergen south to Biscay. Larvae, but not adults, have been recorded from the Mediterranean. There is no further information available on the temperature tolerance of Synarachnactis lloydii.

Nemertesia ramosa is found inshore to deeper water, being common throughout the British Isles and is distributed from Iceland to north-west Africa (Hayward & Ryland, 1994). In the Azores, for example, it is well known in the sublittoral at 15 to 158 m deep in the central group of islands, but historical records exist from seamounts down to bathyal grounds of nearly 1,000 m (Gomes-Pereira & Tempera, 2016).

Gili & Hughes (1995) report that temperature is a critical factor stimulating or preventing reproduction and that most species have an optimal temperature for reproduction. However, limited evidence for thermal thresholds and thermal ranges was available for the characterizing species recorded in this biotope. Berrill (1949) reported that growth in Obelia commissularis (syn. dichotoma) was temperature dependent but ceased at 27°C. Hydranths did not start to develop unless the temperature was less than 20°C, and any hydranths under development would complete their development and rapidly regress at ca 25°C. Berrill (1948) reported that Obelia species were absent from a buoy in July and August during excessively high summer temperatures in Booth Bay Harbour, Maine, USA. Berrill (1948) reported that the abundance of Obelia species and other hydroids fluctuated greatly, disappearing and reappearing as temperatures rose and fell markedly above and below 20°C during this period. The upwelling of cold water (8 to 10°C colder than surface water) allowed colonies of Obelia sp. to form in large numbers. Cantero et al. (2002) describe the presence and year-round fertility of Obelia dichotoma, Kirchenpaureria pinnata, Nemertesia ramosa and Halecium spp. in the Mediterranean, indicating probable tolerance to temperature increases at the benchmark level.

Sensitivity assessment

At the level of the benchmark, a change in temperature is unlikely to have a negative impact on the biotope. Therefore, both resistance and resilience are assessed as ‘High’, and sensitivity is assessed as ‘Not sensitive’ at the benchmark level.

Temperature decrease (local)

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

Evidence

Synarachnactis lloydii adults are locally abundant in many localities on all coasts of the British Isles and, in some areas, are common on the shore. This species occurs on all western coasts of Europe from Greenland and Spitzbergen south to Biscay. No further information is available on the temperature tolerance of Synarachnactis lloydii.

Orejas et al. (2012) describe the feeding ecology of Obelia dichotoma in an Arctic environment (Kongsfjorden, Svalbard) which experiences temperatures of 1-5°C (Beszczynska-Möller & Dye, 2013). Palerud et al. (2004) also describe the presence of Obelia dichotoma, Halecium halecinum and Nemertesia sp. in Svalbard. This suggests that the characterizing hydroids are probably tolerant of the lowest temperatures they are likely to encounter in Britain and Ireland of ca 4°C (Beszczynska-Möller & Dye, 2013). It should be noted that growth rates are reduced at low temperatures. Berrill (1949) reported that for Obelia, stolons grew, under optimal nutritive conditions, at less than 1 mm in 24 hrs at 10-12°C, 10 mm in 24 hrs at 16-17°C, and as much as 15-20 mm in 24 hrs at 20°C. Berrill (1948) also reported that the abundance of Obelia species and other hydroids fluctuated greatly, disappearing and reappearing as temperatures rose and fell markedly above and below 20°C during this period. The upwelling of cold water (8 to 10°C colder than surface water) allowed colonies of Obelia sp. to form in large numbers. In addition, the hydroids Obelia dichotoma, Halecium halecinum and Nemertesia sp. have been recorded in Svalbard (Orjas et al., 2012).

Sensitivity assessment

All species assessed are present in northern/boreal habitats and are unlikely to be affected at the benchmark level. Resistance has been assessed as ‘High’, and resilience as ‘High’. Therefore, sensitivity has been assessed as ‘Not sensitive’ at the benchmark level.

Salinity increase (local)

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

Evidence

No evidence was found for osmoregulation by Synarachnactis (syn. Cerianthus) lloydii.  Synarachnactis lloydii is recorded in biotopes with variable salinity regimes (18-40 psu), such as SS.SMx.CMx.ClloModHo, but most records occur in full salinity. Studies on hydroids, in general, have found that prey capture rates may be affected by salinity and temperature (Gili & Hughes, 1995), although no evidence was found for Nemertesia antennina. However, due to the lack of evidence for the characterizing species within this biotope an assessment of ‘No evidence’ has been given. 

Salinity decrease (local)

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

Evidence

No evidence was found for osmoregulation by Synarachnactis (syn. Cerianthus) lloydii.  Synarachnactis lloydii is recorded in biotopes with variable salinity regimes (18-40 psu), such as SS.SMx.CMx.ClloModHo, but most records occur in full salinity. Studies on hydroids, in general, have found that prey capture rates may be affected by salinity and temperature (Gili & Hughes, 1995), although no evidence was found for Nemertesia antennina. However, due to the lack of evidence for the characterizing species within this biotope an assessment of ‘No evidence’ has been given. 

Water flow (tidal current) changes (local)

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

Evidence

Evidence for the effect of changes in water flow on Synarachnactis lloydii is unavailable.  This species is recorded from biotopes with a wide range of water flow regimes, from very weak to strong flow (Connor et al., 1997b).  Therefore, it is likely to have a high tolerance to changes in water flow regimes.

The characteristic hydroids are typically found in places of low to moderate water movement, although Hayward & Ryland (1995a) note that the abundant communities occur in narrow straits and headlands that may experience high levels of water flow. Hydroids can bend passively with water flow to reduce drag forces, prevent detachment and enhance feeding (Gili & Hughes, 1995). Hydroid growth form also varies to adapt to prevailing conditions, allowing species to occur in a variety of habitats (Gili & Hughes, 1995).  Flow rates are an important factor for feeding in hydroids, and prey capture rates are higher in areas of greater turbulence (Gili & Hughes, 1995).  The capture rate of zooplankton by hydroids is correlated with prey abundance (Gili & Hughes, 1995); thus, prey availability can compensate for sub-optimal flow rates. Water movements are also important to hydroids to prevent siltation, which can cause death (Round et al., 1961). Tillin & Tyler-Walters (2014) suggest that the range of flow speeds experienced by biotopes in which hydroids are found indicates that a change (increase or decrease) in the maximum water flow experienced by mid-range populations for the short periods of peak spring tide flow would not have negative effects on this ecological group.

