Biodiversity & Conservation

SS.SSa.IMuSa.EcorEns

Explanation of sensitivity and recoverability


Physical Factors

Substratum Loss
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Muddy sand communities are highly intolerant of substratum loss because most species are infaunal and so will be removed and die. A few mobile demersal species like the crab Liocarcinus depurator may be able to avoid the factor but even fast moving polychaetes will be removed during substratum loss. Dredging operations, for example, were shown to affect large infaunal and epifaunal species, decrease sessile polychaetes and reduce numbers of burrowing heart urchins. Recovery is dependant on return of suitable sediment and recruitment of individuals but is expected to be moderate (see additional information).
Smothering
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The biotope is characterized by mostly burrowing species such as the heart urchin Echinocardium cordatum, razor shells Ensis sp., polychaete worms and bivalves and is therefore not sensitive to smothering by 5 cm sediment as they should be able to burrow upwards. However, smothering by other material, especially oil, would result in the death of most species in the biotope.
Increase in suspended sediment
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Changes in suspended sediment and siltation rate (resulting from changes in the hydrographic regime, runoff from the land or coastal construction) are likely to result in changes in the sediment composition, certainly of the surface layers and hence the communities present. For example, if suspended sediment levels increase causing a change in sediment type to more silty conditions, Echinocardium cordatum may be replaced by another heart urchin, Brissopsis lyrifera. Increased suspended sediment leads to decreased light penetration, possible clogging of feeding organs of suspension feeders such as Ensis ensis and the possibility of smothering of whole organisms (see Smothering). Species may benefit from increased food supply if suspended sediment has a high organic content. However, the key species in the biotope, Echinocardium cordatum and Ensis ensis, have low intolerance to changes in suspended sediment and the species composition of the biotope is not likely to be drastically affected at the benchmark level, so intolerance is assessed as low. The balance between deposit and suspension feeders may alter with changes in suspended sediment and siltation rate although overall species richness is not likely to be significantly different. On return to normal conditions recovery should be fairly rapid (see additional information).
Decrease in suspended sediment
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Desiccation
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The biotope extends from the very low shore to the shallow subtidal so an increase in desiccation is likely to affect only the upper range of the biotope. Species like Ensis ensis and Echinocardium cordatum have little protection from desiccation when exposed and are likely to be lost if the factor increases. Other species like crabs and errant polychaetes are likely to be able to migrate to unaffected areas. A decrease in desiccation may enable the biotope to extend its range up-shore. An increase in desiccation may cause some individuals to be lost at the extreme upper limit of the biotope although most species are able to reburrow if exposed to air and so will be able to avoid the factor. However, the sublittoral element of the biotope is not likely to be lost and so intolerance is assessed as low. The overall species richness of the biotope is not likely to change and recovery is expected to be high (see additional information).
Increase in emergence regime
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The biotope extends from the intertidal to the sublittoral so a change in emergence will either extend or depress the upper range of the biotope up the shore. Increased emergence (e.g. by tidal and storm surge barrages) is likely to increase the desiccation of the sediment, especially at the top of the biotope. There may be some mortality of individual species and the biotope may take some time to adjust. Mobile species such as the crab Liocarcinus depurator are able to avoid the factor. However, overall species diversity within the biotope is not likely to change and recovery is expected to be high (see additional information). A decrease in emergence may enable the biotope to extend its range up the shore.
Decrease in emergence regime
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Increase in water flow rate
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Changes in the water flow rate will change the sediment structure and have concomitant effects on the community. The biotope is found in sediments of sand or muddy fine sand in sheltered areas. An increase in water flow rate will remove smaller sediment particles leaving coarser elements behind which may be unsuitable for some of the burrowing fauna in the biotope. Individuals of Echinocardium cordatum and Ensis ensis are often washed out by increased water flow (see species reviews). Loss of these species results in the loss of the biotope so intolerance is assessed as high. There will be a decrease in species diversity with the loss of the two key species and the loss of other small invertebrates such as bivalves that inhabit the burrows of Echinocardium cordatum. Providing suitable substratum was available, recovery is expected to be moderate (see additional information).
Decrease in water flow rate
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Increase in temperature
