Biodiversity & Conservation

SS.IGS.FaS.FabMag

Explanation of sensitivity and recoverability


Physical Factors

Substratum Loss
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The majority of species in the biotope are infaunal and would therefore be removed along with the substratum. Some epifaunal and swimming species, such as amphipods and the harbour crab Liocarcinus depurator, may be able to avoid the factor. Because the species which characterize the biotope would be lost, intolerance is assessed as high and there would be a major decline in species richness. Recoverability is recorded as high (see additional information below).
Smothering
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The majority of the species in the biotope are infaunal. Bivalves, such as Fabulina fabula, require their inhalant siphon to be above the sediment surface for feeding and respiration, while the deposit feeding Magelona mirabilis extends its contractile palps to the sediment surface in search of food. Smothering with 5 cm of sediment would temporarily halt feeding and respiration and require the infauna to relocate to their preferred depth. The bivalves, polychaetes and amphipods are active burrowers and would be unlikely to suffer mortality. Kranz (1972) (cited in Maurer et al., 1986) reported that shallow burying siphonate suspension feeders are typically able to escape smothering with 10-50 cm of their native sediment and relocate to their preferred depth by burrowing. However, feeding and respiration may be compromised by smothering and so intolerance is assessed as low. Feeding and respiration would be likely to return to normal soon after relocation and so recoverability is recorded as very high. The epifaunal echinoderms, such as Astropecten irregularis, are probably large, mobile and flexible enough to relocate to the surface following smothering. Species richness is likely to remain unchanged.
Increase in suspended sediment
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Venerid bivalves are active suspension feeders, trapping food particles on the gill filaments (ctenidia). An increase in suspended sediment is therefore likely to affect both feeding and respiration by potentially clogging the ctenidia. In Venerupis corrugatus, increased particle concentrations between low and high tide resulted in increased clearance rates and pseudofaeces production with no significant increase in respiration rate (Stenton-Dozey & Brown, 1994). It seems likely therefore that venerids would be able to clear their feeding and respiration structures, although at high particle concentrations there may be some energetic cost.
An increase in suspended sediment will increase the rate of siltation at the sediment surface, potentially enhancing the food supply of deposit feeders in the biotope, such as Magelona mirabilis.
The bivalve Fabulina fabula has two alternative methods of feeding (Salzwedel, 1979). If the level of suspended sediment becomes so high as to risk clogging the feeding structures, Fabulina fabula could presumably switch to deposit feeding and potentially benefit from the increased deposition of organic matter at the sediment surface.
Based on the intolerance of the suspension feeding bivalves, such as Chamelea gallina, which assumes the likelihood of clogging of feeding structures, biotope intolerance is assessed as low. When suspended sediment returns to original levels, feeding and respiration should quickly return to normal so recoverability is recorded as very high. Species richness is likely to remain unchanged.
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 increase, feeding activity would return to normal and hence recovery is recorded as very high. A decrease in suspended sediment is likely to lead to a decrease in siltation and therefore a reduction in the proportion of fine sediments in the substratum. This would tend to make the substratum less suitable for deposit feeders and their abundance may decrease in the biotope. However, over the benchmark period of one month there is not likely to be any decline in species richness.
Desiccation
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Desiccation would occur if intertidal sediments became 'baked' by sunlight during a prolonged low tide event or the component species in the biotope were to be removed from the sediment and stranded, unable to reburrow. The majority of the fauna in the biotope lives infaunally and so is likely to be protected from desiccation stress. Additionally, bivalves are able to respond to desiccation stress by valve adduction during periods of emersion and it is likely that they would be able to retain enough water within their shells to avoid mortality during the benchmark emersion period of one hour. However, during the period of emersion, the fauna would not be able to feed and respiration would be compromised, so there is likely to be some energetic cost. Intolerance is therefore recorded as low. On immersion, metabolic activity should quickly return to normal and recoverability is therefore recorded as very high. There is unlikely to be a decline in species richness.
Increase in emergence regime
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The biotope occurs on the extreme lower shore (Connor et al., 1997a) and therefore is vulnerable to an increase in emergence. The fact that the biotope does not colonize further up the shore suggests it must be limited by one or more factors including desiccation, temperature and wave exposure. For example, Wilson (1978) noted that the predominantly subtidal Fabulina fabula had a much lower thermal tolerance than the predominantly intertidal Tellina tenuis. The benchmark for emergence is an increase in exposure for one hour every tidal cycle for a year. During this time, individuals in the emersed portion of the biotope will not be able to feed, respiration will be compromised and thermal stress may occur. Assuming migration does not occur, over the course of a year, it is expected that the resultant energetic cost to the individuals highest up the shore will lead to some mortality. Intolerance is therefore assessed as intermediate. Recoverability is recorded as high (see additional information below). The subtidal portion of the biotope will remain immersed and so species richness is likely to remain unchanged.
