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. Further detail.

Evidence

Ophiothrix fragilis is widely distributed in the eastern Atlantic from Norway to South Africa, including the Mediterranean, and Ophiocomina nigra from Norway to the Azores and Mediterranean (Hayward & Ryland, 1995b). Other component species in the biotope also have a widespread distribution in the North East Atlantic. Consequently, these species are exposed to temperatures both above and below those found in the British Isles and their distribution is not limited by temperature. In the Dutch Oosterschelde Estuary, fluctuations in the abundance of Ophiothrix fragilis between 1979 and 1990 appeared to be driven by winter temperatures (Leewis et al., 1994). When winter temperatures increased in 1979-80 and 1987-88, populations of brittlestars increased enormously and occupied 60-90% of the available hard substratum in layers up to 5 cm deep. Populations were reduced to less than 10% following the cold winters in 1978-79, 1984-85 and 1985-86. Thus, ocean warming may be beneficial to populations around the UK.

Sensitivity assessment. Under the middle and high emission and extreme scenarios seawater temperatures are expected to temperatures rise by 3-5°C to potential southern summer temperatures of 22-24°C. There is no experimental evidence of the impact of ocean warming on the characteristic species but biogeographic distribution is often a good predictor of temperature tolerance (Jeffree & Jeffree, 1994). The distribution of Ophiothrix fragilis and Ophiocomina nigra suggests that these species are likely to be tolerant of ocean warming. Dense populations of  Ophiothrix fragilis and Ophiocomina nigra occur in the infralittoral of the southern coast of Spain (Rodriguez, 1980), where seawater temperatures range from 15 - 25°C (www.seatemperature.org). As these species are known to thrive in warmer seawater temperatures than those predicted for the end of this century under all three scenarios, resistance is assessed as ‘High’ and resilience as ‘High’, so the biotope is, therefore,  assessed as ‘Not sensitive’ at levels predicted for the end of this century.

Global warming (high)

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

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

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

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

Evidence

Ophiothrix fragilis is widely distributed in the eastern Atlantic from Norway to South Africa, including the Mediterranean, and Ophiocomina nigra from Norway to the Azores and Mediterranean (Hayward & Ryland, 1995b). Other component species in the biotope also have a widespread distribution in the North East Atlantic. Consequently, these species are exposed to temperatures both above and below those found in the British Isles and their distribution is not limited by temperature. In the Dutch Oosterschelde Estuary, fluctuations in the abundance of Ophiothrix fragilis between 1979 and 1990 appeared to be driven by winter temperatures (Leewis et al., 1994). When winter temperatures increased in 1979-80 and 1987-88, populations of brittlestars increased enormously and occupied 60-90% of the available hard substratum in layers up to 5 cm deep. Populations were reduced to less than 10% following the cold winters in 1978-79, 1984-85 and 1985-86. Thus, ocean warming may be beneficial to populations around the UK.

Sensitivity assessment. Under the middle and high emission and extreme scenarios seawater temperatures are expected to temperatures rise by 3-5°C to potential southern summer temperatures of 22-24°C. There is no experimental evidence of the impact of ocean warming on the characteristic species but biogeographic distribution is often a good predictor of temperature tolerance (Jeffree & Jeffree, 1994). The distribution of Ophiothrix fragilis and Ophiocomina nigra suggests that these species are likely to be tolerant of ocean warming. Dense populations of  Ophiothrix fragilis and Ophiocomina nigra occur in the infralittoral of the southern coast of Spain (Rodriguez, 1980), where seawater temperatures range from 15 - 25°C (www.seatemperature.org). As these species are known to thrive in warmer seawater temperatures than those predicted for the end of this century under all three scenarios, resistance is assessed as ‘High’ and resilience as ‘High’, so the biotope is, therefore,  assessed as ‘Not sensitive’ at levels predicted for the end of this century.

Global warming (middle)

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

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

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

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

Evidence

Ophiothrix fragilis is widely distributed in the eastern Atlantic from Norway to South Africa, including the Mediterranean, and Ophiocomina nigra from Norway to the Azores and Mediterranean (Hayward & Ryland, 1995b). Other component species in the biotope also have a widespread distribution in the North East Atlantic. Consequently, these species are exposed to temperatures both above and below those found in the British Isles and their distribution is not limited by temperature. In the Dutch Oosterschelde Estuary, fluctuations in the abundance of Ophiothrix fragilis between 1979 and 1990 appeared to be driven by winter temperatures (Leewis et al., 1994). When winter temperatures increased in 1979-80 and 1987-88, populations of brittlestars increased enormously and occupied 60-90% of the available hard substratum in layers up to 5 cm deep. Populations were reduced to less than 10% following the cold winters in 1978-79, 1984-85 and 1985-86. Thus, ocean warming may be beneficial to populations around the UK.

Sensitivity assessment. Under the middle and high emission and extreme scenarios seawater temperatures are expected to temperatures rise by 3-5°C to potential southern summer temperatures of 22-24°C. There is no experimental evidence of the impact of ocean warming on the characteristic species but biogeographic distribution is often a good predictor of temperature tolerance (Jeffree & Jeffree, 1994). The distribution of Ophiothrix fragilis and Ophiocomina nigra suggests that these species are likely to be tolerant of ocean warming. Dense populations of  Ophiothrix fragilis and Ophiocomina nigra occur in the infralittoral of the southern coast of Spain (Rodriguez, 1980), where seawater temperatures range from 15 - 25°C (www.seatemperature.org). As these species are known to thrive in warmer seawater temperatures than those predicted for the end of this century under all three scenarios, resistance is assessed as ‘High’ and resilience as ‘High’, so the biotope is, therefore,  assessed as ‘Not sensitive’ at levels predicted for the end of this century.

Marine heatwaves (high)

High emission scenario benchmark: A marine heatwave occurring every two years, with a mean duration of 120 days, and a maximum intensity of 3.5°C. Further detail.

Evidence

Marine heatwaves due to increased air-sea heat flux are predicted to occur more frequently, last for longer and at increased intensity by the end of this century under both middle and high emission scenarios (Frölicher et al., 2018). Whilst there are no laboratory studies on the upper thermal limit of Ophiothrix fragilis or Ophiocomina nigra, these species appear to be tolerant of a wide range of temperatures. Furthermore, Ophiothrix fragilis can be found in the intertidal, where they may be exposed to sharp temperature fluctuations over a short period of time during the tidal cycle. Species that occur in the intertidal are therefore generally able to tolerate a range of temperatures (Davenport & Davenport, 2005). However, short-term acute changes in temperature are noted to cause a reduction in the loading of subcutaneous symbiotic bacteria in echinoderms such as Ophiothrix fragilis.  Reductions in these bacteria are probably indicative of levels of stress and may lead to mortality (Newton & McKenzie, 1995).

