Biodiversity & Conservation

SS.SMx.IMx.VsenAsquAps

Explanation of sensitivity and recoverability


Physical Factors

Substratum Loss
(View Benchmark)
Removal of the substratum would remove entire populations of infauna, epifauna and macroalgae. Intolerance is therefore assessed as high and there would be a major decline in species richness. Recoverability is assessed as high (see additional information below).
Smothering
(View Benchmark)
Venerupis senegalensis typically burrows to a depth of 3-5 cm and is often attached to small stones or shell fragments by byssal threads. It is an active suspension feeder and therefore requires its 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. This is likely to apply to the proportion of the Venerupis senegalensis population which is not firmly attached by byssal threads. However, those individuals which are attached may be inhibited from relocating rapidly following smothering with 5 cm of sediment and some mortality is expected to occur.

Emerson et al. (1990) examined smothering and burrowing of Mya arenaria after clam harvesting. Significant mortality (2 -60%) in small and large clams occurred only at burial depths of 50 cm or more in sandy substrates. However, they suggested that in mud, clams buried under 25 cm of sediment would almost certainly die. Dow & Wallace (1961) noted that large mortalities in clam beds resulted from smothering by blankets of algae (Ulva sp.) or mussels (Mytilus edulis). In addition, clam beds have been lost due to smothering by 6 cm of sawdust, thin layers of eroded clay material, and shifting sand (moved by water flow or storms) in the intertidal. The more mobile burrowing infauna, such as polychaetes, are likely to be able to relocate to their preferred depth following smothering with little or no loss of fitness.

Due to their requirement for light for photosynthesis, macroalgae, and especially the encrusting and low growing species such as the Corallinaceae, are likely to be highly intolerant of smothering.

Due to the intolerance of the important characterizing species, Venerupis senegalensis, intolerance for the biotope is assessed as intermediate. Populations of epifauna and macroalgae may be lost so species richness is expected to decline. Recoverability is recorded as high (see additional information below).

Increase in suspended sediment
(View Benchmark)
Venerupis senegalensis is an active suspension feeder, 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 Venerupis senegalensis would also be able to clear its feeding and respiration structures, although at high particle concentrations there may be some energetic cost. An energetic cost resulting from increased suspended sediment has also been suggested for other bivalves which occur in the biotope, for example Mya arenaria (Grant & Thorpe, 1991) and Cerastoderma edule (Navarro & Widows, 1997). 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 would probably result in an increased rate of siltation. The extent of substratum suitable for the epifauna in the biotope would decrease and encrusting macroalgae would become smothered. There is therefore likely to be a minor decline in species richness.