Decrease in emergence regime
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The biotope is predominantly subtidal and therefore would not be affected by a decrease in emergence regime. A decrease in emergence would potentially allow the biotope to colonize further up the shore.
Increase in water flow rate
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Tidal currents determine to a large degree the nature of the substratum, but in addition, they influence the stability of the sediment, the nature of the food supply for benthic organisms, and, in extreme cases may impose direct physical stresses on the community (Warwick & Uncles, 1980). IGS.FabMag typically occurs in areas of 'weak' water flow, where tidal currents are less than 0.5 m/s (Connor et al., 1997a). An increase in water flow rate to 'strong' (1.5-3 m/s) for 1 year is likely to have profound effects on the biotope. Erosion of fine sand occurs at 0.3 m/s (Elliott et al., 1998) and so substratum characteristics are likely to change significantly. Mackie et al. (1995) noted that the species composition of sandy biotopes varies according to sand grain size and stability. Finer compacted sands favour Fabulina fabula and Magelona sp. whereas generally coarser looser sands influenced by greater water movement tend to have Spisula elliptica and Nephtys cirrosa. Warwick & Uncles (1980) recorded the Tellina subcommunity (characterized by Fabulina fabula and Magelona mirabilis) from sheltered areas of the Bristol Channel with least tidal stress (0-2.5 dynes/cm²). In areas of greater tidal stress the community was replaced, with the Spisula subcommunity occurring at 6-7 dynes/cm². The benchmark increase in water flow, therefore, is likely to result in the loss of the fine sand substratum along with its characteristic species and replacement with a community adapted for life in more mobile, coarser sands. Intolerance is therefore assessed as high. Recoverability is recorded as high (see additional information below). Some species will be lost from the biotope (e.g. Fabulina fabula) while others are ubiquitous (e.g. Spiophanes bombyx and Abra alba). There is therefore expected to be a decline in species richness.
Decrease in water flow rate
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IGS.FabMag typically occurs in areas of 'weak' water flow (Connor et al., 1997a). The benchmark reduction in water flow would place the biotope in the 'very weak' category for 1 year. The likely result would be increased deposition of fine particles altering the substratum characteristics. Deposit feeders tend to dominate over suspension feeders in areas of higher proportions of silt and clay as muddy sediment and high turbidity tend to clog the filtering organs (Elliott et al., 1998). The characterizing species, Fabulina fabula and Magelona mirabilis, are deposit feeders and are not likely to be intolerant of the change. The suspension feeding venerid bivalves, such as Chamelea gallina, are capable of generating their own feeding and respiration currents but may be inhibited by clogging of feeding and respiration structures. They are probably capable of clearing these structures (e.g. Grant & Thorpe, 1991; Navarro & Widows, 1997), but the energetic cost over a year may result in some mortality and so the biotope intolerance is assessed as intermediate with a minor decline in species richness. Recoverability is assessed as high (see additional information below).
Increase in temperature
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The species which characterize the biotope all have wide geographic ranges. Magelona mirabilis and Chamelea gallina both occur in the Mediterranean Sea and Fabulina fabula is found as far south as Morocco (Hayward et al., 1996). The species are therefore likely to be tolerant of higher temperatures than they experience in British and Irish waters and are not likely to be intolerant of chronic rises in temperature. Acute temperature changes, however, may cause physiological stress and inhibit growth and reproduction. Wilson (1978) reported the 24 hour LT50 for Fabulina fabula (studied as Tellina fabula) from Millport in Scotland to be 26.5°C and noted that acclimation to higher temperatures enhanced the species' ability to withstand higher experimental temperatures. Similarly, Ansell et al. (1980) reported the 24 hour LT50 for Fabulina fabula (studied as Tellina fabula and acclimated at 10°C) from Millport to be 27°C. The 96 hour LT50 was 24-27°C depending on acclimation temperature. Growth experiments by Salzwedel (1979) revealed that growth of Fabulina fabula (studied as Tellina fabula) correlated positively with temperature up to about 16°C, after which temperature increase inhibited growth. It seems unlikely therefore that the benchmark increase in temperature would cause mortality of the characterizing species, although growth may be inhibited, so biotope intolerance is assessed as low. Recoverability is recorded as very high. The epifaunal species in the biotope may be more prone to temperature stress. Asterias rubens for example, responds to prolonged high temperatures by arm shedding (autotomy) and, eventually, death (Lawrence, 1995). There may therefore be a minor decline in species richness.
Decrease in temperature
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The species which characterize the biotope all have wide geographic ranges. Fabulina fabula and Chamelea gallina both occur in Norway (Hayward et al., 1996) and Magelona mirabilis occurs in the Baltic Sea (Fiege et al., 2000). The species are therefore likely to be tolerant of lower temperatures than they experience in British and Irish waters and are relatively tolerant of chronic decreases in temperature. Acute temperature changes, however, may cause physiological stress and inhibit growth and reproduction. For example, Salzwedel (1979) noted that growth of Fabulina fabula from the German Bight was minimal at the lowest annual temperature of 3°C. Biotope intolerance is therefore assessed as low. Growth should quickly return to normal when temperatures return to their original levels so recoverability is assessed as very high. Other species in the biotope show greater intolerance to low temperatures. For example, during the severe winter of 1963 Echinocardium cordatum was almost completely eliminated from the German Bight to a depth of about 20m (Lawrence, 1995) and very heavy mortality was observed in the English Channel and North Sea (Crisp (ed.), 1964). There may therefore be a minor decline in species richness.
