Biodiversity & Conservation

SS.SCS.CCS.MedLumVen

Explanation of sensitivity and recoverability


Physical Factors

Substratum Loss
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Removal of the substratum would also remove entire populations of the infauna and sessile epifauna in the biotope. Intolerance is therefore assessed as high and there would be a major decline in species richness. Recoverability is assessed as high (see additional below).
Smothering
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The venerid bivalves are shallow burrowing infauna. They are active suspension feeders and therefore require their siphons to be above the sediment surface in order to maintain a feeding and respiration current. 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. Smothering will result in temporary cessation of feeding and respiration. The energetic cost may impair growth and reproduction but is unlikely to cause mortality. Biotope intolerance is therefore assessed as low. The effect on growth and reproduction will probably not extend beyond 6 months and therefore recoverability is assessed as very high. Similarly, the other infaunal species in the biotope are likely to be able to relocate to their preferred depth with only minor energetic cost. Spatangus purpureus, for example, together with species in similar biotopes (for instance Neopentadactyla mixta and Branchiostoma lanceolatum) are mobile and would burrow upwards. The species which will be most affected by smothering are the sessile epifauna, such as Hydroides norvegica. The species would not be able to relocate following smothering and would not be able to feed or respire. There is therefore likely to be a minor decline in species richness in the biotope.
Increase in suspended sediment
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The venerid bivalves are active suspension feeders, trapping food particles on their gill filaments (ctenidia). An increase in suspended sediment is therefore likely to affect both feeding and respiration by potentially clogging the ctenidia. Other suspension feeding bivalves are able to clear their feeding and respiration structures, although at some energetic cost (e.g. Grant & Thorpe, 1991; Navarro & Widdows, 1997) and it seems likely that the same would apply to the venerids. According to the benchmark, the increase in suspended sediment persists for a month and no mortality of suspension feeders is expected in this time. Intolerance of the biotope is therefore assessed as low. When suspended sediment returns to original levels, metabolic activity should quickly return to normal and recoverability is assessed as very high. An increase in suspended sediment is likely to lead to an increase in siltation and therefore a greater proportion of fine sediments in the substratum. This would tend to favour the deposit feeders in the biotope and there may be a shift in community composition away from suspension feeders. However, over the benchmark period of one month there is not likely to be any decline in species richness.
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 immediate. 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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The biotope occurs below 30 m depth and therefore desiccation is never likely to be a relevant factor.
Increase in emergence regime
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The biotope occurs sublittorally below 30 m depth and therefore change in emergence regime is not a relevant factor.
Decrease in emergence regime
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The biotope occurs sublittorally below 30 m depth and therefore change in emergence regime is not a relevant factor.
Increase in water flow rate
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CGS.Ven occurs in areas of 'moderately strong' or 'weak' flow (see glossary) (Connor et al., 1997a). The benchmark change in water flow rate would place the biotope in areas of 'strong' or 'very strong' flow for one year. The increased water flow rate will change the sediment characteristics in which the biotope occurs, primarily by re-suspending and preventing deposition of finer particles, and may also create a high sediment mobility (Hiscock, 1983). The habitat would therefore become less suitable for burrowing deposit feeders, e.g. Spatangus purpureus, due to the change in substratum characteristics and decreased food supply. The very strong tidal stream would also place the suspension feeders outside their habitat preferences and it is likely that there would be some energetic cost, probably due to interference with respiration and feeding. These changes are likely to result in some mortality, particularly of deposit feeders, and a decline in species richness. Recoverability is assessed as high (see additional information below).
Decrease in water flow rate
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CGS.Ven occurs in areas of 'moderately strong' or 'weak' flow (see glossary) (Connor et al., 1997a). The benchmark change in water flow rate would place the biotope in areas of 'very weak' flow for one year. The venerid bivalves 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). The community is likely to undergo a shift in composition with deposit feeders becoming more prevalent.
Increase in temperature
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All the venerid species in the biotope have a wide geographical range and occur at least as far south as the Mediterranean (Hayward et al., 1996). They must therefore be able to tolerate higher temperatures than are experienced in biotopes in Britain and Ireland and would not be likely to be intolerant of chronic temperature increase. Thouzeau et al. (1996) reported that the Venus fasciata community of the western English channel experienced an annual temperature range of 9 °C to 15.4 °C. The species are more likely to be intolerant of an acute temperature increase which may cause physiological disruption and hence affect growth and reproduction. Biotope intolerance is therefore recorded as low. It is possible that increased temperature will result in enhanced growth rates of species in the biotope, as occurs in the heart urchin, Echinocardium cordatum (Duineveld & Jenness, 1984). As soon as temperatures return to their original levels, physiological function should return to normal and hence recoverability is recorded as immediate. It should be noted that the biotope occurs below 30m depth and so will tend to be buffered from temperature change.
