Biodiversity & Conservation

SS.SMu.CSaMu.VirOphPmax

Explanation of sensitivity and recoverability


Physical Factors

Substratum Loss
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Most species are infaunal or epifaunal and will be lost if the substratum is removed so the overall intolerance of the biotope is high. Although some of the mobile species in the biotope may be able to escape, most, such as the harbour crab Liocarcinus depurator, the common starfish Asterias rubens and the brittlestars are not very fast moving and so are also likely to be removed. Recovery from complete loss of fauna in the sediment is likely to take a long time and so a rank of moderate has been reported - see additional information below for full recovery rationale.
Smothering
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The biotope will have low intolerance to smothering by 5cm of sediment because many of the species are burrowing and live within the sediment anyway. The sea pen Virgularia mirabilis is able to withdraw rapidly into the sediment and appears to be able to recover from smothering (see species review). The brittlestar Amphiura filiformis, which inhabits the top 3-4 cm of sediment, is also not likely to be intolerant of smothering as it is able to move up through sediment. Many of the other infaunal organisms, such as the polychaetes and bivalves, should also survive smothering. However, some species may be unable to self-clean or dig out and so a small decline in species diversity may occur. However, as most species in the biotope are not especially intolerant of smothering by sediment the intolerance of the biotope is recorded as low. Intolerance to other smothering factors, oil for example, may be higher. Recovery should be rapid as species move through the sediment and self clean.
Increase in suspended sediment
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The dominant trophic group associated with this biotope are suspension feeders and these species are likely to have self-cleaning mechanisms and so will be able to deal with certain levels of particulate material. However, suspension of large amounts of fine silt and clay fractions of sediment, resulting from activities such as dredging, may clog feeding structures. For example, there may be some clogging of the feeding organs of the suspension feeding sea pens. However, since these animals are able to self-clean this is not likely to be very energetically costly, particularly at the level of the benchmark. Some species may benefit from increased food supply if suspended sediment has a high organic content. Many species in the biotope are burrowing infauna so will not be affected by an increase in suspended sediment. Intolerance is therefore, assessed as low. Overall species composition and richness is not expected to be affected. On return to normal, suspended sediment levels recovery will be immediate as affected species will be able to self-clean within a few days.
Decrease in suspended sediment
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A decrease in suspended sediment and siltation will reduce the flux of particulate material to the seabed. Since this includes organic matter the supply of food to the biotope may also be reduced. However, the benchmark reduction in suspended sediment of 100mg/l for a month is unlikely to have a significant effect on the biotope and would not alter species composition. Intolerance is therefore, assessed as low. On return to normal conditions, recovery will be rapid and a rank of very high is recorded.
Desiccation
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This biotope is found in the circalittoral and so species are not likely to be affected by desiccation.
Increase in emergence regime
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This biotope is found in the circalittoral, therefore the species in this biotope are not likely to be affected by an emergence regime.
Decrease in emergence regime
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This biotope is found in the circalittoral, therefore the species in this biotope are not likely to be affected by an emergence regime.
Increase in water flow rate
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The biotope is only found in areas of weak or very weak tidal streams and so is likely to be intolerant of increases in water flow. Some tidal flow is necessary for the horizontal supply of small and light nutritious particles by resuspension and advective transport, influencing the growth rate of suspension-feeding benthos (Dauwe, 1998). However, some suspension feeders in the biotope will be unable to feed if the water flow rate increases by two categories in the water flow scale (see benchmarks). The sea pen Virgularia mirabilis for example, will retract into the sediment at water currents speeds greater than 0.5m/s (i.e. 1 knot) (Hiscock, 1983). If water speeds remain at this level or above, sea-pens will be unable to extend above the sediment, will be unable to feed and will probably die. Suspension feeding brittlestars have no self-produced feeding currents and so water flow rate will be of primary importance. For example, individuals of Amphiura filiformis respond rapidly to currents by extending their arms vertically to feed. Under laboratory conditions they were shown to maintain this vertical position at currents of 30 cm/s (approx. 0.6 knots) (Buchanan, 1964). If water movement were to increase to strong (3-6 knots), individuals would be unlikely to maintain this position and so would retract their arms. Other suspension feeders in the biotope will also be unable to feed if the water flow rate increases by two categories in the water flow scale (see benchmarks). A long term increase (i.e. the benchmark level of one year) in water flow will change the nature of the top layers of sediment, becoming coarser and possibly unsuitable for some shallow burrowing species such as the brittle stars Amphiura. Therefore, a long term increase in water flow rates would probably result in the loss of many of the key species, and hence the biotope, so intolerance is reported to be high. Recovery would probably take a long time and is set at moderate - see additional information below.
Decrease in water flow rate
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The biotope exists in habitats such as sea lochs, where tidal streams are already very weak so a decrease in flow rate would result in almost non-moving water. In these enclosed or semi-enclosed water bodies, negligible water flow may result in some deoxygenation of the overlying water and the loss of some intolerant species. The sea pen Virgularia mirabilis for example, has high intolerance to deoxygenation and may die. Tidal currents keep most of the organic particles in the sediment in suspension which can support suspension feeders even in low organic content sediments. Therefore, if water movement becomes negligible suspended organic particles available to filter feeders such as the sea pens will decline. Growth and fecundity will be affected and over a period of a year may result in the death of sea pens. Amphiura filiformis shows a lack of activity in still water and low current speeds can impede feeding because it may reduce the transport of organic particles. Therefore, if water flow rate changes by the benchmark level of two categories for a year feeding would be significantly impaired and viability of the population reduced. The overall impact on the biotope is likely to be the loss of a few key species such as sea pens and so intolerance is assessed as high. Recovery would probably take longer than five years and so is assessed as moderate - see additional information below for rationale.
Increase in temperature