Increase in turbidity
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An increase in turbidity may reduce 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 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.
Decrease in turbidity
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A decrease in turbidity will mean more light is available for photosynthesis by phytoplankton in the water column and microphytobenthos on the sediment surface. This would increase primary production and may mean greater food availability for deposit feeders and suspension feeders. However, primary production is probably not a major source of production in the biotope so the turbidity decrease is not likely to have a significant effect. The benthos is probably supported predominantly by pelagic production and by detrital materials emanating from the coastal fringe (Barnes & Hughes, 1992).
Increase in wave exposure
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The benchmark increase in wave exposure would place the biotope in the 'exposed' and 'very exposed' categories (see glossary) (Connor et al., 1997a). Oscillatory water movement occurs down to about 60m when a force 8 wind is blowing at the sea surface (Hiscock, 1983) and therefore the biotope will definitely experience the effects of increased wave exposure. Hiscock (1983) reviewed the effects:
  • fine sediments would be eroded resulting in the likely reduction of the habitat of many infaunal species and a decrease in food availability for deposit feeders;
  • species may be damaged or dislodged by scouring from sand and gravel mobilized by increased wave action;
  • 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.
Warwick & Uncles (1980) noted that the Tellina subcommunity in Carmarthen Bay only occurs in the sheltered areas. The characterizing species are described as being fragile, for instance, Fabulina fabula with a "thin, brittle shell" and Magelona mirabilis with "long, delicate palps". It is likely that the benchmark increase in wave exposure would precipitate a shift in substratum type and associated community and the development of a more dynamic biotope with high sediment transport and more robust species, such as Spisula elliptica and Nephtys cirrosa. Biotope intolerance is therefore assessed as high with a major decline in species richness. Recoverability is recorded as high (see additional information below).
Decrease in wave exposure
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The benchmark decrease in wave exposure would place the biotope in the 'very sheltered' or 'extremely sheltered' category (see glossary) (Connor et al., 1997a). The decrease in water movement would result in increased siltation and a consequent change in sediment characteristics (Hiscock, 1983). A substratum with a higher proportion of fine sediment would probably result in an increase in abundance of the deposit feeders in the biotope, particularly species which favour finer sediments, such as the polychaete Aphelochaeta marioni and the echinoid Echinocardium cordatum. The increase is likely to be at the expense of suspension feeders, such as the venerid bivalves. There is likely to be some mortality of suspension feeders and hence intolerance is assessed as intermediate with a minor decline in species richness. Recoverability is assessed as high (see additional information below).
Noise
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No information was found concerning the intolerance of the biotope or the characterizing species to noise. The siphons of bivalves and palps of polychaetes are likely to detect vibrations and are probably withdrawn as a predator avoidance mechanism. 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 infaunal and have little or no visual acuity. No evidence was found concerning intolerance to visual presence, but it is unlikely that the biotope will be affected.
Abrasion & physical disturbance
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Despite their robust body form, bivalves are vulnerable to physical abrasion. For example, as a result of dredging activity, mortality and shell damage have been reported in Mya arenaria and Cerastoderma edule (Cotter et al., 1997). Bergman & van Santbrink (2000) suggested that the megafauna such as Echinocardium cordatum, Corystes cassivelaunus, and bivalves such as Phaxas pellucidus, Dosinia lupinus, Mactra corallina, Abra alba, Spisula solida and Spisula subtruncata were amongst the species most vulnerable to direct mortality due to bottom trawling in sandy sediments. Bivalves such as Ensis spp., Corbula gibba and Chamelea gallina together with starfish were relatively resistant (Bergman & van Santbrink, 2000). Bradshaw et al. (2000) suggested that fragile species such a urchins (e.g. Spatangus purpureus and Echinus esculentus), the brittlestar Ophiocomina nigra, starfish Anseropoda placenta and the edible crab Cancer pagurus suffered badly from impact with a passing scallop dredge. More robust bodied or thick shells species were less sensitive, while species with brittle, hard tests are regarded to be sensitive to impact with scallop dredges (Kaiser & Spencer, 1995; Bradshaw et al. , 2000). Venerid bivalves are generally shallow burrowers and Fabulina fabula has a fragile shell (Fish & Fish, 1996). The bivalves that characterize the biotope may therefore be damaged by physical abrasion. The polychaete, Magelona mirabilis, is a soft bodied organism which lives within a few centimetres of the sediment surface and exposes its palps at the surface while feeding. It is, therefore, also likely to be damaged by the benchmark physical abrasion.