Sensitivity assessment

This biotope is recorded from moderately strong (1-3 kn) to very weak flow (negligible) and wave exposed to very wave sheltered conditions from 5 m to 30 m depth.  The biotope probably experiences wave-mediated flow in its more shallow examples, while tidal flow is more important in its deeper examples.  The biotope probably would not occur in areas subject to both strong flow and wave action, nor in areas subject to very weak flow and shelter from wave action. Therefore, the biotope probably experiences a range of water flow and/or wave-mediated flow between the extremes cited above.  Therefore, a change in the flow of 0.1 to 0.2 m/s is probably not significant, and resistance and resilience are assessed as ‘High’, so sensitivity is assessed as ‘Not sensitive’.

Emergence regime changes

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

Evidence

This biotope does not occur in the intertidal, and consequently an increase in emergence is considered 'Not relevant' to this biotope.

Wave exposure changes (local)

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

Evidence

No evidence for the effect of changes in wave exposure on Synarachnactis lloydii was available. However, it is recorded from extremely wave sheltered to wave exposed sites (Connor et al., 1997b). Jackson (2004) reported that Nemertesia ramosa was intolerant of high wave exposure and only found in sheltered areas. Faucci & Boero (2000) recorded hydroid communities at two sites of different wave exposure and recorded the presence of Obelia dichotoma and Halecium spp. in both the exposed and sheltered sites, but only found Kirchenpaueria sp. in the sheltered site.

Sensitivity assessment

This biotope is recorded from moderately strong to very weak flow and wave exposed to very wave sheltered conditions from 5 m to 30 m depth.  The biotope probably experiences wave-mediated flow in its more shallow examples, while tidal flow is more important in its deeper examples.  The biotope probably would not occur in areas subject to both strong flow and wave action, nor in areas subject to very weak flow and shelter from wave action. Therefore, the biotope probably experiences a range of water flow and/or wave-mediated flow between the extremes cited above. Storms may mobilise the surface of the substratum and may explain the sparse fauna. An increase in wave exposure (e.g. to very exposed) may mobilise the sediment further, reducing the abundance of epifauna, but the important characteristic burrowing anemone would probably remain. Similarly, a further decrease in wave exposure may increase the abundance of epifauna, such as found in the sub-biotope SS.SMx.CMx.ClloMx.Nem. Therefore, resistance is assessed as 'Medium' to represent changes in the community while the biotope will probably remain. Hence, resilience is assessed as 'High,' and sensitivity as 'Low'. Confidence in the assessment is 'Low' due to the lack of direct evidence.

Transition elements & organo-metal contamination

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

Evidence

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

Hydrocarbon & PAH contamination

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

Evidence

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

Synthetic compound contamination

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

Evidence

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

Radionuclide contamination

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

Evidence

No evidence.

Introduction of other substances

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

Evidence

This pressure is Not assessed.

De-oxygenation

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

Evidence

In general, respiration in most marine invertebrates does not appear to be significantly affected until extremely low concentrations are reached. For many benthic invertebrates, this concentration is about 2 ml/l (Herreid, 1980; Rosenberg et al., 1991; Diaz & Rosenberg, 1995).  Cole et al. (1999) suggest possible adverse effects on marine species below 4 mg/l and probable adverse effects below 2mg/l.

Hydroids mainly inhabit environments in which the oxygen concentration exceeds 5 ml/l (Gili & Hughes, 1995).  Diaz & Rosenberg (1995) noted that anemones include species that were reported to be particularly tolerant of hypoxia (e.g. Cerianthus sp and Epizoanthus erinaceus). A major hypoxic event due to a pycnocline in the Gulf of Trieste resulted in a mass mortality of benthos between 12 and 26^th September 1983 (Stachowitsch, 1992), during which the oxygen levels fell below 4.2 mg/l, became anoxic, and hydrogen sulphide and ammonia were released (Faganeli et al., 1985). Amongst the epifauna, even hypoxia resistant polychaetes and bivalves died after 4 to 5 days, and the only organisms to survive after one week were the anemones Cerianthus sp and Epizoanthus erinaceus, the gastropods Aporrhais pespelecani and Trunculariopsis trunculus and the sphinuculid Sipunculus nudis (Stachowitsch, 1992).

Sensitivity assessment

The above evidence suggests that Synarachnactis (syn. Cerianthus) lloydii would probably survive for a week at or below 2 mg O₂/l while the hydroids would probably be reduced to resting stages. Therefore, a resistance of 'Low' is recorded to represent the probable significant mortality of hydroids in the community. However, the hydroids would recover rapidly, so that resilience is likely to be 'High' and sensitivity is assessed as 'Low'.

Nutrient enrichment

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

Evidence

No information was available on the effect of nutrient enrichment on Synarachnactis (syn. Cerianthus) lloydii.  Witt et al. (2004) found that the hydroid Obelia sp. was more abundant in a sewage disposal area in the Weser estuary (Germany), which experienced sedimentation of 1 cm for more than 25 days. It should be noted that another hydroid (Sertularia cupressina) was reduced in abundance when compared with unimpacted reference areas. However, there is no direct evidence for the characterizing hydroid species Nemertesia anteninna.  Hence, no evidence was found on which to base an assessment.  

Organic enrichment

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

Evidence

Borja et al. (2000) and Gittenberger & van Loon (2011), in the development of the AZTI Marine Biotic Index (AMBI) to assess disturbance (including organic enrichment), both assigned Synarachnactis (syn. Cerianthus) lloydii to their Ecological Group I, ‘species very sensitive to organic enrichment and present under unpolluted conditions (initial state)’.  The basis for their assessment and relation to the pressure benchmark was not clear (Tillin & Tyler-Walters, 2014).