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In shallow sea lochs where the biotope CMS.VirOph is typically found, there are seasonal changes in temperature of about 10 °C. Therefore, the biotope may be tolerant of long term increases although growth and fecundity of some species could be affected. No information was found on the upper limit of sea pens tolerance to temperature increases. However, the distribution of Virgularia mirabilis extends south into the warmer waters of the Mediterranean suggesting they may be able to tolerate a long term increase in temperature of 2 °C. Muus (1981) showed that juvenile Amphiura filiformis are capable of much higher growth rates in experiments with temperatures between 12 and 17°C. However, most of the species in the biotope are subtidal animals where wide and rapid variations in temperature, such as experienced in the intertidal, are not likely to occur and so the biotope may be more intolerant of a rapid increase of 5 °C. The reported intolerance to changes in temperature for Virgularia mirabilis is intermediate (see species review). Since the loss of sea pens changes the biotope the intolerance of the biotope to increased temperature is also recorded as intermediate. Recovery of sea pens may take a long time so a rank of moderate is reported - see additional information below for full recovery rationale.
Decrease in temperature
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In shallow sea lochs, sedimentary biotopes typically experience seasonal changes in temperature of about 10 °C and so CMS.VirOph may be tolerant of long term decreases although growth and fecundity of some species may be affected. No information was found on the lower limit of sea pens tolerance to temperature decreases. However, the distribution of Virgularia mirabilis extends into the northern North Atlantic where waters are colder than in the UK suggesting they may be able to tolerate a long term decrease in temperature of 2°C. However, sea pens and other species in the biotope are subtidal where wide and rapid variations in temperature, such as experienced in the intertidal, are not so common and so may be more intolerant of a short term decrease in temperature of 5°C. Echinoderms, including Amphiura filiformis, of the North Sea seem periodically affected by winter cold with mortalities during cold winters. Low temperatures are a limiting factor for breeding which takes place during the warmest months in the UK. Therefore, population viability of one of the key species may be reduced and so the intolerance of the biotope is reported to be intermediate. Recovery should be high - see additional information below.
Increase in turbidity
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This biotope is found in the circalittoral so light levels will be naturally low. Virgularia mirabilis is insensitive to light (Hoare & Wilson, 1977), therefore, an increase or decrease in light levels caused by changing turbidity levels will have little or no effect on the sea pen population. An increase in turbidity, reducing light availability may reduce primary production by phytoplankton in the water column. However, productivity in the CMU.SpMeg biotope is secondary (detritus) and is not likely to be significantly affected by changes in turbidity for a period of a month and so intolerance of the biotope is assessed as low. Recovery will be very rapid.
Decrease in turbidity
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A decrease in turbidity, increasing light availability may increase primary production by phytoplankton in the water column. However, productivity in the CMS.VirOph biotope is secondary (detritus) and is not likely to be significantly affected by changes in turbidity and so intolerance is assessed as low. Nevertheless, primary production by pelagic phytoplankton and microphytobenthos do contribute to benthic communities and long term decreases in turbidity may increase the overall organic input to the detritus. Increased food supply may increase growth rates and fecundity of some species in the biotope.
Increase in wave exposure
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The biotope exists in sheltered areas with low wave exposure and weak tidal currents. Sea pens, for example, may be unable to feed and may be damaged or broken by increased wave exposure. However, Virgularia mirabilis is able to withdraw into the sediment to avoid strong wave oscillations but if wave exposure increases are long term feeding will stop and individuals will be likely to die. If uprooted by wave exposure Virgularia mirabilis can reburrow provided it has not been damaged. Strong wave action can resuspend the sediment and break up and scatter Amphiura filiformis. An increase in wave exposure is likely to change the composition of species present in the biotope because it is likely to disrupt feeding and burrowing and may also have an impact on reproduction and recruitment. An increase in the factor can also change the sediment characteristics which may result in a change in the proportion of suspension to deposit feeders within it. Intolerance of the biotope to an increase in wave exposure is therefore reported to be high. See additional information below for recovery.
Decrease in wave exposure
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The biotope occurs in areas of very low or no wave exposure so a decrease is not relevant.
Noise
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Some of the important characterizing species associated with this biotope, in particular the sea pens, may respond to sound vibrations and can withdraw into the sediment. Feeding will resume once the disturbing factor has passed. However, most of the species are infaunal and not likely to be sensitive to noise disturbance at the benchmark level. It is possible that predator avoidance behaviour in Liocarcinus depurator and other species may be triggered by noise vibrations although this has not been recorded. Therefore, unless predation pressure is reduced increased noise disturbance is not likely to have an impact on the nature and function of the biotope and a rank of not sensitive is recorded. Feeding will resume once the disturbing factor has passed.
Visual Presence
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Most species within the biotope are burrowing and have no or poor visual perception and are unlikely to be affected by visual disturbance such as shading. Epifauna such as crabs have well developed visual acuity and are likely to respond to movement in order to avoid predators. However, it is unlikely that the species will be affected by visual disturbance at the benchmark level. The biotope is therefore, not sensitive to the factor.
Abrasion & physical disturbance
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Virgularia mirabilis is able to retract into the sediment and so some individuals may be able to avoid some forms of abrasion or physical disturbance. However, sea pens retract slowly and are likely to be sensitive to abrasion by trawling for instance, that is likely to break the rachis of Virgularia mirabilis. Species obtained by dredges were invariably damaged (Hoare & Wilson, 1977). Displaced individuals that are not damaged will reburrow but those that are damaged are likely to die. However, the densities of Virgularia mirabilis were similar in trawled and un-trawled sites in Loch Fyne and no changes in sea pen density was observed after experimental trawling over a 18 month period in another loch (Howson & Davies, 1991; Tuck et al. , 1998; Hughes, 1998b). Hughes (1998b) concluded that Virgularia mirabilis and Pennatula phosphorea, which can withdrawn into the sediment, were probably less susceptible to the effects of damage by fishing gear than Funiculina quadrangularis, which is unable to withdraw.

