Biodiversity & Conservation

LR.MLR.MytPid

Explanation of sensitivity and recoverability


Physical Factors

Substratum Loss
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Piddocks live permanently in burrows excavated into the substratum. Loss of the substratum will result in loss of the piddocks and loss of the biotope. Mussels will be similarly affected by the loss of substratum and an intolerance of high has been recorded. Recolonization by pelagic larvae is likely within five years although under certain conditions this may take significantly longer (see additional information below). A recoverability of moderate has been recorded assuming that clay remains after the removal event.
Smothering
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Pholas dactylus have been found living under layers of sand in Aberystwyth, Wales, (Knight, 1984) and in Eastbourne, with their siphons protruding at the surface (Pinn et al., in press). Barnea candida has also been found to survive being covered by shallow layers of sand in Merseyside (Wallace & Wallace, 1983). Wallace & Wallace (1983) were unsure as to how long the Barnea candida could survive smothering but noted that, on the coast of the Wirral, the piddocks have survived smothering after periods of rough weather. Where smothering is constant, survival can be more difficult. The redistribution of loose material following storms off Whitstable Street, in the Thames Estuary, is thought to be responsible for the suffocation of many Petricola pholadiformis and it is possible that this species may be the most intolerant of the three piddock species associated with this biotope. However, it was not known how deep the layer of 'loose material' was, nor how long it lasted for or what type of material it was made up of.

Intertidal Mytilus edulis beds have been reported to suffer mortalities as a result on smothering by large scale movements of sand or sand scour (Daly & Mathieson, 1977; Holt et al., 1998). Similarly, biodeposition within a mussel bed results in suffocation or starvation of individuals that cannot re-surface. Young mussels have been shown to move up through a bed, avoiding smothering, while many others were suffocated (Dare, 1976; Holt et al., 1998).

Some gastropods including Littorina littorea have been recorded as being highly intolerant of smothering and may be suffocated by the sediment. A decrease in the number of Littorina littorea individuals, a prey species for Carcinus maenas, may increase predation pressure on the mussels. Smothering may also adversely affect interstitial fauna and epifauna, resulting in a decrease in species richness and an increase of infaunal species (Tsuchiya & Nishihira, 1985, 1986).

However, on moderately wave exposed to wave exposed coasts sediment is unlikely to remain in place resulting in scour which may remove a proportion of the mussels and possible adversely affect the stability of the sediment surrounding piddock burrows in extensively bored clay. Scouring may be more detrimental to patches of mussels, as opposed to beds, since the patches will have a greater total patch edge:area ratio. Consequently, there will be relatively fewer mussels protected by the inside of the patch / bed.

Overall it is suggested that a proportion of the Mytilus edulis population and of the piddock populations may be able to survive smothering. Therefore, an intolerance of intermediate has been recorded. Smothering by impermeable or immobile materials, e.g. oil, is likely to result in a higher intolerance (see hydrocarbons). Recoverability has been recorded as moderate (see additional information below).
Increase in suspended sediment
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This biotope (MLR.MytPid) occurs in areas associated with turbid water and the associated fauna are likely to be adapted to a certain degree of suspended sediment. An increase in the organic content of suspended sediment is likely to be beneficial to both suspension feeders and deposit feeders.

Piddocks must function in burrows into which sediment is continuously deposited from the both particle-laden water above the burrow to the by-products of the piddock's own mechanical boring (Fankboner, 1971). The piddocks have efficient mechanisms to remove sediment via pseudofaeces. Experimental work on Pholas dactylus showed that large particles can either be rejected immediately in the pseudofaeces or passed very quickly through the gut (Knight, 1984). Petricola pholadiformis is able to cope in water laden with much suspended material by binding the material in mucus and using the palps to reject it (Purchon, 1955).

Mytilus edulis has been reported to be relatively tolerant of suspended sediment and siltation and survived over 25 days at 440 mg/l and on average 13 days at 1200mg/l (Purchon, 1937; Moore, 1977). Mytilus edulis also has efficient pseudofaeces discharge mechanisms (Moore, 1977; de Vooys, 1987), although increased suspended sediment may reduce feeding efficiency (Widdows et al., 1998). In addition, super-abundant accumulation of sediments prevents the Mytilus larvae from settling to patches (Field, 1982, in Tsuchiya & Nishihira, 1986).

Increased siltation may also interfere with larval recruitment in some macroalgae species. Increased siltation may fill the mussel matrix, resulting in increased abundance of infauna but loss of more mobile species and species richness (Tsuchiya & Nishihira, 1985, 1986). Certainly, the space for mussel spat and other small invertebrates to shelter between mussels will be reduced. Overall, the biotope will be little affected but species richness will probably decline and an intolerance of low has been recorded.

