Biodiversity & Conservation

SS.IMX.EstMx.CreAph

Explanation of sensitivity and recoverability


Physical Factors

Substratum Loss
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The majority of species in the biotope live either permanently attached to the substratum (epifauna) or buried in the underlying sediment (infauna). The physical removal of the substratum, e.g. as a result of channel dredging activities, would also remove the associated populations. Therefore, intolerance is recorded as high. For example, Ismail (1985) demonstrated that following suction dredging of the top few centimetres of sediment on oyster grounds in Delaware Bay, the Crepidula fornicata population was removed. Substratum loss is likely to result in the complete eradication of most species and therefore species richness in the biotope will experience major decline. Hall & Harding (1997) reported that following suction dredging in soft sediments, the species richness of infaunal communities was reduced by up to 30% and the numbers of individuals by up to 50%. Recoverability is recorded as high (see additional information below).
Smothering
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The majority of species in the biotope live either infaunally or are capable of burrowing. They would be expected to tolerate an additional 5 cm layer of sediment and relocate to their preferred position. Aphelochaeta marioni, for example, deposit feeds at the surface by extending contractile palps from its burrow. The additional layer of sediment would result in a temporary cessation of feeding activity, and therefore growth and reproduction are likely to be compromised. However, Aphelochaeta marioni would be expected to quickly relocate to its favoured depth, with no mortality. The immobile epifauna in the biotope are likely to be more intolerant of smothering. Ascidians are active suspension feeders and rely on a through current of water for feeding and respiration. Smothering would be likely to cause severe inhibition of these activities and mortality would be expected to result within a few days. However, larger species such as Ascidiella aspersa would probably not be affected as they attach to protuberant surfaces and their siphons are a few centimetres clear of the sediment surface. Crepidula fornicata is also an active suspension feeder and it would be expected that the feeding and respiration structures would be susceptible to smothering. However, it has been demonstrated that Crepidula fornicata is capable of clearing its feeding structures at some energetic cost (Johnson, 1972). Furthermore, areas with large Crepidula fornicata populations do tend to become silted up through deposition of pseudofaeces, apparently with little effect on the species (Thouzeau et al., 2000) and the fact that Crepidula fornicata lives in chains of up to 12 individuals means that at least some of the chain would avoid the effects of smothering. Therefore, although there may be some energetic cost as a result of smothering, probably resulting in decreased growth and reproductive output, there is unlikely to be mortality.
Given the intolerance of the characterizing species, the overall intolerance for the biotope is recorded as low but there is likely to be a minor decline in species richness due to mortality of the smaller ascidian species. Once the infaunal species have relocated to the surface and feeding and respiration structures have been cleared, activity should return to normal and therefore a recoverability of very high is recorded.
Increase in suspended sediment
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The epifauna in the biotope are most likely to be affected by an increase in suspended sediment. Crepidula fornicata is an active suspension feeder, trapping food particles on a mucous sheet lying across the front surface of the gill filament. An increase in suspended sediment is therefore likely to interfere with the feeding and respiration structures. Johnson (1972) transplanted individual Crepidula fornicata to environments of varying turbidity and measured their shell growth rates. Growth rate was found to decrease as turbidity increased. These observations were verified in laboratory conditions by measuring water filtration rate at different turbidities. Filtration rate was found to decrease as turbidity increased with the greatest reduction in filtration occurring between 140-200 mg per litre. Decreased filtration rate was associated with increased production of pseudofaeces in order to keep the filtering mechanism clear of debris. Increased pseudofaeces production coupled with decreased food intake would lead to increased energy consumption that is likely to impair the survival of the species. The infauna and deposit feeders, such as Aphelochaeta marioni, are unlikely to be negatively affected by an increase in suspended sediment (Brenchley, 1981). An increased rate of siltation may result in an increase in food availability and therefore growth and reproduction may be enhanced. However, food availability would only increase if the additional suspended sediment contained a significant proportion of organic matter and the population would only be enhanced if food was previously limiting. Due to the intolerance of the suspension feeders, biotope intolerance is recorded as low. When suspended sediment returns to normal levels, feeding and respiration will return to normal and the only likely lag will be in reproductive output, i.e. it will take a period of time to replenish food reserves, during which reproductive output will not be at maximum levels. A recoverability of very high is therefore recorded.
Decrease in suspended sediment
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The majority of species in the biotope are either suspension feeders or deposit feeders and therefore rely on a supply of nutrients in the water column and at the sediment surface. A decrease in the suspended sediment would result in decreased food availability for suspension feeders. It would also result in a decreased rate of deposition on the substratum surface and therefore a reduction in food availability for deposit feeders. This would be likely to impair growth and reproduction. The benchmark states that this change would occur for one month and therefore would be unlikely to cause mortality. An intolerance of low is therefore recorded. As soon as suspended sediment levels increased, feeding activity would return to normal and hence recovery is recorded as immediate.
