Biodiversity & Conservation

CR.HCR.XFa.Mol

Explanation of sensitivity and recoverability


Physical Factors

Substratum Loss
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Removal of the substratum will result in loss of the community. Therefore, an intolerance of high has been recorded.
Recoverability will depend on recruitment from adjacent or nearby populations and may take many years (see additional information below).
Smothering
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The biotope occurs in habitats that are silted and so are likely to be subject to cover by silt from time-to-time. The dominant ascidians should be capable of expanding their siphons to an extent and possibly clearing a passage through silt. Nevertheless, some mortality may occur in the dominant species and in some associated species. Large epifauna such as Alcyonium digitatum, Nemertesia antennina, large branching or cushion sponges and anemones (e.g. Urticina felina) are unlikely to be adversely affected by smothering with 5cm of sediment. The species that are believed to be long-lived and slow growing (branching sponges especially) are likely to protrude above the sediment or have efficient mucus cleaning systems. Recoverability will depend on recruitment from survivors, from adjacent or nearby population and may take many years to be complete (see additional information below).
Recoverability relates to the species that form a layer over the rock and which are likely to recolonize rapidly and grow quickly. Smothering by impermeable material will have a greater effect.
Increase in suspended sediment
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The biotope occurs in locations (for instance at the entrances to harbours and estuaries) where suspended sediment levels are often high. Increased suspended sediment concentrations may clog suspension feeding apparatus. However, production of mucus in hydroids, sponges and other species is likely to clear deposited silt whilst Molgula manhattensis seems well able to cope with increased level of suspended sediment being ingested (Naranjo et al., 1996). Some loss of condition may occur and an overall biotope intolerance of low has been recorded.