Eleftheriou & Robertson (1992) performed experimental scallop dredging in a sandy bay in Scotland. They observed that the action of the dredge resulted in damage and mortality of Echinocardium cordatum, Asterias rubens, Astropecten irregularis, Cancer pagurus and Ammodytes sp. The authors suggested that the infaunal invertebrates with behavioural or morphological adaptations to the rigours of life in high energy environments, such as amphipods, were not affected by dredging operations in any significant way. The sessile infauna, however, along with large infaunal and epifaunal forms, such as molluscs, decapods, echinoderms and some polychaetes, demonstrated their vulnerability.

It seems likely that the characterizing species will suffer some mortality due to physical abrasion and so intolerance is assessed as intermediate. Recoverability is recorded as high (see additional information below). Particularly vulnerable forms, such as the epifaunal echinoderms, may be eliminated so there may be a minor decline in species richness in the biotope.

Displacement
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Fabulina fabula, Chamelea gallina and Magelona mirabilis are all active burrowers and are capable of reburying themselves if displaced to the surface of a suitable substratum (Jones, 1968; Salzwedel, 1979). However, while at the sediment surface they are vulnerable to predation from echinoderms (Aberkali & Trueman, 1985) and bottom feeding fish (Hunt, 1925; Hayward & Ryland, 1995) so there is likely to be some mortality. Intolerance is therefore assessed as intermediate. Recoverability is recorded as high (see additional information below).

