Biodiversity & Conservation

CR.SCR.BrAs.AntAsH

Explanation of sensitivity and recoverability


Physical Factors

Substratum Loss
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Many of the species within the biotope are attached to the substratum or are slow moving so that substratum removal would result in loss of most faunal populations and so the intolerance of the biotope is set at high. With the loss of adult populations, recovery depends upon recolonization by larvae during the breeding season. In the SCR.AntAsH biotope recovery by recolonization is likely to be slow and a recoverability of moderate is reported - see additional information for full rationale.
Smothering
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Although the biotope occurs in fairly silty environments, such as sheltered sea lochs, the biotope is usually clear of silt as it occurs on vertical surfaces and/or in strong tidal streams. Smothering by 5cm of sediment is likely to result in the death of many species. Feeding structures of the feather-stars and sea-squirts will probably become clogged and suffocate the organisms. Some species, such as the hydroid Nemertesia ramosa, will extend above the surface sediment and may survive. However, many species are likely to be smothered and die so intolerance is assessed as high. In the sheltered conditions in which this biotope is found smothering material is not likely to be removed very rapidly. Recovery is likely to take many years - see additional information below for rationale.
Increase in suspended sediment
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The biotope is found in silty areas such as may be found in sheltered parts of sea lochs where levels of suspended sediment may increase at times. An increase in suspended sediment may clog respiratory and feeding organs of the suspension feeders in the biotope although food supplies may improve if organic content is high. Most species in the biotope can probably tolerate an increase in suspended sediment at the benchmark level although cleaning costs may increase. Intolerance of the biotope is, therefore, recorded as low. Recovery from the energetic expenditure of clearing the feeding apparatus is likely to take only a few days and so on return to normal conditions recovery will probably be immediate.
Decrease in suspended sediment
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Many of the species in the biotope are suspension feeders so a supply of particles is necessary for feeding. Therefore, a decrease in suspended sediment will reduce food supply which may impair growth. However, effects resulting from a reduction for a month are not likely to be significant so intolerance is assessed as low. On return to normal conditions recovery should be rapid.
Desiccation
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SCR.AntAsH is a circalittoral biotope so desiccation will not be relevant.
Increase in emergence regime
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SCR.AntAsH is a circalittoral biotope so and increase in emergence will not be relevant.
Decrease in emergence regime
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SCR.AntAsH is a circalittoral biotope so a decrease in emergence will not be relevant.
Increase in water flow rate
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The biotope is found in areas of weak or very weak tidal streams. However, many of the species in the biotope are present in stronger water flows than this so can probably tolerate an increase. Increased water flow rates may actually improve the supply of particles, and hence food, to the biotope possibly resulting in increased biomass and abundance of species. However, an increase in water flow at the benchmark level is likely to prevent many of the species feeding and may sweep some unattached species away. Over a period of a year many species are likely to die resulting in an impoverished biotope, or change to another biotope, and so intolerance is assessed as high. For recovery see additional information.
Decrease in water flow rate
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Moving water is essential for many reasons, including supplying food and dissolved gases, preventing the accumulation of sediments, and dispersing waste products. The biotope is found in areas of weak or very weak tidal streams so a decrease would effectively reduce water movement to nothing. Over a period of a year such a reduction in the supply of food particles and oxygenated water would probably result in the death of most species and so intolerance of the biotope is reported to be high. See additional information for recovery.
Increase in temperature
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There was no information found on the impact of an increase in temperature on the biotope. Most species in the biotope have a distribution that extends further south than the British Isles and towards the Mediterranean, so that long term chronic increases in temperature can probably be accommodated. Antedon bifida is more commonly found in shallow waters and so may be more tolerant of temperature changes than the other feather-star species. Of the other species particularly common in the biotope there is little information relating to responses to temperature increases. For hydroids temperature is a critical factor in stimulating or preventing hydroid reproduction and most species have an optimum temperature range for reproduction. However, it seems likely that the biotope could tolerate an increase in temperature at the level of the benchmark. Although some very intolerant species may be lost they are likely to be replaced by similar but more tolerant fauna and because no species are particularly dominant in the biotope a slight change of species composition would probably not drastically alter the nature of the community and so intolerance assessed as low.