Desiccation
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The infaunal species in the biotope are likely to be able to avoid desiccation stress by burrowing into the sediment. Fine sediment contains a high silt content and retains a large amount of water. The epifauna, such as Crepidula fornicata, Mytilus edulis and the ascidians, are more likely to be affected by desiccation. Both the mollusc species would probably be able to retain water for extended periods by firm adherence to the substratum in the case of Crepidula fornicata and closure of the valves in the case of Mytilus edulis. The benchmark for desiccation is exposure to the air for one hour. It is likely that Crepidula fornicata and Mytilus edulis would be able to survive this exposure with only some loss of water. During the period of exposure they would not be able to feed and respiration would be compromised so there is likely to be some energetic cost. Intolerance for the biotope is therefore recorded as low. The solitary ascidians, however, are soft bodied and unlikely to be tolerant of desiccation. Exposure to the air for one hour would probably cause significant mortality, resulting in a minor decline in species richness.
Increase in emergence regime
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IMX.CreAph predominantly occurs subtidally. However, the upper part of the biotope is exposed at low water spring tides and therefore an increase in emergence regime is relevant. The benchmark is an additional one hour of emergence every tidal cycle. During this time, exposed individuals of all species will not be able to feed and respiration of most will be compromised. Over the period of a year, the resultant energetic cost may cause the mortality of individuals exposed for the longest time. The overall intolerance of the biotope is therefore recorded as intermediate. Particularly intolerant species, such as ascidians, would be expected to suffer total mortality and therefore there would be a minor decline in species richness. Recoverability is recorded as high (see additional information below).
Decrease in emergence regime
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IMX.CreAph occurs in the subtidal zone and therefore would not be intolerant of a decreased emergence regime. It is possible that decreased emergence would allow the biotope to colonize further up the shore and extend its range.
Increase in water flow rate
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IMX.CreAph occurs in wave protected areas where water flow is typically "moderately strong" or weaker (see glossary). An increase in water flow rate of two categories for one year would place the biotope in areas of strong or very strong flow. Increased water flow rate will change the sediment characteristics in which the biotope occurs, primarily by re-suspending and preventing deposition of finer particles (Hiscock, 1983). The underlying sediment in the biotope has a high silt content; a substratum which would not occur in very strong tidal streams. Therefore, the infaunal species, such as Aphelochaeta marioni, would be outside their habitat preferences and some mortality would be likely to occur. Additionally, the consequent lack of deposition of particulate matter at the sediment surface would reduce food availability for deposit feeders. The resultant energetic cost over one year would also be likely to result in some mortality. An intolerance of intermediate is therefore recorded and species richness is expected to decline. Recoverability is recorded as high (see additional information below).
Decrease in water flow rate
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IMX.CreAph occurs in areas of low water flow including the lowest category on the water flow scale ('very weak' - see glossary) (Connor et al., 1997a). Therefore, the biotope would be unlikely to be intolerant of decreases in water flow regime. However, it should be noted that decreased water flow rate could result in an increased settlement of suspended sediment (Hiscock, 1983) and deoxygenation. These factors are covered in their relevant sections.
Increase in temperature
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Both the characterizing species in the biotope occur over a very wide geographic range. On the east coast of the Americas, Crepidula fornicata is found as far south as Mexico and therefore it must be able to tolerate higher temperatures than it experiences in northern Europe. The effect of temperature on larval development was investigated by Lucas & Costlow (1979). Larvae were found to tolerate daily temperature cycles of 5°C between 15°C and 30°C with little mortality. Over a 12 day period there was 0% mortality at 30°C but 100% mortality occurred by day 6 at 35°C. Thus, it seems that adult Crepidula fornicata are able to tolerate chronic change over time and larvae are able to tolerate acute change in the short term. Aphelochaeta marioni has been recorded from the Mediterranean Sea and Indian Ocean (Hartmann-Schröder, 1974; Rogall, 1977; both cited in Farke, 1979) and therefore must also be capable of tolerating higher temperatures than experienced in Northern Europe. Furthermore, Aphelochaeta marioni lives infaunally and so is likely to be insulated from rapid temperature change. For both the characterizing species, an increase in temperature would be expected to cause some physiological stress but no mortality and therefore an intolerance of low is recorded for the biotope. Metabolic activity should quickly return to normal when temperatures decrease and so a recoverability of very high is recorded. The majority of species in the biotope either live infaunally or are capable of burrowing and therefore would be insulated from rapid temperature change. Of the epifaunal species, Mytilus edulis is generally regarded as being eurythermal and the ascidians have a wide geographic range so are expected to tolerate variations in temperature. Hence, no decline in species richness is expected.
Decrease in temperature