In an investigation into the effect of shellfish traps on benthic habitats (Eno et al. , 1996), creels were dropped on sea pens and left for extended periods to simulate the effects of smothering which could occur during commercial operations. The sea pens consistently righted themselves following removal of the pots.

Ramsay et al. (1998) suggest that Amphiura spp. may be less susceptible to beam trawl damage than other species like echinoids or tube dwelling amphipods and polychaetes. Bergman & Hup (1992) for example, found that beam trawling in the North Sea had no significant direct effect on small brittle stars. Bradshaw et al. (2002) noted that the brittlestars Ophiocomina nigra, Ophiura albida and Amphiura filiformis had increased in abundance in a long-term study of the effects of scallop dredging in the Irish Sea. Brittlestars can tolerate considerable damage to arms and even the disk without suffering mortality and are capable of arm and even some disk regeneration.

Overall, the dominant species are likely to be relatively tolerate of or avoid physical disturbance at the benchmark level and an intolerance of low has been recorded. Recoverability will be dependent on repair and regeneration of damage and is likely to be rapid.

Displacement
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Displaced individuals of Virgularia mirabilis, which are not damaged (see Abrasion above for damage), will re-burrow (Jones et al., 2000) and recover completely within 72 hours, provided the basal peduncle remains in contact with the sediment surface. The other important characterizing species associated with this biotope, such as brittlestars, also have the ability to reburrow, provided they have not been damaged. Displaced individuals of Amphiura filiformis that are not damaged (see Abrasion above) can right themselves if displacement caused them to be inverted and they can rapidly re-burrow into the sediment. Intolerance of the biotope to displacement is therefore low and recovery is likely to only take a short time and so recovery is recorded as immediate.

