|Researched by||Jacqueline Hill||Refereed by||Dr Eunice Pinn|
|Other common names||-||Synonyms||-|
Pholas dactylus is a boring bivalve, approximately elliptical in outline with a beaked anterior end, up to 12 cm long. The shell is thin and brittle with a sculpture of concentric ridges and radiating lines. The shell is dull white or grey in colour, the periostracum yellowish and often discoloured. The siphons are joined and at least one to two times the length of the shell, white to light ivory in colour. Pholas dactylus has phosphorescent properties, the outlines of the animal glowing with a green-blue light in the dark.
- none -
|Phylum||Mollusca||Snails, slugs, mussels, cockles, clams & squid|
|Class||Bivalvia||Clams, cockles, mussels, oysters, and scallops|
|Order||Myida||Gapers, piddocks, and shipworms|
|Male size range||up to 120mm|
|Male size at maturity|
|Female size range||Medium(11-20 cm)|
|Female size at maturity|
|Growth rate||Data deficient|
|Body flexibility||Low (10-45 degrees)|
|Characteristic feeding method||Active suspension feeder, No information|
|Typically feeds on||Suspended organic particles|
|Is the species harmful?||No|
Pholas dactylus is an edible species. However, it is rarely collected for food in Britain.
Live individuals do not support other species but old burrows provide refugia for other species and this has an influence on overall diversity.
|Physiographic preferences||Open coast, Strait / sound, Enclosed coast / Embayment|
|Biological zone preferences||Lower eulittoral, Sublittoral fringe|
|Substratum / habitat preferences||Bedrock|
|Tidal strength preferences||No information|
|Wave exposure preferences||No information|
|Salinity preferences||Full (30-40 psu)|
|Depth range||To 35m|
|Other preferences||No text entered|
|Migration Pattern||No information found|
|Reproductive type||Gonochoristic (dioecious)|
|Reproductive frequency||Annual episodic|
|Fecundity (number of eggs)||No information|
|Generation time||Insufficient information|
|Age at maturity||Insufficient information|
|Season||June - August|
|Life span||up to 14 years|
|Duration of larval stage||No information|
|Larval dispersal potential||No information|
|Larval settlement period||Insufficient information|
This MarLIN sensitivity assessment has been superseded by the MarESA approach to sensitivity assessment. MarLIN assessments used an approach that has now been modified to reflect the most recent conservation imperatives and terminology and are due to be updated by 2016/17.
|Pholas dactylus lives permanently in a burrow excavated in soft rock, peat or similar substrata. Substratum loss will result in the death of the animal because when removed from its burrow and placed on the surface, it cannot excavate a new chamber (Barnes, 1980) and will be at risk from desiccation and predation. Provided a similar substratum remains and there is larval availability, recolonization is likely to occur and so recovery within five years should be possible, though maybe not to previous abundance.|
|Intolerance to smothering is expected to be low because feeding apparatus can be cleared of particles although this will be energetically costly. Experimental work with Pholas dactylus showed that large particles can either be rejected immediately in the pseudofaeces or passed very quickly through the gut (Knight, 1984). In Exmouth, Knight (1984) found Pholas dactylus covered in a layer of sand and in Eastbourne individuals live under a layer of sand with siphons protruding at the surface (E. Pinn pers. comm.). However, smothering by impermeable material such as oil or tar is likely to result in the death of individuals. On return to normal conditions recolonization by pelagic larvae is likely and recovery within five years should be possible.|
|Intolerance to siltation is likely to be low because Pholas dactylus produces sediment in the process of burrow drilling. This sediment is eliminated by taking it into the mantle cavity and then ejecting it with the pseudofaeces through the gut. Experimental work with Pholas dactylus showed that large fragments are either rejected immediately in the pseudofaeces or passed very quickly through the gut (Knight, 1984). An increase in the organic content of suspended sediment is likely to be beneficial to suspension feeders such as the common piddock. Occurrence of Pholas dactylus has been recorded from silty habitats in north Yorkshire (JNCC, 1999).|