Recovery of epifaunal species is likely to be rapid once the prior conditions return.
Decrease in suspended sediment
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A decrease in suspended sediment levels may lead to reduction in food supply as some will be associated with silt. However, the energy expenditure in clearing silt (production of mucus, unclogging of feeding nets) will be advantageous to organisms in the biotope. Overall, there will be favourable and adverse effects of decrease in turbidity and not sensitive is indicated.
Desiccation
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Molgula manhattensis was assessed as intermediate intolerance to desiccation (see review). Subtidal epifaunal bryozoans, hydroids, ascidians and sponges are likely to be highly intolerant of desiccation. However, this biotope is subtidal and circalittoral and is unlikely to be exposed to the air. The shallower water extent of this biotope may be exposed for short periods during on extreme low water tides. However, overall this factor is not relevant.
Increase in emergence regime
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Subtidal epifaunal bryozoans, hydroids, ascidians and sponges are likely to be highly intolerant of desiccation resulting from increased emergence. However, this biotope is subtidal and circalittoral and is unlikely to be exposed to the air. The shallower water extent of this biotope may be exposed for short periods during on extreme low water tides. However, overall this factor is not relevant.
Decrease in emergence regime
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The biotope is subtidal and, since it does not occur on the shore, not relevant is appropriate.
Increase in water flow rate
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The biotope occurs in strong or moderately strong tidal currents and many of the sponges and hydroids present in the biotope are found in stronger currents. Ascidians, which are the most important characterizing species, may be adversely affected by increase in water flow. High water flow rates may 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. Tidal currents alone are not believed to result in detachment of the main species present in this biotope. However, if tidal flow increased from Moderate or Strong to Very Strong (>6 knots) for one year, there is a possibility that encrusting sponges such as Halichondria panicea and Esperiopsis fucorum would smother the ascidians and the biotope would perhaps become £ECR.BalHpan£ (Balanus crenatus, Halichondria panicea and Alcyonidium diaphanum on extremely tide-swept sheltered circalittoral rock). The biotope MCR.MolPol would therefore be lost and intolerance would be high. ECR.BalHpan has a lower species diversity. On return to previous conditions, the species requiring strong currents would take some time to regress and settlement of lost characterizing species is likely to be within a few years so that a recoverability of moderate is likely
Decrease in water flow rate
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In the absence of wave action, water flow is likely to be very important in preventing siltation and stagnation and in bringing food. The species in this biotope require a reasonable water flow rate in order to ensure sufficient food availability and oxygen supply. However, one of the characterizing ascidians at least, Molgula manhattensis, is frequently found in areas with minimal water exchange and renewal such as harbours, marinas and docks. Sagasti et al. (2000) demonstrated that Molgula manhattensis can withstand episodes of hypoxia and so, even if stagnation occurs for short periods, mortality is unlikely to occur. Many of the sessile species in the biotope (mainly sponges and ascidians) are active suspension feeders and do not rely on external currents to bring them food. Some of the species in the biotopes, especially hydroids, may suffer from shortage of food and lose condition. However, overall, the biotope is expected to persist albeit with the loss of some species. On resumption of normal energy expenditure and feeding, condition should be restored rapidly.
Increase in temperature
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Species that characterize the biotope are widely distributed in the north Atlantic. However, the biotope appears to be restricted to Wales and south-west England and to Kent. If temperature rather than geographical location is important, then increased temperature would be expected to result in an expansion of the distribution of the biotope northwards. However, since the key characterizing species including dominant species have a wide geographical distribution in Britain and Ireland, it seems that some features of habitat and physical environmental conditions are important in determining distribution. Intolerance is, therefore, indicated as not sensitive but with a low confidence.
Decrease in temperature
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Species that characterize the biotope are widely distributed in the north Atlantic. However, the biotope appears to be restricted to Wales and south-west England and to Kent. If temperature rather than geographical location is important, then decreased temperature would be expected to result in an reduction of the distribution of the biotope northwards. However, since the key characterizing species including dominant species have a wide geographical distribution in Britain and Ireland, it seems that some features of habitat and physical environmental conditions are important in determining distribution. Intolerance is therefore indicated as not sensitive but with a low confidence.
Increase in turbidity
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The biotope occurs in harbours and the entrances to estuaries where turbidity may increase to high levels. Some clogging of feeding structures in many of the species that characterize this biotope may occur as a result of increased turbidity due to increased suspended sediment levels. Nevertheless, the likely outcome of clogging will be of decreased feeding and loss of condition.
Decrease in turbidity
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The majority of species in the biotope are suspension feeders and, although there may be some reliance on the organic material associated with increased turbidity for nutrition, the reduced need for energy expenditure to clear silt may be beneficial. Decreased turbidity will result in increased light penetration and algal growth which may adversely affect the biotope at its shallowest extent. There may be some reduction in the shallow-water penetration of the biotope. However, it seems most likely that, at the level of the benchmark, species characteristic of the biotope will persist albeit overgrown by algae. Loss of some species characteristic of the biotope may occur but the biotope will still be recognised where algae colonize. Recoverability will be rapid as characterizing species will persist.
Increase in wave exposure
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This biotope has been reported from areas that are exposed or moderately exposed to wave action, whereas Molgula manhattensis has been mainly reported from wave sheltered habitats and may, therefore, be intolerant of increase in wave exposure together with some other species that characterize this biotope. Should the wave exposure increase from exposed to extremely exposed, Molgula manhattensis and other ascidians in the biotope may be removed from examples of the biotope in shallow depths where wave action is most severe. Similarly, a proportion of the associated epifaunal species are also likely to be removed, being replaced by more wave tolerant species such as Alcyonium digitatum and erect bryozoans. There is a possibility that the biotope will change to one that is not characterized by solitary ascidians such as £ECR.Alc.MaS£ (Alcyonium digitatum with massive sponges and Nemertesia antennina on moderately tide-swept exposed circalittoral rock). Therefore, an intolerance of high has been recorded. Recovery will depend on space being available for settlement but, bearing in mind the rapidity with which Molgula manhattensis at least settles and grows, a recoverability of high is suggested (see additional information below).
Decrease in wave exposure
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This biotope has been reported from areas of moderate wave exposure, whereas Molgula manhattensis has been reported from wave exposed to extremely wave sheltered habitats and is therefore relatively insensitive to changes in wave exposure within this range. Overall, the distribution of this biotope seems to be mainly influenced by tidal stream strength and to reflect the presence of silty conditions. A decrease in wave exposure, e.g. from moderately exposed to very sheltered is unlikely to increase siltation as tidal flow is the most important form of water movement to this biotope. However, the species composition of the epifauna is likely to change, favouring species tolerant of reduced wave action or water movement. Indeed, area subject to moderate tidal flow but little wave action are often much richer in species. Overall, the biotope is likely to remain as MCR.MolPol and not sensitive* has been recorded to reflect the possibility that the species composition may increase in richness and some of the more scarce species such as Axinella dissimilis and Axinella infundibuliformis might be favoured by calmer conditions. Recoverability has been recorded as not relevant as the biotope is expected to become more species rich and remain MCR.MolPol.
Noise
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Few species within the biotope are likely to respond to noise or vibration at the benchmark level.
Visual Presence
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Few, if any, species within the biotope have a significant visual acuity, and are unlikely to respond to visual disturbance at the benchmark level.
Abrasion & physical disturbance
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It is likely that physical disturbance at the benchmark level would remove individuals from their substratum and damage others. Disturbance of the cohesive groups of individuals may strip away tracts of the biotope or create gaps or 'edges' that may allow peeling away of the ascidians by tidal streams or wave action.

