Pholas dactylus

Researched By	Jacqueline Hill	Data Supplied By	MarLIN
Refereed by	Dr Eunice Pinn
Taxonomy
Scientific name	Pholas dactylus	Common name	Common piddock
MCS Code	W2177	Recent Synonyms	None
Phylum	Mollusca	Subphylum
Superclass		Class	Pelecypoda
Subclass		Order	Myoida
Suborder	Pholadina	Family	Pholadidae
Genus	Pholus	Species	dactylus
Subspecies
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 form	Bivalved	Feeding method	Active suspension feeder
Mobility/Movement	Burrower	Environmental position	Lithotomous
Typical food types	Suspended organic particles	Habit	Burrow dwelling
Bioturbator		Flexibility	Low (10-45 degrees)
Fragility	Intermediate	Size	Medium(11-20 cm)
Height	Not relevant	Growth Rate	Insufficient information
Adult dispersal potential	Very limited (<1m)	Dependency	Independent
Sociability	Gregarious
Toxic/Poisonous?	No
Additional Information	Live 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 & 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.
Biogeographic range	Not researched	Depth range	To 35 m
Migratory	Insufficient information
Distribution Additional 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.).
Substratum preferences	Bedrock, Artificial (e.g. metal/wood/concrete)	Physiographic preferences	Open coast, Strait / sound, Enclosed coast / Embayment
Biological zone	Lower Eulittoral, Sublittoral Fringe	Wave exposure	Insufficient information
Tidal stream strength/Water flow	Insufficient information	Salinity	Full (30-40 psu)
Habitat Additional Information
Distribution References	Hayward & Ryland, 1995b, Tebble, 1976, Barnes, 1980, Wood, 1984, Seaward, 1990, Seaward, 1993, Seaward, 1982
Reproduction/Life History
Reproductive type	Gonochoristic	Developmental mechanism	Planktotrophic
Reproductive Season	June to August	Reproductive Location	Insufficient information
Reproductive frequency	Annual episodic	Regeneration potential	No
Life span	11-20 years	Age at reproductive maturity	Insufficient information
Generation time	Insufficient information	Fecundity	Insufficient information
Egg/propagule size	Insufficient information	Fertilization type	External
Larvae/Juveniles Larval/Juvenile dispersal potential Insufficient information Larval settlement period Insufficient information Duration of larval stage Insufficient information
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