Common piddock (Pholas dactylus)

Large specimen of Pholas dactylus from off Mumbles Head.

Photographer: Jon Moore Copyright: Jon Moore

The common piddock Pholas dactylus, South Wales.

Photographer: Judith Oakley Copyright: Judith Oakley

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

Researched by	Jacqueline Hill	Refereed by	Dr Eunice Pinn
Researched by Jacqueline Hill	Refereed by Dr Eunice Pinn
Authority	Linnaeus, 1758
Authority Linnaeus, 1758
Other common names	-	Synonyms	-
Other common names -	Synonyms -

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 -

Biology review

Taxonomy

Level	Scientific name	Common name
Phylum	Mollusca	Snails, slugs, mussels, cockles, clams & squid
Snails, slugs, mussels, cockles, clams & squid
Class	Bivalvia	Clams, cockles, mussels, oysters, and scallops
Clams, cockles, mussels, oysters, and scallops
Order	Myida	Gapers, piddocks, and shipworms
Gapers, piddocks, and shipworms
Family	Pholadidae
Genus	Pholas
Authority	Linnaeus, 1758
Recent Synonyms

Biology

Parameter	Data
Typical abundance
Male size range	up to 120mm
Male size at maturity
Female size range	Medium(11-20 cm)
Female size at maturity
Growth form	Bivalved
Growth rate	Data deficient
Body flexibility	Low (10-45 degrees)
Mobility
Characteristic feeding method	Active suspension feeder, No information
Diet/food source
Typically feeds on	Suspended organic particles
Sociability
Environmental position	Lithotomous
Dependency	Independent.
Supports	None
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

Parameter	Data
Physiographic preferences	Open coast, Strait or Sound, Enclosed coast or 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

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

Parameter	Data
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

Larval characteristics

Parameter	Data
Larval/propagule type	-
Larval/juvenile development	Planktotrophic
Duration of larval stage	No information
Larval dispersal potential	No information
Larval settlement period	Insufficient 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?

The MarLIN sensitivity assessment approach used below has been superseded by the MarESA (Marine Evidence-based Sensitivity Assessment) approach (see menu). The MarLIN approach was used for assessments from 1999-2010. The MarESA approach reflects the recent conservation imperatives and terminology and is used for sensitivity assessments from 2014 onwards.