Decrease in temperature
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All the venerid species in the biotope have a wide geographical range and occur at least as far north as Norway (Hayward et al., 1996). They must therefore be able to tolerate lower temperatures than are experienced in biotopes in Britain and Ireland and would not be likely to be intolerant of chronic temperature decrease. The species are more likely to be intolerant of acute temperature decreases. Partial mortality of Venus striatula was reported from Wales in the harsh winter of 1962-3 when temperatures were 5-6 °C below normal for a period of 2 months (Crisp (ed.), 1964). However, the affected populations were likely to be from shallow water. During the same period, the heart urchin, Echinocardium cordatum, was almost completely eliminated from the German Bight to a depth of about 20 m (Lawrence, 1996) and very heavy mortality was observed in the English Channel and North Sea (Crisp (ed.), 1964). The biotope intolerance is therefore recorded as intermediate and there is likely to be a minor decline in species richness. Recoverability is recorded as high (see additional information below). Again, it should be noted that the depth at which the biotope occurs means it is likely to be buffered from temperature change.
Increase in turbidity
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CGS.Ven occurs below 30 m depth and so there is unlikely to be significant primary production in the biotope. An increase in turbidity is therefore not likely to have a significant effect on the biotope directly. The benthic fauna rely on nutrient input from pelagic and coastal fringe production (Barnes & Hughes, 1992). Increased turbidity in these areas may reduce primary production and consequently reduce the food supply to the circalittoral benthos. The fauna in the CGS.Ven biotope may therefore suffer decreased growth and reproduction. However, the nutrient input to the biotope originates from a very wide area and the decrease in food supply is not likely to cause mortality over a year so the biotope intolerance is assessed as low. Primary production will quickly return to normal levels when turbidity decreases so recoverability is assessed as very high.
Decrease in turbidity
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CGS.Ven occurs below 30 m depth and so there is unlikely to be significant primary production in the biotope. A decrease in turbidity in the water column above the biotope may result in increased production by phytoplankton and therefore a potential increase in food supply to the benthic suspension and deposit feeders. However, it is not likely that there would be any significant effect over a year and so the biotope is assessed as not sensitive.
Increase in wave exposure
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The benchmark increase in wave exposure would place some of the biotope in the 'extremely exposed' category (see glossary) (Connor et al., 1997a). Oscillatory water movement occurs down to about 60 m 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, e.g. Spatangus purpureus, and a decrease in food availability for deposit feeders;
  • gravel is likely to be moved by strong wave action resulting in damage and displacement of infauna and epifauna;
  • 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.
The above considerations are likely to result in some mortality of many species, including the venerid bivalves and therefore biotope intolerance is assessed as intermediate with a 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 'sheltered' or 'very 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 at the expense of the suspension feeders, such as the venerid bivalves and epifaunal tubeworms. 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. 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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Ramsay et al. (2000b) investigated physical disturbance using dog cockles, Glycymeris glycymeris, as indicators. They reported that the incidence of scars on the shells was significantly higher in areas heavily exploited by beam trawlers and concluded that trawling causes damage and possibly mortality of these robust bivalves. 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. Overall, species with brittle, hard tests are regarded to be sensitive to impact with scallop dredges (Kaiser & Spencer, 1995; Bradshaw et al., 2000).The echinoid, Spatangus purpureus, is particularly susceptible to physical abrasion. Damage and mortality caused by beam trawling has been reported by Kaiser & Spencer (1994b) and Evans et al. (1996a). In both reports, damaged urchins were opportunistically predated by fish and mobile epifauna. Biotope intolerance is therefore recorded as intermediate. Recoverability is assessed as high (see additional information below). It is unlikely that any species would be eradicated from the biotope and hence there would be no change in species richness.
Displacement
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The majority of the infauna in the biotope is likely to be able to rebury following displacement to the sediment surface, for example, following washing out by a storm event. This has been observed in the venerid bivalve, Venerupis senegalensis (Kaschl & Carballeira, 1999), and the heart urchin, Echinocardium cordatum. However, while at the sediment surface, the infauna are vulnerable to predation and it is likely that some mortality would occur. Biotope intolerance is therefore recorded as intermediate. Recoverability is assessed as high (see additional information below). Permanently attached species, such as the tube worm Hydroides norvegica, would not be able to reattach following displacement and hence there would be a minor decline in species richness in the biotope.