Erect epifaunal species are particularly vulnerable to physical disturbance. Sponges, hydroids and bryozoans are likely to be uprooted or damaged by bottom trawling or dredging and some bryozoans repair damage slowly (Holt et al., 1995). Veale et al. (2000) reported that the abundance, biomass and production of epifaunal assemblages decreased with increasing fishing effort. The characteristic and dominant species in the biotope, the ascidians, are likely to colonize rapidly, other species may settle and establish, preventing re-establishment of the biotope. More importantly, some species in the biotope, for instance, Axinella dissimilisand Axinella infundibuliformis are long-lived and slow growing and would not be replaced rapidly.

Overall, physical disturbance at the benchmark level may remove or damage a proportion of the ascidian bed and its associated epifauna. Therefore, an intolerance of intermediate has been recorded. For recoverability, see additional information below.
Displacement
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Most of the characteristic species in this biotope are sessile and will not reattach after displacement so that the biotope will not recover through re-attachment and new settlement will be needed.

Chemical Factors

Synthetic compound contamination
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intolerance of component species of the biotope ranges greatly. One dominant species, Molgula manhattensis, is most likely tolerant of synthetic chemical pesticides. Weis & Weis (1992) found that the ascidian was commonly present, although in small numbers, on wood treated with chromated arsenate. In mesocosm experiments, Flemer et al. (1995) studied the effect of the pesticide endosulfan and found that the average abundance of Molgula manhattensis increased with increasing concentration of the pesticide possibly as a result of reduced competition with more susceptible organisms. The high abundance of the species in harbours where levels of tributyl tin are or were likely to be high also suggests tolerance. Molgula manhattensis may benefit from tolerance to synthetic pollutants by occupying space that would have been colonized by less tolerant species. On the other hand, Rees et al. (2001) reported that the abundance of epifauna (including bryozoans) had increased in the Crouch estuary in the five years since TBT was banned from use on small vessels. This last report suggests that bryozoans and ascidians may be at least inhibited by the presence of TBT. Hoare & Hiscock (1974) suggested that Polyzoa (Bryozoa) were amongst the most intolerant species to acidified halogenated effluents in Amlwch Bay, Anglesey and reported that Flustra foliacea did not occur less than 165m from the effluent source. Smith (1968) reported dead colonies of Alcyonium digitatum at a depth of 16m in the locality of Sennen Cove (Cornwall) resulting from the offshore spread and toxic effect of detergents sprayed along the shoreline to disperse oil from the Torrey Canyon tanker spill. At the level of the benchmark, it might be expected that some species would be adversely affected but that the biotope would persist albeit in a possibly impoverished form. On removal of contaminants, recovery to full species richness would depend on the individual tolerance of species and whether any long-lived and slow growing ones had been killed. Intolerance is assessed as intermediate but with low confidence. For recoverability, see additional information. Since information has not been found for species such as Axinella dissimilis and Axinella infundibuliformis, confidence in assessing recoverability is low.
Heavy metal contamination
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Very little information has been found regarding the effects of heavy metals on species in the biotope. One of the dominant species, Molgula manhattensis, commonly lives in the entrance to harbours and estuaries so that it might be expected to be tolerant of heavy metals. Bryozoans are common members of the fouling community, and amongst those organisms most resistant to anti-fouling measures, such as copper containing anti-fouling paints (Soule & Soule, 1979; Holt et al, 1995). Amongst the large number of species that occur in this biotope, some at least are likely to be intolerant of heavy metal pollution and so an intolerance of intermediate is suggested by with a very low confidence in the absence of evidence. The biotope will still be recognised and recoverability (see additional information below) refers to species richness in the biotope.
Hydrocarbon contamination
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Subtidal populations are protected from the direct effects of oil spills by their depth but are likely to be exposed to the water soluble fraction of oils and hydrocarbons, or hydrocarbons adsorbed onto particulates. During the Torrey Canyon oil spill, Urticina felina were found to be alive even in intertidal areas (Smith, 1968). However, Smith (1968) reported dead colonies of Alcyonium digitatum at a depth of 16m in the locality of Sennen Cove (Cornwall) resulting from the offshore spread and toxic effect of detergents sprayed along the shoreline to disperse oil from the Torrey Cannon tanker spill (see synthetic chemicals). During the Sea Empress oil spill, surveys of the open Pembrokeshire coast (where this biotope commonly occurs) found no signs of adverse effects (SEEEC, 1998). Overall, because the biotope is circalittoral and because of lack of evidence of impacts from spills, an intolerance of intermediate is suggested indicating that some stress and possibly loss of intolerant crustaceans may occur. Recoverability is suggested to be high as the most intolerant species would most likely recolonize rapidly.
Radionuclide contamination
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No information found.
Changes in nutrient levels
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Moderate increases in nutrient levels may benefit suspension feeding members of the community by increasing macroalgal and phytoplankton productivity, increasing the proportion of organic particulates and hence increasing the food supply. Similarly, increased availability of organic particulates may benefit the community. Nutrient enrichment may however lead to increased turbidity and decreased oxygen levels due to bacterial decomposition of organic material (see below). The species composition of the epifaunal community may also change as a result. However, suspension feeders would probably benefit from increased nutrients at the benchmark level.
Increase in salinity
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The biotope is found on the open coast in full salinity and so increase in salinity is considered not relevant.
Decrease in salinity
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The biotope is found on the open coast and includes some species such as sponges, anthozoans and some ascidians that might be intolerant of decrease in salinity. However, others including the dominant species are tolerant of decreased salinity. Van Name (1945; quoted in Kott, 1985), notes that Molgula manhattensis occurs in salinities equivalent to 20 to 36 psu whilst Hartmeyer (1923) quoted in Tokioka & Kado (1972) records Molgula manhattensis in brackish (16-30 psu) water of the Belt Sea. A fall in salinity from full to reduced would not therefore be expected to have an adverse effect on Molgula manhattensis but would on a significant number of other species in the biotope. Since the biotope is recorded only from open coast locations or the entrance to enclosed areas, it is expected that the biotope would not survive in low salinity conditions or at least would be greatly impoverished. Therefore an intolerance of high is recorded. For recoverability, see additional information below.
Changes in oxygenation
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The biotope occurs on the open coast where oxygen is unlikely to be depleted and some species might be expected to be intolerant of low oxygen. For instance, Alcyonium digitatum mainly inhabits environments in which the oxygen concentration usually exceeds 5 ml l-1 and respiration is aerobic. Assimilation of oxygen occurs simply by diffusion through the epidermis of exposed tissues and transport to tissues is facilitated by hydroplasmic flow and ciliary activity (Hickson, 1901). However, Sagasti et al. (2000) demonstrated that Molgula manhattensis can withstand episodes of hypoxia and so intolerance is likely to be low amounting to some loss in condition. Some of the cushion sponges occur in Abereiddy Quarry in Pembrokeshire near to a zone affected by severe de-oxygenation (Hiscock & Hoare, 1973) and might be able to survive brief episodes of low oxygen conditions. Overall, in conditions of low oxygen, several species in the biotope may die and, whilst the dominant Molgula manhattensis might survive, it would cease to be that biotope. For recoverability, see additional Information below.