Witt et al. (2004) found that the hydroid Obelia spp. was more abundant in a sewage disposal area in the Weser estuary (Germany) which experienced sedimentation of 1 cm for more than 25 days.  It should be noted that another hydroid (Sertularia cupressina) was reduced in abundance when compared with unimpacted reference areas.  As suspension feeders, an increase in organic content at the benchmark is likely to be of benefit to the characterizing hydroids.

Sensitivity assessment

This biotope is considered to have a 'Low' resistance at the pressure benchmark, which refers to enrichment rather than gross organic pollution. Hence, resilience is assessed as 'Medium' and sensitivity as 'Medium'.

Physical loss (to land or freshwater habitat)

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

Evidence

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

Physical change (to another seabed type)

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

Evidence

If the sedimentary habitat was replaced with a rocky habitat, the biotope would be lost because it is characterized by sandy, muddy gravel, on which the community depends. The resistance to this change is ‘None’, and the resilience is assessed as ‘Very low’ due to the permanent nature of the change in the substratum.  The biotope is assessed to have a ‘High’ sensitivity to this pressure at the benchmark.

Physical change (to another sediment type)

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

Evidence

Synarachnactis (syn. Cerianthus) lloydii is found in a very wide range of substrata (Tillin & Tyler-Walters, 2014), but only dominates the fauna in the mixed substrata biotope described in this biotope. A change in Folk class from mixed to coarse or sandy mud substrata would result in loss of the biotope, even though the population of Synarachnactis lloydii may remain. Nemertesia antennina and other hydroids are only found attached to large pebbles and cobbles within SS.SMx.CMx.ClloMx.Nem. A reduction in the presence of cobbles and pebbles due to a change in sediment type would also reduce the abundance of the characteristic hydroids.

Sensitivity assessment

A change in the substratum by one Folk class would result in the loss of the biotope. Therefore, a resistance of ‘None’ is recorded.  As resilience is 'Very low' (the pressure is a permanent change), sensitivity is, therefore, 'High'.

Habitat structure changes - removal of substratum (extraction)

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

Evidence

Resistance is assessed as ‘None’ based on expert judgment but supported by the literature relating to the position of these species on or within the seabed.  At the pressure benchmark, the exposed sediments are considered suitable for recolonization almost immediately following extraction. Recovery will be mediated by the scale of the disturbance and the suitability of the sedimentary habitat.  Recovery is most likely to occur via larval recolonization.  Resilience is probably ‘Low’, so that sensitivity is assessed as ‘High’.

Abrasion / disturbance of the surface of the substratum or seabed

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

Evidence

No direct evidence was found to assess the sensitivity of Synarachnactis lloydii to surface abrasion. The burrowing habit of the species specifically assessed would confer some protection from surface disturbance, although individuals would be more exposed when close to the surface feeding. Synarachnactis lloydii inhabits a soft tube, which can be up to 40 cm long and is permanently buried. The anemone can move freely within the tube and can retract swiftly if required (Tillin & Tyler-Walters, 2014).

The available evidence indicates that hydroids can be entangled and removed by abrasion. Drop-down video surveys of Scottish reefs exposed to trawling showed that visual evidence of damage to bryozoans and hydroids on rock surfaces was generally limited and restricted to scrape scars on boulders (Boulcott & Howell, 2011). The study showed that damage was incremental, with damage increasing with the frequency of trawls rather than a blanket effect occurring on the pass of the first trawls.

Re-sampling of grounds that were historically studied (from the 1930s) indicated that some species have increased in areas subject to scallop fishing (Bradshaw et al., 2002). This study also found (unquantified) increase in abundance of tough-stemmed hydroids including Nemertesia spp.; whose morphology may have prevented excessive damage. Bradshaw et al. (2002) suggested that as well as having high resistance to abrasion pressures, Nemertesia spp. have benthic larvae that could rapidly colonize disturbed areas with newly exposed substrata close to the adult. Hydroids may also recover rapidly as the surface covering of hydrorhizae may remain largely intact, from which new uprights are likely to grow. In addition, the resultant fragments of colonies may be able to develop into new colonies.

Hydroid colonies were still present in the heavily fished area, albeit at lower densities than in the closed area. This may largely be because the Isle of Man scallop fishery is closed from 1^st June to 31^st October (Andrews et al., 2011), so at the time the samples were taken for the study in question, the seabed had not been dredged for at least 3.5 months. The summer period is also the peak growing/breeding season for many marine species (Bradshaw et al., 2003).

Physical disturbance by fishing gear has been shown to adversely affect sessile benthic and emergent epifaunal communities, with hydroid and bryozoan matrices reported to be greatly reduced in fished areas and increase when fishing activity is removed (Jennings & Kaiser, 1998; Sheehan et al., 2017; Kaiser et al., 2018; Long et al., 2021; Langton, Stirling & Boulcott, 2023).

Sensitivity assessment

 Abrasion at the surface only is considered likely to damage and remove epiphytic species. Synarachnactis lloydii can retract into its tube. However, there is the possibility of the tube being damaged, which could affect the health of the organism. Given the sessile, erect nature of hydroids, damage and mortality following a physical disturbance are likely to be significant. However, hydroids have rapid growth rates, and potentially high recruitment and can recover quickly from fragments or dormant resting stages. The resistance of the biotope is assessed as ‘Medium’, although the significance of the impact for the bed will depend on the spatial scale of the pressure footprint. Resilience is assessed as ‘Medium’ (2 to10 years), and sensitivity is assessed as ‘Medium’.

