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Varicorbula gibba lives infaunally in muddy sandy sediments. Removal of the substratum would also remove the entire population of this species, and so the intolerance has been assessed to be high with a moderate recoverability. See additional information for recoverability.

Varicorbula gibba is a burrower in shallow muddy or sandy sediments and uses a byssus thread to attach to pieces of shell or rock in the sediment (CRIMP, 2000). It uses its short inhalant siphon above the sediment for feeding and respiration. If smothered Varicorbula gibba would most likely burrow up through the new sediment. Varicorbula gibba is also considered to be generally tolerant of prolonged oxygen deprivation (see deoxygenation below). Laboratory studies on Varicorbula have shown that they can survive up to 57 days in near anoxic conditions (Jensen, 1990). Therefore Varicorbula could probably survive for 1 month under smothering conditions (see benchmark). However, sudden smothering of the sediment would halt feeding. Therefore, intolerance has been assessed as low with an immediate recoverability level.

Levels of suspended sediment are likely to be most relevant to feeding. An increase in suspended sediment is likely to increase the rate of siltation and the availability of food. This will affect Varicorbula as it is a suspension feeder. However, Varicorbula is one of the most efficient bivalve particle feeders (Kiørboe & Mohlenberg, 1981). In the Limfjord, increased winds caused a greater mixing of the water column and probably greater resuspension of bottom material in 1986. As a result population increases were recorded with higher densities and faster growth than the previous year when wind speeds were not as high (Jensen, 1990). Varicorbula gibba also has a well developed cleansing mechanism to deal with the accumulation of pseudofaeces by posterior and periodic contractions of the quick muscle component in the adductors (Yonge & Thompson, 1976). An increase in sediment may benefit Varicorbula gibba and tolerant* has been recorded.

Levels of suspended sediment are likely to be most relevant to feeding. A decrease in suspended sediment is likely to decrease the availability of food for suspension feeders like Varicorbula gibba. Mortality is unlikely to occur within a month (see benchmark) but growth rates may be slower and so intolerance is assessed as low. When suspended sediment levels return to normal so to will food availability and recoverability is assessed as immediate.

The effect of desiccation stress on Varicorbula gibba is likely to be minimal as it lives infaunally in muddy sand and is able to burrow to avoid or reduce the effects of desiccation. Bivalves are also able to respond to desiccation stress by valve adduction during periods of emersion. Nevertheless, some stress is likely and an intolerance of low has been recorded but with a very high recoverability.

An increase in emergence may cause thermal stress on Varicorbula gibba and increase the risk of dislodgement from the sediments because of the increased strength of wave action, especially in shallow water populations. Therefore, an intolerance of intermediate is given with a high recoverability.

A decrease in emergence is not likely to stress Varicorbula gibba and may benefit the species, allowing Varicorbula gibba to colonize further up the shore and increase its habitat range. Periods of thermal stress, risk of predation and dislodgement would be reduced. Therefore, tolerant* is recorded.

An increase in the water flow rate would increase the availability of food that may increase growth rates and the size of individual Varicorbula gibba. However, increased water flow may cause the substratum to be disturbed and the sediment on the seabed to erode. This scouring of sand and gravel causes coarse sediments to become unstable and difficult to burrow perhaps leading to the dislodgement and abrasion of Corbula gibba. The sediments and the species within may then be transported to another area. High water flow rates may also damage or prevent the settlement of larvae that can lower recruitment levels and lower the population present (Hiscock, 1983). Therefore, an intolerance of intermediate has been assessed with high recoverability.

A decrease in the water flow rate could result in a reduction in food that may be obtained from suspension feeding in Varicorbula gibba, which could lower growth rates and the sizes of individuals within the population. It may also lower the dispersion of planktonic larvae. In areas exposed to less water flow, the sediments will be more stable (Hiscock, 1983) and particles may become finer and the substratum may become more muddy which is the preferred substrata of Varicorbula gibba. However, a decrease in water flow over the benchmark level of 1 year may also cause the substratum to become too muddy for Varicorbula gibba, which prefers sediments that contain between 10 - 15 % mud (Parry & Cohen, 2001). Therefore, intolerance has been assessed as intermediate with a high recoverability level.