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During the severe winter of 1962-63 the British populations of marine invertebrates were subjected to an acute decrease in temperatures. Waugh (1964) recorded 25% mortality of Crepidula fornicata from the south coast and east coast of England where the recorded temperatures were 5-6°C and 3-4°C respectively below normal for a period of 2 months. Aphelochaeta marioni is more tolerant of decreases in temperature, probably because it lives infaunally. For example, in the Wadden Sea, the population was apparently unaffected by a short period of severe frost in I973 (Farke, 1979). The intolerance of Crepidula fornicata is in line with the benchmarks for temperature decrease and hence the intolerance of the biotope is recorded as intermediate. Recoverability is recorded as high (see additional information below). During the cold winter of 1962-63, severe mortalities of Carcinus maenas were recorded, while the infaunal species (e.g. Corophium volutator, Harmothoe impar, Nephtys hombergi) were largely unaffected (Crisp, 1964). Species richness in the biotope is therefore expected to show a minor decline.
Increase in turbidity
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IMX.CreAph occurs in turbid estuarine waters and therefore the species in the biotope are likely to be well adapted to turbid conditions. An increase in turbidity may affect primary production in the water column and therefore reduce the availability of diatom food, both for suspension feeders and deposit feeders. In addition, primary production by the microphytobenthos on the sediment surface may be reduced, further decreasing food availability for deposit feeders. However, primary production is probably not a major source of nutrient input into the system and, furthermore, phytoplankton will also immigrate from distant areas and so the effect may be decreased. As the benchmark turbidity increase only persists for a year, decreased food availability would probably only affect growth and fecundity of the intolerant species so a biotope intolerance of low is recorded. As soon as light levels return to normal, primary production will increase and hence recoverability is recorded as very high. There is not expected to be any mortality due to increased turbidity and hence the species richness is not expected to change.
Decrease in turbidity
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None of the species in the IMX.CreAph biotope require light and so therefore are not likely to be affected by a decrease in turbidity for light attenuation purposes. However, a decrease in turbidity will mean more light is available for photosynthesis by phytoplankton in the water column and phytobenthos on the sediment surface. Over the course of a year, this may lead to the development of a community of macroalgae which could potentially compete with some of the epifaunal species in the biotope, resulting in some mortality. Intolerance is therefore recorded as intermediate and there may be a minor decline in species richness. Recoverability is recorded as high (see additional information below).
Increase in wave exposure
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IMX.CreAph occurs in sheltered areas such as estuaries and is characterized by a mixed substratum (Connor et al., 1997a). This suggests that the biotope would be intolerant of wave exposure to some degree. An increase in wave exposure by two categories for one year would be likely to affect the biotope in several ways. Fine sediments would be eroded (Hiscock, 1983) resulting in the likely reduction of the habitat of the infaunal species and a decrease in food availability for deposit feeders. Gravel and cobbles are likely to be moved by strong wave action resulting in damage and displacement of epifauna. For example, Crepidula fornicata is often found cast ashore following storms (Hayward & Ryland, 1995). Species may be damaged or dislodged by scouring from sand and gravel mobilized by increased wave action. Furthermore, strong wave action is likely to cause damage or withdrawal of delicate feeding and respiration structures of species within the biotope resulting in loss of feeding opportunities and compromised growth. It is likely that high mortality would result and therefore an intolerance of high is recorded and species richness is expected to decline. Recoverability is recorded as high (see additional information below).
Decrease in wave exposure
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IMX.CreAph occurs in 'extremely sheltered' environments (Connor et al., 1997a). The species present thrive in low energy environments and are tolerant of changes in chemical factors such as dissolved oxygen and salinity. The biotope, therefore, is unlikely to be intolerant of a further decrease in wave exposure and species richness is unlikely to change.
Noise
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No information was found concerning the intolerance of the biotope or the characterizing species to noise. However, it is unlikely that the biotope will be affected by noise or vibrations caused by noise at the level of the benchmark.
Visual Presence
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The majority of species in the biotope are unlikely to be affected by visual disturbance. However, Farke (1979) noted the intolerance of Aphelochaeta marioni to visual disturbance in a microsystem in the laboratory. In order to observe feeding and breeding in the microsystem at night, the animals had to be gradually acclimated to lamp light. Even then, additional disturbance, such as an electronic flash, caused the retraction of palps and cirri and cessation of all activity for some minutes. Visual disturbance, in the form of direct illumination during the species' active period at night, may therefore result in loss of feeding opportunities, which may compromise growth and reproduction. On the basis of the reaction of Aphelochaeta marioni, an intolerance of low is recorded. When the visual disturbance is removed feeding activity should return to normal immediately.
Abrasion & physical disturbance
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Both the epifaunal and the infaunal species in the biotope are likely to be sensitive to physical disturbance due to dredging for scallops or oysters. Soft bodied epifauna, such as ascidians, are most vulnerable, and are likely to suffer high mortality. Sponges and hydroids attached to the slipper limpet bed are likely to be removed along the dredge track.