Decrease in suspended sediment
(View Benchmark)
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
(View Benchmark)
The majority of the species in the biotope, including Venerupis senegalensis, live infaunally in muddy sand and gravel, a substratum with a high water content, and are therefore protected from desiccation stress. Additionally, most bivalves, including Venerupis senegalensis, are able to respond to desiccation stress by valve adduction during periods of emersion. It is likely that these species would be able to retain enough water in the shell to prevent mortality during the benchmark exposure period of one hour. However, during the period of emersion, the species would not be able to feed and respiration would be compromised, so there is likely to be some energetic cost. Biotope intolerance is therefore recorded as low. On immersion, metabolic activity should quickly return to normal so recoverability is recorded as very high. Other bivalves, including Mya arenaria and Ensis ensis, have gaping shells and are therefore more intolerant of desiccation.
Increase in emergence regime
(View Benchmark)
The biotope occurs on the extreme lower shore (Connor et al., 1997a) and so is vulnerable to an increase in emergence. The fact that the biotope does not occur further up the shore suggests that the characterizing species must be limited by one or more factors including desiccation, temperature and wave exposure. The benchmark for emergence is an increase in exposure for one hour every tidal cycle for a year. During this time, exposed marine species will not be able to feed and respiration will be compromised. 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 and therefore intolerance is recorded as intermediate. Some species will be more sensitive than others. Littorina littorea, for example, is relatively tolerant of increases in emergence as it is mobile and has behavioural adaptations to counter desiccation. Recoverability is recorded as high (see additional information below).
Decrease in emergence regime
(View Benchmark)
The majority of the biotope occurs in the shallow subtidal (Connor et al., 1997a) and so is not likely to be intolerant of a decrease in emergence regime. It is possible that a decrease in emergence regime would allow the biotope to extend further up the shore.
Increase in water flow rate
(View Benchmark)
IMX.VsenMtru occurs in wave protected areas where water flow is typically "weak" (Connor et al., 1997a). An increase in water flow of 2 categories would place the biotope in areas of "strong" flow. The increase would 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 Venerupis senegalensis, would be outside their habitat preferences and some mortality would be likely to occur, probably due to interference with feeding and respiration. Additionally, the consequent lack of deposition of particulate matter at the sediment surface would reduce food availability for the deposit feeders in the biotope. The resultant energetic cost over one year would also be likely to result in some mortality. A biotope intolerance of intermediate is therefore recorded and species richness is expected to decline. Recoverability is assessed as high (see additional information below). The expected change in sediment composition would favour the epifauna and macroalgae which would probably become more abundant.
Decrease in water flow rate
(View Benchmark)
IMX.VsenMtru occurs in low energy environments such as sheltered beaches where the water flow is typically "weak" (Connor et al., 1997a). The majority of species in the biotope are infaunal and are capable of generating their own respiration and feeding currents. These species are unlikely to be intolerant of a decrease in water flow rate. However, decreased water flow rate is likely to lead to increased deposition of fine sediment (Hiscock, 1983) and therefore decreased availability of suitable substrata for the attachment of macroalgae and epifauna. There may, therefore, be a minor decline in species richness in the biotope.
Increase in temperature
(View Benchmark)
The temperature intolerance of the biotope is largely dependent on the intolerance of the important characterizing species. The geographic range of Venerupis senegalensis extends to northern Africa. Therefore, the species must be capable of surviving in higher temperatures than it experiences in Britain and Ireland and thus would be expected to tolerate temperature change over an extended period. A population of Venerupis corrugatus endured a temperature rise from 13 to 18°C over 5 hours in a rockpool and then a drop to 14°C following inundation by the tide, with no obvious ill effects (Stenton-Dozey & Brown, 1994). Albentosa et al. (1994) found that scope for growth of Venerupis senegalensis increases to an optimum at 20°C and then declines. Hence, it is expected that Venerupis senegalensis would be able to tolerate a long term, chronic temperature increase and a short term acute change with no mortality. However, a rapid increase in temperature may result in sub-optimal conditions for growth and reproduction and therefore intolerance of the biotope is assessed as low. Metabolic activity should return to normal when temperatures decrease and so recoverability is assessed as very high. The intolerance of other species in the biotope is variable. Epifauna and macroalgae which occur in the intertidal tend to be quite tolerant of temperature change. Littorina littorea, for example, occurs in upper shore rockpools where temperatures may exceed 30°C. The infauna may be less tolerant of temperature change per se, e.g. upper median lethal temperature for Cerastoderma edule is 29°C after 96 hrs exposure (Ansell et al., 1981), but are less likely to experience rapid changes in temperature due to being buried in sediment.
Decrease in temperature
(View Benchmark)
The temperature intolerance of the biotope is largely dependent on the intolerance of the important characterizing species. The geographic range of Venerupis senegalensis extends to northern Norway. Therefore, the species must be capable of survival at lower temperatures than it does in Britain and Ireland and would be expected to tolerate a chronic temperature decrease over an extended period. However, in the harsh British winter of 1962-63, when the south coast experienced temperatures 5-6°C below average for a period of 2 months, Venerupis senegalensis (studied as Venerupis pullastra) suffered 50% mortality around the Isle of Wight and near 100% mortality in Poole Harbour (Waugh, 1964). The species is less tolerant therefore of acute decreases in temperature and a biotope intolerance of intermediate is recorded. Recoverability is recorded as high (see additional information below). Other species which suffered significant mortality during the winter of 1962-63 include Cerastoderma edule, Ensis ensis and Gibbula cineraria (Crisp, 1964). It is expected that there will be a minor decline in species richness in the biotope.
Increase in turbidity
(View Benchmark)
IMX.VsenMtru occurs in relatively turbid 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 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. Macroalgae are likely to be most affected by an increase in turbidity. There may therefore be a minor decline in species richness.
Decrease in turbidity
(View Benchmark)
A decrease in turbidity will mean more light is available for photosynthesis by macroalgae, phytoplankton in the water column and microphytobenthos on the sediment surface. This would increase the primary production in the biotope and may mean greater food availability for grazers, suspension feeders and deposit feeders. There may be a consequent proliferation of epifauna and macroalgae at the expense the previously dominant infauna.
Increase in wave exposure
(View Benchmark)
IMX.VsenMtru occurs in sheltered inlets and sea lochs 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, e.g. Venerupis senegalensis, 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. Species may be damaged or dislodged by scouring from sand and gravel mobilized by increased wave action. For example, large macroalgae, such as Fucus serratus, are particularly vulnerable and are likely to suffer damaged fronds and dislodged plants. 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
(View Benchmark)
IMX.VsenMtru occurs in "extremely sheltered" environments (Connor et al., 1997a). The biotope, therefore, is unlikely to be intolerant of a further decrease in wave exposure and species richness is unlikely to change. However, it should be noted that decreased wave exposure will lead to changes in oxygenation and increased risk of smothering due to siltation. These factors are discussed in their relevant sections.
Noise
(View Benchmark)
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
(View Benchmark)
The majority of the species in the biotope, including Venerupis senegalensis, have very little or no visual acuity, and are therefore unlikely to be intolerant of visual disturbance. Some species, however, respond to visual disturbance by withdrawal of feeding structures and are therefore likely to experience some energetic cost through loss of feeding opportunities. Aphelochaeta marioni, for example, feeds only at night, and responds to sudden light pollution by the retraction of palps and cirri and cessation of all activity for some minutes (Farke, 1979).
Abrasion & physical disturbance
(View Benchmark)
Many species in the biotope are vulnerable to physical abrasion. 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 scallop dredge. Despite their robust body form, bivalves are also vulnerable. 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). Robust bodied or thick shelled 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). Epifauna and macroalgae risk being damaged and/or dislodged by physical abrasion. Some mortality is likely to result from physical abrasion so intolerance is recorded as intermediate and species richness may suffer a minor decline. Recoverability is assessed as high (see additional information below).
Displacement
(View Benchmark)
Venerupis senegalensis is the only important characterizing species in the biotope. When displaced and returned to the surface of the substratum, it is able to re-bury itself (e.g. Kaschl & Carballeira, 1999). This probably occurs naturally due to shifting sediments caused by storms. However, while exposed at the sediment surface, the species is more vulnerable to predation and some mortality may occur. Intolerance is therefore recorded as intermediate. Recoverability is recorded as high (see additional information below).