|Pholas dactylus inhabits the shallow sub-tidal and lower shore so is likely to have some tolerance of desiccation. However, the species is fixed in position within its burrow and the shell does not completely close to protect against water loss so intolerance to an increase in desiccation is assessed as intermediate. An increase in desiccation at the level of the benchmark, equivalent to a change in position of one vertical biological zone on the shore is likely to result in the death of many individuals particularly at the top of the populations' range. Pholas dactylus is likely to be tolerant to a decrease in desiccation and may be able to extend its range up-shore. Recolonization by pelagic larvae is likely to occur and recovery within 5 years, though maybe not to previous abundance, is expected.|
|Pholas dactylus is fixed in position within its burrow and so will be exposed to changes in emergence. An increase in emergence may cause the death of some individuals at the upper limit of the species range because of increased desiccation. During an extended period of exposure animals squirt some water from their inhalant siphon and extend their gaping siphons into the air (Knight, 1984). Recolonization by pelagic larvae is likely to occur and recovery of the population within 5 years is expected.|
|Low||Very high||Very Low||Low|
|Pholas dactylus is fixed permanently within a burrow and is unlikely to be washed away by an increase in water flow rate. However, a significant increase in water flow rates may interfere with suspension feeding and may also increase rates of substratum erosion. A change in turbidity associated with changing water flow rate may affect the supply of particulate matter available for suspension feeding (see turbidity). Changes in food supply are likely to have an impact on growth and fecundity. On return to normal water flow rates typical suspension feeding, growth and fecundity should resume.|
|Pholas dactylus is a southern species and occurrence in Britain represents the northern limit of its range. An increase in temperature may allow the species to extend its presence further north. The animals are able to spawn all through the summer and usually have released their gametes by the end of August when the temperature of the water is about 19°C. However, in the summer of 1982 all the animals had spawned by the end of July and this early spawning correlated with an earlier than usual increase in temperature (Knight, 1984). Spawning can be induced by increasing the water temperature. A decrease in temperature will probably have a detrimental effect on colonies because Pholas dactylus is fixed in position and unable to move and may impair the reproductive potential of the species. At a temperature of 7°C Pholas dactylus did not siphon actively and oxygen consumption was much lower than that observed at between 15 and 18°C when the animals were seen to be siphoning actively (Knight, 1984). During the exceptionally cold winter of 1962-3 no living individuals of Pholas dactylus could be found above the low-water mark at Lyme Regis in the southwest of England (Crisp, 1964). Cold certainly kills individuals (E. Pinn pers. comm.) and so intolerance is assessed as intermediate. Recolonization by pelagic larvae is likely to occur and recovery within five years, though maybe not to previous abundance.|
|Pholas dactylus lives in chalk areas where water can be very turbid. A change in light availability due to changes in turbidity is unlikely to affect Pholas dactylus directly because the species is a suspension feeder. However, changes in turbidity determines the amount of light available for primary production by phytoplankton, benthic microalgae and macroalgae and may therefore, affect food availability affecting growth and reproductive potential. At high levels, the suspended sediment that causes turbidity may clog feeding apparatus but this effect is included in siltation'. Therefore, changes in turbidity at the level of the benchmark are unlikely to result in the loss of individuals and so intolerance is assessed as low.|
|Pholas dactylus is fixed permanently within a burrow and so is unlikely to be damaged or removed by exposure to wave action. However, in soft substratum habitats long term increases in wave exposure will cause erosion and a consequent loss of habitat. Changes in wave exposure may influence the supply of particulate matter for suspension feeding.|