Crepidula fornicata has a robust body form and so individuals are likely to be resistant to the benchmark level of physical abrasion. However, the gregarious chain-forming characteristic of the species renders it susceptible to disturbance, as chains are more likely to be broken up, leaving some individuals exposed to predation.

De Montaudouin et al. (2001) (following Sauriau et al. , 1998) suggested that physical disturbance is a factor which could stimulate the presence of Crepidula fornicata. They noted that the species settles preferentially in the trails of trawl fishing gear, and that this may explain why Crepidula fornicata is not very abundant in the Arcachon Basin, France, as bottom trawling activities are prohibited here.

The infaunal annelids are predominantly soft bodied, live within a few centimetres of the sediment surface and may expose feeding or respiration structures where they could easily be damaged by a physical disturbance such as a passing dredge. The species with robust exoskeletons, such as bivalves and crustaceans, are likely to be the most resistant. The overall, a proportion of the slipper limpet bed, and its associated epifauna and infauna are likely to be removed or displaced. Therefore, an overall intolerance of intermediate has been recorded. For recoverability see additional information below.

Displacement
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Effects of displacement on both the characterizing species in this biotope have been studied. Crepidula fornicata live in chains of up to 12 individuals, with the bottom individual being attached permanently to the substratum. Attachment is permanent as the shell takes on the shape of the substratum (Hoagland, 1979). Displacement would almost certainly lead to the mortality of the bottom individual in the chain as it would become very vulnerable to predation. However, other individuals in the chain would be unaffected by the displacement. Johnson (1972) demonstrated that transplanted individuals continue to grow normally. Farke (1979) noted the effects of displacement on Aphelochaeta marioni while performing experiments on intolerance to salinity changes. It was observed that when an individual was removed from its habitat and displaced to a similar habitat, it took approximately one minute to dig itself into the sediment. Aphelochaeta marioni is therefore recorded as not intolerant of displacement. The low level of mortality suffered by Crepidula fornicata suggests that the intolerance of the biotope to displacement is intermediate. Recoverability is recorded as high (see additional information below). Soft bodied, permanently attached epifauna, such as ascidians, are unlikely to survive displacement and therefore there would be a minor decline in species richness.

