Biodiversity & Conservation

SS.SMu.OMu.StyPse

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. Epifaunal species (that constitute a significant part of the biotope) depend on the presence of hard substratum including terrigenous debris and the shells of Pseudamussium septemratiatum. Although recruitment of species can be rapid (see additional information below), the need for hard substratum to re-occur means that recovery might be slow and a recoverability of moderate is suggested .
Smothering
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Smothering by 5 cm of sediment is likely to lead to the death of some of the organisms in the biotope. However, many species such as large solitary ascidians including Styela gelatinosa will most like protrude sufficiently above the sediment to escape mortality. Abra alba and Terebellidae will be able to burrow upwards through silt. Overall, it is expected that some species might be killed but for most there will be expenditure of energy to clear silt or burrow up through silt and intolerance is considered intermediate with recoverability very high. However, impermeable materials would have a more severe effect.
Increase in suspended sediment
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Some species may benefit from increased food supply if suspended sediment has a high organic content. There may be additional cleaning costs for suspension feeders but this will not affect survival of animals. The intolerance of the biotope is therefore reported to be low. Recovery is likely to be very rapid as affected animals clean away sediment particles.
Decrease in suspended sediment
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Food availability for some suspension feeding species may decline but this will not affect survival of animals. The intolerance of the biotope is therefore reported to be low.
Desiccation
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The biotope only occurs in deep water and so desiccation is not relevant.
Increase in emergence regime
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The biotope only occurs in deep water and so emergence is not relevant.
Decrease in emergence regime
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The biotope only occurs in deep water and so desiccation is not relevant.
Increase in water flow rate
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The biotope habitat is fine sediment that only develops in areas of weak tidal streams and, because of the presence of terrigenous debris, the biotope most likely is in an area where currents slacken allowing material to fall to the seabed. A long term increase in water flow rate is likely to affect the nature of the substratum as fine particles and semi-buoyant terrigenous debris are washed away (together with attached fauna) and a coarser sediment type including shell debris remains. Since the majority of characterizing species are epifaunal, the community might benefit from increase in hard substratum. As a general rule, ascidians require a reasonable water flow rate in order to ensure sufficient food availability. High water flow rates may also be detrimental to feeding ability and posture. Hiscock (1983) found that, for the solitary ascidian Ascidia mentula, siphons closed when the current velocity rose above about 15 cm/sec. It seems likely therefore that some reduction in feeding would occur with increased water flow rate although that would result in slower growth and loss of condition but not mortality. The intolerance of the biotope is reported to be intermediate because some species may be swept away with their substratum. However, once loss has occurred, increased water flow might be beneficial by creating more hard substratum. Recovery will occur within a year or slightly longer depending on the time of year an impact occurs.
Decrease in water flow rate
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The biotope occurs in weak tidal streams. A decrease may reduce the supply of particles to the suspension feeders in the biotope. However, effects are only expected to be sub-lethal so intolerance is reported to be low. Normal feeding will resume on return to normal conditions.
Increase in temperature
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There is no information on the response of the biotope to an increase in temperature. The biotope is found in relatively deep sublittoral habitats where the temperature may fluctuate by a maximum of about 10°C over the period of a year. Whilst most species are widely distributed in the north east Atlantic, the important characterizing species Styela gelatinosa is only elsewhere recorded much further north than Loch Goil. Therefore, Styela gelatinosa may be lost and the biotope would no longer be COS.Sty. In the case a catastrophic event that destroyed the entire population of the characteristic species Styela gelatinosa, the biotope would be unlikely to ever recover as the nearest recorded populations of Styela gelatinosa are in deep water off the Faroes and in Norway. However, there are no other biotopes that are very similar (the closest possibly being SCR.Aasp Ascidiella aspersa on sheltered circalittoral rocks on muddy sediment) and a new biotope without Styela gelatinosa would need to be described. An increase in temperature may allow the establishment of some warm waters species and species richness may increase.
Decrease in temperature
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There is no information on the response of the biotope to a decrease in temperature. The biotope is found in relatively deep sublittoral habitats where the temperature may fluctuate by a maximum of about 10°C over the period of a year because of seasonal changes. Species are widely distributed in the north east Atlantic. Therefore, the biotope is likely to be able to tolerate a long term decrease in temperature. Furthermore, the sea squirt Styela gelatinosa is a northern species and its survival prospects would most likely be enhanced by a decrease in temperature ensuring that the biotope survives and possibly thrives. Therefore 'not sensitive*' is suggested.
Increase in turbidity
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An increase in turbidity, reducing light availability may reduce primary production by phytoplankton in the water column and reduce the food available to suspension feeders. However,, if organic material is included in the material increasing turbidity, suspension feeders may benefit. Overall, no net benefit or disbenefit is likely and 'not sensitive' is suggested..
Decrease in turbidity
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An decrease in turbidity, increasing light availability may increase primary production by phytoplankton in the water column and increase the food available to suspension feeders. However, if organic material is part of the material causing turbidity, suspension feeders may have a reduced food supply. Overall, no net benefit or disbenefit is likely and 'not sensitive' is suggested..
Increase in wave exposure
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The biotope occurs in a sea loch where even strong gales cause only limited wave action that, because of its short fetch, does not penetrate to anywhere near the depth of this biotope (see Hiscock, 1983 for details). Therefore, even a significant increase in wave exposure will not result in disturbance to the depths at which the biotope occurs and an assessment of not sensitive is made.
Decrease in wave exposure
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The biotope occurs in depths where wave action has no effect so decrease in wave action is irrelevant.
Noise
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Tunicates are not known to have organs sensitive to noise and most species in the biotope are unlikely to be sensitive to noise and so the biotope is assessed as not sensitive.
Visual Presence
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Tunicates are not known to have organs sensitive to visual presence. Sabella pavonina, recorded as rare in the biotope, reacts to shadows and may be briefly unable to feed as a result of retracting. However, overall not sensitive is suggested.
Abrasion & physical disturbance
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On a sedimentary seabed, physical disturbance may crush a minority of species but is most likely to displace substrata including burying individual in a population. Some species, the burrowing infauna, may recover but damaged or buried sessile fauna may be killed. Intolerance is assessed as intermediate and recovery high (see additional information below).
Displacement
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Epifaunal species are most likely to be displaced together with the substratum to which they are attached and, unless buried, are likely to survive. Infaunal species are likely to be able to re-burrow. Some mortality may occur in species that become buried so intolerance is suggested as intermediate. For recoverability, see additional information below.

