BIOTIC Species Information for Pholas dactylus
Researched byJacqueline Hill Data supplied byMarLIN
Refereed byDr Eunice Pinn
Scientific namePholas dactylus Common nameCommon piddock
MCS CodeW2177 Recent SynonymsNone

PhylumMollusca Subphylum
Superclass ClassPelecypoda
Subclass OrderMyoida
SuborderPholadina FamilyPholadidae
GenusPholus Speciesdactylus

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.
Taxonomy References Hayward et al., 1996, Fish & Fish, 1996, Hayward & Ryland, 1995b, Howson & Picton, 1997, Tebble, 1976, Turner, 1954,
General Biology
Growth formBivalved
Feeding methodActive suspension feeder
Environmental positionLithotomous
Typical food typesSuspended organic particles HabitBurrow dwelling
Bioturbator FlexibilityLow (10-45 degrees)
FragilityIntermediate SizeMedium(11-20 cm)
HeightNot relevant Growth RateInsufficient information
Adult dispersal potentialVery limited (<1m) DependencyIndependent
General Biology Additional InformationLive individuals do not support other species but old burrows provide refugia for other species and this has an influence on overall diversity.
Biology References Knight, 1983, Barnes, 1980,
Distribution and Habitat
Distribution in Britain & IrelandPholas 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 distributionDistributed from Britain south to the Iberian Peninsula, the Mediterranean and Black Sea and the Atlantic coast of Morocco.
Biogeographic rangeNot researched Depth rangeTo 35 m
MigratoryInsufficient information   
Distribution Additional InformationAll 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.).

Substratum preferencesBedrock
Artificial (e.g. metal/wood/concrete)
Physiographic preferencesOpen coast
Strait / sound
Enclosed coast / Embayment
Biological zoneLower Eulittoral
Sublittoral Fringe
Wave exposureInsufficient information
Tidal stream strength/Water flowInsufficient information
SalinityFull (30-40 psu)
Habitat Preferences Additional Information
Distribution References Hayward & Ryland, 1995b, Tebble, 1976, Barnes, 1980, Wood, 1984, Seaward, 1990, Seaward, 1993, Seaward, 1982,
Reproduction/Life History
Reproductive typeGonochoristic
Developmental mechanismPlanktotrophic
Reproductive SeasonJune to August Reproductive LocationInsufficient information
Reproductive frequencyAnnual episodic Regeneration potential No
Life span11-20 years Age at reproductive maturityInsufficient information
Generation timeInsufficient information FecundityInsufficient information
Egg/propagule sizeInsufficient information Fertilization typeExternal
Larval/Juvenile dispersal potentialInsufficient information Larval settlement periodInsufficient information
Duration of larval stageInsufficient information   
Reproduction Preferences Additional Information
  • Acetate peel work with Pholas dactylus indicates that the species has a maximum life span 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.
Reproduction References Fish & Fish, 1996, Knight, 1983,
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