Sensitivity assessment. Under the middle emission scenario, if heatwaves occurred every three years, with a maximum intensity of 2°C for 80 days by the end of this century, this could lead to summer sea temperatures reaching up to 24°C in southern England. Under the high emission scenario, if heatwaves occurred every two years by the end of this century, reaching a maximum intensity of 3.5°C for 120 days, this could lead to the heatwave lasting the entire summer with temperatures reaching up to 26.5°C.

Ophiothrix fragilis is widely distributed across the Mediterranean Sea and both characterizing species of the SS.SMx.CMx.OphMx biotope (Ophiothrix fragilis and Ophiocomina nigra) present in the western Mediterranean (Koukouras et al., 2007), where sea temperatures can reach 28°C in summer months (www.seatemperature.org). Therefore, under both the middle and high emission scenarios, resistance is assessed as ‘High’ and resilience as ‘High’, so the biotope is assessed as ‘Not sensitive’.

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. Further detail.

Evidence

Marine heatwaves due to increased air-sea heat flux are predicted to occur more frequently, last for longer and at increased intensity by the end of this century under both middle and high emission scenarios (Frölicher et al., 2018). Whilst there are no laboratory studies on the upper thermal limit of Ophiothrix fragilis or Ophiocomina nigra, these species appear to be tolerant of a wide range of temperatures. Furthermore, Ophiothrix fragilis can be found in the intertidal, where they may be exposed to sharp temperature fluctuations over a short period of time during the tidal cycle. Species that occur in the intertidal are therefore generally able to tolerate a range of temperatures (Davenport & Davenport, 2005). However, short-term acute changes in temperature are noted to cause a reduction in the loading of subcutaneous symbiotic bacteria in echinoderms such as Ophiothrix fragilis.  Reductions in these bacteria are probably indicative of levels of stress and may lead to mortality (Newton & McKenzie, 1995).

Sensitivity assessment. Under the middle emission scenario, if heatwaves occurred every three years, with a maximum intensity of 2°C for 80 days by the end of this century, this could lead to summer sea temperatures reaching up to 24°C in southern England. Under the high emission scenario, if heatwaves occurred every two years by the end of this century, reaching a maximum intensity of 3.5°C for 120 days, this could lead to the heatwave lasting the entire summer with temperatures reaching up to 26.5°C.

Ophiothrix fragilis is widely distributed across the Mediterranean Sea and both characterizing species of the SS.SMx.CMx.OphMx biotope (Ophiothrix fragilis and Ophiocomina nigra) present in the western Mediterranean (Koukouras et al., 2007), where sea temperatures can reach 28°C in summer months (www.seatemperature.org). Therefore, under both the middle and high emission scenarios, resistance is assessed as ‘High’ and resilience as ‘High’, so the biotope is assessed as ‘Not sensitive’.

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. Further detail 

Evidence

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

The planktonic larval stage is often thought to be the most sensitive stage to ocean acidification in benthic organisms (Kurihara, 2008, Chan et al., 2015), and brittlestar larvae seem to be more sensitive to ocean acidification than sea urchins (Dupont & Thorndyke, 2008, Chan et al., 2015). The larvae of both Ophiothrix fragilis and Ophiocomina nigra have been shown to be sensitive to a small (0.2 unit) experimental pH decrease, leading to a decrease in survival and changes to developmental dynamics (Dupont & Thorndyke, 2008). A 0.2 unit pH decrease led to almost 100% mortality in Ophiothrix fragilis larvae after one week's exposure (Dupont & Thorndyke, 2009). Under low pH conditions, surviving larvae of Ophiothrix fragilis show skeletal malformations (Dupont & Thorndyke, 2008).

Sensitivity Assessment. Whilst evidence of the effect of ocean acidification on the adult stages of Ophiothrix fragilis and Ophiocomina nigra are lacking, the fact that even a small decrease in pH (0.2 units) has such a dramatic effect on the survival and normal growth of these species’ larvae suggests that this may well drive these species responses to future acidification. Unfortunately, at present, there are no multi-generational studies to determine whether these species can adapt or acclimate to future pH conditions, but on the evidence available, under both the middle and high emission scenarios (0.15 and 0.35 pH unit decrease, respectively) the biotope is assessed as having a resistance level of ‘None’, and a resilience level of ‘Very low’ because of the long-term nature of ocean acidification, leading to an assessment of ‘High’ at the benchmark level. 

Ocean acidification (middle)

Middle emission scenario benchmark: a further decrease in pH of 0.15 (annual mean) and 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. Further detail.

Evidence

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

The planktonic larval stage is often thought to be the most sensitive stage to ocean acidification in benthic organisms (Kurihara, 2008, Chan et al., 2015), and brittlestar larvae seem to be more sensitive to ocean acidification than sea urchins (Dupont & Thorndyke, 2008, Chan et al., 2015). The larvae of both Ophiothrix fragilis and Ophiocomina nigra have been shown to be sensitive to a small (0.2 unit) experimental pH decrease, leading to a decrease in survival and changes to developmental dynamics (Dupont & Thorndyke, 2008). A 0.2 unit pH decrease led to almost 100% mortality in Ophiothrix fragilis larvae after one week's exposure (Dupont & Thorndyke, 2009). Under low pH conditions, surviving larvae of Ophiothrix fragilis show skeletal malformations (Dupont & Thorndyke, 2008).

Sensitivity Assessment. Whilst evidence of the effect of ocean acidification on the adult stages of Ophiothrix fragilis and Ophiocomina nigra are lacking, the fact that even a small decrease in pH (0.2 units) has such a dramatic effect on the survival and normal growth of these species’ larvae suggests that this may well drive these species responses to future acidification. Unfortunately, at present, there are no multi-generational studies to determine whether these species can adapt or acclimate to future pH conditions, but on the evidence available, under both the middle and high emission scenarios (0.15 and 0.35 pH unit decrease, respectively) the biotope is assessed as having a resistance level of ‘None’, and a resilience level of ‘Very low’ because of the long-term nature of ocean acidification, leading to an assessment of ‘High’ at the benchmark level. 

Sea level rise (extreme)

Extreme scenario benchmark: a 107 cm rise in average UK by the end of this century (2018-2100). Further detail.