Chemical Factors

Synthetic compound contamination
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Ascidians may be intolerant of synthetic chemicals such as tri-butyl-tin anti-foulants. Rees et al. (2001), working in the Crouch estuary, observed that six ascidian species were recorded at one station in 1997 compared with only two at the same station in 1987, shortly following the banning of TBT in antifouling paints. Also, there was a marked increase in the abundance of ascidians especially Ascidiella aspersa and Ascidia conchilega in the estuary after the ban on TBT was introduced. Although there is no direct evidence for effects of synthetic chemicals on Pseudamussium septemradiatum, TBT-based antifouling paint was shown to be detrimental to growth and survival of juvenile Pecten maximus scallops (Paul & Davies, 1986) and there is some evidence that recruitment to inshore scallop beds may have been affected by TBT used in anti-fouling paints (Minchin et al., 1987). Since solitary ascidians and small scallops are the main characterizing component species, an intolerant of high is suggested but with low confidence. Recoverability of high is according to additional information below but assumes that the population of Styela gelatinosa is not annihilated.
Heavy metal contamination
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No information has been found for effects of heavy metals on ascidian species that characterize the biotope. Although there is no direct evidence for effects of synthetic chemicals on Pseudamussium septemradiatum, Pecten maximus scallops concentrate metals in their tissues with an efficiency greater than that of other bivalves (Gould & Fowler, 1991). When Pecten maximus is grown in close proximity to copper-oxide based antifouling paint high levels of copper may be accumulated in the tissues (Davies & Paul, 1986), growth is also inhibited and mortality increased. Experimental studies with various species suggests that polychaete worms are quite tolerant of heavy metals (Bryan, 1984). However, there is insufficient information on the species present in the biotope to assess the intolerance of the biotope.
Hydrocarbon contamination
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No information has been found.
Radionuclide contamination
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No information has been found.
Changes in nutrient levels
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The biotope is found in areas of fine sediment with significant input of terrigenous debris, mainly organic material. An increase in nutrients in subtidal habitats of this depth will not cause the biotope to become overgrown with ephemeral algae so the smothering effects often associated with eutrophication will not occur. The intolerance of the biotope to a 50% increase in nutrients is expected to be low and recovery will be rapid on return to normal conditions.
Increase in salinity
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Although the biotopes classification suggests that COS.Sty occurs in full salinity, it seems likely that, even at the depth this biotope occurs, some dilution may occur especially during periods of heavy rain and freshwater flow. Since increase in salinity would be unlikely to above full salinity conditions, 'not relevant' is indicated.
Decrease in salinity
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Although the biotopes classification suggests that COS.Sty occurs in full salinity, it seems likely that, even at the depth this biotope occurs, some dilution may occur especially during periods of heavy rain and freshwater flow. A fall in salinity from full to reduced would be unlikely to affect Ascidiella scabra which occurs in reduced salinity conditions. Other ascidian species (with the exception of Styela gelatinosa for which there is no information found) are found in variable salinity as are species such as Sabella pavonina, Asterias rubens and Metridium senile. Overall, the biotope is likely to be tolerant of some lowering of salinity. However, in situations where salinity is already variable, a further lowering is likely to result in mortality. Intolerance is indicated as intermediate but may be high. For recoverability, see additional information.
Changes in oxygenation
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There is no information regarding the effect of deoxygenation on the key species in the biotope or the biotope as a whole. Cole et al. (1999) suggest possible adverse effects on marine species below 4 mg/l and probable adverse effects below 2mg/l. Different species in the biotope will have varying responses to deoxygenation. Ascidians are active suspension feeders that pump water. It seems likely that the effects of lowered oxygenation will be reduced as stagnation can be avoided. Other species such as the scallop Pseudamussium septemradiatum and species burrowing in the sediment are more likely to be adversely affected especially in the still conditions that the biotope lives in. Also, the presence of large amounts of rotting organic debris (terrigenous detritus) in the biotope may increase demand for scant oxygen resources and exacerbate de-oxygenation so that some species perish. An intolerance of intermediate is therefore suggested but with very low confidence. Recovery is likely to be high (see additional information below).