Physical pressures

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	Intolerance	Recoverability	Sensitivity	Evidence / Confidence
Substratum loss [Show more] Substratum loss Benchmark. All of the substratum occupied by the species or biotope under consideration is removed. A single event is assumed for sensitivity assessment. Once the activity or event has stopped (or between regular events) suitable substratum remains or is deposited. Species or community recovery assumes that the substratum within the habitat preferences of the original species or community is present. Further details Evidence 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.	High	High	Moderate	Moderate
Smothering [Show more] Smothering Benchmark. All of the population of a species or an area of a biotope is smothered by sediment to a depth of 5 cm above the substratum for one month. Impermeable materials, such as concrete, oil, or tar, are likely to have a greater effect. Further details. Evidence 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
Increase in suspended sediment [Show more] Increase in suspended sediment Benchmark. An arbitrary short-term, acute change in background suspended sediment concentration e.g., a change of 100 mg/l for one month. The resultant light attenuation effects are addressed under turbidity, and the effects of rapid settling out of suspended sediment are addressed under smothering. Further details Evidence 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).	Low	High	Low	Moderate
Decrease in suspended sediment [Show more] Decrease in suspended sediment Benchmark. An arbitrary short-term, acute change in background suspended sediment concentration e.g., a change of 100 mg/l for one month. The resultant light attenuation effects are addressed under turbidity, and the effects of rapid settling out of suspended sediment are addressed under smothering. Further details Evidence			No information
Desiccation [Show more] Desiccation A normally subtidal, demersal or pelagic species including intertidal migratory or under-boulder species is continuously exposed to air and sunshine for one hour. A normally intertidal species or community is exposed to a change in desiccation equivalent to a change in position of one vertical biological zone on the shore, e.g., from upper eulittoral to the mid eulittoral or from sublittoral fringe to lower eulittoral for a period of one year. Further details. Evidence 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
Increase in emergence regime [Show more] Increase in emergence regime Benchmark. A one hour change in the time covered or not covered by the sea for a period of one year. Further details Evidence 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.	Intermediate	High	Low	Low
Decrease in emergence regime [Show more] Decrease in emergence regime Benchmark. A one hour change in the time covered or not covered by the sea for a period of one year. Further details Evidence			No information
Increase in water flow rate [Show more] Increase in water flow rate A change of two categories in water flow rate (view glossary) for 1 year, for example, from moderately strong (1-3 knots) to very weak (negligible). Further details Evidence 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.	Low	Very high	Very Low	Low
Decrease in water flow rate [Show more] Decrease in water flow rate A change of two categories in water flow rate (view glossary) for 1 year, for example, from moderately strong (1-3 knots) to very weak (negligible). Further details Evidence			No information
Increase in temperature [Show more] Increase in temperature A short-term, acute change in temperature; e.g., a 5°C change in the temperature range for three consecutive days. This definition includes ‘short-term’ thermal discharges. A long-term, chronic change in temperature; e.g. a 2°C change in the temperature range for a year. This definition includes ‘long term’ thermal discharges. For intertidal species or communities, the range of temperatures includes the air temperature regime for that species or community. Further details Evidence 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.	Intermediate	High	Low	High
Decrease in temperature [Show more] Decrease in temperature A short-term, acute change in temperature; e.g., a 5°C change in the temperature range for three consecutive days. This definition includes ‘short-term’ thermal discharges. A long-term, chronic change in temperature; e.g. a 2°C change in the temperature range for a year. This definition includes ‘long term’ thermal discharges. For intertidal species or communities, the range of temperatures includes the air temperature regime for that species or community. Further details Evidence			No information
Increase in turbidity [Show more] Increase in turbidity A short-term, acute change; e.g., two categories of the water clarity scale (see glossary) for one month, such as from medium to extreme turbidity. A long-term, chronic change; e.g., one category of the water clarity scale (see glossary) for one year, such as from low to medium turbidity. Further details Evidence 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.	Low	Immediate	Not sensitive	Moderate
Decrease in turbidity [Show more] Decrease in turbidity A short-term, acute change; e.g., two categories of the water clarity scale (see glossary) for one month, such as from medium to extreme turbidity. A long-term, chronic change; e.g., one category of the water clarity scale (see glossary) for one year, such as from low to medium turbidity. Further details Evidence			No information
Increase in wave exposure [Show more] Increase in wave exposure A change of two ranks on the wave exposure scale (view glossary) e.g., from Exposed to Extremely exposed for a period of one year. Further details Evidence 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.	Low	High	Low	Low
Decrease in wave exposure [Show more] Decrease in wave exposure A change of two ranks on the wave exposure scale (view glossary) e.g., from Exposed to Extremely exposed for a period of one year. Further details Evidence			No information
Noise [Show more] Noise Underwater noise levels e.g., the regular passing of a 30-metre trawler at 100 metres or a working cutter-suction transfer dredge at 100 metres for one month during important feeding or breeding periods. Atmospheric noise levels e.g., the regular passing of a Boeing 737 passenger jet 300 metres overhead for one month during important feeding or breeding periods. Further details Evidence 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	Low
Visual presence [Show more] Visual presence Benchmark. The continuous presence for one month of moving objects not naturally found in the marine environment (e.g., boats, machinery, and humans) within the visual envelope of the species or community under consideration. Further details Evidence 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.	Low	Immediate	Not sensitive	Moderate
Abrasion & physical disturbance [Show more] Abrasion & physical disturbance Benchmark. Force equivalent to a standard scallop dredge landing on or being dragged across the organism. A single event is assumed for assessment. This factor includes mechanical interference, crushing, physical blows against, or rubbing and erosion of the organism or habitat of interest. Where trampling is relevant, the evidence and trampling intensity will be reported in the rationale. Further details. Evidence 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.	Intermediate	High	Low	High
Displacement [Show more] Displacement Benchmark. Removal of the organism from the substratum and displacement from its original position onto a suitable substratum. A single event is assumed for assessment. Further details Evidence 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.	High	High	Moderate	Moderate