Chemical Factors

Synthetic compound contamination
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No information was found concerning the effects of synthetic chemicals specifically on Fabulina fabula or Chamelea gallina. However, inference can be drawn from related species. Beaumont et al. (1989) concluded that bivalves are particularly intolerant of tri-butyl tin (TBT), the toxic component of many antifouling paints. For example, when exposed to 1-3 µg TBT/l, Cerastoderma edule and Scrobicularia plana suffered 100% mortality after 2 weeks and 10 weeks respectively. There is also evidence that TBT causes recruitment failure in bivalves, either due to reproductive failure or larval mortality (Bryan & Gibbs, 1991). Stirling (1975) investigated the effects of phenol, a non-persistent, semi-synthetic organic pollutant, on Tellina tenuis. Exposure to phenol produced a measurable effect on burrowing at all concentrations tested, i.e. 50 mg/l and stronger. Sub-lethal effects of exposure to phenol included delayed burrowing and valve adduction to exclude the pollutant from the mantle cavity. After exposure to 100 mg/l for 24 hours, the majority of animals were extended from their shells and unresponsive to tactile stimulation. Following replacement of the phenol solution with clean seawater, good recovery was exhibited after 2 days for animals exposed to 50 mg/l and some recovery occurred after 4 days for animals exposed to 100 mg/l.
Similarly, no evidence was found directly relating to the effects of synthetic chemicals on Magelona mirabilis. However, there is evidence from other polychaete species. 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 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.
Detergents used to disperse oil from the Torrey Canyon oil spill caused mass mortalities of Echinocardium cordatum (Smith, 1968) and its intolerance to TBT is similar to that of other benthic organisms with LC50 values of 222ng Sn/l in pore water and 1594ng Sn/g dry weight of sediment (Stronkhorst et al., 1999). Gammaridean amphipods have also been reported to be intolerant of TBT with 10 day LC50 values of 1-48 ng/l (Meador et al., 1993).
In light of the likely intolerance of the important characterizing species in the biotope, intolerance is assessed as high and there is likely to be a major decline in species richness. Recoverability is recorded as high (see additional information below).
Heavy metal contamination
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The capacity of bivalves to accumulate heavy metals in their tissues, far in excess of environmental levels, is well known. Reactions to sub-lethal levels of heavy metal stressors include siphon retraction, valve closure, inhibition of byssal thread production, disruption of burrowing behaviour, inhibition of respiration, inhibition of filtration rate, inhibition of protein synthesis and suppressed growth (see review by Aberkali & Trueman, 1985). No evidence was found directly relating to Fabulina fabula. However, inferences may be drawn from studies of a closely related species. Stirling (1975) investigated the effect of exposure to copper on Tellina tenuis. The 96 hour LC50 for Cu was 1000 µg/l. Exposure to Cu concentrations of 250 µg/l and above inhibited burrowing behaviour and would presumably result in greater vulnerability to predators. Similarly, burial of the venerid bivalve, Venerupis senegalensis, was inhibited by copper spiked sediments, and at very high concentrations, clams closed up and did not bury at all (Kaschl & Carballeira, 1999). The copper 10 day LC50 for Venerupis senegalensis was found to be 88 µg/l in sandy sediments (Kaschl & Carballeira, 1999).
Echinoderms are also regarded as being intolerant of heavy metals (e.g. Bryan, 1984; Kinne, 1984) while polychaetes are tolerant (Bryan, 1984). Given the likely intolerance of the bivalves, biotope intolerance is assessed as high and species richness is expected to decline. Recoverability is recorded as high (see additional information below).
It should be noted that experimental exposures to heavy metals in the laboratory are likely to be far higher than those encountered in the sea and therefore the real effect in vivo may be far less.
Hydrocarbon contamination
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Suchanek (1993) reviewed the effects of oil on bivalves. Generally, contact with oil causes an increase in energy expenditure and a decrease in feeding rate, resulting in less energy available for growth and reproduction. Sublethal concentrations of hydrocarbons also reduce byssal thread production (thus weakening attachment) and infaunal burrowing rates.
Conan (1982) investigated the long term effects of the Amoco Cadiz oil spill at St Efflam beach in France. It was estimated that the delayed mortality effects on sand and mud biotas were 1.4 times as large as the immediate effects. Fabulina fabula (studied as Tellina fabula) started to disappear from the intertidal zone a few months after the spill and from then on was restricted to subtidal levels. In the following 2 years, recruitment of Fabulina fabula was very much reduced. The author commented that, in the long term, the biotas most severely affected by oil spills are low energy sandy and muddy shores, bays and estuaries. In such places, populations of species with long and short term life expectancies (e.g. Fabulina fabula, Echinocardium cordatum and Ampelisca sp.) either vanished or displayed long term decline following the Amoco Cadiz oil spill. Polychaetes, however, including Nephtys hombergii, cirratulids and capitellids were largely unaffected.