Biological Factors

Introduction of microbial pathogens/parasites
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There were no records found of any diseases or parasites affecting the species or the biotope.
Introduction of non-native species
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Of the non-native species known from British waters, the sea squirt Styela clava and the slipper limpet Crepidula fornicata are the only ones thought likely to thrive in this biotope. However, at present, no non-native species are known from the biotope and not sensitive is suggested.
Extraction
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It is extremely unlikely that any of the species indicative of sensitivity would be targeted for extraction and we have no evidence for the indirect effects of extraction of other species on this biotope.

Additional information icon Additional information

Recoverability
The biotope is dominated by species, especially the large solitary ascidians, that have mobile larvae and are known to settle readily onto new surfaces. The community probably has a high turnover rate within individuals of the component species reflecting the likely transitory nature of the biogenic hard substrata available for settlement. For instance, Ascidiella scabra has a high fecundity and settles readily, probably for an extended period from spring to autumn. Svane (1988) describes it as "an annual ascidian" and demonstrated recruitment onto artificial and scraped natural substrata. The occurrence and longevity of most large solitary ascidians appears similar to that of Ascidiella scabra although no information has been found of Styela gelatinosa. Allen (1953a) demonstrated that Pseudamussium septemradiatum has a life span of about 3.5 years in populations sampled in the Firth of Clyde and, based on information from other scallops, recruitment is likely to occur readily from long-lived larvae. Other species that are recorded as rare or occasional in the biotope (Abra alba, Ciona intestinalis, Metridium senile, Protanthea simplex, Sabella pavonina) are all known to settle onto new surfaces within a year or a very few years. No information has been found for Styela gelatinosa that can be used to estimate longevity or settlement frequency. Nevertheless, it appears that the community would reach maturity rapidly (possibly within a year or a very few years) after new substrata became available and providing that sources of larvae existed nearby. In the case a catastrophic event that destroyed the entire population of the characteristic species Styela gelatinosa, the biotope would be unlikely to ever recover as the nearest recorded populations of Styela gelatinosa are in deep water off the Faroes and in Norway. However, there are no other biotopes that are very similar (the closest possibly being SCR.Aasp Ascidiella aspersa on sheltered circalittoral rocks on muddy sediment) and a new biotope without Styela gelatinosa would need to be described.

This review can be cited as follows:

Hiscock, K. 2002. Styela gelatinosa and other solitary ascidians on very sheltered deep circalittoral muddy sediment. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 25/10/2014]. Available from: <http://www.marlin.ac.uk/habitatbenchmarks.php?habitatid=274&code=2004>