|Pholas dactylus probably has limited facility for detection of noise. However, the species can probably detect the vibration caused by predators and will withdraw its siphons, ejecting water from the burrow as it does so. Humans walking over piddock grounds often get squirted as the animals pull down into their burrows in response to human movement. On removal of noise or vibration disturbance normal behaviour will resume.|
|Pholas dactylus reacts to changes in light intensity by withdrawing its siphon which may be an adaptive response to avoid predation by shore birds and fish (Knight, 1984). However, the visual presence of boats or humans is not likely to be detrimental to Pholas dactylus communities. On removal of visual disturbance normal behaviour will resume.|
|The shell of Pholas dactylus is 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. However, because the common piddock lives within a burrow in soft rock, generally only those individuals close to the surface will be damaged by an abrasive force or physical disturbance. Individuals living in softer the substrata such as clays or peats may be more vulnerable. Therefore, an intolerance of intermediate has been recorded to represent the possible loss of a proportion of the population. Recolonization of the affected area by pelagic larvae is likely to occur and with several months spawning every year recovery within five years is expected.|
|Intolerance to removal from the substratum and displacement from original position onto a suitable substratum is high because Pholas dactylus cannot excavate a new chamber (Barnes, 1980) and so will die from predation or desiccation. Provided a suitable substratum remains and there is larval availability (the species spawns throughout the summer), recolonization is likely to occur and so recovery within five years should be possible, though maybe not to previous abundance.|
|Although no information on the specific effects of chemicals on Pholas dactylus was found TBT has been found to be toxic to many adult bivalves. Reports of reductions in numbers of bivalves in estuaries with high pleasure craft activity, have provided evidence of the high toxicity of TBT to bivalves (Beaumont et al., 1989). Laboratory toxicity trials have demonstrated that growth in oysters is inhibited by TBT (Waldock & Thain, 1983). In microcosm studies Beaumont et al. (1989) demonstrated that levels of 1-2µg/l TBT can rapidly kill adult bivalves in their natural habitat. For example, all Cerastoderma edule individuals died within two weeks at 1-3µg/l TBT concentrations and 80% died after 17 weeks at a TBT concentration of 0.06-0.17µg/l and Scrobicularia plana (Beaumont et al., 1989). Cerastoderma edule was found to be more intolerant of TBT than Scrobicularia plana in toxicity trials and was thought to be a reflection of the mode of feeding of the two species with filter feeding being a more direct route delivering a higher burden of the toxic material to the animal. Therefore, as a filter feeding bivalve Pholas dactylus it is likely that this species is also highly intolerant of TBT. Pholas dactylus spawns for several months every year, so when normal conditions resume rapid recolonization by the pelagic larvae is likely.|
|Bryan (1984) states that Hg is the most toxic metal to bivalve molluscs while Cu, Cd and Zn seem to be most problematic in the field. In bivalve molluscs Hg was reported to have the highest toxicity, mortalities occurring above 0.1-1 µg/l after 4-14 days exposure (Crompton, 1997), toxicity decreasing from Hg > Cu and Cd > Zn > Pb and As > Cr ( in bivalve larvae, Hg and Cu > Zn > Cd, Pb, As, and Ni > to Cr). In investigations of faunal distribution in the metal contaminated Restronguet Creek in the Fal estuary bivalve molluscs appear to be the most vulnerable (Bryan, 1984). The bivalve Scrobicularia plana, for example, is absent from large areas of the intertidal muds where, under normal conditions, it would account for a large amount of the biomass (Bryan & Gibbs, 1983). Bryan (1984) also reports that metal-contaminated sediments can exert a toxic effect on burrowing bivalves and so intolerance has been assessed as intermediate. The embryonic and larval stages of bivalves are the most intolerant of heavy metals (Bryan, 1994). Pholas dactylus spawns for several months every year, so when normal conditions resume rapid recolonization by the pelagic larvae is likely.|
|No information||No information||No information||Not relevant|