Evidence

Sea-level rise is occurring through a combination of thermal expansion and ice melt.  Sea levels have risen 1-3 mm/yr. in the last century (Cazenave & Nerem, 2004, Church et al., 2004, Church & White, 2006). Evidence appears to suggest that impacts of sea-level rise on exposure or tidal energy will be non-linear and site-specific (Pickering et al., 2012, Li et al., 2016).  This biotope occurs on mixed sediment, in moderately exposed to sheltered areas, subject to strong to weak tidal streams and, therefore, should be reasonably robust to any changes which occur. Furthermore, this biotope occurs at depths of 5-50 m around the UK, and sea-level rises predicted for the end of this century should have limited impact on this biotope.

Sensitivity assessment. As this biotope can occur from 5-50 m depth, in a range of different energy environments, it is assumed that a sea-level rise of 50 cm, 70 cm or 107 cm (middle and high emission, and extreme scenarios) would have limited effect. Therefore, resistance is assessed as ‘High’ under all three scenarios, so that resilience is ‘High’ and sensitivity assessed as ‘Not sensitive’. 

Sea level rise (high)

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

Evidence

Sea-level rise is occurring through a combination of thermal expansion and ice melt.  Sea levels have risen 1-3 mm/yr. in the last century (Cazenave & Nerem, 2004, Church et al., 2004, Church & White, 2006). Evidence appears to suggest that impacts of sea-level rise on exposure or tidal energy will be non-linear and site-specific (Pickering et al., 2012, Li et al., 2016).  This biotope occurs on mixed sediment, in moderately exposed to sheltered areas, subject to strong to weak tidal streams and, therefore, should be reasonably robust to any changes which occur. Furthermore, this biotope occurs at depths of 5-50 m around the UK, and sea-level rises predicted for the end of this century should have limited impact on this biotope.

Sensitivity assessment. As this biotope can occur from 5-50 m depth, in a range of different energy environments, it is assumed that a sea-level rise of 50 cm, 70 cm or 107 cm (middle and high emission, and extreme scenarios) would have limited effect. Therefore, resistance is assessed as ‘High’ under all three scenarios, so that resilience is ‘High’ and sensitivity assessed as ‘Not sensitive’. 

Sea level rise (middle)

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

Evidence

Sea-level rise is occurring through a combination of thermal expansion and ice melt.  Sea levels have risen 1-3 mm/yr. in the last century (Cazenave & Nerem, 2004, Church et al., 2004, Church & White, 2006). Evidence appears to suggest that impacts of sea-level rise on exposure or tidal energy will be non-linear and site-specific (Pickering et al., 2012, Li et al., 2016).  This biotope occurs on mixed sediment, in moderately exposed to sheltered areas, subject to strong to weak tidal streams and, therefore, should be reasonably robust to any changes which occur. Furthermore, this biotope occurs at depths of 5-50 m around the UK, and sea-level rises predicted for the end of this century should have limited impact on this biotope.

Sensitivity assessment. As this biotope can occur from 5-50 m depth, in a range of different energy environments, it is assumed that a sea-level rise of 50 cm, 70 cm or 107 cm (middle and high emission, and extreme scenarios) would have limited effect. Therefore, resistance is assessed as ‘High’ under all three scenarios, so that resilience is ‘High’ and sensitivity assessed as ‘Not sensitive’. 

Temperature increase (local)

Benchmark. A 5°C increase in temperature for one month, or 2°C for one year. Further detail

Evidence

Ophiothrix fragilis is widely distributed in the eastern Atlantic from Norway to South Africa and Ophiocomina nigra from Norway to the Azores and Mediterranean (Hayward & Ryland, 1995b). Other component species in the biotope also have a widespread distribution in the north east Atlantic. Consequently, these species are exposed to temperatures both above and below those found in the British Isles and their distribution is not limited by temperature. In the Dutch Oosterschelde Estuary fluctuations in the abundance of Ophiothrix fragilis between 1979 and 1990 appeared to be driven by winter temperatures (Leewis et al., 1994). When winter temperatures increased in 1979-80 and 1987-88, populations of brittlestars increased enormously, and occupied 60-90% of the available hard substratum in layers up to 5 cm deep. Populations were reduced to less than 10% following the cold winters in 1978-79, 1984-85 and 1985-86. Thus, increases in temperature may be beneficial to populations. However, short-term acute changes in temperature are noted to cause a reduction in the loading of subcutaneous symbiotic bacteria in echinoderms such as Ophiothrix fragilis. Reductions in these bacteria are probably indicative of levels of stress and may lead to mortality (Newton & McKenzie, 1995).

Sensitivity assessment. The distribution of Ophiothrix fragilis and Ophiocomina nigra suggests that they are likely to be tolerant of an acute or chronic temperature increase at the pressure benchmark. Resistance is assessed as ‘High’ and resilience as ‘High’ (by default), so the biotope is therefore considered to be ‘Not sensitive’ at the pressure benchmark.

Temperature decrease (local)

Benchmark. A 5°C decrease in temperature for one month, or 2°C for one year. Further detail

Evidence

Ophiothrix fragilis is widely distributed in the eastern Atlantic from Norway to South Africa and Ophiocomina nigra from Norway to the Azores and Mediterranean (Hayward & Ryland, 1995b). Other component species in the biotope also have a widespread distribution in the north east Atlantic. Consequently, these species are exposed to temperatures both above and below those found in the British Isles with distribution not limited by temperature. In the Dutch Oosterschelde Estuary fluctuations in the abundance of Ophiothrix fragilis between 1979 and 1990 appeared to be driven by winter temperatures (Leewis et al., 1994). When winter temperatures increased in 1979-80 and 1987-88, populations of brittlestars increased enormously, and occupied 60-90% of the available hard substratum in layers up to 5 cm deep. Populations were reduced to less than 10% following the cold winters in 1978-79, 1984-85 and 1985-86. Thus, decreases in temperature may affect population densities. Short-term acute changes in temperature are noted to cause a reduction in the loading of subcutaneous symbiotic bacteria in echinoderms such as Ophiothrix fragilis. Reductions in these bacteria are probably indicative of levels of stress and may lead to mortality (Newton & McKenzie, 1995).

Sensitivity assessment. The distribution of Ophiothrix fragilis and Ophiocomina nigra, the characterizing species of the biotope, suggests that they are likely to be tolerant of an acute or chronic temperature decrease at the pressure benchmark. Thus, resistance is assessed as ‘High’ and resilience as ‘High’ (by default) and the biotope is therefore considered to be ‘Not Sensitive’ at the pressure benchmark. 

