MarLIN

information on the biology of species and the ecology of habitats found around the coasts and seas of the British Isles

Common piddock (Pholas dactylus)

Distribution data supplied by the Ocean Biogeographic Information System (OBIS). To interrogate UK data visit the NBN Atlas.

Summary

Description

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.

Recorded distribution in Britain and Ireland

Pholas dactylus occurs in Britain from Kent along the south and south-west coasts including south Wales, Anglesey and Solway. Also recorded from several sites on the east coasts of Yorkshire and Northumbria and southwest Ireland.

Global distribution

Distributed from Britain south to the Iberian Peninsula, the Mediterranean and Black Sea and the Atlantic coast of Morocco.

Habitat

Pholas dactylus bores into a wide range of substrata including various soft rocks such as chalk and sandstone, clay, peat and very occasionally in waterlogged wood. Found from the lower shore to the shallow sublittoral.

Depth range

To 35m

Identifying features

  • Shell thin and brittle, elliptical in shape with a beaked anterior end and bulbous umbones in anterior third of shell.
  • Anterio-ventral margin deeply concave about a large, elliptical pedal gape, posterior margin regularly rounded, not gaping.
  • Shell sculpture of concentric ridges and radiating lines developed as stout tubercles where they intersect, most pronounced anteriorly.
  • Hinge line with slender projection, the apophysis (the point of attachment of foot muscles) just below the beak of each valve.
  • Interior of shell glossy white with the external sculpture faintly visible.
  • Four dorsal accessory plates.
  • Siphons joined and at least one to two times the length of the shell, white to light ivory in colour, papillose and devoid of periostracum except for small band near the posterior.

Additional information

  • The shell is often thicker in older individuals, up to 2 mm thick in 12 cm specimens (E. Pinn pers. comm.).
  • Although thin and brittle the shell of Pholas dactylus has a cross-lamellar design which efficiently deflects cracks away from the bulk of the shell which gives it the strength to burrow through soft rocks.

Listed by

- none -

Further information sources

Search on:

Biology review

Taxonomy

PhylumMollusca
ClassBivalvia
OrderMyoida
FamilyPholadidae
GenusPholas
AuthorityLinnaeus, 1758
Recent Synonyms

Biology

Typical abundance
Male size rangeup to 120mm
Male size at maturity
Female size rangeMedium(11-20 cm)
Female size at maturity
Growth formBivalved
Growth rateData deficient
Body flexibilityLow (10-45 degrees)
Mobility
Characteristic feeding methodActive suspension feeder, No information
Diet/food source
Typically feeds onSuspended organic particles
Sociability
Environmental positionLithotomous
DependencyIndependent.
SupportsNone
Is the species harmful?No

Pholas dactylus is an edible species. However, it is rarely collected for food in Britain.

Biology information

Live individuals do not support other species but old burrows provide refugia for other species and this has an influence on overall diversity.

Habitat preferences

Physiographic preferencesOpen coast, Strait / sound, Enclosed coast / Embayment
Biological zone preferencesLower eulittoral, Sublittoral fringe
Substratum / habitat preferencesBedrock
Tidal strength preferencesNo information
Wave exposure preferencesNo information
Salinity preferencesFull (30-40 psu)
Depth rangeTo 35m
Other preferencesNo text entered
Migration PatternNo information found

Habitat Information

All boring bivalves begin excavation following settling of the larva and slowly enlarge and deepen the burrow with growth. They are forever locked within their burrows, and only the siphons project to the small surface opening (Barnes, 1980). Individuals in waterlogged wood are quite rare and often deformed (E. Pinn pers. comm.).

Life history

Adult characteristics

Reproductive typeGonochoristic (dioecious)
Reproductive frequency Annual episodic
Fecundity (number of eggs)No information
Generation timeInsufficient information
Age at maturityInsufficient information
SeasonJune - August
Life spanup to 14 years

Larval characteristics

Larval/propagule type-
Larval/juvenile development Planktotrophic
Duration of larval stageNo information
Larval dispersal potential No information
Larval settlement periodInsufficient information

Life history information

  • Acetate peel work with Pholas dactylus indicates that the species has a maximum lifespan of 14 years (E. Pinn pers. comm.).
  • There is a free swimming veliger larva which attaches by a byssus at settlement, the byssus later being lost (Fish & Fish, 1996).
  • The gonads start to develop in February or March and are fully mature by the beginning of June. 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).
  • Fertilization is thought to be external and no evidence was found by Knight (1984) to support earlier suggestions that brooding occurs in this species.

Sensitivity reviewHow is sensitivity assessed?