Chemical Factors

Synthetic compound contamination
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There was no information found on the effect of chemical contaminants on the biotope. However, effects on some of the individual species in the biotope have been reported. Dahllöf et al. (1999) studied the long term effects of tri-n-butyl-tin (TBT) on the function of a marine sediment system. TBT spiked sediment was added to a sediment that already had a TBT background level of approximately 27ng g-1 (83 pmol TBT g-1) and contained the following fauna: Amphiura spp., the bivalve Abra alba and several species of polychaete. Within two days of treatment with a TBT concentration above 13.7 µmol / mē all species except the polychaetes had crept up to the surface and after six weeks these fauna had started to decay. Thus, increased contamination from TBT is likely to result in the death of some intolerant species such as brittle stars and heart urchins. Bryan & Gibbs (1991) report that crabs appear to be relatively resistant to TBT although some deformity of regenerated limbs has been observed. However, arthropods are very intolerant of the insecticide carbaryl (1-napthol n-methyl carbamate; sold under the trade name Sevin®) which has been used to control burrowing shrimp in oyster farms (Feldman et al., 2000). There is no information available on the possible consequences of chemicals to British sea pens. Different species will be affected by different chemicals but a general trend in areas of increasing pollution is a reduction in species diversity with habitats becoming dominated by pollution tolerant polychaete worms. However, Ivermectin, an anti-louse treatment coming into use in the salmon fish farming industry, has been shown to be highly toxic to sediment dwelling polychaetes (Hughes, 1998(b)). The dominant trophic group associated with this biotope are suspension feeders and therefore have the ability to accumulate pollutants although effects are uncertain. Growth and regeneration are decreased in species such as Pecten maximus and Amphiura filiformis. Intolerance of the biotope is reported to be high.
Heavy metal contamination
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There was no information found on the effect of heavy metals on sea pens. In Norwegian fjords Rygg (1985) found a relationship between species diversity in benthic fauna communities and sediment concentrations of heavy metals Cu, Pb and Zn. Copper in particular showed a strong negative correlation and the author suggested a cause-effect relationship. Those species not present at sites where Cu concentrations were greater than ten times higher than the background level, such as Amphiura filiformis and several bivalves including Nucula sulcata and Thyasira equalis, were assessed as non-tolerant species. The tolerant species were all polychaete worms. Therefore, increased heavy metal contamination in sediments may change the faunal composition of the community and decrease overall species diversity. However, effects of heavy metals are generally sub-lethal so an intolerance rank of intermediate is recorded.
Hydrocarbon contamination
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There is very little information available on the impact of hydrocarbons on the species in the biotope. Nothing could be found for Virgularia mirabilis or other sea pens. In a study of the effects of oil exploration and production on benthic communities. Olsgard & Gray (1995) found Amphiura filiformis to be very intolerant of oil pollution. The overall impact of oil contamination on the biotope is likely to be a loss of species diversity as very intolerant species are lost and so intolerance of the biotope is reported to be intermediate but with a very low confidence. However, the biotope is found in the circalittoral and so any oil from spills would have to be dispersed deep into the water column to affect them. In addition the biotope occurs in sheltered locations and storms would be unlikely to disperse oils to these depths and so the biotope is not particularly vulnerable to this particular factor.
Radionuclide contamination
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In an investigation of bioturbation in the north-eastern Irish Sea Hughes & Atkinson (1997) surveyed several sites close to the Sellafield nuclear reprocessing plant. At a station immediately offshore from the Sellafield outfall pipeline a community similar to the CMS.VirOph biotope was present. Epifauna were abundant, particularly Ophiura ophiura and Asterias rubens. The sea pen Virgularia mirabilis occurred at high density. Dragonets and small gobies were also common. Thus, the key species in the biotope occur in bottom sediments that contain particles of long half-life radionuclides derived from the liquid effluent released from the reprocessing plant at Sellafield and so intolerance is assessed as low. However, species diversity may be slightly reduced compared to unpolluted sites.
Changes in nutrient levels
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Although absent from the most enriched areas the sea pen Virgularia mirabilis was present at organic contents of 4.5% (Atkinson, 1989). Very large increases in organic content can result in significant changes in community composition of sedimentary habitats. Typically an increasing gradient of organic enrichment results in a decline in the suspension feeding fauna and an increase in the number of deposit feeders, in particular polychaete worms (Pearson & Rosenberg, 1978). For example, in areas under fish farm cages gross organic pollution has been observed to result in the loss of megafaunal burrowers. However, these changes generally refer to gross nutrient enrichment. At the level of the benchmark, a 50% increase in nutrients is likely to impact only the most intolerant species and may result in a reduction in the number of sea pens. In places where oxygen concentrations are still sufficiently high, the suspension feeding Amphiura filiformis reacts positively to organic enrichment in terms of increasing abundance and biomass (Josefson & Smith, 1984; Rosenberg et al., 1987). Overall the intolerance of the biotope to a benchmark increase in nutrients is likely to be low.
Increase in salinity
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The biotope is found in fully marine conditions so is likely to be intolerant of increases in salinity. The overall effect on the biotope of a chronic decrease in salinity for a period of a year is likely to be the loss of most species and so intolerance is reported as high.
Decrease in salinity
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The biotope is found in fully marine conditions and does not extend into estuaries so is likely to be intolerant of decreases in salinity. The key species are highly intolerant of salinity changes although Jones et al. (2000) suggest that Virgularia mirabilis appears to be somewhat tolerant of occasional lowering of salinity. Ophiura albida was found in the Baltic Sea at salinities of 8psu although circumstantial evidence suggests that adaptation is probably genetic in this species (Stickle & Diehl, 1987). In the laboratory Ophiura albida tolerated a salinity of 17psu for 22 days (Pagett, 1980). However, the species are likely to be intolerant of a long term, chronic decrease; e.g., a change of one category from the MNCR salinity scale for one year. The overall effect on the biotope of a chronic decrease in salinity for a period of a year is likely to be the loss of most species and so intolerance is reported as high. Recovery from loss of most species is likely to take many years and so is assessed as moderate - see additional information below for rationale.
Changes in oxygenation
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Virgularia mirabilis, the main important characterizing species in this biotope, is often found in sea lochs so may be able to tolerate some reduction in oxygenation. However, Jones et al., (2000) found sea pen communities to be absent from areas which are deoxygenated and characterized by a distinctive bacterial community. Hoare & Wilson (1977) reported Virgularia mirabilis absent from sewage related anoxic areas of Holyhead harbour. The brittlestars Ophiura albida and Amphiura filiformis are tolerant of low oxygen concentrations. Ophiura albida shows a definite resistance to low oxygen levels with 50% of individuals still surviving after 32 hours in seawater with an oxygen concentration of 0.21mg/l (Theede et al., 1969). Rosenberg et al. (1991) suggests that some part of the benthic community, including Amphiura filiformis, can withstand oxygen concentrations of around 1mg/l for several weeks. Therefore, the benchmark level of 2mg/l of oxygenation for one week will result in the death of only the most intolerant species and maybe some individual sea pens. The total loss of populations of the key species is not likely to occur at the benchmark level and since the faunal composition of the overall biotope is unlikely to change to any great extent intolerance is assessed as low. For most species on return to normal oxygenation recovery will be immediate as respiratory rates return to pre-impact levels.