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The two key species in the biotope are distributed to the north and south of Britain and Ireland where seasonal changes mean that water temperatures may vary by as much as 10°C from summer to winter. Therefore, these species are likely to be tolerant of long term changes in temperature although growth and fecundity would probably be affected. Both species are likely to be more intolerant of rapid changes in temperature of 5°C outside their range. Mortality of both species was observed in the very cold winter of 1962-63 (Crisp (ed.), 1964) and so intolerance has been assessed as intermediate. Ensis spp. are known to emerge from the sediment when shallow inshore waters become warm such as occurred in Torbay in 1999 (K. Hiscock pers. comm.). Infaunal species are not subjected to the larger temperature variations experienced in the intertidal and so many of the other species in the biotope may be intolerant of changes leading to a decline in species diversity. Temperature may also affect microbial activity within the sediment which could alter the depth at which the anoxic layer appears. Overall an intermediate intolerance has been suggested with a high recovery (see additional information).
Decrease in temperature
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Increase in turbidity
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An increase in turbidity, reducing light availability will reduce primary production in the biotope. However, the majority of productivity in the IMS.EcorEns biotope is secondary (detritus) and so is not likely to be significantly affected by changes in turbidity. Nevertheless, primary production by pelagic phytoplankton and microphytobenthos do contribute to benthic communities and long term increases in turbidity may reduce the overall organic input to the detritus. In estuaries and surf zones on the lower shore turbidity can be measured in g/l so the benchmark level is low in comparison. Recovery will probably be very rapid (see additional information)
Decrease in turbidity
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Increase in wave exposure
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The biotope is found in sheltered to moderately exposed areas. An increase in wave exposure is likely to change the composition of species present in the biotope. Several species, including the burrowing Echinocardium cordatum and Ensis ensis are likely to be washed out of the sediment by strong wave action such as experienced during winter storms. The biotope would be subject to higher levels of disturbance if wave exposure increases with the likely result of a significantly reduced faunal diversity. Actively swimming amphipods and disturbance tolerant polychaetes may come to dominate resulting in a change of biotope. On return to normal conditions recovery is expected to be achievable within five years from migration of individuals from adjacent areas and settlement of pelagic larvae (see additional information).
Decrease in wave exposure
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Noise
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Some of the important characterizing species associated with this biotope, such as Ensis ensis, may respond to sound vibrations and can retract into the sediment. Feeding will resume once the disturbing factor has passed. However, none of the characterizing species are especially sensitive to noise disturbance at the level of the benchmark such as boats etc. passing overhead. Therefore, the biotope is recorded as not sensitive to noise disturbance.
Visual Presence
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With the exception of mobile predators most of the species in this biotope are infaunal and therefore, not likely to be sensitive to visual disturbance. Mobile predators, like Liocarcinus depurator, are likely to be very sensitive and respond to shadows and movement as an adaptive response to predation. However, at the level of the benchmark this is unlikely to significantly affect the nature of the biotope and a rank of not sensitive is recorded.
Abrasion & physical disturbance
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Eleftheriou & Robertson (1992) performed experimental scallop dredging in a sandy bay in Scotland. They observed that the action of the dredge resulted in damage and mortality of Echinocardium cordatum, Asterias rubens, Astropecten irregularis, Cancer pagurus and Ammodytes sp. Houghton et al., (1971), Graham (1955), de Groot & Apeldoorn (1971) and Rauck (1988) refer to significant trawl-induced mortality of Echinocardium cordatum. Bergman & van Santbrink (2000) suggested that the megafauna such as Echinocardium cordatum, Corystes cassivelaunus, and bivalves such as Phaxas pellucidus, Dosinia lupinus, Mactra corallina, Abra alba, Spisula solida and Spisula subtruncata were amongst the species most vulnerable to direct mortality due to bottom trawling in sandy sediments. Bivalves such as Ensis spp., Corbula gibba and Chamelea gallina together with starfish were relatively resistant (Bergman & van Santbrink, 2000). Bradshaw et al. (2000) suggested that fragile species such as urchins (e.g. Spatangus purpureus and Echinus esculentus), the brittlestar Ophiocomina nigra, starfish Anseropoda placenta and the edible crab Cancer pagurus suffered badly from impact with a passing scallop dredge. More robust bodied or thick shelled species were less sensitive. The sessile infauna, however, along with large infaunal and epifaunal forms, such as molluscs, decapods, echinoderms and some polychaetes, demonstrated their vulnerability (Eleftheriou & Robertson, 1992). Overall, species with brittle, hard tests are regarded to be sensitive to impact with scallop dredges (Kaiser & Spencer, 1995; Bradshaw et al., 2000).