Decrease in suspended sediment
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A decrease in the volume of suspended sediment, especially organic particles, may reduce the amount of available food to Pholas dactylus, Petricola pholadiformis, Barnea candida, Mytilus edulis and other suspension feeders. A decrease in food consumption may have a temporary deleterious effect on growth rate and fecundity however, on resumption of normal levels of suspended sediment, the suspension feeders are likely to recover rapidly. The sediments within mussel patches are an important component that contributes to the heterogeneity of the environment and therefore to its diversity (Tsuchiya & Nishihira, 1986). A reduction in suspended sediment may therefore temporarily decrease the diversity of infaunal invertebrates within the mussel matrix. Overall, the biotope will be little affected and accordingly, an intolerance of low has been recorded.
Desiccation
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Pholas dactylus inhabits the shallow sub-tidal and lower shore and is therefore likely to have some tolerance to desiccation due to periodic immersion and emersion. Barnea candida and Petricola pholadiformis live slightly higher up the shore than Pholas dactylus (Duval, 1977) and are also likely to have a degree of tolerance. The fact that the piddocks live in burrows that are likely to retain some water when the tide falls will offer them further protection from desiccation. However, the shells of the piddocks are unable to close completely and this renders them susceptible to desiccation to a certain degree. An increase in desiccation at the level of the benchmark, equivalent to a change in position of one vertical biological zone is likely to result in the death of many individuals particularly at the upper shore extent of the population.

The upper limit of Mytilus edulis population is primarily controlled by the synergistic effects of temperature and desiccation (Suchanek, 1978; Seed & Suchanek, 1992; Holt et al., 1998). For example, on extremely hot days in the summers of 1974 -1976 on Strawberry Island, Washington State, Suchanek (1978) reported mass mortality of mussels at the upper edge of the mussel bed. Mortality decreased down the shore. The upper limit of mussels fluctuated, increasing up the shore in winter and decreasing again in summer (Suchanek, 1978). Therefore, an increase in desiccation at the benchmark level is likely to result in mortality of mussels at the upper limit of the bed, and loss of their associated organisms, with patches of mussels restricted to depressions and rocks pools at their upper limit.

Similarly the upper limit of most intertidal species is partly determined by desiccation. Ceramium virgatum, for example, has been recorded as being highly intolerant to desiccation. Small invertebrates using small patches of this species for shelter may therefore be displaced or lost altogether. The entire biotope is likely to become 'squeezed' between a reduced upper limit and its lower limit. The biotope is likely to be more vulnerable to desiccation in moderately wave exposed conditions than in wave exposed conditions, since the latter tend to exhibit a greater humidity due to wash, spray and wave crash.

Overall, the extent of the biotope is likely to be reduced and an intolerance of intermediate has been recorded. Recoverability is probably high (see additional information below).

Increase in emergence regime
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Piddocks are fixed in position in their burrows and are therefore unable to escape any changes in emergence through migration. The burrows are likely to retain some water when the tide falls and this will offer them some protection from desiccation. However, the shells of the piddocks are unable to close completely. During extended periods of exposure, Pholas dactylus squirt some water from their inhalant siphon and extend their gaping siphons into the air (Knight, 1984). This may result in increased predation by birds. An increase in emergence, equivalent to a one hour change in the time not covered by the sea for one year, is likely to result in the death of many individuals, particularly at the upper shore extent of the population.

Mytilus edulis can only feed when immersed, therefore, changes in emergence regime will affect individuals ability to feed and their energy metabolism. Growth rates decrease with increasing shore height and tidal exposure, due to reduced time available for feeding and reduced food availability, although longevity increases (Seed & Suchanek, 1992; Holt et al., 1998). Concomitant with the cessation of feeding during periods of exposure is the cessation of biodeposit production (Tsuchiya, 1980). Consequently, the amount of available food for deposit feeders such as Hediste diversicolor will be decreased. Increased emergence will expose mussel populations to increased risk of desiccation (see above) and increased vulnerability to extreme temperatures, potentially reducing their upper limit on the shore, and reducing their extent in the intertidal. Predation from birds such as the oyster catcher can reduce the number of mussel eggs released into the plankton and decrease the amount of byssal settlement space for the protection of the spat (McGrorty et al., 1990). The result is likely to be a fall in recruitment to the mussel patches.

Overall, the upper limit of the biotope and its associated community will probably decrease and an intolerance of intermediate has been recorded. Recoverability will probably be high (see additional information below).
Decrease in emergence regime
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A decrease in exposure to air will reduce exposure to desiccation and extremes of temperature and allow the resident Pholas dactylus, Barnea candida, Petricola pholadiformis and Mytilus edulis to feed for longer periods and hence grow faster. Piddocks and mussels are therefore likely to be tolerant of a decrease in emergence and as a result, the biotope may be able to colonize further up the shore, providing a suitable substrate was available.

No information was found on factors controlling the lower limit of piddock populations and it is possible, for example, that predation may increase at the lower edge of the biotope. The lower limit of the mussel in the biotope may become susceptible to greater predation pressure from crabs resulting in greater turnover of individuals and a reduced number of size classes, and reduced age of mussels.