Varicorbula is present in Mediterranean and Australian waters. Growth has been recorded at the following temperatures:

• 11.3 - 24.3 °C in Elefsis Bay, Greece (Theodorou, 1994);
• 9.18 - 21.45 °C in the Adriatic Sea (Moodley et al., 1998);
• 8 - 26 °C in Port Phillip Bay (Talman, 2000; cited in NIMPIS, 2002).

Therefore, Varicorbula gibba has been assessed as tolerant to increases in temperature at the benchmark level.

Varicorbula gibba has a wide geographic range, occurring in waters throughout the British Isles and is likely to be tolerant of lower temperatures than it experiences in Britain and Ireland. Varicorbula gibba would probably tolerate a decrease in temperature (see benchmark) as it has been found at temperatures between - 1 to 16 °C in the Limfjord (Jensen, 1990), and also at 7.5 °C in the Kattegat (Christensen, 1970). Therefore, Varicorbula gibba has been assessed to be tolerant to decreases in temperature.

Varicorbula gibba does not require light therefore, the effects of increased turbidity on light attenuation are not directly relevant. An increase in turbidity may, however, affect primary production in the water column that would lower phytoplankton availability for Varicorbula gibba, as it is a suspension feeder, but it can use other food sources e.g. particulate organic matter. Therefore, intolerance has been assessed as low with a very high recoverability.

Varicorbula gibba does not require light therefore, the effects of a decrease in turbidity on light attenuation are not directly relevant. A decrease in turbidity may however, affect primary production in the water column that would increase phytoplankton availability for Varicorbula gibba. That could improve the growth rates of Corbula gibba and also increase their abundance. A decrease in turbidity is unlikely to affect Varicorbula gibba. Therefore not relevant has been recorded.

Varicorbula gibba inhabits coarse muddy / sandy environments. This preference for coarse muddy sands was observed offshore of Port Erin on the Isle of Man where significant differences in the numbers of Varicorbula gibba was recorded. In areas where the substratum was coarse, the numbers of Corbula gibba were abundant. Whereas in areas where the substratum was fine the abundance of Varicorbula gibba was low (Jones, 1956).
An increase in wave exposure is likely to change the nature of the sediment in shallow depths making it less muddy and less suitable for Varicorbula gibba. The dispersion and settlement of larval and juvenile stages may also be disrupted. Damage or the withdrawal of the siphons, which reduces the ability of Varicorbula gibba to feed could occur. Increased wave exposure may also be detrimental to predators of Corbula gibba and prevent them from feeding. Intolerance has been assessed as intermediate with a high recoverability.

Changes in wave exposure are likely to have marked effects on the sediment dynamics. If the wave exposure is decreased sediments that are deposited will slowly consolidate becoming more fine and muddy and can increase the substratum. Decreased exposure could increase siltation and the risk of smothering. Varicorbula gibba is specialized for its preferred habitat of muddy sand, however, a decrease in wave exposure over the benchmark period of 1 year may cause the substrata to become too muddy for Varicorbula gibba. Therefore, intolerance has been assessed as intermediate with a high recoverability.

No information was found concerning the intolerance levels of Varicorbula gibba to noise. This species is not expected to be sensitive to the level of the benchmark.

Varicorbula gibba probably has little visual acuity and was recorded to be not sensitive to this factor.