Biological Factors

Introduction of microbial pathogens/parasites
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Microbial pathogens are not known to have a severe adverse effect (causing extensive mortality) on any of the characterizing species but, in the absence of information, insufficient information is recorded.
Introduction of non-native species
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There are no non-native species known to colonize this biotope although the author of this review speculates that Molgula manhattensis may have been a very early introduction to north-western Europe (see the species review) .
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
Whilst the dominant species in the community (the ascidians) are likely to recolonize and grow very quickly (re-establishment within one year) if propagules are present at the time that bare rock is available, a different dominant species (for instance, Musculus discors) may settle and dominate the area for at least many years transforming the biotope to a different one. Some of the associated species (for instance, Nemertesia spp., Alcyonium digitatum, Pomatoceros triqueter, other ascidians) will colonize and grow within a few years. However, some of the species associated with the community most likely reproduce and settle infrequently and are slow growing (especially cushion and branching sponges). Little evidence is available for the longevity and growth of these sponges although Hiscock (1994) observes that Axinella dissimilis has a growth rate of less than 1 mm a year. Therefore, although recoverability to the same biotope is assessed as moderate (5-10 years) following a catastrophic event, recovery of full species richness may take much longer and will be very low.

This review can be cited as follows:

Hiscock, K. 2002. Molgula manhattensis and Polycarpa spp. with erect sponges on tide-swept moderately exposed circalittoral rock. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 22/09/2014]. Available from: <http://www.marlin.ac.uk/habitatbenchmarks.php?habitatid=305&code=2004>