Penetration or disturbance of the substratum subsurface

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

Evidence

Penetration and disturbance of the substratum would result in similar effects as abrasion or removal of this biotope.  Damage to Synarachnactis lloydii would be greater within this pressure, as their ability to retract within their tubes would be limited. Therefore, the resistance of the biotope is assessed as ‘Low’, although the significance of the impact for the bed will depend on the spatial scale of the pressure footprint.  Resilience is assessed as ‘Medium’, and sensitivity is assessed as ‘Medium’.

Changes in suspended solids (water clarity)

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

Evidence

An increase in suspended sediment may have a deleterious effect on the suspension-feeding community.  It is likely to clog their feeding apparatus to some degree, resulting in reduced ingestion over the benchmark period and, subsequently, a decrease in growth rate (Jackson, 2004). As the hydroids capture small prey in suspension (Gili & Hughes, 1995), a reduction in feeding efficiency could potentially lead to a reduction in overall biomass.

No evidence on the effect of a change in turbidity on Synarachnactis lloydii could be found. Nemertesia ramosa is a passive suspension feeder, extracting seston from the water column. Increased siltation may clog up the feeding apparatus, requiring energetic expenditure to clear. Recovery is likely to take only a few days. (Jackson, 2004).  A decrease in suspended sediment is likely to benefit the community associated with this biotope. The suspension feeders may be able to feed more efficiently due to a reduction in time and energy spent cleaning feeding apparatus. Over the course of the benchmark, the hydroids may increase in abundance.

Sensitivity assessment.

No directly relevant evidence was found to assess the effect of pressure.  Resistance to this pressure is assessed as 'High' as an increase in turbidity may influence feeding and growth rates but not result in mortality of adults.  Resilience is assessed as 'High' (by default), and the biotope is assessed as 'Not sensitive' to changes in turbidity at the benchmark level.

Smothering and siltation rate changes (light)

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

Evidence

In normal accretion, Synarachnactis lloydii keeps pace with the accretion and, as a result, develops burrows much larger than the animal itself (Schäfer, 1972; Bromley, 2012). Schäfer (1972) reported that an increase in depositional rate led to avoidance behaviour in Synarachnactis lloydii.  The organism ceases tube building activity, and instead the animal bunches its tentacles and intrudes its way up to the new surface, where it establishes a new burrow. However, no information on the depth of material through which it can burrow was given.

In general, it appears that hydroids are sensitive to silting (Boero, 1984; Gili & Hughes, 1995) and the decline of beds in the Wadden Sea has been linked to environmental changes including siltation. Round et al. (1961) reported that the hydroid Sertularia (now Amphisbetia) operculata died when covered with a layer of silt after being transplanted to sheltered conditions.  Boero (1984) suggested that deepwater hydroid species develop upright, thin colonies that accumulate little sediment, while species in turbulent water movement were adequately cleaned of silt by water movement. Hughes (1977) found that maturing hydroids that were smothered with detritus and silt lost most of the hydrocladia and hydranths. After one month, the hydroids were seen to have recovered, but although neither the growth rate nor the reproductive potential appeared to have been affected, the viability of the planulae may have been affected. Nemertesia ramosa is an upright hydroid with a height of up to 15 cm. The colony structure is fairly tough and flexible. Smothering with 5 cm of sediment may cover over some individuals; others may just have the lower section of the main stem covered (Hayward & Ryland, 1994). Halecium halecinum can grow up to 25 cm, and Kirchenpaueria pinnata can grow to ca 10 cm (Hayward & Ryland, 1994). Some of the hydroid community is therefore likely to survive smothering by 5 cm.

Sensitivity assessment. Synarachnactis lloydii will actively burrow up through sediment that has smothered the entrance to its burrow. The thickness of sediment through which Synarachnactis lloydii is able to burrow is not known. However, it can grow up to 15 cm in length, and its burrows can reach 40 cm in depth. Hence, smothering by 5 cm of sediment is unlikely to cause mortality of Synarachnactis lloydii. This pressure will also influence the hydroid species within SS.SMx.CMx.ClloMx.Nem. Given the information available, the resistance to this pressure is considered to be ‘High’, as is the resilience, so sensitivity is assessed as ‘Not sensitive’ at the benchmark level.

Smothering and siltation rate changes (heavy)

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

Evidence

In normal accretion, Synarachnactis lloydii keeps pace with the accretion and, as a result, develops burrows much larger than the animal itself (Schäfer, 1972; Bromley, 2012). Schäfer (1972) reported that an increase in depositional rate led to avoidance behaviour in Synarachnactis lloydii.  The organism ceases tube building activity, and instead the animal bunches its tentacles and intrudes its way up to the new surface, where it establishes a new burrow. However, no information on the depth of material through which it can burrow was given.

In general, it appears that hydroids are sensitive to silting (Boero, 1984; Gili & Hughes, 1995) and the decline of beds in the Wadden Sea has been linked to environmental changes including siltation. Round et al. (1961) reported that the hydroid Sertularia (now Amphisbetia) operculata died when covered with a layer of silt after being transplanted to sheltered conditions.  Boero (1984) suggested that deepwater hydroid species develop upright, thin colonies that accumulate little sediment, while species in turbulent water movement were adequately cleaned of silt by water movement. Hughes (1977) found that maturing hydroids that were smothered with detritus and silt lost most of the hydrocladia and hydranths. After one month, the hydroids were seen to have recovered, but although neither the growth rate nor the reproductive potential appeared to have been affected, the viability of the planulae may have been affected. Nemertesia ramosa is an upright hydroid with a height of up to 15 cm. The colony structure is fairly tough and flexible. Smothering with 5 cm of sediment may cover over some individuals; others may just have the lower section of the main stem covered (Hayward & Ryland, 1994). Halecium halecinum can grow up to 25 cm, and Kirchenpaueria pinnata can grow to ca 10 cm (Hayward & Ryland, 1994). Some of the hydroid community is therefore likely to survive smothering by 5 cm.