Chemical Factors

Synthetic compound contamination
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No evidence was found concerning the intolerance of the venerid bivalves in the biotope. However, Beaumont et al. (1989) concluded that bivalves in general 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. Furthermore, there is evidence that TBT causes recruitment failure in bivalves, either due to reproductive failure or larval mortality (Bryan & Gibbs, 1991). The intolerance of Spatangus purpureus, would be expected to be similar to another heart urchin, Echinocardium cordatum. 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 222 ng Sn/l in pore water and 1594ng Sn/g dry weight of sediment (Stronkhorst et al., 1999). Given the likely intolerance of the venerid bivalves, biotope intolerance is assessed as high and species richness is expected to decline. Recoverability is recorded as high (see additional information below).
Heavy metal contamination
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Bryan (1984) suggests that the larval and embryonic stages of bivalves are particularly intolerant of heavy metal contamination. Kaschl & Carballeira (1999) investigated the effect of sediment contamination on the venerid bivalve, Venerupis senegalensis by exposing the species to sediments spiked with copper sulphate. Slowing of clam burial correlated positively with copper concentration and at very high concentrations, clams closed up and did not bury at all. Spiking of the sediments with copper also resulted in re-emergence between 24 and 120 hours after burial, a behaviour not observed in controls. 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 venerid bivalves, biotope intolerance is assessed as high and species richness is expected to decline. Recoverability is recorded as high (see additional information below).
Hydrocarbon contamination
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Suchanek (1993) reviewed the effects of oil on bivalves. Sublethal concentrations may produce substantially reduced feeding rates and/or food detection ability, probably due to ciliary inhibition. Respiration rates have increased at low concentrations and decreased at high concentrations. 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. Axiak et al. (1988) investigated the physiological response of Venus verrucosa to oil contamination. Long term exposure (150 days) to low levels (100 µg/l) of the water accommodated fraction of oil resulted in atrophy of digestive cells and reduced membrane stability. These responses caused a significant reduction in the scope for growth. Exposure to high concentrations of oil (420 µg/l) resulted in increased cellular volume, damage to the epithelial lining of the foot, stomach and style sac and atrophy of digestive cells. These responses were detected after 144 hours of exposure and would be expected to result in mortality.
Dauvin (1998) reported the effects of the Amoco Cadiz oil spill on the fine sand Abra alba 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 sensitive to oil contamination. The spill occurred in 1978 and after 2 weeks, 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 sensitive to 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, shortly after the Amoco Cadiz oil spill (Cabioch et al., 1978).
The biotope generally appears to be very intolerant of pollution by hydrocarbons and so intolerance is assessed as high and there would be a decline in species richness. The majority of the species are likely to recover relatively quickly, with the exception of the amphipods Ampelisca sp., and so recoverability is recorded as high (see additional information below).
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 Venerupis 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 Venerupis sp., 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 capitellid polychaete Notomastus latericeus (Pearson & Rosenberg, 1978). It is likely that such species would increase in abundance following nutrient enrichment, with an associated decline in suspension feeding species such as the venerid bivalves. 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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CGS.Ven occurs in the open sea in fully saline conditions. An increase in salinity is therefore not likely to be a relevant factor. No information was found concerning the reaction of the characterizing species to hypersaline conditions.
Decrease in salinity
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Little evidence was found concerning the salinity tolerance of the venerid bivalves which occur in the biotope. Mohan & Velayudhan (1998) reported that the venerid, Paphia malabarica, tolerates salinities between 12 and 40 psu, although the lower limit was based on 50% mortality. Lange (1972) reported that the muscle volume of Venerupis rhomboides, a stenohaline species, increased as salinity decreased, and hence concluded that the species was unable to regulate its muscle volume. Venerupis japonica was active down to 20 psu and suffered mortality if salinity remained below 14 psu for an extended period (Yaroslavtseva & Fedoseeva, 1978).
Binyon (1981) reported that the cephalochordate, Branchiostoma lanceolatum, was intolerant of decreases in salinity. At 18 psu, flagella activity and gill cilia activity ceased abruptly, but the animal was able to remain alive for several weeks, presumably by mobilizing food reserves to maintain a depressed metabolic state. Binyon concluded that Branchiostoma lanceolatum was a fully oceanic species that would be unable to colonize estuarine environments.
The benchmark reduction in salinity would be to 'variable' salinity for a year or 'reduced' salinity for a week. Given that the biotope only occurs in fully saline conditions and considering the likely intolerance of the characterizing species, it is likely that significant mortality would result from a reduction in salinity and it is unlikely that the biotope would persist. Intolerance is therefore assessed as high, with a major decline in species richness. Recoverability is recorded as high (see additional information below).
Changes in oxygenation
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No information was found concerning the intolerance of the venerid bivalves in the biotope to reductions in dissolved oxygen. Brand & Morris (1984) recorded that Glycymeris glycymeris was very tolerant of anoxia. Individuals survived for more than 4 weeks in anoxic water and under normoxic conditions spent long periods buried where aerobic metabolism was reduced or absent. Glycymeris glycymeris responded to hypoxia by increased ventilation rate and a large increase in oxygen utilization. Oeschger (1990) recorded a similar tolerance of Astarte borealis to anoxia. In the Kiel Bight, it experienced long periods of oxygen deficiency and was one of the only species which did not experience mortality. Astarte borealis survived anoxia for 60 days by switching to anaerobiosis and is among the species with the highest LT50 values known in marine invertebrates.
The spatangoid echinoids, on the other hand, are very intolerant of deoxygenation. In the south-eastern North Sea a period of reduced oxygen resulted in the death of many individuals of Echinocardium cordatum (Niermann, 1997) and during periods of hypoxia the species migrates to the surface of the sediment (Diaz & Rosenberg, 1995). At 4mg/l individuals appeared on the sediment surface and many were dead at a concentration of 2.4mg/l (Nilsson & Rosenberg, 1994).
Despite the tolerance of the larger bivalves in the biotope to deoxygenation, growth and reproduction are still likely to be compromised and so intolerance is assessed as low. Growth and reproduction should rapidly return to normal when normoxic conditions are restored so recoverability is recorded as very high. Very intolerant species, such as Spatangus purpureus, are likely to be eliminated from the biotope.