Salinity increase (local)

Benchmark. A increase in one MNCR salinity category above the usual range of the biotope or habitat. Further detail

Evidence

Echinoderms are stenohaline owing to the lack of an excretory organ and a poor ability to osmo- and ion-regulate (Stickle & Diehl, 1987; Russell, 2013) and unable to tolerate wide fluctuations in salinity. A review by Russell (2013) confirmed that Ophiothrix fragilis and Ophiocomina nigra have not been previously recorded in hypersaline conditions, although Pagett (1981) suggested that echinoderms may exhibit localised physiological adaption to reduced or variable salinities in near shore areas subject to freshwater runoffs. However, a circalittoral habitat is less likely to experience variable salinities, and resident species, therefore, less likely to be adapted to variation in salinity, as suggested by the results given by Pagett (1981).

Sensitivity assessment. There is little direct evidence of the effects of hypersaline conditions on Ophiothrix fragilis and Ophiocomina nigra. However, echinoderms are generally considered to be stenohaline (Stickle & Diehl, 1987; Russell, 2013). Therefore, an increase in salinity to >40 psu is likely to result in mortality and resistance is assessed as ‘Low’ but with low confidence. Resilience is probably ‘Medium’ so that sensitivity is therefore assessed as ‘Medium’.

Salinity decrease (local)

Benchmark. A decrease in one MNCR salinity category above the usual range of the biotope or habitat. Further detail

Evidence

Echinoderms are stenohaline owing to the lack of an excretory organ and a poor ability to osmo- and ion-regulate (Stickle & Diehl, 1987; Russell, 2013). This means that they are unable to tolerate wide fluctuations in salinity. Although brittlestar beds are generally found in fully marine conditions, Wolff (1968) observed dense aggregations of Ophiothrix fragilis occurring in salinities of 16 psu and even persisting down to 10 psu in the Oosterschelde Estuary. Russell (2013) reported that Ophiocomina nigra and Ophiura albida can tolerate 27.6‰ and 20‰ in experiments, respectively. Pagett (1981) suggested that localised physiological adaption to reduced or variable salinities may occur in near shore areas subject to freshwater runoffs However, a circalittoral habitat is less likely to experience variable salinities, and resident species, therefore, less likely to adapt to variation in salinity, as suggested by the results given by Pagett (1981).

Sensitivity assessment. The evidence suggests that a decrease in salinity may result in significant mortality of the biotope's defining species, Ophiothrix fragilis, especially as populations in the circalittoral may not be adapted to tolerate variations in salinity. Resistance is therefore assessed as ‘Low’ and resilience as ‘Medium’. Sensitivity is therefore assessed as ‘Medium’.

Water flow (tidal current) changes (local)

Benchmark. A change in peak mean spring bed flow velocity of between 0.1 m/s to 0.2 m/s for more than one year. Further detail

Evidence

Dense brittlestar beds are found in a range of water flows from sea lochs with restricted water flows to higher-energy environments on open coastlines (Connor et al., 2004). In the Dover Strait, Ophiothrix beds experience current speeds of up to 1.5 m/s during average spring tides (Davoult & Gounin, 1995). Davoult & Gounin (1995) found that current speeds below 0.2 m/s were optimal for suspension feeding by Ophiothrix fragilis. If the velocity exceeded 0.3 m/s the animals ceased feeding, flattening themselves against the substratum and linking arms, so increasing their collective stability in the current. These values agree with those found by Warner (1971).

Similarly strong tidal streams (1.0-1.2 m/s) were also recorded over beds in the Isle of Man (Brun, 1969). In both locations (Isle of Man and the Dover Strait), Ophiothrix densities of up to 2000 individuals/m² were recorded. Hughes (1998b) suggested that high density aggregations could probably only be maintained where strong currents can supply enough suspended food. Food requirements probably set a lower limit on the current regime of areas able to support brittlestar beds. However, above a certain water speed (0.25 m/s) the feeding arms are withdrawn from the water column (Warner & Woodley, 1975; Hiscock, 1983). At water speeds above about 0.28 m/s individuals or even small groups may be displaced from the substratum and they have been observed being rolled along the seabed by the current (Warner, 1971). Living in dense aggregations may reduce displacement by strong currents (Warner & Woodley, 1975). Ophiocomina nigra is usually found in fairly sheltered sites with some water movement, and tends to become more dominant in deeper water than Ophiothrix (Connor et al., 2004), which suggests a lower resistance to changes in water flow.

Sensitivity assessment. The evidence available suggests that brittlestars have behavioural adaptations to changes in water flow (Tillin & Tyler-Walters, 2014). Increased flow rates, increases suspension and transport of organic particles and can enhance feeding rates. If the flow is too strong, brittlestars may flatten, link arms, or withdraw arms into sediment. At lower flow rates species may switch to deposit feeding. Thus, although brittlestar beds can tolerate increased water flow over tidal cycles a long-term increase will probably prevent the population feeding and over a period of a year this is likely to cause the loss of the population. A decrease in water flow could potentially have an effect on some of the characterizing species of the biotope and may alter species richness as a result of sediment deposition. These are not considered to alter the character of the biotope. However, this brittlestar dominated biotope occurs in a range of water flows, so the change in the water flow experienced by mid-range populations of the characterizing species is unlikely to have an impact at the pressure benchmark. The biotope is considered to have ‘High’ resistance and ‘High’ resilience, and are therefore assessed as ‘Not Sensitive’ at the benchmark level.

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. Further detail

Evidence

Changes in emergence are 'Not Relevant' to this biotope, which is restricted to fully subtidal/circalittoral conditions. The pressure benchmark is relevant only to littoral and shallow sublittoral fringe biotopes. 

Wave exposure changes (local)

Benchmark. A change in near shore significant wave height of >3% but <5% for more than one year. Further detail

Evidence

Records indicate that SS.SMx.CMx.OphMx occurs in moderately exposed and sheltered areas (Connor et al., 2004). Connor et al. (2004) also suggested that the biotope occurs over a wide range of depths, although mainly in bands 10-20 m and 20-30 m, and that this means that wave action was not severe on the seabed as to displace the dense mat of brittlestars. A decrease may increase siltation and limit food supply for the dominant suspension feeding community but only where tidal flow is also reduced.

Sensitivity assessment: An increase or decrease in wave height at the pressure benchmark is unlikely to be significant in wave exposed examples of the biotope. As brittlestar beds require strong to moderate water movements, water flow is probably a more important source of water movement in sheltered examples of the biotope. Therefore, brittlestar beds probably have a 'High' resistance to a change in significant wave height at the pressure benchmark. Resilience is assessed as ‘High’, by default, and the biotope is considered ‘Not Sensitive’ at the benchmark level.