Therefore, in the short term, a decrease in emergence is likely to change the population structure of the mussel bed and, possibly, the piddock populations at their lower limits, probably reducing the species richness of the biotope. Although the mussel patches and piddock populations will effectively survive, the lower limit of the biotope as described may be lost and an intolerance of intermediate has been recorded. This biotope will probably colonize further up the shore and recovery is likely to be high (see additional information below).
Increase in water flow rate
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This biotope occurs in moderately wave exposed and wave exposed habitats where water movement from wave action will probably exceed the strength of any water flow rate. At any rate, piddocks live fixed permanently in burrows and are therefore unlikely to be washed away or adversely affected by an increase in water flow rate. However, if physical erosion due to an increase in flow rate occurs at such a rate that they will become exposed or are removed from their burrows, they will die. Mussels are firmly attached to the substratum but can be dislodged by storms and strong surges (see MarLIN review of Mytilus edulis). Overall, the biotope is considered to be tolerant of an increase in water flow rate.
Decrease in water flow rate
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This biotope occurs in moderately wave exposed and wave exposed habitats where water movement from wave action will probably exceed the strength of any water flow rate. If the biotope occurred in areas where water flow was more important to provide an adequate supply of food and prevent siltation some adverse effects on feeding and reproduction may occur. An intolerance of low has been recorded.
Increase in temperature
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Little information was found about the specific effects of increasing temperature on any of the three species of piddocks. However, the occurrence of Pholas dactylus in Britain is known to represent the northern limit of its distribution and an increase in temperature may therefore facilitate the extension of this species' presence further north. Temperature has also been implicated in the timing of reproduction in Pholas dactylus, which usually spawns between July and August. Increased summer temperatures in 1982 induced spawning in July on the south coast of England (Knight, 1984). Spawning of the piddock Petricola pholadiformis is initiated by increasing water temperature (>18 °C) (Duval, 1963a), so elevated temperatures outside of usual seasons may disrupt normal spawning periods. The spawning of Barnea candida was also reported to be disrupted by changes in temperature. Barnea candida normally spawns in September when temperatures are dropping (El-Maghraby, 1955). However, a rise in temperature in late June of 1956, induced spawning in some specimens of Barnea candida (Duval, 1963b). Disruption from established spawning periods, caused by temperature changes, may be detrimental to the survival of recruits as other factors influencing their survival may not be optimal, and some mortality may result. Established populations may otherwise remain unaffected by elevated temperatures. In laboratory studies, the majority of Petricola pholadiformis larvae did not survive beyond the veliger stage at 22 °C. However, it is unlikely that the species will experience these temperatures in British waters at the level of the benchmark.

In the British Isles, an upper, sustained thermal tolerance limit of about 29 °C was reported in Mytilus edulis (Read & Cumming, 1967; Almada-Villela et al., 1982). However, Seed & Suchanek (1992) noted that European populations were unlikely to experience temperatures greater than about 25 °C. Mytilus edulis is generally considered to be eurythermal. Temperature has been found to affect the amount of biodeposit produced by this animal. The optimum temperature for maximum biodeposit production was found to be approximately 20 °C (Tsuchiya, 1980). An increased amount of biodeposit produced by the mussels, associated with an increase in temperature, could increase the amount of potential habitat for infaunal species and increase the amount of available food for deposit feeders.

Overall, the dominant characterizing species will probably survive an increase in temperature at the benchmark level and it is possible that the diversity of associated fauna may even increase slightly. In balance, tolerant has been recorded.
Decrease in temperature
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Little information concerning the effects of a decrease in temperature on piddock species was found. In Pholas dactylus, the siphon activity and oxygen consumption was considerably reduced at a temperature of 7 °C when compared to behaviour at 15 and 18 °C where the animals were observed to be siphoning actively (Knight, 1984). Crisp (1964a) reported that no living individuals of Pholas dactylus could be found above low-water mark in Lyme Regis on the south coast of England during the extreme winter of 1962-3. The piddock populations at Lyme Regis are now fully recovered (Pinn et al., in press). In the same winter, J. Taylor (pers. comm. to Knight, 1984) mentioned a number of Pholas dactylus were found frozen in the chalk at Margate in Kent.

Overall, Mytilus edulis is considered to be eurythermal. Mytilus edulis can withstand extreme cold and freezing, surviving when its tissue temperature drops to -10 °C (Williams, 1970; Seed & Suchanek, 1992) or exposed to -30 °C for as long as six hours twice a day (Loomis, 1995). Bourget (1983) also reported that cyclic exposure to otherwise sublethal temperatures, e.g. -8 °C every 12.4 hrs resulted in significant damage and death after 3-4 cycles. This suggests that Mytilus edulis can survive occasional, sharp frost events but may succumb to consistent very low temperatures over a few days. Mytilus edulis was relatively little affected by the severe winter of 1962/63, with 30% mortality reported from south-east coasts of England (Whitstable area) and ca. 2% from Rhosilli in south Wales (Crisp, 1964) mainly due to predation on individuals weakened or moribund due to the low temperatures rather than the temperature itself.