Dauvin (1998) reported the effects of the Amoco Cadiz spill on the fine sand community in the Bay of Morlaix. Reductions in abundance, biomass and production of the community were very evident through the disappearance of the dominant populations of the amphipods Ampelisca sp. which are very intolerant of oil contamination. 2 weeks after the spill, the level of hydrocarbons in subtidal sediments reached 200 ppm (Dauvin, 1984; cited in Poggiale & Dauvin, 2001). This caused the disappearance of the Ampelisca populations, leaving behind a single species, Ampelisca sarsi, in very low densities. The sediment rapidly depolluted and in 1981 benthic recruitment occurred in normal conditions (Dauvin, 1998). However, the recovery of the Ampelisca populations took up to 15 years. This was probably due to the amphipods' low fecundity, lack of pelagic larvae and the absence of local unperturbed source populations (Poggiale & Dauvin, 2001).
Echinoderms also seem to be especially intolerant of the toxic effects of oil, probably because of the large amount of exposed epidermis (Suchanek, 1993). The high intolerance of Echinocardium cordatum to hydrocarbons was seen by the mass mortality of animals, down to about 20m depth, shortly after the Amoco Cadiz oil spill (Cabioch et al., 1978).
Many species in the biotope are highly intolerant of hydrocarbon contamination. However, the biotope occurs subtidally in low energy environments and so the majority of the biotope is likely to remain unaffected. Biotope intolerance is therefore assessed as intermediate with no change in species richness. Recoverability is likely to be limited by slow recovering species such as the amphipods. However, persistence of local populations should ensure that recovery occurs within 5 years and so recoverability is recorded as high.
Radionuclide contamination
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Very little information exists concerning the effects of radioactivity on marine species. Stamouli & Papadapoulou (1990) investigated bioaccumulation of radioactive trivalent Chromium 51 (Cr-51) in a venerid bivalve species from Greece. Cr-51 is derived from nuclear tests, disposal of radioactive waste and is one of the principal corrosion products of nuclear powered ships. Cr-51 was found to rapidly accumulate in the venerid species, predominantly in the shell, and reached a stable level in 8 days. No mortality was reported after 20 days. Similar accumulation has also been described for Venus verrucosa (Stamouli & Papadapoulou, 1990). There is insufficient information to assess the intolerance of 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 some species tolerant of nutrient enrichment, such as the polychaete Capitella capitata (Pearson & Rosenberg, 1978). It is likely that such species would increase in abundance following nutrient enrichment (Elliott, 1994), with an associated decline in suspension feeding species, such as the venerid bivalves, and organisms adapted to low nutrient levels, such as Magelona mirabilis (Niermann, 1996). In extreme cases of eutrophication, sediments may become anoxic and defaunated (Elliott, 1994). Biotope intolerance is therefore assessed as intermediate. Recoverability is recorded as high (see additional information below). As suggested above, the major change is likely to be a shift in community composition but there may also be a minor decline in species richness, with the intolerant suspension feeders most likely to be eradicated.
Increase in salinity
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IGS.FabMag occurs in 'full' salinity conditions (Connor et al., 1997a) and therefore increase in salinity is not a relevant factor. No information was found concerning the intolerance of the characterizing species to hypersaline conditions.
Decrease in salinity
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IGS.FabMag occurs in 'full' salinity conditions (Connor et al., 1997a) and therefore is likely to be intolerant of salinity decreases in some way. The benchmark decrease in salinity would place the biotope in areas of variable salinity for one year or reduced salinity for one week. Biotope intolerance is likely to be dependent on the intolerance of the characterizing species. Soemodinoto et al. (1995) reported that Magelona sp. from a brackish lagoon in Java showed a positive correlation with salinity, i.e. the species decreased in abundance with increased freshwater influx. The population did not survive a reduction in salinity from 20 psu to 3 psu over a 5 month period. Salzwedel (1979) reported that the Fabulina fabula does occur in variable salinity conditions (down to 20 psu) but that growth is inhibited. It seems likely that the benchmark reduction in salinity would result in some mortality of the characterizing species and so intolerance is assessed as intermediate. Recoverability is recorded as high (see additional information below). Echinoderms are considered to be stenohaline animals that lack the ability to osmoregulate (Stickle & Diehl, 1987) and there may therefore be some decline in species richness.
Changes in oxygenation
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Niermann et al. (1990) reported the changes in a fine sand community from the German Bight in an area with regular seasonal hypoxia. In 1983, oxygen tension fell to exceptionally low levels; < 3 mg O2/dm3 in large areas and < 1 mg O2/dm3 in some places. Species richness was reduced by 30-50% following this event and overall biomass was reduced. Niermann et al. (1990) reported that Fabulina fabula and Magelona sp. remained abundant during the period of hypoxia, and, in fact, decreased slightly in abundance on resumption of normoxia. The important characterizing species appear, therefore, to be 'not sensitive' to the benchmark level of hypoxia of 2 mg O2/l for one week, although there is likely to be a decline in species richness. Species which were reduced in abundance significantly by the hypoxia included the polychaetes Owenia fusiformis, Lanice conchilega and Spio filicornis and the echinoderms Ophiura albida and Echinocardium cordatum.