|No information||No information||No information||Not relevant|
|No information||No information||No information||Not relevant|
|The species inhabits the lower intertidal zone and so will be exposed to some changes in salinity due to precipitation. However, Pholas dactylus is a marine species, permanently fixed within its burrow and unable to avoid changes in salinity. A change in salinity at the level of the benchmark is likely to result in the species being outside its habitat preference so intolerance has been assessed as intermediate. Pholas dactylus spawns for several months every year, so when normal conditions resume rapid recolonization by the pelagic larvae is likely.|
|There is no information regarding the tolerance of Pholas dactylus to changes in oxygen concentration. Cole et al., (1999) suggest possible adverse effects on marine species below 4 mg/l and probable adverse effects below 2mg/l. However, as an intertidal species Pholas dactylus is able to gain oxygen from the air during periods of emersion. In experiments with oxygen levels the species was able to tolerate water oxygen saturation of only 5% for about 17 hours by 'air gaping', that is extending the inhalent siphon into air (Knight, 1984). Therefore, intolerance has been assessed as low. Knight (1984) found Pholas dactylus living in peat with a very high concentration of hydrogen sulphide suggesting a tolerance to low oxygenation. On return to normal conditions recovery should be rapid.|
|No information||Not relevant||No information||Not relevant|
|A ciliated protozoon, Syncilancistrumina elegantissima, has been found associated with Pholas dactylus and may be specific to this host (Knight & Thorne, 1982). However, the effect of the protozoon, which inhabits the gills and mantle cavity of Pholas dactylus is unknown.|
|Tolerant||Not relevant||Not sensitive||Moderate|
|The American piddock Petricola pholadiformis has become established along south and east coasts of England from Lyme Regis in Dorset to the Humber. It is most common off Essex and the Thames estuary and is more similar to the hyposaline tolerant white piddock, Barnea candida. There is no documentary evidence, however, that Petricola pholadiformis has displaced any native piddocks (Eno et al., 1997). There may however, be some competition between Pholas dactylus and Petricola pholadiformis for substratum (E. Pinn pers. comm.).|
|Although Pholas dactylus is edible it is not widely harvested in Britain. In Italy, harvesting of piddocks has had a destructive impact on habitats and has now been banned (E. Pinn pers. comm.). Farming methods are being investigated as an alternative. It is possible therefore, that targeted extraction could be a future possibility. However, if extracted recovery should be high because recolonization by pelagic larvae should be rapid and return to normal population levels possible within five years.|
|Tolerant||No information||Not sensitive||Very low|
|Pholas dactylus has no known obligate relationships. Extraction of other species is not likely to have any effect on a Pholas dactylus habitat.|
- no data -
|National (GB) importance||-||Global red list (IUCN) category||-|
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Beaumont, A.R., Newman, P.B., Mills, D.K., Waldock, M.J., Miller, D. & Waite, M.E., 1989. Sandy-substrate microcosm studies on tributyl tin (TBT) toxicity to marine organisms. Scientia Marina, 53, 737-743.
Bryan, G.W. & Gibbs, P.E., 1983. Heavy metals from the Fal estuary, Cornwall: a study of long-term contamination by mining waste and its effects on estuarine organisms. Plymouth: Marine Biological Association of the United Kingdom. [Occasional Publication, no. 2.]
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Knight, R. & Thorne, J., 1982. Syncilancistrumina elegantissima (Scuticociliatida: Thigmotrichina), a new genus and species of ciliated protozoon from Pholas dactylus (Mollusca: Bivalvia), the common piddock. Protistologica, 18, 53-66.
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Waldock, M.J. & Thain, J.E., 1983. Shell thickening in Crassostrea gigas: organotin antifouling or sediment induced? Marine Pollution Bulletin, 14, 411-415.
Wood, C., 1984. Sussex sublittoral survey. Selsey Bill to Beachy Head. (Contractor: Marine Conservation Society, South East Branch), unpublished report to Nature Conservancy Council, CSD Report, no. 527.
This review can be cited as:
Last Updated: 07/09/2006