Chemical Factors

Synthetic compound contamination
(View Benchmark)
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. Furthermore, there is evidence that TBT causes recruitment failure in bivalves, either due to reproductive failure or larval mortality (Bryan & Gibbs, 1991). Beaumont et al. (1989) also concluded that TBT had a detrimental effect on the larval and/or juvenile stages of infaunal polychaetes. Collier & Pinn (1998) investigated the effect on the benthos of ivermectin, a feed additive treatment for infestations of sea-lice on farmed salmonids and a common contaminant in sea lochs. 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. Given the intolerance of infaunal bivalves and polychaetes, overall biotope intolerance is assessed as high and there is likely to be a decline in species richness in the biotope. Recoverability is assessed as high (see additional information below).
Heavy metal contamination
(View Benchmark)
Kaschl & Carballeira (1999) investigated the effect of sediment contamination on Venerupis senegalensis (studied as Venerupis pullastra) by exposing the species to sediments spiked with copper sulphate. Following placement of clams on the sediment surface, slowing of burial was observed in proportion to the concentration of copper added to the sediment. The effect was detectable at a pore water concentration of 95 µg Cu/l. At the highest copper concentrations (spiking solution concentration > 125 mg Cu/l), the majority of clams closed up and did not bury. 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 proportion of clams re-emerging increased with the copper concentration in the sediment, and was concluded to be an avoidance behaviour. Kaschl & Carballeira (1999) suggested that the delay in burial at low copper concentrations was due to physiological disruption as it did not avoid exposure to the toxin and further increased the risk of predation. At higher concentrations, there was a payoff between toxin avoidance (by valve closure or re-emergence) and predator avoidance. The copper 10 day LC50 for Venerupis senegalensis was found to be 88 µg/l in sandy sediments (Kaschl & Carballeira, 1999). For reference to polluted UK sediments, copper concentration in the interstitial water of Restronguet Creek sediments has been measured at 100 µg/l (Bryan & Langston, 1992).