Decrease in temperature
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There was no information found on the impact of a decrease in temperature on the biotope. Most species present in the biotope have a distribution that extends to the north of Britain, so that long term chronic decreases in temperature can probably be accommodated. The distribution of Antedon spp. only extends to the Shetlands so feather-stars may be less able to tolerate long term decreases in temperature. Of the other species particularly common in the biotope there is little information relating to the response to decreasing temperature. For hydroids temperature is a critical factor in stimulating or preventing hydroid reproduction and most species have an optimum temperature range for reproduction. It seems possible that feather-stars may be lost from the biotope should temperatures fall so intolerance is reported to be intermediate.
Increase in turbidity
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The biotope is an animal dominated community, dependent on secondary production rather than light and photosynthesis. Therefore, the biotope is probably not sensitive to changes in turbidity and light attenuation. However, light may be important in determining the distributions of a few species through the responses of larvae prior to settlement. Some hydroids, for example, are positively phototactic. There is a general tendency for most hydroids, especially large ones, to be less abundant in well lit situations where competition with algae for the substratum would be greatest although whether this is a direct effect of competition or an evolved avoidance response is not known. However, a slight change in the species composition in the biotope will not radically alter the nature of the community.
Decrease in turbidity
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The biotope is an animal dominated community, dependent on secondary production rather than light dependent photosynthesis. Therefore, the biotope is probably not sensitive to changes in turbidity and light attenuation. However, light may be important in determining the distributions of some species through the responses of larvae prior to settlement. Some hydroids, for example, are positively phototactic. There is a general tendency for most hydroids, especially large ones, to be less abundant in well lit situations where competition with algae for the substratum would be greatest although whether this is a direct effect of competition or an evolved avoidance response is not known. However, a slight change in the species composition in the biotope will not radically alter the nature of the community.
Increase in wave exposure
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The biotope occurs in very sheltered conditions so is likely to be intolerant of increased wave exposure. For example, Antedon bifida may become detached by strong wave action. Some of the fine hydroids in the biotope are also likely to be damaged. Therefore, an increase in wave exposure for a period of a year is likely to significantly change the nature of the biotope, leading to an increase in wave tolerant species. This will probably result in the development of another biotope and so intolerance is reported to be high. See additional information for recovery.
Decrease in wave exposure
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The biotope occurs in very sheltered conditions where wave action is minimal or absent. As long as tidal flows provide a supply of oxygenated water a decrease in wave exposure is not likely to have an impact and a rank of not sensitive is reported.
Noise
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Some of the important characterizing species associated with this biotope may respond to vibrations associated with sound. For example, the feather-star Antedon bifida curl up their arms in response to vibrations. 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 and so the biotope is recorded as being not sensitive.
Visual Presence
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Most species within the biotope have no or poor visual perception and are unlikely to be affected by visual disturbance such as shading. The biotope is therefore reported as not sensitive to the factor.
Abrasion & physical disturbance
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Erect epifaunal species are particularly vulnerable to physical disturbance. Hydroids and bryozoans are likely to be removed or damaged by bottom trawling or dredging (Holt et al., 1995). Veale et al. (2000) reported that the abundance, biomass and production of epifaunal assemblages decreased with increasing fishing effort. Hydroid and bryozoan matrices were reported to be greatly reduced in fished areas (Jennings & Kaiser, 1998 and references therein). The removal of rocks or boulders to which species are attached by the passage of mobile fishing gears (Bullimore, 1985; Jennings & Kaiser, 1998) results in substratum loss (see above).