Biological Factors

Introduction of microbial pathogens/parasites
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Individuals of Fabulina fabula from Boulogne-sur-Mer (studied as Angulus fabula) were infected with the trematode parasite Gymnophallus strigatus, causing erosion of the shell (Giard, 1897, cited in Kinne, 1983). No indication of the possible effects of the infection were given, but it would be likely to increase mortality through shell weakening. Dolgikh (1968a, cited in Kinne, 1983) reported infection of Chamelea gallina from the Black Sea by the digenean trematode Bacciger bacciger, which is known to cause castration and depletion of body reserves. Chamelea gallina is a host for a number of other trematode species, including Gymnophallus rostratus. Pathological effects of infection include shell erosion, paralysis of adductor muscles, general debilitation and death.
No information was found concerning infection of the polychaete, Magelona mirabilis. The bivalves present in the biotope appear to be vulnerable to infection by parasites and are likely to suffer some mortality. Biotope intolerance is therefore assessed as intermediate. Recoverability is recorded as high (see additional information below).
Introduction of non-native species
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No information was found concerning the intolerance of the biotope to invasion by alien species. However, no species have been identified which pose a particular threat to the biotope and so it is likely to be 'not sensitive'.
Extraction
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It is extremely unlikely that any of the species indicative of sensitivity would be targeted for extraction. However, the biotope is potentially at risk from fishing activities on sandy substrata, e.g. dredging for scallops (Eleftheriou & Robertson, 1992), beam trawling for flatfish, and extraction of sand by the aggregate industry (Eno, 1991). Venerid bivalves are generally shallow burrowers and Fabulina fabula has a fragile shell (Fish & Fish, 1996). The bivalves that characterize the biotope may therefore be damaged by bottom fishing (see Physical Disturbance for further details). The polychaete, Magelona mirabilis, is a soft bodied organism which lives within a few centimetres of the sediment surface and exposes its palps at the surface while feeding. It is, therefore, also likely to be damaged by the benchmark physical abrasion.

It seems likely that the characterizing species will suffer some mortality and intolerance is assessed as intermediate. Recoverability is recorded as high (see additional information below). Particularly vulnerable forms, such as the epifaunal echinoderms, may be eliminated so there may be a minor decline in species richness in the biotope.

Additional information icon Additional information

Recoverability
Niermann et al. (1990) studied the recovery of a fine sand Fabulina fabula community from the German Bight following a severe hypoxia event. Re-establishment of faunal composition took approximately 8 months, but biomass did not fully recover for approximately 2 years. However, some of the climax species, including Fabulina fabula, were least affected by the hypoxia and therefore did not limit the recovery of the biotope.

Diaz-Castaneda et al. (1989) studied the colonization of defaunated sediments from a Venus community in Dunkerque Harbour, France. The number of species in the experimental substrata increased progressively and reached a stabilized value similar to the number in the surrounding community within 13 to 17 weeks in spring and summer and 16 to 24 weeks in autumn and winter. It was noted that biomass took much longer to recover than species richness as most colonizers were young and small. Indeed, larval recruitment accounted for 70% of colonizers, suggesting that biotope recoverability is likely to be governed by larval dispersal rather than migration of adults. The last species in the successional sequence to establish themselves were equilibrium species such as Fabulina fabula, Nephtys hombergii and venerid bivalves.

The life history characteristics of the species which characterize the biotope suggest that the biotope would recover from major perturbations within 5 years. Experimental studies support this conclusion and hence biotope recoverability is assessed as high. As biotope recoverability is largely dependent on larval recruitment, recoverability is not likely to be significantly more rapid in instances of intermediate biotope intolerance versus high intolerance.


This review can be cited as follows:

Rayment, W.J. 2006. Fabulina fabula and Magelona mirabilis with venerid bivalves in infralittoral compacted fine sand. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 18/04/2014]. Available from: <http://www.marlin.ac.uk/habitatbenchmarks.php?habitatid=142&code=1997>