Brittlestars such as Ophiura albida may be more tolerant of abrasion. Bergman & Hup (1992) for example, found that beam trawling in the North Sea had no significant direct effect on small brittlestars. Brittlestars can tolerate considerable damage to arms and even the disk without suffering mortality and are capable of arm and even some disk regeneration. 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. In an analysis of long-term effects of scallop dredging on benthic communities in the Irish Sea, Bradshaw et al., (2002) noted a decline in the sedentary, filter feeding brittlestars Ophiothrix fragilis and Ophiopholis aculeata but an increase in surface detritivores or scavenging brittlestars such as Amphiura filiformis, Ophiocomina nigra and Ophiura albida.

The two key species in the biotope, Echinocardium cordatum and Ensis ensis, are probably highly intolerant of physical disturbance. Recovery is likely to be moderate because, although the individual key species may recolonize the area within five years, several of the species are very long-lived and so the biotope may take longer to return to original age-structure and species diversity. For example, it takes approximately 3 years for Echinocardium cordatum to grow to maturity. The first re-population of Echinocardium cordatum after the Torrey Canyon accident was recorded two years after the oil spill off Northumberland (Southward & Southward, 1978). However, it has been observed that subtidal populations of Echinocardium cordatum appear never to reach sexual maturity off Northumberland (Buchanan, 1967) and recruitment is often sporadic, with reports the species recruiting in only 3 years over a 10 year period (Buchanan, 1966).

Displacement
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Muddy sand communities are likely to be intolerant of displacement as infaunal species may be disturbed by a storm, or passing trawls. The washing out of individuals of Echinocardium cordatum and Ensis ensis, as well as other bivalve molluscs that characterize the biotope, from the sediment by storms, for example, has been frequently observed (see individual reviews). Most animals in the biotope are burrowing, and provided they have not been damaged in the process of displacement, are able to re-establish themselves in the sediment. However, species such as heart urchins, molluscs and crustaceans are likely to be damaged or killed in dredging operations (Elliot et al., 1998) and animals, damaged or undamaged are likely to experience increased predation pressure either at low (birds) or high tide (fish and crabs) and so intolerance has been assessed as intermediate. See additional information for details on recovery, which is expected to be high.