Eisler (1977) exposed Mya arenaria to a mixture of heavy metals in solution at concentrations equivalent to the highest recorded concentrations in interstitial waters in the study area. At 0°C and 11°C (winter temperatures) 100% mortality occurred after 4-10 weeks. At 16-22°C (summer temperatures) 100% mortality occurred after 6-14 days, indicating greater intolerance at higher temperatures.

Generally, polychaetes (e.g. Bryan, 1984), gastropods (e.g. Bryan, 1984) and macroalgae (e.g. Strömgren, 1979a,b) are regarded as being tolerant of heavy metal contamination. In light of the high intolerance of bivalves, including the important characterizing species, overall biotope intolerance is assessed as high and species richness is expected to decline. Recoverability is recorded as high (see additional information below).
Hydrocarbon contamination
(View Benchmark)
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).

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. Mortality following oil spills has been recorded in Mya arenaria (Dow, 1978; Johnston, 1984), Ensis sp. (SEEC, 1998) and Cerastoderma edule (SEEEC, 1998). Suchanek (1993) reported that infaunal polychaetes were also vulnerable to hydrocarbon contamination. For example, high mortality has been demonstrated in Arenicola marina (Levell, 1976). However, deposit feeders, such as Aphelochaeta marioni, are likely to be less vulnerable due to the feeding tentacles being covered with a heavy secretion of mucus (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. Some of the more intolerant species are likely to be eradicated so there may be a minor decline in species richness. Oil has the capacity to persist for a long time in soft sediments and so recoverability is assessed as moderate.
Radionuclide contamination
(View Benchmark)
Stamouli & Papadapoulou (1990) investigated bioaccumulation of radioactive trivalent Chromium 51 (51Cr) in a Venerupis species from Greece. 51Cr is derived from nuclear tests, disposal of radioactive waste and is one of the principal corrosion products of nuclear powered ships. 51Cr 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. Chassaud-Bouchard (1992) reported accumulation of Americium 241, Plutonium 239 and Uranium 238 in Cerastoderma edule associated with damage to the nucleus, increased activity of the golgi apparatus and decreased numbers of mitochondria in the cells. The ecological significance of these findings is unclear. No further information was found concerning the uptake of radionuclides by species in the biotope.
Changes in nutrient levels
(View Benchmark)
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 species tolerant of nutrient enrichment and typical of enriched habitats (e.g. Tubificoides benedii) (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 bivalves.

No information regarding the specific effects of nutrients on Venerupis senegalensis, the important characterizing species, was found. However, increased nutrients are likely to enhance ephemeral algal and phytoplankton growth, increase organic material deposition and enhance bacterial growth. At low levels, an increase in phytoplankton and benthic diatoms may increase food availability for Venerupis senegalensis, thus enhancing growth and reproductive potential (e.g. Beiras et al., 1993). However, increased levels of nutrient (beyond the carrying capacity of the environment) may result in eutrophication, algal blooms and reductions in oxygen concentrations (e.g. Rosenberg & Loo, 1988). Rosenberg & Loo (1988) reported mass mortalities of Mya arenaria and Cerastoderma edule following a eutrophication event in Sweden, although no direct causal link was established. It is likely therefore that a dramatic increase in nutrient levels would cause some mortality of Venerupis senegalensis and so a biotope intolerance of intermediate is recorded. Recoverability is recorded as high (see additional information below). As explained above, community composition may change but a decline in species richness is not expected.

Increase in salinity
(View Benchmark)
The biotope occurs in fully saline conditions (Connor et al., 1997a) and therefore is not likely to be intolerant of increases in salinity. No information was found concerning the intolerance of the important characterizing species, Venerupis senegalensis, to hypersaline conditions. However, the intolerance to hypersalinity of some other species which occur in the biotope has been researched. For Cerastoderma edule, Russell & Peterson (1973) reported an upper median salinity limit of 38.5 psu. Rygg (1970) noted that a population of Cerastoderma edule did not survive 23 days exposure at 60 psu, although they did survive at 46 psu. When exposed to hyper-osmotic shock (47 psu), Arenicola marina lost weight, but were able to regulate and gain weight within 7-10 days (Zebe & Schiedek, 1996).
Decrease in salinity
(View Benchmark)
The biotope occurs in variable salinity conditions (Connor et al., 1997a) and therefore the majority of species are unlikely to be intolerant of a reduction in salinity. In assessing biotope intolerance, the most important species to consider is Venerupis senegalensis. No information was found concerning the effects of decreasing salinity on the species specifically. However, 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. Euryhaline bivalve species, however, e.g. Mya arenaria, Cerastoderma edule, were able to regulate muscle volume with changing salinity. Venerupis japonica displayed a variety of behavioural reactions in response to reduced salinity in the Sea of Japan (Yaroslavtseva & Fedoseeva, 1978). Salinities typically encountered ranged from 11-30 psu over the course of a day. Venerupis japonica was active down to 20 psu, below which it reacted with siphon withdrawal and valve closure. Mortality occurred if salinity remained below 14 psu for an extended period. The benchmark includes a change of 2 categories on the salinity scale for a week (see glossary). This would place some of the biotope in a reduced salinity environment (<18 psu) and it is likely that some mortality of Venerupis senegalensis would occur. A biotope intolerance of intermediate is therefore recorded. Recoverability is recorded as high (see additional information below).