Varicorbula gibba has a small solid shell. The shells of Varicorbula gibba may be vulnerable to physical damage (from e.g. otter boards) (Rumohr & Krost, 1991). However, the size of Varicorbula gibba relative to the meshes of commercial trawls may ensure survival of a moderate proportion of disturbed individuals that pass through them. Specimens exposed on the sediment surface would be at risk of predation. Bergmann & van Santbrink (2000) reported direct mortalities of <0.5%, 9% and 14% from the passage of an experimental beam trawl, depending on the type of trawl used and sampling method employed. They noted that smaller species or smaller individuals of larger species suffered lower mortalities. Overall, they concluded that Varicorbula gibba was amongst the species studied that were relatively resistant to bottom trawling (Bergmann & van Santbrink, 2000).
Ball et al. (2000) noted that Varicorbula gibba was not found at their offshore experimental otter trawling site but was present at an untrawled, shipwreck site. In a further study in Loch Gareloch, Varicorbula gibba was identified as one of the species sensitive to fishing disturbance. The Gareloch study carried out otter trawls at monthly intervals for 16 months in a previously undisturbed area, sheltered sea loch. The experimental trawling resulted in changes in the sediment and the associated community due to an increase in opportunistic polychaetes, resulting in a 45% decrease in the abundance of Varicorbula gibba with respect to reference sites. However, the Gareloch study represents a level of impact greater than the benchmark. Nevertheless, both of the experimental trawling studies result in mortality. Therefore, an intolerance of intermediate is recorded with a high recovery level.

Fishing for demersal species will disturb the surface layer of sediment and any protruding or shallow burrowing species. The small size of Corbula gibba may ensure that individuals are sieved over the mesh of fishing nets. Once through the net Varicorbula gibba are then able to resettle in the substrata. Displacement may also occur during storms if the sediment is mobilized. The increased wave action may cause whole populations to be lifted along with the substratum and transported by sediment bedload transport to a different area. Varicorbula gibba can burrow back down into the sediment when it is displaced to the surface therefore, it is probably relatively tolerant of displacement. However, it burrows slowly and when displaced the risk of predation by predators is increased which can lead to some mortalities of Varicorbula gibba. Therefore, an intolerance of intermediate has been recorded with a high recoverability.

No specific information was found concerning the effect of synthetic chemicals on Varicorbula  gibba. However, an inference may be drawn from related species. Burrowing and avoidance behaviour in the bivalves Tellina tenuis and Macoma balthica becomes impaired when they are exposed to phenol but no deaths occurred. Impairment of burrowing can leave bivalves vulnerable to predation and wave action (Møhlen & Kiørboe, 1983). High levels of tributyltin (TBT), the toxic component of many antifouling paints, has been implicated in the slow growth and shell malformation 'balling' in the oyster Magallana gigas and larval mortality in Mytilus edulis (Beaumont et al., 1989). Overall, an intolerance of intermediate has been suggested, albeit with very low confidence.

Heavy metals can inhibit the activity of many enzymes and affect the function of several cellular constituents such as membranes, they can also inhibit growth, the production of the byssal thread, respiration, filtration rate, protein synthesis, the uptake of amino acids by various tissues and compromise reproduction in bivalves (full review by Aberkali & Trueman, 1985). The embryonic and larval stages of bivalves are the most vulnerable to heavy metals (Bryan, 1984).

Bryan (1984), states that Hg is the most toxic metal to bivalve molluscs in experimental studies while copper (Cu), cadmium (Cd) and zinc (Zn) seem to be most problematic for bivalves in the field. For example:

• exposure to 15 parts per billion (ppb) of copper was found to produce deformed embryos in Crassostrea virginicaand 33 ppb proved lethal to their larvae (Bryan, 1984). The adults, on the other hand can withstand exposure to such levels although, through the immobilization of copper, they become green and unpalatable (Bryan, 1984);
• exposure to 100 ppb of cadmium for 15 weeks induced poor conditions and mortalities in adult Crassostrea virginica (Bryan, 1984).

Rygg (1985) used 71 sampling stations in a dozen fjord areas with varying degrees of pollution to examine the effects of pollution on benthic fauna. He noted that benthic faunal biodiversity decreased with increasing Cu concentrations in the sediment. Corbula gibba was reported to be present at some but not all of the stations where sediment copper concentrations were above 200 ppm in the sediments and was classified as one of the moderately tolerant species (Rygg, 1985). A concentration of 200 ppm was approximately 10 tens background values (Rygg, 1985). Corbula gibba was more tolerant than the bivalves, Ennucula tenuis, which was absent at all the sampling stations and Thyasira equalis, which was occasionally present at the sampling stations (Rygg, 1985). Overall, an intolerance of intermediate has been recorded, since Corbula gibba was exclude from some of the polluted sites examined..