Chemical Factors

Synthetic compound contamination
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High levels of mortality of both Echinocardium cordatum and Ensis spp. resulted from the use of dispersants following the Torrey Canyon oil spill (Smith, 1968). Almost complete mortality of razor shells was found at stations more than a kilometre from the shore at a depth of about 20m. The toxicity of TBT to Echinocardium cordatum is similar to that of other benthic organisms and echinoderms tend to be very intolerant of various types of marine pollution (Newton & McKenzie, 1995). Other species in the biotope, in particular polychaete worms, are generally less intolerant of a range of marine pollutants so a change in the faunal composition may be expected if chemical pollution increases. Polluted areas would be characterized by biotopes with lower species diversity and a higher abundance and density of pollution tolerant species such as polychaetes On return to normal conditions recovery may occur within a year if recruitment is good. However, recruitment of Echinocardium cordatum is sporadic and recovery may take longer but should be complete within five years (see additional information).
Heavy metal contamination
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Different members of the same community are likely to vary in their intolerance to heavy metals. Bryan (1984) suggests that polychaetes were fairly resistant to heavy metals, while the larval and embryonic stages of bivalve molluscs are the most intolerant. However, the viability and reproductive potential of polychaetes is probably reduced by heavy metal pollution and contaminated sediments can exert a toxic effect on burrowing bivalves. Reduced growth, abundance and abnormalities have been observed in echinoderms (see Psammechinus miliaris for example) in areas of heavy pollution. The likely impact on the biotope of an increase in heavy metals is reduced growth, fecundity and abundance of the key species, the probable loss of some species and an increase in pollution tolerant polychaete worms. Intolerance is therefore, assessed as intermediate. On return to normal conditions recovery should be rapid although most contaminants will have a long-life in sedimentary systems (see additional information).
Hydrocarbon contamination
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Oil spills resulting from tanker accidents can cause large-scale deterioration of communities in intertidal and shallow subtidal sedimentary systems. The two key species in the biotope, Echinocardium cordatum and Ensis ensis are highly intolerant of oil pollution and suffered mass mortality after the Torrey Canyon and Amoco Cadiz oil spills. Many other species in the biotope are also likely to be affected. For example, after the West Falmouth, Florida spill of 1969 the entire benthic fauna was eradicated immediately following the spill and populations of the opportunistic polychaete Capitella capitata increased to abundances of over 200,000/m² (Sanders, 1978). Echinocardium cordatum has high fecundity, reproduces every year and has a pelagic larvae so recovery should be good on return to normal conditions. Many other components of the biotope have planktonic larvae and are likely to recolonize rapidly. The first repopulation of Echinocardium cordatum after the Torrey Canyon accident was noticed two years after the oil spill (Southward & Southward, 1978). Although recruitment of Ensis ensis is sporadic recovery should be complete within five years. However, invertebrate communities respond to severe chronic oil pollution in much the same way. Initial massive mortality and lowered community diversity is followed by extreme fluctuations in populations of opportunistic mobile and sessile fauna (Suchanek, 1993). Oscillations in population numbers slowly dampen over time and diversity slowly increases to original levels. Thus, although the individual key species may recolonize the area within five years the biotope may take longer to return to original species diversity and abundance and so recovery is assessed as moderate (see additional information).
Radionuclide contamination
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Insufficient information.
Changes in nutrient levels
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Increases in organic content can result in significant change in the 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). An increasing level of nutrients in the sediment will result in a reduction in the abundance of the key species, in particular the heart urchin Echinocardium cordatum, which is generally associated with sediments of low organic content. The overall species diversity will decline and the habitat becomes modified as numbers of bioturbating species decline. The community, and hence the biotope, may change to one dominated by nutrient tolerant species, in particular polychaete worms such as Capitella capitata. However, these changes generally refer to gross nutrient enrichment. At the level of the benchmark, a 50% increase in nutrients is not likely to result in such drastic changes so intolerance is assessed as intermediate. A decrease in nutrient availability may result in impaired growth and fecundity although species diversity is not likely to be affected significantly . On return to normal conditions recovery within five years should occur (see additional information).
Increase in salinity
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The biotope is found in fully marine conditions so is likely to have some intolerance to changes in salinity. Although the biotope also occurs in the intertidal where it will be subject to precipitation long term changes in salinity are likely to result in the loss of some species. The representative species suggested for this biotope are assessed as intermediate intolerance to reduced salinity. Intolerance of the biotope is similarly assessed as intermediate (see additional information on recovery).
Decrease in salinity
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Changes in oxygenation
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Oxygen-deficient marine areas are characterized by a decline in the number and diversity of species. A decrease in oxygenation is likely to see the loss of the key species in the biotope. Echinocardium cordatum in particular has high intolerance to changes in oxygenation. For example, in the south-eastern North Sea a period of reduced oxygen resulted in the death of many individuals of Echinocardium cordatum (Niermann, 1997) and during periods of hypoxia the species migrates to the surface of the sediment (Diaz & Rosenberg, 1995). Ensis ensis has intermediate intolerance, being able to tolerate sands which are slightly reducing, in which there is a grey layer below the surface, but will eventually die in low oxygenated areas. With the loss of these two key species the biotope would also be lost so intolerance is assessed as high. Community composition will become dominated by fewer species that are tolerant of hypoxic conditions, such as some polychaete worms, so that the overall species richness will decline significantly. On return to normal conditions recovery should be rapid as recolonization can take place by migration or recruitment of new individuals (see additional information).