Transition elements & organo-metal contamination

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

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

Adult echinoderms such as Ophiothrix fragilis are known to be efficient concentrators of heavy metals including those that are biologically active and toxic (Ag, Zn, Cd and Co) (Hutchins et al., 1996). There is no information available regarding the effects of this bioaccumulation. Gounin et al. (1995) studied the transfer of heavy metals (Fe, Mn, Pb, Cu and Cd) through Ophiothrix beds. They concluded that heavy metals ingested or absorbed by the animals transited rapidly through the body and were expelled in the faeces and did not appear to accumulate in their tissues. Studies by Deheyn & Latz (2006) at the Bay of San Diego found that heavy metal accumulation in brittlestars occurs both through dissolved metals as well as through diet, to the arms and disc, respectively. Similarly, Sbaihat et al. (2013) measured concentrations of heavy metals (Cu, Ni, Cd, Co, Cr and Pb) in the body of Ophiocoma scolopendrina collected from the Gulf of Aqaba, and found that most concentration was found in the central disc rather than arms and no simple correlations could be found between contaminant and body length. It is logical to suppose that brittlestar beds would be adversely affected by major pollution incidents such as oil spills, or by continuous exposure to toxic metals, pesticides, or the antiparasite chemicals used in cage aquaculture. So far, however, there are no field observations of epifaunal brittlestar beds being damaged by any of these forms of pollution, and there seems to be no evidence of the toxicity effects of heavy metal accumulation on brittlestars.

Hydrocarbon & PAH contamination

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

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

Echinoderms tend to be very sensitive to various types of marine pollution (Newton & McKenzie, 1995). Adult Ophiothrix fragilis have been documented to be intolerant to hydrocarbons (Newton & McKenzie, 1995). The sub-cuticular bacteria that are symbiotic with Ophiothrix fragilis are reduced in number following exposure to hydrocarbons. Exposure to 30,000 ppm oil reduces the bacterial load by 50% and brittlestars begin to die (Newton & McKenzie, 1995). The water-accumulated fraction of diesel oil has been found to be acutely toxic to Ophiothrix fragilis and Ophiocomina nigra, although no field observations of beds being damaged by hydrocarbon pollution have been found (Hughes, 1998b).

Untreated oil (e.g. from oil spills) is not a risk, since it is concentrated mainly at the surface, and circalittoral biotopes are likely to be protected by their depth. If oil is treated by dispersant, the resulting emulsion will penetrate down the water column, especially under the influence of turbulence (Hartnoll, 1998).

Synthetic compound contamination

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

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

Echinoderms tend to be very sensitive to various types of marine pollution (Newton & McKenzie, 1995) but there is no more detailed information than this broad statement. In laboratory experiments Smith (1968) found the concentration of BP1002 (the detergent used in the Torrey Canyon oil spill clean-up) needed to kill the majority of Ophiocomina nigra was 5 ppm. Although there are no known examples of brittlestar beds being damaged by chemical pollutants such as pesticides or anti-parasite chemicals used in aquaculture, it is logical to suppose they would be adversely affected.

Radionuclide contamination

Benchmark. An increase in 10µGy/h above background levels. Further detail

Evidence

Adult echinoderms such as Ophiothrix fragilis are known to be efficient concentrators of radionuclides (Hutchins et al., 1996). However, there was no information available about the effect of this bioaccumulation.

Introduction of other substances

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

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). Further detail

Evidence

Cole et al. (1999) suggested possible adverse effects on marine species exposed to dissolved oxygen concentrations below 4 mg/l and probable adverse effects below 2mg/l. However, Ophiothrix fragilis is known to have a low respiration rate (Migné & Davoult, 1997b), and experiments by Rosenberg et al. (1991, cited in Diaz & Rosenberg, 1995) suggested that the higher tolerance to hypoxia shown by Amphiura chiajei compared to Amphiura filiformis could also be linked to lower respirations rates, although both these brittlestars species were considered to be resistant to moderate hypoxia (Diaz & Rosenberg, 1995, references therein).

Stachowitsch (1984) observed a mass mortality of benthic organisms in the Gulf of Trieste, northern Adriatic Sea, caused by the onset of severe hypoxia (oxygen depletion) in the near-bottom water. A wide variety of organisms were affected, including burrowing invertebrates, sponges, and the brittlestar Ophiothrix quinquemaculata, a dominant component of the local epifaunal community. This event was likely caused by a combination of unfavourable weather and tidal conditions, at the same time as a period of maximal organic input from coastal pollution and dying phytoplankton. Water exchange in the Gulf was poor, and the area tended to accumulate sediment and suspended organic material. Very high productivity in the water column, combined with sewage input throughout the summer tourist season, probably led to the consumption of most of the dissolved oxygen by microbial activity. Mortality occurred when the oxygen-deficient water mass extended to the sea floor (Stachowitsch, 1984).

Sensitivity assessment: The evidence presented suggests that some species of brittlestar are likely to tolerate moderate levels of hypoxia. However, Stachowitsch (1984) observed a mass mortality of brittlestar Ophiothrix quinquemaculata within 2-3 days of the onset of a hypoxia event. Resistance of the biotope is assessed as ‘Medium’ and recovery assessed as ‘High’ resulting in the sensitivity of the biotope being considered ‘Low’ at the pressure benchmark.

Nutrient enrichment

Benchmark. Compliance with WFD criteria for good status. Further detail

Evidence

It is thought that dense Ophiothrix beds may play an important role in local nutrient cycles by filtration and concentration of suspended particulate matter and by excretion of nitrogenous waste (Hughes, 1998b). Brittlestar beds are therefore likely to be able to resiste increased nutrient levels in the form of dissolved nutrients or particulate matter. For example, in the Bay of Brest in Brittany, Hily (1991) estimated that Ophiothrix beds with over 400 individuals/m² could filter the equivalent of 30% of the total water volume of the bay daily. The inflow of nutrient-rich stream water into the bay led to very high primary productivity, but eutrophication did not occur, because of the removal of particulate matter by the benthic community of brittlestars. A dense aggregation of Ophiothrix and Ophiocomina appeared to be unaffected by the presence of a salmon farm within 100 m (B. Ball pers. comm. in Hughes, 1998b). Since such farms often result in an increase in nutrients to the sea bed, brittlestar beds appear to be able to resiste some increase in nutrient levels (Hughes, 1998b). Raymont (1950) recorded an increase in populations of Ophiocomina nigra following the addition of fertilizers to the waters of an enclosed basin of Loch Sween, Argyll.