Damage to emergent epifauna was the first sign of damage from scallop dredging on horse mussel beds (see Modiolus modiolus) (Service & Magorrian, 1997; Magorrian & Service, 1998; Service 1998). However, while several species of upright hydroids and bryozoans were adversely affected by bottom fishing, some species increased in abundance after fishing disturbance either due to their ability to rapidly colonize space (e.g. Nemertesia sp.) and/or their ability to recover from fragments or budding (e.g. small ascidians, especially Ascidiella spp. and Alcyonium digitatum) (Bradshaw et al., 2000; 2002).

Ciona intestinalis is a large ascidian, with a soft, retractile body. Physical disturbance by a passing scallop dredge is likely to cause physical damage and death. The light bulb tunicate is permanently attached to the substratum and is unable to move out of the way from abrasive objects. The body of the species is soft and delicate, so abrasion is likely to cause physical damage and possibly death.

The majority of the characterizing species in this biotope are sessile, attached to the substratum and so are unable to move away and are likely to be physically removed or damaged by a passing scallop dredge. Therefore, given the evidence above, an overall intolerance of high has been recorded. Recovery of the biotope is likely to be high as effects are local and partial - see additional information below.

Displacement
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Many species in the biotope are sessile organisms attached to the substratum that cannot re-attach once removed and so intolerance is reported to be high. Some species such as the feather-stars are not sensitive to displacement. See additional information for recovery.

Chemical Factors

Synthetic compound contamination
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There is very little information regarding the effect of synthetic chemicals on either the biotope or the individual species in the biotope. Prolonged exposure to low concentrations of polychlorinated biphenyls (PCB's) has been shown to result in growth and regenerative abnormalities in the feather-star Antedon mediterranea (Carnevali et al., 2001). Therefore, it seems likely that the congeneric Antedon spp. occurring in British and Irish waters would also be intolerant of PCBs. However, no reports of the death of individuals was found. Hoare & Hiscock (1974) reported that Antedon bifida appeared to be completely intolerant of conditions within the vicinity of an acidified, halogenated effluent discharge. Torrey Canyon in 1967 off Land's End of Cornwall, and the detergent used to disperse it caused mass mortalities of echinoderms. Hydroid populations often show sublethal effects of exposure to potential pollutants such as morphological and growth changes (Gili & Hughes, 1995). Intolerance is therefore reported to be high. See additional information for recovery.
Heavy metal contamination
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Information about the effects of heavy metals on the biotope, or the individual species in the biotope, were not found although some general work is available. For example, Bryan (1984) reports that early work has shown that echinoderm larvae are intolerant of heavy metals. In experiments hydroids often show sub-lethal effects of exposure to potential pollutants, such as morphological changes or changes in growth rates. Low concentrations of metal ions may inhibit growth regulators of hydroids and increase growth rates (Gili & Hughes, 1995). It is also well recognised that ascidians are capable of accumulating trace elements such as heavy metals. However, most evidence for heavy metals points only to sublethal effects so it is unlikely that many species would be lost and so intolerance is set to low.
Hydrocarbon contamination
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Although there is no information available on the effect of hydrocarbons on the biotope there is a little information regarding different phyla. For example, echinoderms appear to be highly intolerant of hydrocarbon pollution. Long term chronic pollution is thought to be responsible for reduced abundance of Asterias rubens (Bokn et al., 1993) and Echinocardium cordatum (Daan & Mulder, 1996). Crude oil from the Torrey Canyon in 1967 off Land's End of Cornwall, and the detergent used to disperse it caused mass mortalities of echinoderms. Thus it seems likely that one of the key species, the feather stars Antedon spp., will also result in the loss of the biotope so intolerance is reported to be high.
Radionuclide contamination
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Insufficient information.
Changes in nutrient levels
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There was no information found on the effect of an increase in nutrients on the biotope. However, the general response of communities to an increasing gradient of organic enrichment is one of falling species diversity and an increasing dominance of organisms such as polychaete worms. Such drastic changes however, are usually in response to chronic pollution. At the level of the benchmark, a 50% increase in the nutrients in the biotope, some intolerant species may be lost and growth and reproduction rates of others may be enhanced. Although the presence of algae is limited to crustal species, growth may increase because algae are often nutrient limited. If light conditions permit there may be an increase in the abundance of ephemeral algae. Intolerance of the biotope is set to low because at the benchmark level impacts are expected to be predominantly sublethal. On return to normal conditions previous growth levels will return very rapidly.
Increase in salinity
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The species in the biotope are predominantly subtidal species not exposed to increased salinity such as can be experienced in rock pools in the intertidal. Echinoderms, for example, are considered to be stenohaline animals that lack the ability to osmo- and ion-regulate (Stickle & Diehl, 1987). It is expected that many species in the biotope will be intolerant of increased salinity and will be lost and so intolerance is reported to be high.
Decrease in salinity
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The biotope is a marine community although some of the species in the biotope, such as Clavelina lepadiformis and Ciona intestinalis, are found in areas of reduced salinity. However, a decrease in salinity at the level of the benchmark for a period of a year is likely to result in the loss of many species, changing the composition of the biotope. For example, echinoderms such as the feather-star Antedon bifida, are considered to be stenohaline animals that lack the ability to osmo- and ion-regulate (Stickle & Diehl, 1987). Echinoderm larvae have a narrow range of salinity tolerance and will develop abnormally and die if exposed to reduced salinity. The species is therefore likely to be intolerant of a decrease in salinity for a period of a year. Thus, the overall impact on the biotope could be the loss of some key species so intolerance is reported to be intermediate. On return to normal conditions recovery is expected to be moderate - see additional information.
Changes in oxygenation
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Cole et al. (1999) suggest possible adverse effects on marine species below 4 mg/l and probable adverse effects below 2 mg/l. Although no information could be found on the effects of reduced oxygen on the biotope it is possible that some species will be affected and the biotope degraded so intolerance is reported to be intermediate.