As mentioned above, some species in the biotope are more tolerant of reduced salinity than others:
  • Mya arenaria, for example, is a euryhaline osmoconformer and has been reported from the west Atlantic coast in salinities of 4 psu (Strasser, 1999).
  • Arenicola marina is unable to tolerate salinities below 24 psu and is excluded from areas influenced by freshwater runoff or input (e.g. the head end of estuaries) (Hayward, 1994).
The more intolerant species are unlikely to be able to tolerate salinities below 18 psu so there is expected to be a minor decline in species richness.
Changes in oxygenation
(View Benchmark)
The fauna in the biotope are all aerobic organisms and are therefore likely to be intolerant in some degree to lack of oxygen. 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 bivalves, Cerastoderma edule and Mya arenaria, suffered mortality between 2 and 7 days. Rosenberg et al. (1991) reported 100% mortality of Cerastoderma edule exposed to 0.5 - 1.0 ml/l oxygen for 43 days and 98% mortality after 32 days. Intertidal and infaunal organisms tend to be more tolerant of anoxia. Zebe & Schiedek (1996) reported that Arenicola marina is able to respire anaerobically and survived 72 hrs of anoxia at 16°C. Littorina littorea can endure long periods of oxygen deprivation. The snails can tolerate anoxia by drastically reducing their metabolic rate (down to 20% of normal) (MacDonald & Storey, 1999). At the bench mark level of hypoxia (2 mg/l for 1 week) it is expected that some mortality of the more intolerant species, such as bivalves, would occur and therefore biotope intolerance is assessed 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
(View Benchmark)
Navas et al. (1992) investigated the parasites of Venerupis senegalensis (studied as Venerupis pullastra), from a population in south west Spain. The following were recorded:
  • 36.6% prevalence of Perkinsus atlanticus; trophozoites found in the connective tissue of different organs with a very intensive hemocytic response, encysting the parasite and destroying tissue structure.
  • 96.6% prevalence of ciliates in gills, including Trichodina sp.
  • 11.8% prevalence of turbellarians.
  • 11.1% prevalence of trematodes.
Perkinsus atlanticus was also recorded as causing mortality in Venerupis decussata and Venerupis aureus. Freire-Santos et al. (2000) recorded the presence of oocysts of Cryptosporidium sp. in Venerupis senegalensis (studied as Venerupis pullastra) collected from north west Spain and destined for human consumption.

Several parasites occur in Mya arenaria, e.g. cercaria of Himasthla leptosoma, the nemertean parasite Malacobdella sp. and the copepod Myicola metisciensis may be commensal (Clay, 1966). The protozoan, Perkinsus sp. has recently been isolated from Mya arenaria in Chesapeake Bay, USA (McLaughlin & Faisal, 2000). Mya arenaria is also known to suffer from cancers, disseminated neoplasia and gonadal tumours. Disseminated neoplasia, for example, has been reported to occur in 20% of the population in north eastern United States and Canada, and caused up to 78% mortalities in New England (Brousseau & Baglivo, 1991; Landsberg, 1996). Little information was found regarding microbial infection of polychaetes, although 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.