Sensitivity assessment. Due to the resistance of high levels of nutrient input demonstrated by species of brittlestars, brittlestar beds may be able to resiste nutrient enrichment. Resistance is assessed as 'High' and resilience as 'High' (by default) and the biotope is considered as ‘Not Sensitive’ at the pressure benchmark that assumes compliance with good status as defined by the WFD.

Organic enrichment

Benchmark. A deposit of 100 gC/m²/yr. Further detail

Evidence

Organic enrichment may be beneficial to suspension feeders as a direct source of food and may indirectly enhance food supply where enrichment stimulates local growth of phytoplankton and diatoms. Raymont (1950) recorded an increase in Ophiocomina nigra populations following the addition of fertilizers to the waters of an enclosed basin of Loch Sween, Argyll. A dense aggregation of Ophiothrix and Ophiocomina was recorded in 1974 from a site at the mouth of Killary Harbour, western Ireland, and reported unchanged following subsequent establishment of a salmon farm within 100 m of the main beds (B. Ball, pers. comm. cited in Hughes, 1998b). However, high levels of organic enrichment would be expected to result in excessive sedimentation and hypoxia having deleterious effects on brittlestars and other suspension feeders. Stachowitsch (1984) reported that organic pollution may well have contributed to the environmental oxygen depletion causing mass mortality of brittlestar Ophiothrix quinquemaculata in the Gulf of Trieste.

The AZTI Marine Biotic Index (AMBI) is a biotic index to assess disturbance (including organic enrichment). Borja et al. (2000) assigned Ophiothrix fragilis to Ecological Group I (Species very sensitive to organic enrichment and present under unpolluted conditions (initial state) whereas Gittenberger & Van Loon (2011) assigned this species to Ecological Group II (Species indifferent to enrichment, always present in low densities with non-significant variations with time) (from initial state, to slight unbalance). Ophiocomina nigra has not been assigned an AMBI category. Although the unpublished Gittenberger & Van Loon (2011) report is an update on Borja et al. (2000), the former is a peer reviewed publication. Given that the evidence used in both cases is unclear, the degree of confidence is assessed as medium.

Sensitivity assessment: The evidence presented based on the AMBI scores conflicts and considered with caution. This biotope is generally found in areas with some water movement and this is likely to disperse organic matter reducing organic material load. Resistance to organic enrichment is assessed as ‘High’ and resilience as ‘High’. The biotope is therefore assessed as ‘Not Sensitive’ to organic enrichment, and the animals found within the biotope may be able to utilize the input of organic matter as food, or are likely to be resistant of inputs at the benchmark level.

Physical loss (to land or freshwater habitat)

Benchmark. A permanent loss of existing saline habitat within the site. Further detail

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. Further detail

Evidence

If the mixed sediments were replaced with rock substrata, this would represent a fundamental change to the physical character of the biotope. The biotope would be lost and/or re-classified.

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

Physical change (to another sediment type)

Benchmark. Permanent change in one Folk class (based on UK SeaMap simplified classification). Further detail

Evidence

Records indicate that SS.SMx.CMx.OphMx occurs on a range of substrata, often including boulders, pebbles, cobbles, gravel, sand and mud (Connor et al., 2004).The characterizing species of this biotope, the brittlestars, are epifaunal and not attached to the substratum, and therefore unlikely to be adversely affected by a change in one Folk class from mixed sediment to mud and sandy mud, for example.

Sensitivity assessment: A change in the seabed type at the benchmark level is unlikely to affect the characterizing species, which have been recorded on a wide variety of substrata, ranging from bedrock, through boulders and cobbles to gravel, sand and mud (Hughes, 1998b). Resistance is therefore assessed as ’High’ and resilience as ‘High’ (by default), and the biotope is considered ‘Not Sensitive’ at the pressure benchmark.

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). Further detail

Evidence

Extraction of substratum to 30 cm is likely to result in the removal of the biological community along with the substrata, including the characterizing species, the brittlestars, and other component infaunal and epifaunal species.

Sensitivity assessment. Due to the nature of this pressure it is highly likely that a large amount of the sediment would be removed along with the biological community, resulting in the removal of the biotope. Resistance is assessed as ‘None’ and resilience as ‘Medium’ with a sensitivity of ‘Medium’

Abrasion / disturbance of the surface of the substratum or seabed

Benchmark. Damage to surface features (e.g. species and physical structures within the habitat). Further detail

Evidence

A review by Jennings & Kaiser (1998) suggested that the main direct effects of fishing on marine ecosystems usually include scraping, scouring and re-suspension of substratum. Brittlestars are epifaunal and have fragile arms so are likely to be directly exposed and damaged by abrasion. Brittlestars can tolerate considerable damage to arms and even the disk without suffering mortality and are capable of arm and even some disk regeneration (Sköld, 1998). Fishermen tend to avoid brittlestar beds since the animals clog their nets (Jones et al., 2000). However, a passing scallop dredge is likely to remove, displace, or damage brittlestars caught in its path. Although several species of brittlestar were reported to increase in abundance in trawled areas (including Ophiocomina nigra), Bradshaw et al. (2002) noted that the relatively sessile Ophiothrix fragilis decreased in the long-term in areas subject to scallop dredging. Overall, a proportion of the population is likely to be damaged or removed. An average of 36% of individuals in five British brittlestar beds were regenerating arms (Aronson, 1989) showing that the beds can persist following exposure to a pressure.

SS.SMx.CMx.OphMx occurs in a variety of substrata including boulders, cobbles, pebbles, gravels, sand and mud (Connor et al., 2004). Sediment re-suspension is therefore likely to occur, with associated consequences for the biological community (see suspended solids and siltation pressures), as well as potential changes in the sediment characteristics of the seabed as a result of the repeated non-targeted removal of sediment (Bradshaw et al., 2002, references therein). Furthermore, e.g. cobbles and pebbles present, are likely to be moved and turned as a result of the passing dredge, leading to further damage to the epifaunal communities.

Sensitivity assessment. Epifaunal species and communities are considered to be amongst the most vulnerable to bottom gears (Jennings & Kaiser, 1998) and the impact of surface abrasion will depend on the footprint, duration and magnitude of the pressure. Based on the evidence, resistance to a single abrasion event is assessed as ‘Low’ and resilience as ‘Medium’, so that sensitivity is assessed as ‘Medium’. However, Veale et al. (2000) suggested that the abundance, biomass and production of epifaunal assemblages decreased with increasing fishing effort suggesting that, resistance and recovery of the biotope’s species are likely to vary with pressure intensity. Resistance and resilience will therefore be lower (and hence sensitivity greater) to repeated abrasion events.