Chemical Factors

Synthetic compound contamination
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Toxins, including synthetic chemicals such as dieldrin and poly-chlorinated biphenyls, tend to accumulate in low energy areas such as estuaries where the IMX.CreAph biotope occurs. Dispersion is low in these areas and the fine substrata act as a sink, retaining toxins for long periods of time (see review by Elliot et al., 1998). Therefore, the species which live infaunally in fine sediments, such as polychaetes, would be expected to be most vulnerable. Collier & Pinn (1998) investigated the effect on the benthos of ivermectin, a feed additive treatment for infestations of sea-lice on farmed salmonids. The polychaete Hediste diversicolor was particularly susceptible, exhibiting 100% mortality within 14 days when exposed to 8 mg/m² of ivermectin in a microcosm. Arenicola marina was also intolerant of ivermectin through the ingestion of contaminated sediment (Thain et al., 1998; cited in Collier & Pinn, 1998) and it was suggested that deposit feeding was an important route for exposure to toxins. Beaumont et al. (1989) investigated the effects of tri-butyl tin (TBT) on benthic organisms. At concentrations of 1-3 µg/l there was no significant effect on the abundance of Hediste diversicolor or Cirratulus cirratus after 9 weeks in a microcosm. However, no juvenile polychaetes were retrieved from the substratum and hence there is some evidence that TBT had an effect on the larval and/or juvenile stages of these polychaetes. No evidence was found on the effects of synthetic compounds specifically on Crepidula fornicata. However, there is a wealth of evidence concerning effects on related molluscs. For example, the effect of TBT from anti-fouling paints on gastropods is very well documented. Imposex, female mortality and the subsequent decline in population, has been described in Nucella lapillus (e.g. Bryan et al., 1986), Littorina littorea (Bauer et al., 1995), Ilyanassa obsoleta and Urosalpinx cinerea (Matthiessen & Gibbs, 1998). Limpets (Patellidae) are extremely intolerant of aromatic solvent based dispersants used in oil spill clean-up. During the clean-up response to the Torrey Canyon oil spill nearly all the limpets were killed in areas close to dispersant spraying. Viscous oil will not be readily drawn in under the edge of the shell by ciliary currents in the mantle cavity, whereas detergent, alone or diluted in sea water, would creep in much more readily and be liable to kill the limpet (Smith, 1968). For example, a concentration of 5ppm of dispersant killed half the patellid limpets tested in 24 hours (Southward & Southward, 1978; Hawkins & Southward, 1992). Thus, although no evidence was found concerning effects of synthetic chemicals on the biotope specifically, the intolerance of infaunal polychaetes and the likely intolerance of Crepidula fornicata, suggests that biotope intolerance is high and species richness will decline. Recoverability is recorded as high (see additional information below).
Heavy metal contamination
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As with synthetic chemicals, heavy metals tend to accumulate in the fine sediments present in biotopes such as IMX.CreAph (see review by Elliot et al., 1998). The intolerance of the characterizing species, Crepidula fornicata, has been well studied. In the Fal Estuary, Crepidula fornicata does occur in the Carrick Roads, an area where creek water polluted with heavy metals mixes with the open ocean (Bryan & Gibbs, 1983). In this area, concentrations of silver, cadmium, copper, lead and zinc were found to be higher than in 'control' estuaries (Bryan & Gibbs, 1983). This suggests that Crepidula fornicata is at least partially tolerant to heavy metal contamination. Laboratory trials have revealed specific responses to heavy metals. Thain (1984) investigated the effects of exposure to mercury. The adult and larval 96 hour LC50s (concentrations at which half the organisms die after 96 hours) were 330 and 60 µg/l respectively. As a reference, levels of mercury in UK waters at the time of these experiments were 104 to 105 below the 96 hour LC50 for adult Crepidula fornicata. Furthermore, sub-lethal concentrations of mercury were shown to impair growth and condition of young adult Crepidula fornicata and impair reproductive capacity at 0.25 µg/l. Nelson et al. (1983) investigated the effects of exposure to silver. Reproductive output was found to be impaired following exposure to the highest concentration of silver nitrate (10 µg/l) for 24 months. The evidence suggests that high concentrations of heavy metals will cause mortality in Crepidula fornicata. However, lower concentrations, which could realistically occur in situ impair growth, condition and reproductive output and will therefore affect the long term health of the population. The species intolerance is therefore recorded as low.