Overall, the dominant key structural species, namely the piddocks, are likely to experience some mortality. A long term, chronic change in temperature similar to that of the benchmark could ultimately result in the loss of a significant proportion of the biotope. Accordingly, an intolerance of intermediate has been recorded. Recolonization by pelagic larvae is likely within five years although under certain conditions this may take significantly longer (see additional information below). A recoverability of moderate has been recorded.
Increase in turbidity
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Increased turbidity may reduce phytoplankton primary productivity, therefore reducing the food available to Pholas dactylus, Petricola pholadiformis, Barnea candida, Mytilus edulis and other suspension feeders. However, mussels use a variety of food sources and the effects are likely to be minimal, and this species is probably not sensitive to changes in turbidity. No information concerning the effects of an increase in turbidity on piddocks was found. Increased turbidity will decrease photosynthesis and primary productivity in seaweeds when immersed but they will probably be able to compensate when emersed. In any case, the seaweeds in this biotope only represent a small part of the total flora and fauna. An intolerance of low has been recorded.
Decrease in turbidity
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Decreased turbidity may increase phytoplankton primary productivity, therefore potentially increasing the food available to Pholas dactylus, Petricola pholadiformis, Barnea candida, Mytilus edulis and other suspension feeders. Macroalgae may benefit from decreased turbidity resulting in rapid growth, especially of ephemeral green algae. This will, in turn, increase the available food for grazers including Littorina littorea. Therefore, tolerant has been recorded.
Increase in wave exposure
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This biotope occurs in moderately wave exposed to wave exposed locations. Pholas dactylus, Petricola pholadiformis and Barnea candida are protected to a certain degree due to their fixed position within the burrow. A substantial increase in wave exposure that could lead to the piddocks becoming exposed or removed from their burrows would, however, result in their death. Wallace & Wallace (1983) reported that the clay crumbled easily around the piddocks where there were dense aggregations of the animals, as would certainly be the case in this biotope. Wallace & Wallace (1983) reported densities of 30-60 Barnea candida siphon holes per square foot in Merseyside and burrows up to 6 inches in length. Duval (1977) found that the depth of the boring depended on the size of the animal. For example, an animal with a shell length of 1.2 cm could bore a 2.7 cm burrow whereas animals 4.8 cm long could bore burrows of 12 cm. An increase from, for example, wave exposed to extremely wave exposed may threaten the viability of the sediment in which the piddocks dwell. If the clay then started to erode it may result in loss of the piddock burrows, especially those of smaller animals, and ultimately the loss of the biotope. Duval (1963a) found that displaced piddocks made no attempt at a second boring when placed on a consolidated substratum.

Mussels are tolerant of wave exposure and increase their byssus thread production (and hence attachment) with increased by water agitation (Young, 1985). However, Young (1985) concluded that mussels would be susceptible to sudden squalls and surges. Fouling organisms, e.g. barnacles and seaweeds, may also increase mussel mortality by increasing weight and drag, resulting in an increased risk of removal by wave action and tidal scour (Suchanek, 1985; Seed & Suchanek, 1992). Winter storms and increased wave exposure are likely to result in removal of patches of mussels. Loss of patches may lead to further sediment instability and piddock loss. In Mytilus californianus gaps were enlarged during winter, while recolonization and recovery rates increased in summer (Seed & Suchanek, 1992). Sediment and decaying matter may be 'flushed' from remaining mussel patches which will lead to a reduction in numbers of infauna and scavengers such as the common ragworm Hediste diversicolor. Mussel spat, that use the byssal threads of the adults in the patches as refuge from the common shore crabs, Carcinus maenas, may also be dislodged leaving them more susceptible to predation.

A reduction in macroalgae will result in loss of associated mesoherbivores. Similarly, mobile gastropods such as littorinids are likely to be lost.

Overall, an increase in wave exposure is likely to result in major decline in key structural species and an intolerance of high has been recorded. Recoverability will probably be moderate (see additional information below).

Decrease in wave exposure
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Mytilus edulis is tolerant of sheltered conditions and a reduced wave exposure may leave to a build-up of sediment and biodeposits within the mussel patches. This could result in increased scavenger and infauna diversity. Alternatively, the reduced wave exposure may lead to dominance by certain invertebrate species and therefore a decrease in species diversity.
It is unlikely that piddocks will be adversely affected by a decrease in wave exposure. However, the supply of particulate matter for suspension feeding may be reduced and therefore an intolerance of low has been recorded.
Noise
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Most of the invertebrates within the community are probably not sensitive to noise at the benchmark level. Pholas dactylus can probably detect the vibration caused by predators and will withdraw its siphons, ejecting water from the burrow as it does so (Knight, 1984). Mytilus edulis can probably detect slight vibrations in its immediate vicinity, however, it probably detects predators by touch (on the shell) or by scent. Therefore, it is probably insensitive to noise disturbance at the levels of the benchmark. Birds are major predators of mussels and piddocks, and several species are highly intolerant of noise. Therefore, noise at the level of the benchmark may disturb predatory birds, so that the mussel and piddock populations may benefit indirectly.
Visual Presence
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Pholas dactylus reacts quickly to changes in light intensity, after a couple of seconds, by withdrawing its siphon (Knight, 1984). This reaction is ultimately an adaptation to reduce the risk of predation by, for example, approaching birds (Knight, 1984). Mytilus edulis can probably detect changes in light commensurate with shading by predators as well. However, its visual acuity is probably very limited and it is unlikely to be sensitive to visual disturbance. Birds are highly intolerant of visual presence and are likely to be scared away by increased human activity, therefore reducing the predation pressure on the mussels and piddocks. Therefore, visual disturbance may be of indirect benefit to mussel and piddock populations.
Abrasion & physical disturbance
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Although, like most shells, the shell of Pholas dactylus is strong in one particular direction, it is predominantly thin and brittle so a force, equivalent to a 5-10 kg anchor and its chain being dropped or a passing scallop dredge, is likely to result in death. Although the piddocks are afforded some protection by living in their burrows, the clay is soft which leaves many individuals, especially those near the surface of the clay, vulnerable to damage and death. Duval (1977) found that the depth of the boring depended on the size of the animal. For example, an animal with a shell length of 1.2 cm could bore a 2.7 cm burrow whereas animals 4.8 cm long could bore burrows of 12 cm.