Biological Factors

Introduction of microbial pathogens/parasites
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Insufficient information
Introduction of non-native species
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There are no records of any non-native species invading the biotope (Eno et al., 1997) and so the biotope is assessed as not sensitive. However, as several species have become established in British waters there is always the potential for new introduced non-native species to have an effect on the biotope.
Extraction
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The two important characterizing species are not subjected to targeted extraction. Pecten maximus, on the other hand, is a valuable commercial species. However, it is unlikely that they are collected from this biotope and not relevant has been suggested.

Additional information icon Additional information

Recoverability
No evidence on community development was found. Very little is known about the population dynamics and longevity of Virgularia mirabilis in Britain. However, information from other species suggest that this species is likely to be slow growing with patchy and intermittent recruitment and so recovery from loss of this species is likely to take longer than five years. The other key species, Amphiura filiformis and Pecten maximums are also long lived and take a relatively long time to reach reproductive maturity. It takes approximately 5-6 years for Amphiura filiformis to grow to maturity so population structure will probably not reach maturity for at least this length of time. In addition, Muus (1981) shows the mortality of new settling Amphiura filiformis to be extremely high with less than 5% contributing to the adult population in any given year. Pecten maximums reaches sexual maturity within the first two to three years and has a life span of 10-20 years. The suggested life span for Ophiura ophiura in the west of Scotland was 5-6 years (Gage, 1990). Many of the other species in the biotope, such as polychaetes and bivalves, are likely to reproduce annually, be shorter lived and reach maturity much more rapidly. However, because the key species in the biotope, Virgularia mirabilis and Amphiura filiformis are long lived and take several years to reach maturity the time for the overall community to reach maturity is also likely to be several years, possibly in the region of 5-10 years. Thus, a rank of moderate is reported for recovery from loss of key species in the biotope.

This review can be cited as follows:

Hill, J.M. & Wilson, E. 2004. Virgularia mirabilis and Ophiura spp. on circalittoral sandy or shelly mud. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 19/12/2014]. Available from: <http://www.marlin.ac.uk/habitatbenchmarks.php?habitatid=66&code=2004>