Evidence suggests that the other characterizing species in the biotope, Aphelochaeta marioni, is more tolerant of heavy metal contamination. It occurs in the heavily polluted Restronguet Creek (Bryan & Gibbs, 1983) and also is an accumulator of arsenic (Gibbs et al., 1983). Based on the intolerance of Crepidula fornicata, the intolerance of the biotope is recorded as low and there is likely to be an associated minor decline in species richness. Recoverability is recorded as high (see additional information below).
Hydrocarbon contamination
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Oil spills resulting from tanker accidents can cause large-scale deterioration of communities in shallow subtidal sedimentary systems. The majority of benthic species often suffer high mortality, allowing a few tolerant opportunistic species to proliferate. For example, after the Florida spill of 1969 in Massachusetts, 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). The two characterizing species in the IMX.CreAph biotope are good examples of how species react differently to hydrocarbon pollution. No evidence could be found for the effect of hydrocarbons on Crepidula fornicata specifically. However, inferences can be drawn from other gastropods. Following the Torrey Canyon oil spill in 1967, total mortality of 3 Patella species was reported after one month of oil coming ashore at Porthleven reef (Smith, 1968). Other gastropod mortalities included Nucella lapillus, Nassarius incrassatus and Gibbula sp. Therefore, it is suggested that Crepidula fornicata would suffer high mortality when exposed to hydrocarbon contamination. Aphelochaeta marioni, however, seems to be mostly immune to oil spills, probably because the feeding tentacles are protected by a heavy secretion of mucus (Suchanek, 1993). This is supported by observations of the species following the Amoco Cadiz oil spill in March, 1978 (Dauvin, 1982, 2000). Prior to the spill, Aphelochaeta marioni was present in very low numbers in the Bay of Morlaix, western English Channel. Following the spill, the level of hydrocarbons in the sediment increased from 10 mg/kg dry sediment to 1443 mg/kg dry sediment 6 months afterwards. In the same period, Aphelochaeta marioni increased in abundance to a mean of 76 individuals/m², which placed it among the top five dominant species in the faunal assemblage. It was suggested that the population explosion occurred due to the increased food availability because of accumulation of organic matter resulting from high mortality of browsers. Six years later, abundance of Aphelochaeta marioni began to fall away again, accompanied by gradual decontamination of the sediments. This is an example of how invertebrate communities often react to oil spills. Initial massive mortality and lowered community diversity is followed by extreme fluctuations in populations of opportunistic mobile and sessile fauna (Suchanek, 1993).
As the biotope occurs subtidally, it is likely to avoid the worst impact of an oil spill and therefore the intolerance is recorded as intermediate. Recoverability of Crepidula fornicata is likely to be rapid (see additional information below), but recovery of the biotope to original species diversity and abundance may take longer and therefore, biotope recoverability is recorded as moderate.
Radionuclide contamination
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Information on intolerance to nuclear radiation is generally scarce. Greenberber et al. (1986) exposed larval Crepidula fornicata to doses of X-ray radiation between 500 and 20,000 Rad in total. After 20 days, there was a dose dependent decrease in larval shell growth rate and a significant increase in larval mortality following doses above 2000 Rad. These levels of radiation are extremely high compared to background levels in the environment. For reference, Polykarpov (1998) (cited in Cole et al., 1999) describes the natural levels of background radiation being equivalent to a dose of 0.005 Gy per year (equivalent to 0.5 Rad per year). Hence, high doses of radiation have been shown to significantly increase mortality while lower levels have sub-lethal effects on growth and reproduction. Based on the intolerance of Crepidula fornicata, the overall biotope intolerance is recorded as intermediate. Recoverability is recorded as high (see additional information below). There is little evidence concerning other species in the biotope.
Changes in nutrient levels