Daly & Mathieson (1977) reported that the lower limit of Mytilus edulis populations at Bound Rock, USA, was determined by burial or abrasion by shifting sands. Wave driven logs have been reported to influence Mytilus edulis populations causing the removal of patches. It is likely that abrasion or impact at the level of the benchmark would also damage or remove patches of the population.
The effects of trampling on Mytilus californianus beds in Australia were studied by Brosnan & Cumrine (1994). They concluded that mussel beds were intolerant of trampling, depending on bed thickness, and noted that in heavily tramped site mussels were uncommon and restricted to crevices. Trampling also inhibited subsequent recovery. Trampling pressure was most intense in spring and summer, so that gaps and patches created by storms in winter were not repaired but exacerbated. Patches are likely to be even more susceptible to damage than the beds mentioned in the above study.

Therefore, it is likely that abrasion and physical disturbance at the benchmark level will result in loss of a proportion of the piddocks populations and mussel patches and their associated species and an intolerance of intermediate has been recorded. Recoverability is likely to be moderate (see additional information below). The effects of large scale abrasion e.g. due to a vessel grounding, is likely to be similar to substratum loss.
Displacement
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The key structural species in this biotope are sessile. Duval (1963a) reported that neither juvenile or adult Petricola pholadiformis attempted to re-bore after being left on the surface of consolidated substrata. Pholas dactylus has also been reported to be incapable of excavating a new chamber (Barnes, 1980). The piddocks, therefore, are likely to be highly intolerant to displacement due to increased risk of predation and desiccation.

Dare (1976) reported that individual mussels swept or displaced from mussel beds rarely survived, since they either became buried in sand or mud, or were scattered and eaten by oystercatchers. However, mussels can attach to a wide range of substrata and, should a mussel be displaced to a suitable substratum, it is likely to be able to attach itself using byssus threads quickly.

Mobile gastropods may be washed to deeper water, only to return, while mobile crustaceans are unlikely to be adversely affected.

However, displacement would result in removal of the mussel patches and piddocks, and loss of their associated community, and hence the biotope. Intolerance has, therefore, been recorded as high. Recoverability is likely to be moderate (see additional information below).

Chemical Factors

Synthetic compound contamination
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Although no information on the specific effects of chemicals on the three piddock species was found, tributyl tin (TBT) has been found to be toxic to many bivalves. See, for example, Beaumont et al, 1989. Michelson (1978, cited in Knight, 1984) suggested that the dispersants used to clean up oil spills may have been responsible for the disappearance of the French colonies of Pholas dactylus.

Mytilus edulis species were extensively reviewed by Widdows & Donkin, (1992) and Livingstone & Pipe (1992), and summarized in the MarLIN review and Holt et al. (1998). A variety of chemical contaminants have been shown to produce sublethal effects and reduce scope for growth (e.g. PCBs, and organo-chlorides) (Widdows et al., 1995), while others (e.g. the detergent BP1002, the herbicide trifluralin and TBT) cause mortalities.

Similarly, most pesticides and herbicides were suggested to be very toxic for invertebrates, especially crustaceans (amphipods, isopods, mysids, shrimp and crabs) (Cole et al., 1999). The pesticide Ivermectin is very toxic to crustaceans, and has been found to be toxic towards some benthic infauna (Cole et al., 1999). The common shore crabs Carcinus maenas are susceptible to various synthetic compounds including cypermethrin, a chemical used to treat salmon for fish lice, and DDT (see review of Carcinus maenas).

Laboratory studies of the effects of oil and dispersants on several red algae species (Grandy 1984 cited in Holt et al. 1995) concluded that they were all sensitive to oil/ dispersant mixtures, with little differences between adults, sporelings, diploid or haploid life stages. O'Brien & Dixon (1976) suggested that red algae were the most sensitive group of algae to oil or dispersant contamination. Ceramium virgatum, for example, has been recorded as being highly intolerant to synthetic chemicals (see MarLIN review).