Sensitivity assessment. Synarachnactis lloydii will actively burrow up through sediment that has smothered the entrance to its burrow. The thickness of sediment through which Synarachnactis lloydii is able to burrow is not known. However, it can grow up to 15 cm in length, and its burrows can reach 40 cm in depth. Smothering by 30 cm of sediment may cause some mortality of Synarachnactis lloydii. This pressure will also influence the hydroid species within SS.SMx.CMx.ClloMx.Nem. Given the information available, the resistance to this pressure is considered to be ‘Medium’, as is the resilience, so sensitivity is assessed as ‘Medium’ at the benchmark level but with 'Low' confidence.

Litter

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

Evidence

Not assessed.

Electromagnetic changes

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

Evidence

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

Sensitivity assessment. No studies have examined the effect of EMFs on burrowing anemones or hydroids. Therefore, there is ‘Insufficient evidence’ on which to base an assessment of the likely sensitivity of Synarachnactis lloydii to EMFs.

Underwater noise changes

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

Evidence

Species characterizing this habitat do not have hearing perception but vibrations may cause a response. However, noise, as defined by the pressure, is probably 'Not relevant'.

Introduction of light or shading

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

Evidence

Since 2016, research on artificial light at night (ALAN) has expanded considerably in the marine and coastal environment. Light was previously assumed to be of low ecological significance in subtidal and intertidal habitats, but there is now evidence that ALAN is widespread in the marine environment, with biologically relevant levels of light penetrating to depths of up to 50 m (Davies et al., 2020; Smyth et al., 2021). ALAN can alter biological processes across taxa and at multiple levels of organisation. Documented responses include disruption of diel and circalunar rhythms, changes in activity and foraging, altered predator–prey interactions, shifts in community composition, and impacts on algal growth and phenology (Davies et al., 2014, 2015; Gaston et al., 2017; Tidau et al., 2021; Lynn et al., 2022; Marangoni et al., 2022; Miller & Rice, 2023; Ferretti et al., 2025). Evidence for benthic habitats and assemblages specifically is beginning to emerge (e.g. Trethewy et al., 2023; Schaefer et al., 2025), but remains limited and fragmented, often focusing on single taxa or short-term experiments. Mortality thresholds, long-term consequences, and responses at the biotope scale are rarely addressed, and there are major gaps around indirect effects such as trophic cascades or habitat modification. Therefore, given the rapid expansion of the evidence base but the continuing lack of data at the level of individual biotopes, resistance and resilience cannot be robustly assessed. Sensitivity is therefore recorded as 'Insufficient evidence'.

Barrier to species movement

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

Evidence

Not relevant – this pressure is considered applicable to mobile species, e.g. fish and marine mammals rather than seabed habitats. Physical and hydrographic barriers may limit propagule dispersal.  But propagule dispersal is not considered under the pressure definition and benchmark.

Death or injury by collision

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

Evidence

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

Visual disturbance

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

Evidence

Not relevant.

Genetic modification & translocation of indigenous species

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

Evidence

Habitat restoration projects may translocate stock to repopulate areas of suitable habitat (Elsäßer et al., 2013).  No evidence was found for detrimental effects arising from this practice in the habitat, although there is potential for the movement of pathogens and non-indigenous, invasive species.

Introduction of microbial pathogens

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

Evidence

No evidence was available on the effect of microbial pathogens on Synarachnactis (syn. Cerianthus) lloydii. Hydroids exhibit astonishing regeneration and rapid recovery from injury (Sparks, 1972), and the only inflammatory response is active phagocytosis (Tokin & Yaricheva, 1959, 1961, as cited in Sparks, 1972).  No record of diseases in the characterizing hydroids could be found. Therefore, there was 'Insufficient evidence' to assess the effect of this pressure on the biotope.

Removal of target species

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

Evidence

None of the characterizing species within this biotope are currently directly targeted in the UK and hence this pressure is considered to be ‘Not relevant’.

Removal of non-target species

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

Evidence

Direct, physical impacts from harvesting are assessed under the abrasion and penetration of the seabed pressures.  The characterizing species within this biotope could easily be incidentally removed from this biotope as by-catch when other species are being targeted.  The loss of these species and other associated species would decrease species richness and negatively impact ecosystem function.

Removal of a large percentage of the characterizing species would alter the character of the biotope.  The resistance to removal is ‘Low’ due to the easy accessibility of the biotopes' location and the inability of these species to evade collection. The resilience is ‘Medium’, with recovery only being able to begin when the harvesting pressure is removed altogether. This gives an overall sensitivity score of ‘Medium’.

The American slipper limpet, Crepidula fornicata

Evidence

The American slipper limpet Crepidula fornicata was introduced to the UK and Europe in the 1870s from the Atlantic coasts of North America with imports of the eastern oyster Crassostrea virginica. It was recorded in Liverpool in 1870 and on the Essex coast in 1887-1890. It has spread through expansion and introductions along the full extent of the English Channel and into the European mainland (Blanchard, 1997, 2009; Bohn et al., 2012, 2013a, 2013b, 2015; De Montaudouin et al., 2018; Helmer et al., 2019; Hinz et al., 2011; McNeill et al., 2010; Powell-Jennings & Calloway, 2018; Preston et al., 2020; Stiger-Pouvreau & Thouzeau, 2015).

Crepidula fornicata is recorded from shallow, sheltered bays, lagoons and estuaries or the sheltered sides of islands, in variable salinity (18 to 40), although it prefers ca 30 (Tillin et al., 2020). Larvae require hard substrata for settlement. It prefers muddy, gravelly, shell-rich substrata that include gravel, or shells of other Crepidula, or other species, e.g., oysters, and mussels. It is highly gregarious and seeks out adult shells for settlement, forming characteristic ‘stacks’ of adults. But it also recorded in a wide variety of habitats including clean sands, artificial substrata, Sabellaria alveolata reefs and areas subject to moderately strong tidal streams (Blanchard, 1997, 2009; Bohn et al., 2012, 2013a, 2013b, 2015; De Montaudouin et al., 2018; Hinz et al., 2011; Powell-Jennings & Calloway, 2018; Preston et al., 2020; Stiger-Pouvreau & Thouzeau, 2015; Tillin et al., 2020).