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Nutrient enrichment can lead to significant shifts in community composition in sedimentary habitats. Typically the community moves towards one dominated by deposit feeders and detritivores, such as polychaete worms (see review by Pearson & Rosenberg, 1978). The biotope includes several species tolerant of nutrient enrichment (e.g. Nephtys hombergi, Eteone longa, Corophium volutator) and typical of enriched habitats (e.g. Tubificoides sp., Mediomastus fragilis) (Pearson & Rosenberg, 1978). It is likely that these species would increase in abundance following nutrient enrichment, with an associated decline in suspension feeding species such as ascidians. The intolerance of the characterizing species Aphelochaeta marioni is difficult to ascertain from the available evidence. Raman & Ganapati (1983) presented evidence that Aphelochaeta marioni is not tolerant of eutrophication. However, nutrient enrichment would lead to increased food availability, the species is tolerant of low oxygen conditions (Broom et al., 1991) and has been recorded as proliferating following an oil spill which resulted in eutrophic conditions (Dauvin 1982, 2000). No information was found for the intolerance of Crepidula fornicata to nutrient enrichment. It seems likely that nutrient enrichment would result in a shift in community structure rather than a gross change in species composition and so biotope intolerance is recorded as intermediate, with a minor decline in species richness. Recoverability is recorded as high (see additional information below).
Increase in salinity
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IMX.CreAph occurs in estuaries and so the community is likely to be tolerant of variable salinities. Both characterizing species and the majority of other species in the biotope also occur on the open coast where sea water is at full salinity. Therefore the biotope is not likely to be intolerant of increases in salinity. No evidence was found concerning the reaction of the characterizing species to hypersaline conditions.
Decrease in salinity
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IMX.CreAph occurs in estuaries and so the community is likely to be tolerant of variable salinities. Aphelochaeta marioni, for example, has been recorded from brackish inland waters in the southern Netherlands with a salinity of 16 psu, but not in areas permanently exposed to lower salinities (Wolff, 1973). It also penetrates into areas exposed to salinities as low as 4 psu for short periods at low tide when fresh water discharge from rivers is high (Farke, 1979). The other species in the biotope do not display such tolerance. Despite being described as euryhaline (Blanchard, 1997), Crepidula fornicata is a marine organism and a drop in salinity to levels below 18 psu would be likely to cause water balance stress and therefore impair growth and reproduction. The same is likely to be true for the majority of the species in the biotope and hence an intolerance of low is recorded. Growth and reproduction should very quickly return to normal when salinity increases so recoverability is recorded as very high.
Changes in oxygenation
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The fauna in the biotope are all aerobic organisms and are therefore likely to be intolerant in some degree to lack of oxygen. No evidence was found for specific effects of reduced oxygenation on Crepidula fornicata but inferences can be drawn from the effects on other species. Jorgensen (1980) recorded the effects of low oxygen levels on benthic fauna in a Danish fjord. At dissolved oxygen concentrations of 0.2-1.0 mg/l the gastropod Hydrobia ulvae suffered mortality unless able to crawl to areas of higher oxygen concentration and the bivalves, Cardium edule and Mya arenaria, suffered mortality between 2 and 7 days. As Crepidula fornicata is not mobile, it is expected that some mortality would occur within a week at the benchmark level of 2 mg/l. Infaunal species which typically tolerate lower oxygen tensions than occur in the water column are likely to be less intolerant of reductions in dissolved oxygen. For example, Broom et al. (1991) recorded that Aphelochaeta marioni characterized the faunal assemblage of very poorly oxygenated mud in the Severn Estuary. They found Aphelochaeta marioni to be dominant where the redox potential at 4 cm sediment depth was 56 mV and, therefore, concluded that the species was tolerant of very low oxygen tensions. On the basis of the intolerance of epifauna such as Crepidula fornicata, the intolerance of the biotope is recorded as intermediate, with a minor decline in species richness. Recoverability is recorded as high (see additional information below).