Biological Factors

Introduction of microbial pathogens/parasites
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There is little information on microbial pathogen effects on the characterizing species in this biotope. The occurrence of several parasitic gregarine protozoans, such as Urospora neapolitana, have been observed in the body cavity of Echinocardium cordatum (Coulon & Jangoux, 1987) although no reports of disease related mortalities were found. Liocarcinus depurator is known to suffer from Black Necrotic disease caused by a bacteria although no information was available on mortality effects. However, if numbers are reduced predatory effects may decrease but this is not likely to have an impact on overall species diversity. No evidence of losses of this biotope due to disease were found and it is likely that microbial pathogens will have only a minor possible impact on this biotope. Intolerance has been assessed as low with a high recovery.
Introduction of non-native species
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The north American razor shell Ensis americanus was introduced into Britain via Europe and was found in Norfolk in 1989. Although it is currently found at sites along the British east coast south from the Humber and along the English Channel west as far as East Sussex (Eno et al., 1997). Armonies & Reise (1999) report that there are no significant interactions between Ensis americanus and resident species. Nevertheless, should local species be replaced by the American razor shell the nature of the biotope should be little altered. No other alien species are known to represent a threat to characterizing species in the biotope. Intolerance has been assessed as low with a high recovery.
Extraction
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In Scotland, some subtidal razor shell beds are dense enough to be exploited commercially and recently the species has been harvested by suction dredger (Fowler, 1999). Dredging operations may also adversely affect the heart urchin Echinocardium cordatum. Loss of either or both species from a habitat will result in the loss of the biotope. Crabs such as Liocarcinus depurator are often extracted as a by-catch species in benthic trawling. A reduction in the density of predators may affect species abundance but is not likely to have a significant effect on overall species diversity. Overall, intolerance has been assessed as intermediate. Recovery is expected to be high (see additional information).

Additional information icon Additional information

Recovery
Echinocardium cordatum has high fecundity, reproduces every year and has a pelagic larvae so recovery should be good on return to normal conditions. The first re-population of Echinocardium cordatum after the Torrey Canyon accident was recorded two years after the oil spill (Southward & Southward, 1978). Although recruitment of Ensis ensis is sporadic, recovery should be complete within five years. Populations may be skewed towards smaller and younger individuals. However, all invertebrate communities respond to perturbations in a similar way. Initial massive mortality and lowered community diversity is followed by extreme fluctuations in populations of opportunistic mobile and sessile fauna (Suchanek, 1993). Oscillations in population numbers slowly dampen over time and diversity slowly increases to original levels. Thus, although the individual key species may recolonize the area within five years the biotope may take longer to return to original species diversity and abundance and so recovery from factors to which the biotope is highly intolerant is assessed as moderate.

This review can be cited as follows:

Hill, J.M. 2007. Echinocardium cordatum and Ensis spp. in lower shore or shallow sublittoral muddy fine sand.. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 02/10/2014]. Available from: <http://www.marlin.ac.uk/habitatbenchmarks.php?habitatid=124&code=2004>