High densities of Crepidula fornicata cause ecological impacts on sedimentary habitats. The species can form dense carpets that can smother the seabed in shallow bays, changing and modifying the habitat structure. At high densities, the species physically smothers the sediment, and the resultant build-up of silt, pseudofaeces, and faeces is deposited and trapped within the bed (Tillin et al., 2020; Fitzgerald, 2007; Blanchard, 2009; Stiger-Pouvreau & Thouzeau, 2015). The biodeposition rates of Crepidula are extremely high and, once deposited, form an anoxic mud, making the environment suitable for other species, including most infauna (Stiger-Pouvreau & Thouzeau, 2015; Blanchard, 2009). For example, in fine sands, the community is replaced by a reef of slipper limpets that provide hard substrata for sessile suspension-feeders (e.g., sea squirts, tube worms and fixed shellfish), while mobile carnivorous microfauna occupy spaces between or within shells, resulting in a homogeneous Crepidula dominated habitat (Blanchard, 2009). Blanchard (2009) suggested the transition occurred and became irreversible at 50% cover of the limpet. De Montaudouin et al. (2018) suggested that homogenization occurred above a threshold of 20-50 Crepidula /m².

Impacts on the structure of benthic communities will depend on the type of habitat that Crepidula colonizes. De Montaudouin & Sauriau (1999) reported that in muddy sediment dominated by deposit-feeders, species richness, abundance and biomass increased in the presence of high densities of Crepidula (ca 562 to 4772 ind./m²), in the Bay of Marennes-Oléron, presumably because the Crepidula bed provided hard substrata in an otherwise sedimentary habitat. In medium sands, Crepidula density was moderate (330-1300 ind./m²), but there was no significant difference between communities in the presence of Crepidula. Intertidal coarse sediment was less suitable for Crepidula, with only moderate or low abundances (11 ind./m²), and its presence did not affect the abundance or diversity of macrofauna. However, there was a higher abundance of suspension–feeders and mobile Crustacea in the absence of Crepidula (De Montaudouin & Sauriau, 1999). The presence of Crepidula as an ecosystem engineer has created a range of new niche habitats, reducing biodiversity as it modifies habitats (Fitzgerald, 2007). De Montaudouin et al. (1999) concluded that Crepidula did not influence macroinvertebrate diversity or density significantly under experimental conditions, on fine sands in Arcachon Bay, France. De Montaudouin et al. (2018) noted that the limpet reef increased the species diversity in the bed, but homogenised diversity compared to areas where the limpets were absent. In the Milford Haven Waterway (MHW), the highest densities of Crepidula were found in areas of sediment with hard substrata, e.g., mixed fine sediment with shell or gravel or both (grain sizes 16-256 mm), but while Crepidula density increased as gravel cover increased in the subtidal, the reverse was found in the intertidal (Bohn et al., 2015). Bohn et al. (2015) suggested that high densities of Crepidula in high-energy environments were possible in the subtidal but not the intertidal, suggesting the availability of this substratum type is beneficial for its establishment. Hinz et al. (2011) reported a substantial increase in the occurrence of Crepidula off the Isle of Wight, between 1958 and 2006, at a depth of ca 60 m, on hard substrata (gravel, cobbles, and boulders), swept by strong tidal streams. Presumably, Crepidula is more tolerant of tidal flow than the oscillatory flow caused by wave action, which may be less suitable (Tillin et al., 2020).

Sensitivity assessment

The evidence suggests that Crepidula could colonize mixed sediment habitats in the subtidal, typical of this biotope, due to the presence of gravel or any other hard substrata that can be used for larval settlement (Tillin et al., 2020). Bohn et al. (2015) demonstrated that Crepidula had a preference for gravelly habitats, while De Montaudouin & Sauriau (1999) and Bohn et al. (2015) noted that Crepidula densities were low in intertidal coarse sediments. Therefore, Crepidula has the potential to colonize, and modify the habitat and its associated community due to the introduction of Crepidula shell biomass, silt, pseudofaeces and faeces (Blanchard, 2009; Tillin et al., 2020), as occurs in maerl gravels (Grall & Hall-Spencer, 2003), resulting in the loss of the biotope. This is a wave exposed to very sheltered habitat, so storms may mobilise the sediment (JNCC, 2022), which may also mitigate or prevent colonization by Crepidula at high densities, although it has been recorded from areas of strong tidal streams (Hinz et al., 2011). Therefore, the habitat may be more suitable for Crepidula in wave sheltered areas of the biotope and where water movement is mediated by tidal flow rather than wave action, e.g., the deeper examples of the biotope. Therefore, resistance is assessed as 'Medium' in examples where wave action is high and subject to storms, but 'Low' in wave sheltered areas dominated by tidal flow. Resilience is assessed as 'Very low' as it would require the removal of Crepidula, probably by artificial means. Hence, sensitivity is assessed as 'High' based on the worst-case scenario. Crepidula has not yet been reported to occur in this biotope, so the confidence in the assessment is 'Low' and further evidence is required.

The carpet sea squirt, Didemnum vexillum

Evidence

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

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

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

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

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

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

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

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

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

Kaplan et al. (2018) reported that Cerianthus tube anemones in the Georges Bank were less abundant in areas with higher per cent cover Didemnum vexillum, indicating a negative association between the two species.