Loss of intolerant epifaunal and epifloral grazers such as gastropods may result in an increase in fouling of the mussels themselves by algae. Overall, a number of chemical contaminants are likely to result in reduced growth and condition and loss of a proportion of the mussel and piddock populations while the species richness may show a marked decline. Therefore an intolerance of intermediate has been recorded. Recoverability is probably high (see additional information below).
Heavy metal contamination
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Heavy metal contamination affects different taxonomic groups and species to varying degrees.
  • The effects of contaminants on Mytilus edulis were extensively reviewed by Widdows & Donkin, (1992) and Livingstone & Pipe (1992), and summarized in the MarLIN review. Heavy metals were reported to cause sublethal effects and occasionally mortalities in mixed effluents.
  • Bryan (1984) suggested that adult gastropod molluscs (e.g. Littorina littorea) were relatively tolerant of heavy metal pollution.
  • Crustaceans are generally regarded to be intolerant of cadmium (McLusky et al., 1986).
  • Bryan (1984) suggested that the general order for heavy metal toxicity in seaweeds is: Organic Hg > inorganic Hg > Cu > Ag > Zn > Cd > Pb. Cole et al. (1999) reported that Hg was very toxic to macrophytes.
No information concerning the effects of heavy metals on piddocks was found. However, Bryan (1984) stated that Hg is the most toxic metal to bivalve molluscs and that mortalities occurred above 0.1-1 µg/l after 4-14 days of exposure (Crompton, 1997). Scrobicularia plana, another burrowing bivalve, was absent from large areas of metal polluted intertidal mud in the Fal estuary, Cornwall, where under normal conditions it would account for a large amount of the biomass (Bryan & Gibbs, 1983). This was thought to be a consequence of the mussels spending so much time avoiding contact with the metals, via valve closure, that they died of anoxia and starvation. Overall, a proportion of the mussel bed and piddock population is likely to be lost. A loss of mesoherbivores, crabs and red algal species will lead to a decline in species richness. Therefore, an intolerance of intermediate has been recorded but with very low confidence. Recoverability will probably be high (see additional information below).
Hydrocarbon contamination
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Hydrocarbon contamination, e.g. from spills of fresh crude oil or petroleum products, may cause significant loss of component species in the biotope, through impacts on individual species viability or mortality, and resultant effects on the structure of the community.
  • Michelson (1978) suggested that oil pollution, and the dispersants used to clean it up, may have been responsible for the disappearance of populations of Pholas dactylus in Brittany, France. However, no other information concerning the specific effects of hydrocarbon was found.
  • The effects of contaminants on Mytilus edulis species were extensively reviewed by Widdows & Donkin, (1992) and Livingstone & Pipe (1992), and summarized in the MarLIN review and Holt et al. (1998). Overall, hydrocarbon tissue burden results in decreased scope for growth and in some circumstances may result in mortalities, reduced abundance or extent of Mytilus edulis (see review).
  • Littorina littorea has been recorded as being highly intolerant to hydrocarbon contamination (see Suchanek, 1993). The abundance of Littorina littorea and other littorinids were reduced after the Esso Bernica oil spill in Sullom Voe in December 1978 but had returned to pre-spill levels by May 1979 (Moore et al., 1995). In heavily impacted sites, subjected to clean-up where communities were destroyed in the process, Littorina littorea took ca 7 years to recover prior abundance (Moore et al., 1995). Widdows et al. (1981) found Littorina littorea surviving in a rockpool, exposed to chronic hydrocarbon contamination due to the presence of oil from the Esso Bernica oil spill. A decrease in abundance of Littorina littorea, a prey species for Carcinus maenas, may increase predation pressure on the mussels.
  • Ulva intestinalis and Ceramium virgatum have also recorded as being highly intolerant to hydrocarbon contamination. Laboratory studies of the effects of oil and dispersants on several red algae species (Grandy 1984 cited in Holt et al. 1995) concluded that they were all intolerant of oil/ dispersant mixtures, with little differences between adults, sporelings, diploid or haploid life stages. O'Brien & Dixon (1976) suggested that red algae were the most sensitive group of algae to oil or dispersant contamination. The loss of these species may reduce the abundance of cryptic fauna and herbivores.
  • The common shore crab Carcinus maenas has been recorded as having a high intolerance to hydrocarbon contamination (see MarLIN review of this species).
The mussels may succumb directly to smothering by oil which is likely to be retained within the mussel matrix resulting is additional mortality to interstitial and infauna species. Although a proportion of the mussel population may survive hydrocarbon contamination, the additional effects on the community and potential for smothering suggest that the biotope will be lost. Similarly, oil on the shore is likely to collect in the vertical burrows of the piddocks which may lead to suffocation and increased mortality. Therefore, an intolerance of high has been recorded.

On wave exposed rocky coasts oil will be removed relatively quickly. Recovery of rocky shore populations was intensively studied after the Torrey Canyon oil spill in March 1967. On shores that were not subject to clean-up procedures, the community recovered within ca 3 years, however, in shores treated with dispersants recovery took 5-8 years but was estimated to take up to 15 years on the worst affected shores (Southward & Southward, 1978; Hawkins & Southward, 1992; Raffaelli & Hawkins, 1999). Therefore, a recoverability of moderate has been recorded (see additional information below).

Radionuclide contamination
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Insufficient information
Changes in nutrient levels
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Pholas dactylus, Petricola pholadiformis, Barnea candida and Mytilus edulis may benefit from moderate nutrient enrichment, especially in the form of organic particulates and dissolved organic matter. The resultant increased food availability may increase growth rates and reproductive potential, and reduce vulnerability to predators.
However, filter feeders are likely to accumulate toxins from toxic algae which may be associated with areas of nutrient enrichment and algal blooms. The accumulation of such toxins in mussels has resulted in the closure of shellfish beds (Shumway, 1992). The toxic algal blooms themselves (or deoxygenation resulting from their death) have been shown to cause tumours, sublethal effects, reproductive failure and to be highly toxic to Mytilus edulis (Pieters et al., 1980; Shumway, 1990; Landsberg, 1996).