Hydrocarbons may produce substantially reduced feeding, respiration and energy metabolism rates that reduce growth and reproduction as observed in Mya arenaria (Cooper & Cristini, 1994) and Mytilus edulis (Moore et al., 1987). Reproduction may also be compromised on exposure to hydrocarbons, for example Macoma balthica showed gamete resorption and abnormal gamete development. Additional effects of hydrocarbons on bivalves include a decline in tissue and shell growth, increased susceptibility to predation, parasitism and disease (Moore et al., 1987).

However, oil spills may benefit some bivalve molluscs. For instance, the 1978 Amoco Cadiz oil spill may have benefited the population of Abra alba present due to the nutrient enrichment that was caused by the oil spill. The biomass of the community doubled as a result of an increase in Abra alba abundance following the oil spill. Throughout the 20 years of monitoring the community's recovery, Abra alba has been one of the dominant species recorded (Dauvin, 1998). Corbula gibba has been noted as being indifferent to organic pollution (Pearson & Rosenberg, 1978) and has been recorded to often thrive in nutrient enriched waters (Crema et al., 1991) (see nutrients below). However, no information on the effects of hydrocarbon contamination on Corbula gibba was found, and intolerance has not been assessed.

No specific information was found concerning the effects of radionuclides on Varicorbula gibba.

Varicorbula gibba is known to be a pioneer species in recolonization of defaunated seabeds and prominent in subnormal zones in areas polluted or enriched by organic material (Pearson & Rosenberg, 1978; Jensen 1990). It has also been suggested that Corbula gibba are indicative of unstable environments such as ones with low oxygen levels and areas of eutrophication (Crema et al., 1991). For example, samples were taken of the macrozoobenthic community in Elefsis Bay in the northern Adriatic in 1985. Elefsis Bay suffered from nutrient enrichment where nutrient pollution mainly came from the disposal of untreated waste waters of about 600,000 m³ / day at the entrance of the bay. The major nutrient inputs recorded were from phosphates, silicates, nitrites, nitrates and ammonium. Eutrophication of the northern Adriatic Sea was marked by red tides, extensive mucus aggregates, anoxic bottom conditions and mass mortalities. Despite this anoxia, large abundance's of Varicorbula gibba were recorded. The abundance of Varicorbula gibba at one sampling station was 1396 ind/m² and during the summer of 1989 Varicorbula gibba was the only living species recorded (Theodorou, 1994). Nutrient enrichments appear to benefit Corbula gibba by allowing it to increase its population size and to further colonize an area. It was suggested that as the amount of organic material reaching sediments increases, the larger species and deeper burrowing species are gradually eliminated and replaced by greater numbers of bivalves like Varicorbula gibba (Pearson & Rosenberg, 1978). Therefore, nutrient enrichment may benefit Varicorbula gibba and tolerant* has been recorded.

Varicorbula gibba are mainly found at oceanic salinities but have also been recorded in 26 - 39 ppt in Port Phillip Bay (Talman, 2000: cited in NIMPIS, 2002). Therefore, it is likely that Varicorbula gibba would tolerate an increase in salinity at the benchmark level.

Varicorbula gibba are found at oceanic salinities and in estuarine waters showing a tolerance for a reduction in salinity. In Elefsis Bay, Corbula gibba can be found at salinities as low as 8.2 ppt (Theodorou, 1994). Therefore Corbula gibba is likely to be tolerant of decreases in salinity.

Diaz & Rosenberg (1995) state that Varicorbula gibba is resistant to severe hypoxia. Varicorbula gibba is often found at the edge of anoxic and azoic areas (Pearson & Rosenberg, 1978) and it has been suggested that it is highly tolerant to environmental variability (Rosenberg, 1997).