Penetration or disturbance of the substratum subsurface

Benchmark. Damage to sub-surface features (e.g. species and physical structures within the habitat). Further detail

Evidence

Damage to the seabed’s sub-surface is likely to remove both the infaunal and epifaunal communities that occur in this biotope. Additionally, penetrative activities (e.g. anchoring, scallop or suction dredging) are likely to remove or displace the cobbles, pebbles, or small boulders that occur in this biotope. As a result the biotope could be lost or severely damaged, depending on the scale of the activity (see abrasion above). Therefore, a resistance of 'None' is suggested. Resilience is probably 'Medium' therefore the biotope’s sensitive to this pressure if likely to be ‘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. Further detail

Evidence

Ophiothrix fragilis and Ophiocomina nigra are passive suspension feeders and a significant supply of suspended organic material is needed to meet the energetic costs of the great numbers of individuals in a brittlestar bed. Brittlestar beds occur in a variety of water flow regimes from sea lochs to more energetic coastal sites (Connor et al., 2004) so are likely to tolerate a variety of different suspended sediment concentrations. For example, dense britlestar beds occur in the Dover Straits, where the concentration of suspended particles in the water column changes between 18-32 mg/l annually (Davoult & Gounin, 1995). Although some brittlestar species are able to perceive differences in light and dark, visual perception is limited (Tillin & Tyler-Walters, 2014) and brittlestars are unlikely to be directly affected by changes in light resulting from a change in turbidity and suspended solids.

However, local increases in turbidity in waters previously within the photic zone, may alter local abundances of phytoplankton and surface diatoms and the zooplankton and other small invertebrates that feed on them. Davoult & Gounin (1995) found that the growth rate of Ophiothrix in the Dover Strait was maximal in April/May, coinciding with the spring phytoplankton bloom, which suggests that an increase in suspended solids and resulting increase in turbidity may indirectly reduce feeding efficiency in brittlestars. Nonetheless, since phytoplankton may arrive from distant sources and brittlestars may also feed on organic detritus any effects are expected to be small. Additionally, Ophiothrix fragilis has a low respiration rate and can tolerate considerable loss of body mass during reproductive periods (Davoult et al., 1990) suggesting that this species may tolerate feeding restrictions.

Sensitivity Assessment: The evidence presented suggests that an increase in suspended organic matter may be beneficial by providing increased food material while a decrease in suspended sediment may reduce food supplies to brittlestar beds. Additionally, increases in suspended solids that involve increase of inorganic particles may interfere with the feeding of brittlestars (Aronson, 1992 cited in Hughes, 1998b), particularly in non-current swept areas. However, the biotope occurs in a wide range of conditions and are likely to be adapted to respond to changes in suspended solids at the pressure benchmark and the overall species richness in the biotope is not likely to change. Resistance is therefore assessed as ‘High’ and resilience as ‘High’, so the biotope is assessed as ‘Not Sensitive’ to a change in turbidity at the pressure benchmark. 

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. Further detail

Evidence

Material in suspension can affect the efficiency of filter and suspension feeding (Sherk & Cronin, 1970; Morton, 1976). Effects can include abrasion and clogging of gills, impaired respiration, clogging of filter mechanisms, and reduced feeding and pumping rates. Dense beds of brittlestars tend not to persist in areas of excessive sedimentation, because high levels of sediment foul the brittlestars feeding apparatus (tube feet and arm spines), and ultimately suffocates them (Schäfer, 1962 cited in Aronson, 1992). Aronson (1989) referred to the demise of Warner's (1971) Ophiothrix bed in Torbay, and tentatively suggested it was due to increased sedimentation caused by the localised dumping of construction materials (Aronson, 1989).

In areas of high water flow dispersion of fine sediments may be rapid and this could mitigate the magnitude of this pressure by reducing the time exposed, where ‘light’ deposition of sediments is likely to be cleared in a few tidal cycles.

In exposed situations suspended material can cause scour, but this is normally a result of the temporary re-suspension of relatively coarse bottom material rather than of fine material in long-term suspension.

Sensitivity assessment. This pressure is not considered to alter the physical reef habitat but there may be effects on the biological community. Habitat resistance is assessed as ‘Low’ given that the key characterizing species of brittlestars are likely to be affected by a single discrete event of light deposition of fine materials that is not removed in the short-term by water movement. Resilience is likely to be ‘Medium’ and the habitat sensitivity is assessed as ‘Medium’.

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. Further detail

Evidence

Material in suspension can affect the efficiency of filter feeding (Sherk & Cronin, 1970; Morton, 1976). Effects can include abrasion and clogging of gills, impaired respiration, clogging of filter mechanisms, and reduced feeding and pumping rates. Dense beds of brittlestars tend not to persist in areas of excessive sedimentation, because high levels of sediment foul the brittlestars feeding apparatus (tube feet and arm spines), and ultimately suffocates them (Schäfer, 1962 cited in Aronson, 1992). Aronson (1989) referred to the demise of Warner's (1971) Ophiothrix bed in Torbay, and tentatively suggested it was due to increased sedimentation caused by the localised dumping of construction materials (Aronson, 1989).

In areas of high water flow dispersion of fine sediments may be rapid and this could mitigate the magnitude of this pressure by reducing the time exposed, where ‘heavy’ deposition of sediments is likely to be cleared in a few tidal cycles.

In exposed situations suspended material can cause scour, but this is normally a result of the temporary re-suspension of relatively coarse bottom material rather than of fine material in long-term suspension.

Sensitivity assessment. The brittlestars forming the dense beds characterizing this biotope are likely to be adversely affected by the smothering effect of a ‘heavy’ deposition of 30 cm of sediment in a single discrete event. Habitat resistance is assessed as ‘Low’ and recovery is probably ‘Medium’ and the habitat sensitivity is assessed as ‘Medium’.

Litter

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

Evidence

Not assessed.

Electromagnetic changes

Benchmark. A local electric field of 1 V/m or a local magnetic field of 10 µT. Further detail

Evidence

‘No Evidence’ is available on which to assess this pressure. 

Underwater noise changes

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

Evidence

There is little known about the effects of underwater sound on marine invertebrates. Although there are no records of brittlestars reacting to noise, sound vibrations may trigger some response. However, at the level of the benchmark the biotope is not likely to be sensitive to noise pollution. For example, brittlestar beds have been recorded from Kinsale Harbour (Hughes, 1998b) on the south coast of Ireland where there is likely to be noise disturbance from passing boat traffic.

Sensitivity assessment: There is not enough evidence to assess this pressure. 