Sensitivity assessment

Didemnum vexillum has been recorded in the sublittoral to depths of 81 m in Georges Bank and 30 m in Long Island, USA (Bullard et al., 2007; Valentine et al., 2007b; Mercer et al., 2009). This biotope occurs on sandy muddy gravel, containing cobbles, pebbles and shells which could provide a suitable hard substratum for colonization by Didemnum sp (Tillin et al., 2020). Didemnum vexillum is reported to prefer sheltered conditions but has also been recorded in moderately strong currents (Valentine et al., 2007b; Mercer et al., 2009; Tillin et al., 2020) and is predicted to survive stronger currents, as the current velocity which will dislodge Didemnum vexillum is around 7.6 m/s (Reinhardt et al., 2012). This biotope experiences very weak to moderately strong water flow (1 to 3 m/s) and very sheltered to moderately exposed wave exposure. However, the effect of wave action reduces with depth, so it is possible that only the most wave-exposed examples of the biotope could be unsuitable for Didemnum. Didemnum vexillum regresses as temperatures decline in winter, so shallow examples may be able to recover their condition in winter (Gittenberger, 2007; Valentine et al., 2007a; Herborg et al., 2009). However, deeper examples may not experience enough temperature change to trigger the decline in Didemnum vexillum (Valentine et al., 2007a). If Didemnum sp. could gain a 'foothold', it might overgrow, smother or cause mortality of epifauna. Holt (2024) noted that Didemnum vexillum had not spread as far as feared in the UK since it was first recorded. Therefore, a resistance of 'Medium' (some, <25% mortality) is suggested as a precaution in case Didemnum vexillum could colonize the biotope, but with 'Low' confidence due to the lack of direct evidence. Resilience is assessed as 'Very low' as recovery would require the physical removal of Didemnum sp., so sensitivity is assessed as 'Medium'. 

The Pacific oyster, Magallana gigas

Evidence

The Pacific oyster, Magallana (syn. Crassostrea) gigas, is native to warm temperate regions from the northwest Pacific to Japan and northeast Asia, including Cape Mariya (Russia) to Hong Kong (China) (Carrasco & Baron, 2010; GBNNSIP, 2011, 2012). It is a fast-growing and tolerant species that has become a successful invader in the coastal waters of all continents, aside from Antarctica (Wrange et al., 2010; Carrasco & Baron, 2010; Padilla, 2010). Magallana gigas is recognised as a beneficial and important species in aquaculture worldwide (Padilla, 2010). It was initially introduced for aquaculture in Europe and the UK in the 1960s due to a decline in the Portuguese oyster (Crassostrea angulata) and the European flat oyster (Ostrea edulis) (Spencer et al., 1994; GBNNSIP, 2011, 2012; Humphreys et al., 2014 cited in Alves et al., 2021; Hansen et al., 2023).

Since its introduction, the species has invaded and established self-sustaining natural populations throughout Europe from the North Sea, Wadden Sea and Scandinavian coastlines to the Atlantic coastlines of Spain and Portugal, as well as the Mediterranean and Adriatic Sea (Wrange et al., 2010; GBNNSIP, 2011, 2012; Ezgeta-Balic et al., 2019; Spagnolo et al., 2019; Bergstrom et al., 2021; Hansen et al., 2023). In the UK, the species predominantly occurs around the southern and western coastlines (OBIS, 2024; NBN, 2024).

Shipping activity has also been associated with the introduction of Magallana gigas in the north-eastern Adriatic Sea, where it was not introduced for aquaculture (Ezgeta-Balic et al., 2019). It was also suggested that some Magallana gigas populations were established in south-west England from France, possibly via fouling on ships (GBNNSIP, 2011, 2012; Padilla, 2010; Ezgeta-Balic et al., 2019).

Magallana gigas requires hard substrata for successful settlement and establishment, including littoral rock, bedrock, chalk, bare boulders, cobbles and pebbles and shells (Kochmann et al., 2012, 2013; McKinstry & Jensen, 2013; Herbert et al., 2016; Tillin et al., 2020) because its larvae require hard substrata for successful settlement and development (McKinstry & Jensen, 2013; Tillin et al., 2020). It also prefers mudflats with mixed sediment composed of shingle and sand, attaching to whatever hard substrata are available within otherwise unsuitable fine muddy sediment (Spencer et al., 1994; McKinstry & Jensen, 2013; Tillin et al., 2020). Invasive populations of Magallana gigas have been found on wave-exposed rocky shores to wave-sheltered soft sediment environments, and it has been described as a habitat generalist (Troost, 2010; Kochmann et al., 2012, 2013). For example, in Scotland, wild Magallana gigas are mainly located in the lower intertidal on bedrock, bedrock encrusted with barnacles, within bedrock crevices, and large and small boulders (Cook et al., 2014). They are unlikely to occur under boulders as they require access to the water column (Tillin et al., 2020). Patches of Pacific oyster reefs have been recorded on littoral rock in Kent, southern England and on littoral sediments in southern England, the North Sea, and the English Channel (Herbert et al., 2012, 2016; Morgan et al., 2021).

Magallana gigas typically occurs in the lower intertidal and shallow subtidal, mostly abundant at depths 1 m to 10 m (Tillin et al., 2020). However, the preferred depth range varies with substratum. On littoral rock in Brittany, the Pacific oyster colonizes all intertidal levels from Mean High Water to Mean Low Water on sheltered (low energy), moderately exposed (moderate energy) and high energy rock shores (Herbert et al., 2012). However, in the Northwest Pacific, Magallana gigas is commonly found on sheltered low energy littoral rock and has less than 10% cover on exposed high energy littoral rock shores (Herbert et al., 2012, 2016). Magallana gigas was recorded in the mid intertidal on hard rock and artificial harbour structures in northern and eastern Adriatic, but absent from beam trawl surveys off the coast of Istria (Ezgeta-Balic et al., 2019). Ezgeta-Balic et al. (2019) noted that the only oysters found in their trawls were large Ostrea edulis, often confused for Magallana gigas by local fishermen. Magallana gigas has not been found at extreme low water levels or subtidally beneath rocky habitats, as it has been in soft sediment areas (Herbert et al., 2012).

Sensitivity assessment.