Nutrient enrichment may lead to an increase in algal growth but also lead to eutrophication and associated increases in turbidity and suspended sediments (see above), deoxygenation (see below) and the risk of algal blooms. Increased nutrients may increase growth in fast growing species (e.g. Ulva spp.) to the detriment of slower growing species of macroalgae. An increase in ephemeral algae may be detrimental to the mussel bed due to smothering of the mussels and increased drag which will render the mussels more susceptible to dislodgement during increased wave exposure (see above).

Therefore, algal blooms may result in loss of a proportion of the biotope and its associated species and an intolerance of intermediate has been recorded. Recoverability is probably high (see additional information).
Increase in salinity
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This biotope occurs in intertidal areas and will therefore be exposed to some changes in salinity due to precipitation and evaporation. It is possible that piddock burrows may act as a buffer to protect them against large changes in salinity although when the water eventually evaporates from burrows exposed to air at low tide, hypersaline conditions may occur in the burrow. However, the piddocks experience this change on a cyclical basis. Mytilus edulis is considered tolerant of a wide range of salinities (Holt et al., 1998). Therefore tolerant has been suggested.
Decrease in salinity
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This biotope occurs in intertidal areas and will therefore be exposed to some changes in salinity due to precipitation and evaporation. Only limited information was found on the effects of a decrease in salinity on piddock populations. It is possible that their burrows may act as a buffer to protect them against large changes in salinity. Fish & Fish (1996) stated that Barnea candida can live in water with a salinity as low as 20 ppt in estuaries. Petricola pholadiformis is particularly common off the Essex and Thames estuary, e.g. the River Medway (Bamber, 1985) suggesting tolerance of brackish waters. Hyposaline conditions (20% of normal seawater) adversely affected the uptake of glycine in Pholas dactylus (Knight, 1984). It is possible that if fresh water collects in the vertical burrows after, for example, an extended period of rain at low tide, hyposaline conditions may occur temporarily. However, given the short amount of time in which the piddocks could be affected, i.e. low tide, it is unlikely that the piddocks will suffer an enduring adverse effect. Mytilus edulis is considered to be tolerant of a wide range of salinity (see MarLIN reviews for details). The intertidal interstitial invertebrates and epifauna probably experience short term fluctuating salinities, with increased salinity due to evaporation or reduced salinities due to rainfall and freshwater runoff when emersed.

Prolonged reduction in salinity, e.g. from full to reduced may reduce species richness of the biotope. However, the dominant species will probably survive and accordingly, an intolerance of low has been suggested, together with a decline in species richness. Recoverability is likely to be high (see additional information below).

Changes in oxygenation
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Duval (1963a) observed that conditions within the burrows of Petricola pholadiformis were anaerobic and lined with a loose blue/black sludge, suggesting that the species may be relatively tolerant to conditions of reduced oxygen. Eunice Pinn (pers. Comm. To MarLIN) noted that generally when black sludge has been found in piddock burrows, the piddocks were either dead or dying. Knight (1984) reported that Pholas dactylus could survive for 17 hours at 5% oxygen saturation of seawater (approximately 0.33 ml/l dissolved oxygen) and, on return to 90% oxygen saturated seawater, functioned entirely as normal. However, decreasing oxygen levels could also result in water being squirted from the inhalant siphon in this species and the siphon may also be extended into the air during periods of low oxygen (Knight, 1984). This could ultimately lead to an increase in predation. However, in moderately wave exposed to exposed habitats the resultant water movement and turbulence probably provides adequate oxygenation so that deoxygenation at the benchmark is unlikely to occur except under extreme circumstances. Accordingly, intermediate tolerance has been suggested.

Biological Factors

Introduction of microbial pathogens/parasites
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A ciliated protozoan, Syncilancistrumina elegantissima, has been found associated with Pholas dactylus and may be specific to this host (Knight & Thorne, 1982). However, the effects of this parasite are unknown. Information concerning the effects of diseases or parasites on the other two piddock species was not available.

Mytilus spp. host a wide variety of disease organisms and parasites from many animal and plant groups including bacteria, blue green algae, protozoa, boring sponges, boring polychaetes, boring lichen, the intermediary life stages of several trematodes, the copepod Mytilicola intestinalis (red worm disease) and decapods e.g. the pea crab Pinnotheres pisum (Bower, 1992; Bower & McGladdery, 1996). Bower (1992) noted that mortality from parasitic infestation in Mytilus sp. was lower than in other shellfish in which the same parasites or diseases occurred. Mortality may result from the shell boring species such as the polychaete Polydora ciliata or sponge Cliona celata, which weaken the shell increasing the mussels vulnerability to predation (see MarLIN review for details).