Introduction of light or shading

Benchmark. A change in incident light via anthropogenic means. Further detail

Evidence

SS.SMx.CMx.OphMx is a circalittoral biotope (Connor et al., 2004) and therefore, not directly dependent on sunlight. Although some brittlestar species are able to perceive differences in light and dark, visual perception is limited (Tillin & Tyler-Walters, 2014) and this suggests that the brittlestars are unlikely to be directly affected by change in light.

Sensitivity assessment. The biotope is considered to have ‘High’ resistance and, by default, ‘High’ resilience and therefore is ‘Not Sensitive’ to this pressure.

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. Further detail

Evidence

‘Not Relevant’ to biotopes restricted to open waters.

Death or injury by collision

Benchmark. Injury or mortality from collisions of biota with both static or moving structures due to 0.1% of tidal volume on an average tide, passing through an artificial structure. Further detail

Evidence

‘Not Relevant’ to seabed habitats.

Visual disturbance

Benchmark. The daily duration of transient visual cues exceeds 10% of the period of site occupancy by the feature. Further detail

Evidence

Ophiothrix fragilis and other brittlestars and starfish are likely to have poor facility for visual perception and consequently are probably not sensitive to visual disturbance. Movement of a hand near to Ophiothrix fragilis, for example, elicits no escape response (Sköld, 1998). Although some other species, such as crabs and fish, may respond to visual disturbance such behaviour is not likely to have an impact on the nature and function of a brittlestar bed so the biotope is expected to be not sensitive to the factor. Therefore, this pressure is considered '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 that may result in changes in the genetic structure of local populations, hybridization, or change in community structure. Further detail

Evidence

The key characterizing species in the biotope are not cultivated or likely to be translocated. This pressure is therefore considered ‘Not Relevant’. 

Introduction or spread of invasive non-indigenous species

Benchmark. The introduction of one or more invasive non-indigenous species (INIS). Further detail

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 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 species 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 above evidence suggests that Crepidula could colonize mixed sediment habitats in the subtidal, typical of this biotope, due to the presence of pebbles, cobbles, shells, or any other hard substrata that can be used for larvae settlement (Tillin et al., 2020). This habitat is moderately wave exposed to sheltered, in which wave action and possibly storms may mobilise the sediment (JNCC, 2022). Sediment mobility may 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 suitable for Crepidula in wave sheltered examples 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, Crepidula has the potential to colonize, and modify the more sheltered examples of 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.  However, the density of brittlestar beds may prevent the colonization of Crepidula, as the brittlestars may feed on their larvae but if settlement is successful Crepidula may compete for space. It is unclear if the brittlestar beds could occupy the same space, and develop a bed on top of the Crepidula, possibly using the slipper limpet stacks to keep in the currents for feeding. 

Therefore, resistance is assessed as 'Medium', due to wave action, the possibility of storms, and high densities of brittlestars. Resilience is assessed as 'Very low', as it would require the removal of Crepidula, probably by artificial means. Hence, the biotope sensitivity is assessed as 'Medium' based on the worst-case scenario. Crepidula has not yet been reported to occur in this biotope and there is a lack of direct evidence so the confidence in the assessment is 'Low' and further evidence is required. 

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). Further detail

Evidence

Introduced organisms (especially parasites or pathogens) are a potential threat in all coastal ecosystems. So far, brittlestar beds have not been affected. Dense aggregations of brittlestars would offer ideal conditions for the rapid spread of pathogenic organisms or parasites, but so far no examples of this have been recorded. However, several examples are known of echinoderm populations that have been massively reduced by sudden outbreaks of epidemic disease. Cases include the mass mortality of the sea urchin Diadema antillarum throughout the Caribbean as a result of infection by a water-borne pathogen (Lessios, 1988), and the decimation of urchin populations in the North Atlantic by parasitic amoebae and nematodes (Hagen, 1997). Epidemic disease should therefore be considered as having the potential to significantly affect populations of bed-forming brittlestars (Hughes, 1998b), as even widespread and abundant species can be vulnerable.

Lynch et al. (2007) investigated the possible role of benthic macroinvertebrates and zooplankton in the life cycle of Bonamia ostrea, a parasite the European flat oyster Ostrea edulis. Their laboratory studies found that the brittlestar Ophiothrix fragilis was a passive carrier of the parasite but is not infected. Brittlestar mortality in their treatments was not explained, and it was uncertain if parasite infection was to blame. However, they found where the oysters co-habited with the brittlestars, oyster infection by the parasite was lower. It is, however, unlikely that the oyster-specific parasite would be responsible for the brittlestar mortalities recorded (Lynch et al., 2007).

Sensitivity assessment.The evidence suggests that brittlestars may be exposed to pathogens, but that no mortality has been reported. Therefore brittlestar beds probably have ‘High’ resistance to this pressure. By default resilience is assessed as ‘High’ and the biotope is classed as ‘Not Sensitive’. 

Removal of target species

Benchmark. Removal of species targeted by fishery, shellfishery or harvesting at a commercial or recreational scale. Further detail

Evidence

Brittlestar beds are currently not targeted by commercial fisheries and hence not directly affected by this pressure (Tillin & Tyler-Walters, 2014). This pressure is therefore considered ‘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. Further detail

Evidence

Fisheries tend to avoid brittlestar beds since the animals clog nets (Jones et al., 2000). However, brittlestars may be damaged or directly removed by static or mobile gears that are targeting other species. Direct, physical removal is assessed under abrasion and penetration pressures. The sensitivity assessment for this pressure considers any biological and ecological effects resulting from the removal of non-target species.

Although several species of brittlestar (including Ophiocomina nigra) were reported to have increased in abundance in trawled areas, Bradshaw et al. (2002) noted that the relatively sessile Ophiothrix fragilis decreased in the long term in areas subject to scallop dredging. The bed may contract in size as individual brittlestars move to re-establish contact with neighbours or the number of low density patches could increase. If water currents were very strong some animals may be washed away as the support provided by other individuals in dense aggregations decreases. In addition, commercial fisheries may discard damaged or dead non-target species. This could result in increased available food supply to scavenging brittlestars but may also attract mobile predators and scavengers including fish and crustaceans to habitats supporting brittlestars, which may alter predation rates.

Sensitivity assessment: Once extraction or fishing has stopped, brittlestars that remain in the bed are likely to be able to re-establish the density observed prior to the event. Based on the evidence presented, the resistance of the biotope is considered to be ‘Low’, with ‘Medium’ resilience and therefore the biotope is considered to have ‘Medium’ sensitivity to this pressure.