Biological Factors

Introduction of microbial pathogens/parasites
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Although many species may be host to symbiotic or parasitic organisms there was no information found on any diseases affecting the biotope. However, there is always the potential for this to occur. Echinoderms in other parts of the world, for example, have been severely affected by epidemic type diseases drastically reducing population numbers.
Introduction of non-native species
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Of the non-native species known from British waters, only the ascidian Styela clava is recorded as occurring in the biotope. Styela clava was first recorded in the UK at Plymouth in 1952 (Eno et al. 1997). Where Styela clava and Ciona intestinalis co-occur they may compete for space and food. However, this is unlikely to change the overall nature of the biotope. There are no records of any other non-native species invading the biotope and so intolerance is reported to be low. However, as several species have become established in British waters there is always the potential for new non-native species to be introduced.
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, especially since the biotope may often occur in areas where dredging may be difficult because bedrock predominates.

Additional information icon Additional information

Recoverability
Judging from life-history traits of the key species recovery to previous abundances from local populations should be possible within a few years. For example, Antedon bifida reaches sexual maturity within the first or second year and is iteroparous, spawning for 2-3 months every year. Eggs are brooded on the arms of the feather-star and pelagic larvae are then released into the water column. Many of the attached species, especially hydroids and ascidians, are likely to settle quite rapidly (within a year or so) whilst mobile species will migrate in. Most of the species in the biotope have pelagic larvae so recruitment will be largely from the water column. However, the pelagic phase for Antedon bifida and for some hydroids and ascidians is fairly short so dispersal distances may not be great and recruitment may rely on relatively local populations. In sheltered locations such as sea lochs recruitment from the plankton may take longer because tidal flows are very weak which reduces the supply of planktonic larvae from coastal waters. Therefore, if populations are completely removed by a factor recovery may take up to or even longer than five years. Other species such as the anthozoan, Alcyonium digitatum, have long lived and potentially dispersive pelagic larvae and so may recolonize more rapidly. Slow growing species such as encrusting coralline algae will take many years to regain former abundance. Therefore, a qualitative recovery will occur within 5 years although further changes will occur after that. Thus, it appears that the time for the biotope to recover will largely depend on whether populations remain. Where neighbouring populations are present recruitment and recovery may be rapid, but if local populations of many species are lost recruitment from distant populations may take a long time and recovery may be more than five years.

This review can be cited as follows:

Hill, J.M. 2001. Antedon spp., solitary ascidians and fine hydroids on sheltered circalittoral rock. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 22/10/2014]. Available from: <http://www.marlin.ac.uk/habitatbenchmarks.php?habitatid=313&code=1997>