Overall, the occurrence of diseases and parasites are probably highly variable but significant infestations may result in loss of a proportion of the mussel population, either through mortality or reproductive failure. Therefore, an intolerance of intermediate has been recorded. Recovery is likely to be high (see additional information below).
Introduction of non-native species
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The American piddock, Petricola pholadiformis is a non-native, boring piddock that was unintentionally introduced from America with the American oyster, Crassostrea virginica, not later than 1890 (Naylor, 1957). Rosenthal (1980) suggested that, from the British Isles, the species has colonized several northern European countries by means of its pelagic larva and may also spread via driftwood, although it usually bores into clay, peat or soft rock shores. In Belgium and The Netherlands Petricola pholadiformis has almost completely displaced the native piddock, Barnea candida (ICES, 1972). However, there is no documentary evidence to suggest that Barnea candida has been displaced in the British Isles (J. Light & I. Kileen, pers. comm. to Eno et al., 1997). The Australian barnacle Elminius modestus may also occur in this biotope. Overall, there is little evidence of this biotope being adversely affected by non-native species. Petricola pholadiformis and Barnea candida co-occur in this biotope, therefore an assessment of tolerant has been made.
Extraction
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The only regularly harvested key species to occur in this biotope is Mytilus edulis. Pholas dactylus is also known to be harvested in Britain but not to the same extent. In Italy, harvesting of piddocks has had a destructive impact on habitats and has now been banned (E. Pinn, pers. Comm. To MarLIN). In Britain, collection of piddocks is thought to have a similarly destructive effect. People have been known to go out onto the shore and, with the use of a hammer and chisel, excavate the piddocks from the soft rock (K. Hiscock, pers. Comm.). This would be catastrophic for the biotope. The stability of the soft rock would be reduced and potentially lead to the loss of the vast majority of piddocks that inhabit the top ten centimetres of the substratum. Farming methods are being investigated as an alternative and it is therefore possible that further targeted extraction could be a future possibility.

Holt et al. (1998) suggest that when collected by hand at moderate levels using traditional skills mussel beds will probably retain most of their biodiversity. They also cite incidences of over-exploitation of easily accessible small beds by anglers for bait. Holt et al., (1998) suggest that in particular embayments over-exploitation may reduce subsequent recruitment leading to long term reduction in the population or stock. Due to the position of this biotope within the eulittoral and the fact that the mussel patches are likely to be small, removal of a significant proportion of the Mytilus edulis is unlikely. Collection of Littorina littorea is likely to reduce its abundance although the effects on the biotope as a whole are likely to be slight.

However, even a small amount of piddock exploitation would lead to the loss of a proportion of the biotope and accordingly, intolerance has been assessed as intermediate. Recovery is likely to be moderate (see additional information).

Additional information icon Additional information

Richter & Sarnthein (1976) looked at the re-colonization of different sediments by various molluscs on suspended platforms in Kiel Bay, Germany. The platforms were suspended at 11, 15 and 19 m water depth, each containing three round containers filled with clay, sand, or gravel. Substratum type was found to be the most important factor for the piddock Barnea candida, although for all other species it was depth. This highlights the significance of the availability of a suitable substratum to the recovery of piddock species. Richter & Sarnthein (1976) found that within the two year study period the piddocks grew to represent up to 98% of molluscan fauna on clay platforms. Piddock species have also shown very high growth rates of up to 54 mm in 30 months in the laboratory (Arntz & Rumohr, 1973). However, the process of colonization on clay at 15 and 19 m was found to be highly discontinuous, as reflected by the repeated growth and decrease of specimen numbers. In addition, Mytilus edulis showed temporary dominance on clay, linked to heavy settlement by Mytilus edulis larvae. Duval (1977) proposed that it was as a result of the extensive borings of Barnea candida that facilitated the colonization of an area in the Thames Estuary by the introduced American piddock, Petricola pholadiformis. This suggests that Barnea candida is a more competitive colonizing species in clay environments than the American piddock and it is possible that this species will appear first on cleared substrates. Recolonization of the piddock component by pelagic larvae is likely to occur within five years, although possibly not to its original abundance. The sporadic colonization seen in the Kiel Bay experiment above should also be noted.

Mytilus edulis is highly fecund but larval mortality is high. Indeed this is probably true of most bivalves. Larval development occurs within the plankton over ca 1 month (or more), with high dispersal potential. Recruitment within the population is possible when larvae may be entrained within enclosed coasts but it is likely that larval produced in open coast examples of the biotope are swept away from the biotope to settle elsewhere. Larval supply and settlement could potentially occur annually. However, settlement is sporadic with unpredictable pulses of recruitment (Lutz & Kennish, 1992; Seed & Suchanek, 1992). Once settled, Mytilus edulis can reproduce within its first year if growth conditions allow. High intertidal and less exposed sites recovered slower than low shore, more exposed sites. Mytilus spp. populations were considered to have a strong ability to recover from environmental disturbance (Seed & Suchanek, 1992; Holt et al., 1998). While good annual recruitment is possible, recovery of gaps in the mussel population may take up to 5 years. However, where the biotope is significantly damaged, recovery of the mussel population may be delayed by 1-7 years.

Therefore, a recognizable biotope may take between 5 -10 years to recover depending on local conditions. However, it should be noted that in certain circumstances and under some environmental conditions recovery may take significantly longer.

This review can be cited as follows:

Marshall, C.E. 2008. Mytilus edulis and piddocks on eulittoral firm clay. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 21/09/2014]. Available from: <http://www.marlin.ac.uk/habitatbenchmarks.php?habitatid=95&code=1997>