Biological Factors

Introduction of microbial pathogens/parasites
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Gibbs (1971) recorded that nearly all of the population of Aphelochaeta marioni in Stonehouse Pool, Plymouth Sound, was infected with a sporozoan parasite belonging to the acephaline gregarine genus Gonospora, which inhabits the coelom of the host. No evidence was found to suggest that gametogenesis was affected by Gonospora infection and there was no apparent reduction in fecundity. However, any parasitic infection is likely to impair the host in some way so the intolerance of the species is recorded as low. If the parasite were to be removed, the host would be likely to return to normal health quickly so a recoverability of very high is recorded. No information was found concerning infection of the other characterizing species, Crepidula fornicata, by microbial pathogens.
Introduction of non-native species
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The biotope is dominated by Crepidula fornicata which is itself an alien species. It has spread widely through Europe following introduction from North America at the end of the 19th century (Fretter & Graham, 1981; Eno et al., 1997).
Extraction
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IMX.CreAph is associated with oyster beds and relict oyster beds, (£IMX.Ost£), in southern England and Wales, separated from these by the superabundance of Crepidula fornicata (Connor et al., 1997b). Crepidula fornicata is a serious pest on oyster beds (Fretter & Graham, 1981) and therefore extraction of the species has occurred in an attempt to reduce the negative impact on the shellfishery in these areas. Cole & Hancock (1956) reported that over 8 tonnes/ha of slipper limpets were removed from oyster beds by dredging and that it takes up to 10 years to return to pre-clearance levels. Extraction of Crepidula fornicata would therefore be responsible for shifting the IMX.CreAph biotope back towards the IMX.Ost biotope from which it usually develops. Extent of the biotope would be expected to decrease and intolerance has therefore been recorded as intermediate. In this specific case, given the evidence for recovery time, recoverability is recorded as moderate. The effect of dredging for slipper limpets would be similar to removing the upper layer of the substratum and therefore a decline in species richness is expected.

Additional information icon Additional information

Recoverability
The recoverability of the important characterizing species is the principal factor in assessing the recoverability of the biotope.
  • The mode of reproduction of Crepidula fornicata gives the species strong powers of recoverability. Adults spawn at least once a year, large numbers of eggs are produced, there is a long planktotrophic larval stage and adults reach maturity within a year (Fretter & Graham, 1981; Deslou-Paoli & Heral, 1996). The ability of Crepidula fornicata to colonize new areas has been demonstrated by its spread through Europe following introduction from North America at the end of the 19th century (Fretter & Graham, 1981; Blanchard, 1997). Cole & Hancock (1956) reported that following clearance of slipper limpets from oyster beds by dredging, populations took up to 10 years to regain pre-clearance levels. However, given the species' reproductive characteristics and invasive record, it is likely that in most situations, populations would recover within 5 years and therefore recoverability is assessed as high.
  • Aphelochaeta marioni has no pelagic phase in its lifecycle, and dispersal is limited to the slow burrowing of the adults and juveniles (Farke, 1979). The blow lug, Arenicola marina, has similar dispersal capabilities and its recoverability has been well studied. It is therefore a suitable species to act as a guide for the recoverability of infaunal polychaetes. Heavy commercial exploitation in Budle Bay in winter 1984 removed 4 million worms in 6 weeks, reducing the population from 40 to <1 per m². Recovery occurred within a few months by recolonization from surrounding sediment (Fowler, 1999). However, Cryer et al. (1987) reported no recovery for 6 months over summer after mortalities due to bait digging. Beukema (1995) noted that the lugworm stock recovered slowly after mechanical dredging, reaching its original level in at least three years. Fowler (1999) pointed out that recovery may take a long time on a small pocket beach with limited possibility of recolonization from surrounding areas. Therefore, if adjacent populations are available recovery will be rapid. However where the affected population is isolated or severely reduced, recovery may be extended. Recoverability for Aphelochaeta marioni is therefore assessed as high.
  • As abundant epifauna in the biotope, the recoverability of ascidians should also be considered. Ascidians are fast growing, breed annually and disperse over short distances via a brief planktonic larval stage. Long distance dispersal may occur via drifting of adults attached to free floating objects. Ascidians are generally regarded to have low recoverability due to the brief larval stage. However, recoverability may be rapid if populations exist nearby and the hydrographic regime allows.
In light of the above information it is expected that the overall recoverability of the biotope would be high.

This review can be cited as follows:

Rayment, W.J. 2001. Crepidula fornicata and Aphelochaeta marioni in variable salinity infralittoral mixed sediment. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 26/11/2014]. Available from: <http://www.marlin.ac.uk/habitatbenchmarks.php?habitatid=52&code=1997>