Biological Factors

Introduction of microbial pathogens/parasites
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Clausinella fasciata (Venus fasciata) has been recorded as a host for the trematode Cercaria longicaudata which develops in colourless immobile sporocysts in the clam's visceral mass, causing parasitic castration (Kinne, 1983). The ciliate Spenophyra dosiniae has been recorded from Timoclea ovata (Venus ovata) (Chatton & Lwoff, 1950; cited in Kinne, 1983). Other venerids, such as Venerupis senegalensis, often have high loads of parasites, including flagellate and ciliate microbes, turbellarians and trematodes, which have been proven to cause mortality (Navas et al., 1992). It is likely that the venerid bivalves will suffer some mortality due to microbial infection and so biotope intolerance is recorded as intermediate with a minor decline in species richness. Recoverability is assessed 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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Some of the bivalves, e.g. Glycymeris glycymeris, are exported to mainland Europe for human consumption (Hayward et al., 1996) but it is unclear whether their extraction is targeted or as a result of bycatch from beam trawling. Fishing activity targeting fish species which may occur in the biotope is likely to result in incidental damage to the biotope's characterizing species. For example, damage and mortality of Glycymeris glycymeris (Ramsay et al., 2000b) and Spatangus purpureus (Kaiser & Spencer, 1994; Evans et al., 1996) have been reported as a result of beam trawling. Ramsay et al. (2000b) investigated physical disturbance using dog cockles, Glycymeris glycymeris, as indicators. They reported that the incidence of scars on the shells was significantly higher in areas heavily exploited by beam trawlers and concluded that trawling causes damage and possibly mortality of these robust bivalves. 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. Overall, species with brittle, hard tests are regarded to be sensitive to impact with scallop dredges (Kaiser & Spencer, 1995; Bradshaw et al., 2000).The echinoid, Spatangus purpureus, is particularly susceptible to physical abrasion. Damage and mortality caused by beam trawling has been reported by Kaiser & Spencer (1994b) and Evans et al. (1996a). In both reports, damaged urchins were opportunistically predated by fish and mobile epifauna. Biotope intolerance is therefore recorded as intermediate. Recoverability is assessed as high (see additional information below). It is unlikely that any species would be eradicated from the biotope and hence there would be no change in species richness.

Additional information icon Additional information

Recoverability
  • The venerid bivalves in the biotope reach sexual maturity within 2 years, spawn at least once a year and have a pelagic dispersal phase (Guillou & Sauriau, 1985; Dauvin, 1985). No information was found concerning number of gametes produced, but the number is likely to be high as with other bivalves exhibiting planktotrophic development (Olafsson et al., 1994). Their powers of recoverability are therefore likely to be high and it is expected that, following mortality, the populations would recover within 5 years.
  • The purple heart urchin, Spatangus purpureus, is a broadcast spawner and disperses via a pelagic larva (Fish & Fish, 1996). It is likely that recoverability would be similar to another heart urchin, Echinocardium cordatum. The first re-population of Echinocardium cordatum following the Torrey Canyon oil spill was recorded after two years (Southward & Southward, 1978). However, Buchanan (1967) observed that recruitment was sporadic, occurring in only 3 years out of 10.
  • For all the shallow burrowing infauna, an important factor contributing to recoverability may be bedload sediment transport (Emerson & Grant, 1991). It has been demonstrated to account for changes in densities of the clam, Mya arenaria, and suggested that it may affect recruitment in other infaunal bivalves and polychaetes (Emerson & Grant, 1991).
  • Based on the likely recoverability of the venerid bivalves, recoverability of the biotope is assessed as high.

This review can be cited as follows:

Rayment, W.J. 2001. Venerid bivalves in circalittoral coarse sand or gravel. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 01/09/2014]. Available from: <http://www.marlin.ac.uk/habitatbenchmarks.php?habitatid=63&code=2004>