• In the western trough of Lough Ine in Ireland, a summer thermocline develops and the oxygen content falls. Oxygen levels were so low that the muddy substrata turned black. During the summer months, no species were found below 40 m except for Varicorbula gibba and it was reported to be the most hypoxic tolerant species in the Lough. However, by autumn no macrofauna and no populations of Varicorbula gibba were present in the Lough as conditions became too hypoxic (Kitching et al., 1976).
• The Little Belt near the coast of Denmark showed diminishing oxygen concentrations in the bottom waters that resulted in a 5 fold increase of areas with oxygen depletion. A decline in species sensitive to oxygen declines was found. Whereas, those species that were less sensitive to oxygen depletion for example Varicorbula gibba increased by a factor of two to five times (from about 400 - 500 ind.² to > 2000 ind.m²) (Karlson et al., 2002).
• Kiel Bay in the Baltic Sea has also seen significant declines in deep water oxygen concentration since the 1950's. In 1981 the salinity was 20-26 psu, and temperatures of 10-14°C were recorded. Anoxia and hydrogen sulphide were widespread below the halocline at a depth of >20 m (Rosenberg & Loo, 1988). The anoxic event lasted several weeks and during that time, 30,000 t of macrofauna was lost over 750 km². Varicorbula gibba was one of the few species that survived this event.
• Another area that has recorded severe hypoxic events is Kattegat, Sweden. The worst year recorded was 1988, when approximately 30,000 km² of the bottom water was hypoxic. Oxygen concentration recorded were 3.1 ml/l in June, 1.0 ml/l in August, 0.9 ml/l in September, and Varicorbula gibba was amongst the surviving species.
• Laboratory studies have shown that specimens of Varicorbula gibba were able to survive long periods of near anoxic conditions. Results showed that 9 out of 14 specimens survived after 57 days at anoxic conditions (10 - 11 °C and 0.18 - 0.37 mg of oxygen per dm ³) (Jensen, 1990).

Varicorbula gibba has shown tolerance to severe decreases in oxygenation therefore, an intolerance assessment of low has been given with a recoverability assessment of immediate.

The ciliate Sphenophrya dosiniae has been found living in specimens of Varicorbula gibba. If a lamellibranch is infected with Sphenophrya dosiniae, the ciliates will always occur in great numbers in the mantle cavity of their host (Fenchel, 1965). Sphenophrya dosiniae was found in 40 % of the specimens of Varicorbula gibba in the Gullmarfjord (Fenchel, 1965). No specific information concerning the effects of these ciliates on Varicorbula gibba was found.

However, in the bivalve Crassostrea virginica, Sphenophrya dosiniae induced the formation of a lump known as a 'xenoma' that contains hundreds of ciliates (Weissenberg, 1922; cited in Laucker, 1983). Neither the ciliates or the xenomas appeared to distress Crassostrea virginica. However, parasitic infections are likely to result in sub-lethal effects and an intolerance of low has been recorded with a very high recoverability.

There is no evidence of adverse effects or competition from non-native species on Varicorbula gibba. Therefore, an intolerance and recoverability assessment could not be made.

Varicorbula gibba are not targeted for extraction. Therefore, an intolerance assessment is not relevant.

Bergmann & van Santbrink (2000) reported direct mortalities of <0.5%, 9% and 14%from the passage of an experimental beam trawl, depending on the type of trawl used and sampling method employed. They noted that smaller species or smaller individuals of larger species suffered lower mortalities. Overall, they concluded that Varicorbula gibba was amongst the species studied that were relatively resistant to bottom trawling (Bergmann & van Santbrink, 2000).
However, Ball et al. (2000) noted that Varicorbula gibba was not found at their offshore experimental otter trawling site but was present at an untrawled, shipwreck site. In a further study in Loch Gareloch, Varicorbula gibba was identified as one of the species sensitive to fishing disturbance. The Gareloch study carried out otter trawls at monthly intervals for 16 months in a previously undisturbed area, sheltered sea loch. The experimental trawling resulted in changes in the sediment and the associated community due to increase in opportunistic polychaetes, resulting in a 45% decrease in the abundance of Varicorbula gibba with respect to reference sites. Varicorbula gibba can burrow back down into the sediment when it is displaced to the surface but burrows slowly. While displaced onto the sediment surface the risk of predation by predators is increased which can lead to additional mortalities of Varicorbula gibba. Therefore, an intolerance of intermediate has been recorded. Varicorbula gibba has also been found to dominate during the first stages of post dredging recolonization (Talman & Keough, 2001). Therefore, recovery is likely to be rapid.