The parasite loads of the bivalves discussed above have been proven to cause mortality and therefore a biotope intolerance of intermediate is recorded and there may be a minor decline in species richness in the biotope. Recoverability is recorded as high (see additional information below).
Introduction of non-native species
(View Benchmark)
No information was found concerning the susceptibility of Venerupis senegalensis to invasive species. However, the American hard-shelled clam, Mercenaria mercenaria, colonized the niche left by Mya arenaria killed after the cold winters of 1947 and 1962/63 in Southampton Water (Eno et al., 1997). The Mya arenaria populations had not recovered in this area by 1997 (Eno et al., 1997). Mya arenaria often occurs in the IMX.VsenMtru biotope and therefore Mercenaria mercenaria may pose a threat of invasion. Biotope intolerance is therefore recorded as intermediate with a minor decline in species richness. Once Mercenaria mercenaria has invaded, displaced bivalve populations may never re-establish and hence recoverability is recorded as very low.
Extraction
(View Benchmark)
Venerupis senegalensis is a very important commercial shellfish in Spain. It is harvested from the wild and raised in aquaculture (Jara-Jara et al., 2000). No information was found concerning the effect of harvesting on wild populations but it can be assumed that high mortality would occur in the intertidal where populations are more accessible to harvesters. The majority of the biotope occurs subtidally where it is less likely to be exploited. Dredging for Venerupis senegalensis may affect other species such as Mya arenaria. As a result of dredging activity, mortality and shell damage have been reported in Mya arenaria and Cerastoderma edule (Cotter et al., 1997). Other species in the biotope which are exploited commercially include Arenicola marina (Fowler, 1999), Cerastoderma edule (Hall & Harding, 1997), Ensis ensis (Fowler, 1999) and Mya arenaria (Emerson et al., 1990). Overall, an intolerance of intermediate is therefore recorded. Recoverability is recorded as high (see additional information below).

Additional information icon Additional information

Recoverability
The recoverability of the important characterizing species in this biotope, Venerupis senegalensis, is the principal factor in assessing the recoverability of the biotope.
  • Venerupis senegalensis is a long lived, fast growing species that reaches maturity within one year and spawns several times in one season (Johannessen, 1973; Perez Camacho, 1980). 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). The larvae remain in the plankton for up to 30 days (Fish & Fish, 1996) and hence have a high potential for dispersal. Given these life history features, it is expected that Venerupis senegalensis would have strong powers of recoverability. However, recoverability will be influenced by pre and post recruitment processes. The species exhibits pronounced year class variability in abundance (Johannessen, 1973; Perez Camacho, 1980) which suggests that recruitment is patchy and/or post settlement processes are highly variable. Olafsson et al. (1994) reviewed the potential effects of pre and post recruitment processes. Recruitment may be limited by predation of the larval stage or inhibition of settlement due to intraspecific density dependent competition. Post settlement processes affecting survivability include predation by epibenthic consumers, physical disturbance of the substratum and density dependent starvation of recent recruits. Hence, for Venerupis senegalensis, an annual predictable population recovery is not certain. However, given the life history characteristics discussed above it is expected that recovery would occur within 5 years and therefore recoverability for Venerupis senegalensis is assessed as high.
  • The infaunal deposit feeding polychaetes, such as Arenicola marina and Aphelochaeta marioni, have similar recoverability characteristics. Neither species has a pelagic phase in its lifecycle, and dispersal is limited to the slow burrowing of the adults and juveniles. The dispersal and recoverability of Arenicola marina have been well studied. 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.
  • 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).
  • The grazing gastropods in the biotope are likely to have strong powers of recoverability. Littorina littorea, for example, is an iteroparous breeder with high fecundity that lives for up to 4 years. Breeding can occur throughout the year. The planktonic larval stage lasts for up to 6 weeks although larvae do tend to remain in waters close to the shore. Recolonization, recruitment and recovery rates are therefore likely to be high.
  • Among the macroalgae in the biotope, recovery rates are likely to vary according to life history characteristics. Fast growing species, such as Fucus serratus, are iteroparous, highly fecund and survive and breed for protracted periods over 3-4 years. The eggs are broadcast into the water column allowing a potentially large dispersal distance.
The majority of guilds in the biotope are likely to have high recoverability. In light of this, and particularly the recoverability of the important characterizing species, Venerupis senegalensis, recoverability of the biotope as a whole is assessed as high.

This review can be cited as follows:

Rayment, W.J. 2001. Venerupis senegalensis and Mya truncata in lower shore or infralittoral muddy gravel. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 24/11/2014]. Available from: <http://www.marlin.ac.uk/habitatbenchmarks.php?habitatid=354&code=2004>