Chemical pressures

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	Intolerance	Recoverability	Sensitivity	Evidence / Confidence
Synthetic compound contamination [Show more] Synthetic compound contamination Sensitivity is assessed against the available evidence for the effects of contaminants on the species (or closely related species at low confidence) or community of interest. For example: evidence of mass mortality of a population of the species or community of interest (either short or long term) in response to a contaminant will be ranked as high sensitivity; evidence of reduced abundance, or extent of a population of the species or community of interest (either short or long term) in response to a contaminant will be ranked as intermediate sensitivity; evidence of sub-lethal effects or reduced reproductive potential of a population of the species or community of interest will be assessed as low sensitivity. The evidence used is stated in the rationale. Where the assessment can be based on a known activity then this is stated. The tolerance to contaminants of species of interest will be included in the rationale when available; together with relevant supporting material. Further details. Evidence 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.	High	High	Moderate	Moderate
Heavy metal contamination [Show more] Heavy metal contamination Evidence 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.	Intermediate	High	Low	Moderate
Hydrocarbon contamination [Show more] Hydrocarbon contamination Evidence Insufficient information.	No information	No information	No information	Not relevant
Radionuclide contamination [Show more] Radionuclide contamination Evidence Insufficient information.	No information	No information	No information	Not relevant
Changes in nutrient levels [Show more] Changes in nutrient levels Evidence Insufficient information.	No information	No information	No information	Not relevant
Increase in salinity [Show more] Increase in salinity A short-term, acute change; e.g., a change of two categories from the MNCR salinity scale for one week (view glossary) such as from full to reduced. A long-term, chronic change; e.g., a change of one category from the MNCR salinity scale for one year (view glossary) such as from reduced to low. Further details. Evidence 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.	Intermediate	High	Low	Low
Decrease in salinity [Show more] Decrease in salinity A short-term, acute change; e.g., a change of two categories from the MNCR salinity scale for one week (view glossary) such as from full to reduced. A long-term, chronic change; e.g., a change of one category from the MNCR salinity scale for one year (view glossary) such as from reduced to low. Further details. Evidence			No information
Changes in oxygenation [Show more] Changes in oxygenation Benchmark. Exposure to a dissolved oxygen concentration of 2 mg/l for one week. Further details. Evidence 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.	Low	High	Low	Moderate

Biological pressures

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	Intolerance	Recoverability	Sensitivity	Evidence / Confidence
Introduction of microbial pathogens/parasites [Show more] Introduction of microbial pathogens/parasites Benchmark. Sensitivity can only be assessed relative to a known, named disease, likely to cause partial loss of a species population or community. Further details. Evidence 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.	No information	Not relevant	No information	Not relevant
Introduction of non-native species [Show more] Introduction of non-native species Sensitivity assessed against the likely effect of the introduction of alien or non-native species in Britain or Ireland. Further details. Evidence 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.).	Tolerant	Not relevant	Not sensitive	Moderate
Extraction of this species [Show more] Extraction of this species Benchmark. Extraction removes 50% of the species or community from the area under consideration. Sensitivity will be assessed as 'intermediate'. The habitat remains intact or recovers rapidly. Any effects of the extraction process on the habitat itself are addressed under other factors, e.g. displacement, abrasion and physical disturbance, and substratum loss. Further details. Evidence 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.	Intermediate	High	Low	Low
Extraction of other species [Show more] Extraction of other species Benchmark. A species that is a required host or prey for the species under consideration (and assuming that no alternative host exists) or a keystone species in a biotope is removed. Any effects of the extraction process on the habitat itself are addressed under other factors, e.g. displacement, abrasion and physical disturbance, and substratum loss. Further details. Evidence Pholas dactylus has no known obligate relationships. Extraction of other species is not likely to have any effect on a Pholas dactylus habitat.	Tolerant	No information	Not sensitive	Very low

Additional information

Importance review

Policy/legislation

- no data -

Status

National (GB) importance	-	Global red list (IUCN) category	-
National (GB) importance -
Global red list (IUCN) category -

Non-native

Parameter	Data
Native	-
Origin	-
Date Arrived	-

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

Barnes, R.D., 1980. Invertebrate Zoology, 4th ed. Philadelphia: Holt-Saunders International Editions.
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.]
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.
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.
Crompton, T.R., 1997. Toxicants in the aqueous ecosystem. New York: John Wiley & Sons.
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.
Fish, J.D. & Fish, S., 1996. A student's guide to the seashore. Cambridge: Cambridge University Press.
Hayward, P., Nelson-Smith, T. & Shields, C. 1996. Collins pocket guide. Sea shore of Britain and northern Europe. London: HarperCollins.
Hayward, P.J. & Ryland, J.S. (ed.) 1995b. Handbook of the marine fauna of North-West Europe. Oxford: Oxford University Press.
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.]
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
Knight, J.H., 1984. Studies on the biology and biochemistry of Pholas dactylus L.. , PhD thesis. London, University of London.
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.
Seaward, D.R., 1982. Sea area atlas of the marine molluscs of Britain and Ireland. Peterborough: Nature Conservancy Council.
Seaward, D.R., 1990. Distribution of marine molluscs of north west Europe. Peterborough: Nature Conservancy Council.
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].
Tebble, N., 1976. British Bivalve Seashells. A Handbook for Identification, 2nd ed. Edinburgh: British Museum (Natural History), Her Majesty's Stationary Office.
Turner, R.D., 1954. The family Pholadidae in the western Atlantic and the eastern Pacific Part 1 - Pholadinae. Johnsonia, 3, 1-64.
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.