Physical pressures

 IntoleranceRecoverabilitySensitivityEvidence/Confidence
High High Moderate Moderate
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.
Low High Low Moderate
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.
Low High Low Moderate
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).
No information
Intermediate High Low Low
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.
Intermediate High Low Low
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.
No information
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.
No information
Intermediate High Low High
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.
No information
Low Immediate Not sensitive Moderate
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.
No information
Low High Low 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.
No information
Low Immediate Not sensitive Low
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.
Low Immediate Not sensitive Moderate
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.
Intermediate High Low High
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.
High High Moderate Moderate
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.

Chemical pressures

 IntoleranceRecoverabilitySensitivityEvidence/Confidence
High High Moderate Moderate
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.
Heavy metal contamination
Intermediate High Low Moderate
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.
Hydrocarbon contamination
No information No information No information Not relevant
Insufficient
information.
Radionuclide contamination
No information No information No information Not relevant
Insufficient
information.
Changes in nutrient levels
No information No information No information Not relevant
Insufficient
information.
Intermediate High Low Low
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.
No information
Low High Low Moderate
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.

Biological pressures

 IntoleranceRecoverabilitySensitivityEvidence/Confidence
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.).
Intermediate High Low Low
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.

Additional information

Importance review

Policy/legislation

- no data -

Status

Non-native

Importance information

  • May provide a food source for shore birds such as oystercatchers that are frequently seen pecking at rocks as the tide recedes (Knight, 1984).
  • Empty piddock burrows can influence the abundance of species by providing additional habitats and refuges. Eunice Pinn (pers. comm.) found a statistically significant increase in species diversity in areas where old burrows were present compared to where they were absent.

Bibliography

  1. Barnes, R.D., 1980. Invertebrate Zoology, 4th ed. Philadelphia: Holt-Saunders International Editions.

  2. 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.

  3. 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.]

  4. Bryan, G.W., 1984. Pollution due to heavy metals and their compounds. In Marine Ecology: A Comprehensive, Integrated Treatise on Life in the Oceans and Coastal Waters, vol. 5. Ocean Management, part 3, (ed. O. Kinne), pp.1289-1431. New York: John Wiley & Sons.

  5. Crisp, D.J. (ed.), 1964. The effects of the severe winter of 1962-63 on marine life in Britain. Journal of Animal Ecology, 33, 165-210.

  6. Crompton, T.R., 1997. Toxicants in the aqueous ecosystem. New York: John Wiley & Sons.

  7. Eno, N.C., Clark, R.A. & Sanderson, W.G. (ed.) 1997. Non-native marine species in British waters: a review and directory. Peterborough: Joint Nature Conservation Committee.

  8. Fish, J.D. & Fish, S., 1996. A student's guide to the seashore. Cambridge: Cambridge University Press.

  9. Hayward, P., Nelson-Smith, T. & Shields, C. 1996. Collins pocket guide. Sea shore of Britain and northern Europe. London: HarperCollins.

  10. Hayward, P.J. & Ryland, J.S. (ed.) 1995b. Handbook of the marine fauna of North-West Europe. Oxford: Oxford University Press.

  11. Howson, C.M. & Picton, B.E., 1997. The species directory of the marine fauna and flora of the British Isles and surrounding seas. Belfast: Ulster Museum. [Ulster Museum publication, no. 276.]

  12. JNCC (Joint Nature Conservation Committee), 1999. Marine Environment Resource Mapping And Information Database (MERMAID): Marine Nature Conservation Review Survey Database. [on-line] http://www.jncc.gov.uk/mermaid

  13. Knight, J.H., 1984. Studies on the biology and biochemistry of Pholas dactylus L.. , PhD thesis. London, University of London.

  14. 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.

  15. Seaward, D.R., 1982. Sea area atlas of the marine molluscs of Britain and Ireland. Peterborough: Nature Conservancy Council.

  16. Seaward, D.R., 1990. Distribution of marine molluscs of north west Europe. Peterborough: Nature Conservancy Council.

  17. Seaward, D.R., 1993. Additions and amendments to the Distribution of the marine Molluscs of north west Europe. , Joint Nature Conservation Committee, Peterborough. [JNCC Report no. 165].

  18. Tebble, N., 1976. British Bivalve Seashells. A Handbook for Identification, 2nd ed. Edinburgh: British Museum (Natural History), Her Majesty's Stationary Office.

  19. Turner, R.D., 1954. The family Pholadidae in the western Atlantic and the eastern Pacific Part 1 - Pholadinae. Johnsonia, 3, 1-64.

  20. Waldock, M.J. & Thain, J.E., 1983. Shell thickening in Crassostrea gigas: organotin antifouling or sediment induced? Marine Pollution Bulletin, 14, 411-415.

  21. 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.

Citation

This review can be cited as:

Hill, J.M. 2006. Pholas dactylus Common piddock. In Tyler-Walters H. and Hiscock K. (eds) Marine Life Information Network: Biology and Sensitivity Key Information Reviews, [on-line]. Plymouth: Marine Biological Association of the United Kingdom. Available from: http://www.marlin.ac.uk/species/detail/1403

Last Updated: 07/09/2006