No reports were found of Magallana gigas existing with Synarachnactis lloydii. The sandy muddy gravel found in this biotope would provide suitable attachment for the species due to the presence of hard substrata (shells, pebbles and cobbles) that occur in this biotope. Most of the evidence suggests that Magallana gigas is limited to 10 m, but they can form beds down to 42 m where conditions allow (Smaal et al., 2009; Herbert et al., 2012, 2016; Tillin et al., 2020).  Shallow examples of this biotope (5 to 10 m) may, therefore, be susceptible to invasion by Magallana gigas, given the sheltered nature in which they are found and the provision of suitable attachment substrata by both the mixed sediment. Therefore, a precautionary worst-case resistance of ‘Medium’ is suggested for this biotope. Resilience is 'Very low' as the Magallana gigas population would need to be removed for recovery to occur. Therefore, sensitivity is assessed as ‘Medium’. As Magallana gigas has not yet been reported to interact with either characterizing species from this biotope (2026), confidence in the assessment is ‘Low’.

Wireweed, Sargassum muticum

Evidence

Sargassum muticum is a circumglobal invasive species (Engelen et al., 2015). It is recorded (2015) from Norway to Morocco and into the Mediterranean in the eastern Atlantic and from Alaska to Baja California in the eastern Pacific and from southern Russia to southern China in the western Pacific (Engelen et al., 2015). It colonizes a variety of habitats and can tolerate -1°C to 30°C and survive salinities below 10 ppt, but has a preference for full salinity ranges, 30 to 34 psu. Although fertilization does not occur below 15 ppt and growth of germlings is limited below 10°C, it can complete its life cycle as long as temperatures are over 8°C for at least four months of the year (Engelen et al., 2015). However, its distribution is limited by the availability of hard substrata (e.g., stones >10 cm) and light (Staehr et al., 2000; Strong & Dring, 2011; Engelen et al., 2015). It is most abundant between 1 and 3 m below mean water. But it has been recorded at 18 m or 30 m in the clear waters of California. However, it is a poor competitor under low light and only develops dense canopies in shallow areas (Engelen et al., 2015).

Sensitivity assessment

The depth (5 to 30 m) and lack of large enough attachment substrata likely exclude macroalgae from this biotope. Hence, it is unlikely to be colonized by Sargassum. Therefore, resistance and resilience are ‘High’, and this biotope is assessed as 'Not sensitive’ to Sargassum muticum.

Wakame, Undaria pinnatifida

Evidence

Undaria pinnatifida (Wakame or Asian kelp) is a large brown seaweed and an Invasive Non-Indigenous Species (INIS) that could out-compete native UK macroalgae species (Farrell & Fletcher, 2006; Thompson & Schiel, 2012; Brodie et al., 2014; Heiser et al., 2014; Arnold et al., 2016; Epstein & Smale, 2017, 2018; Kraan, 2017; Epstein et al., 2019a,b; Tidbury, 2020). Undaria pinnatifida originates from Japan but is currently established on the coastlines of New Zealand, Australia, Northern France, Spain, Italy, the UK, Portugal, Belgium, Holland, Argentina, Mexico, and the USA (De Leij et al., 2017). Undaria pinnatifida was first recorded in the UK in the Hamble Estuary in 1994 (Macleod et al., 2016) and has since proliferated along UK coastlines. One year after its discovery at the Queen Anne Battery marina, Plymouth, it became a major fouling plant on pontoons (Minchin & Nunn, 2014). Although initially restricted to artificial habitats, such as marinas and ports, it is now widespread in natural habitats in several areas, including Plymouth Sound. In Plymouth Sound, Epstein et al. (2019b) found that within its depth range (+1 to –4 m), Undaria pinnatifida co-existed with seven species of canopy-forming brown macroalgae, including Laminaria hyperborea. Undaria pinnatifida seems to settle better on artificial substrata (e.g., floats, marinas or piers) than on natural rocky shores among local kelps (Vaz-Pinto et al., 2014). It is found predominantly in low intertidal to shallow subtidal habitats (Epstein et al., 2019b) and is significantly more abundant on artificial substrata compared to natural rocky substrata (Heiser et al., 2014; Epstein & Smale, 2018).

Undaria pinnatifida has a wide physiological niche, meaning it can occur in both coastal and estuarine environments, but has a preference for full salinity ranges, 27 to 33 psu, and displays tolerance for varying salinities, turbidity and siltation (Heiser et al., 2014; Epstein & Smale, 2018). Undaria pinnatifida has a greater preference for sites sheltered with low wave exposure and weak tidal streams (Heiser et al., 2014; Epstein & Smale, 2018). In natural habitats, Undaria pinnatifida was not recorded if the wave fetch was greater than 642 km and increased in abundance and cover in very sheltered sites (Epstein & Smale, 2018).

Sensitivity assessment

The depth (5 to 30 m) and lack of large enough attachment substrata likely exclude macroalgae from this biotope. Hence, it is unlikely to be colonized by Undaria. Therefore, resistance and resilience are ‘High’, and this biotope is assessed as 'Not sensitive’ to Undaria pinnatifida.

Other INIS

Evidence

Compass sea squirt, Asterocarpa humilis. This species has been observed colonizing intertidal and shallow subtidal habitats down to 26 m. It can tolerate fully marine to low estuarine salinities and appears to prefer sheltered sites such as harbours and marinas (Tillin et al., 2020). Given the overlap between depth ranges (5 to 30 m) and the availability of suitable attachment substrata (pebbles, cobbles, and gravel), this biotope is considered potentially suitable habitat for Asterocarpa humilis (Tillin et al., 2020). However, little is known about the potential impacts of Asterocapa humilis colonization, and no reports of it interacting with Synarachnactis lloydii were found.

American jack knife clam, Ensis leei. This burrowing infaunal species can be found in coarse to muddy sediments, at depths from 5 to 100 m, and can reportedly withstand salinities of 7 to 32 ppt (Tillin et al., 2020). Given its preference for the soft substrata found in this biotope and the suitable depth and salinity range, it has been suggested (albeit with low confidence) as potentially suitable habitat for Ensis leei (Tillin et al., 2020); however, no reports of its colonization have been found.