Datasets

Bristol Regional Environmental Records Centre, 2017. BRERC species records recorded over 15 years ago. Occurrence dataset: https://doi.org/10.15468/h1ln5p accessed via GBIF.org on 2018-09-25.
Centre for Environmental Data and Recording, 2018. Ulster Museum Marine Surveys of Northern Ireland Coastal Waters. Occurrence dataset https://www.nmni.com/CEDaR/CEDaR-Centre-for-Environmental-Data-and-Recording.aspx accessed via NBNAtlas.org on 2018-09-25.
Conchological Society of Great Britain & Ireland, 2018. Mollusc (marine) data for Great Britain and Ireland - restricted access. Occurrence dataset: https://doi.org/10.15468/4bsawx accessed via GBIF.org on 2018-09-25.
Conchological Society of Great Britain & Ireland, 2023. Mollusc (marine) records for Great Britain and Ireland. Occurrence dataset: https://doi.org/10.15468/aurwcz accessed via GBIF.org on 2024-09-27.
Fenwick, 2018. Aphotomarine. Occurrence dataset http://www.aphotomarine.com/index.html Accessed via NBNAtlas.org on 2018-10-01
Kent Wildlife Trust, 2018. Biological survey of the intertidal chalk reefs between Folkestone Warren and Kingsdown, Kent 2009-2011. Occurrence dataset: https://www.kentwildlifetrust.org.uk/ accessed via NBNAtlas.org on 2018-10-01.
Kent Wildlife Trust, 2018. Kent Wildlife Trust Shoresearch Intertidal Survey 2004 onwards. Occurrence dataset: https://www.kentwildlifetrust.org.uk/ accessed via NBNAtlas.org on 2018-10-01.
Manx Biological Recording Partnership, 2018. Isle of Man historical wildlife records 1990 to 1994. Occurrence dataset: https://doi.org/10.15468/aru16v accessed via GBIF.org on 2018-10-01.
Merseyside BioBank., 2018. Merseyside BioBank (unverified). Occurrence dataset: https://doi.org/10.15468/iou2ld accessed via GBIF.org on 2018-10-01.
Merseyside BioBank., 2018. Merseyside BioBank Active Naturalists (unverified). Occurrence dataset: https://doi.org/10.15468/smzyqf accessed via GBIF.org on 2018-10-01.
National Trust, 2017. National Trust Species Records. Occurrence dataset: https://doi.org/10.15468/opc6g1 accessed via GBIF.org on 2018-10-01.
NBN (National Biodiversity Network) Atlas. Available from: https://www.nbnatlas.org.
Norfolk Biodiversity Information Service, 2017. NBIS Records to December 2016. Occurrence dataset: https://doi.org/10.15468/jca5lo accessed via GBIF.org on 2018-10-01.
OBIS (Ocean Biodiversity Information System), 2025. Global map of species distribution using gridded data. Available from: Ocean Biogeographic Information System. www.iobis.org. Accessed: 2025-05-21
South East Wales Biodiversity Records Centre, 2018. SEWBReC Molluscs (South East Wales). Occurrence dataset: https://doi.org/10.15468/jos5ga accessed via GBIF.org on 2018-10-02.
South East Wales Biodiversity Records Centre, 2018. Dr Mary Gillham Archive Project. Occurance dataset: http://www.sewbrec.org.uk/ accessed via NBNAtlas.org on 2018-10-02

Citation

This review can be cited as:

Hill, J.M. 2006. Pholas dactylus Common piddock. In Tyler-Walters H. and Hiscock K. Marine Life Information Network: Biology and Sensitivity Key Information Reviews, [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 21-05-2025]. Available from: https://www.marlin.ac.uk/species/detail/1403

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Last Updated: 07/09/2006

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