Biodiversity & Conservation

American piddock - Petricola pholadiformis

Petricola pholadiformis

Image National Museum Wales - External view of Petricola pholadiformis valve. Image width ca 6 cm.
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Distribution map

Petricola pholadiformis recorded (dark blue bullet) and expected (light blue bullet) distribution in Britain and Ireland (see below)

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Petricola pholadiformis is not listed under any importance categories.

Taxonomy icon Taxonomy Taxon English term
Phylum Mollusca Snails, slugs, mussels, cockles, clams & squid
Class Bivalvia Clams, cockles, mussels, oysters, and scallops
Authority Lamarck, 1818
Recent synonyms Petricolaria pholadiformis
Map icon Recorded Distribution in Britain and Ireland Present along south and east coasts of England from Lyme Regis, Dorset, to the Humber. It is most common off the coast of Essex and the Thames estuary (River Medway) (Duval, 1963a; Bamber, 1985). Isolated records from north Wales and Cornwall.
Habitat information icon Habitat information Petricola pholadiformis is a mechanical borer into hard clay, chalk, solid mud, peat-moss and limestone from the midtide level to low water. Although dredged from a depth of 8 m, its occurrence in deeper water is not known (Tebble, 1967).
Text page icon Description The piddock has a thin, brittle shell that is similar in shape and sculpturing on both sides. The beaks (tips) of each valve are in the anterior half and turn downward and inwards. The whole shell appears in outline to be elongate and oval with a maximum length of ca 6.35 cm. The surface of the shell is sculptured with numerous concentric lines crossed by >40 radiating ribs, which in the anterior half of the shell are large and coarse, bearing prominent spines. Outer surface of shell is off-white or fawn in colour with a dark brown periostracum. The ligament (which opens the shell) is external and prominent, stretching a quarter of the way between the beaks to the posterior margin. Petricola pholadiformis may be confused with Barnea candida, the native white piddock of the Pholadacea (Tebble, 1976).
Identifying features
  • Thin, brittle, equivalve and inequilateral shell.
  • Beaks in anterior half, turned downwards and inwards, sometimes set back from dorsal edge.
  • Shell elongate and oval in outline.
  • Shell, off-white or fawn in colour, with dark brown periostracum.
  • Sculpturing of concentric rings crossed anteriorly by about 40 ribs, that are spined at the anterior end.
  • Ligament prominent.
  • Interior of shell white, with the coarse anterior ribs showing through.
  • Anterior margin of shell crenulate where the large ribs meet it, elsewhere smooth.
  • Petricola pholadiformis bears a superficial resemblance to Barnea candida, but has 2 cardinal teeth in the right valve and 3 in the left valve where Barnea candida has none.
Additional information icon Additional information Common names
Petricola pholadiformis may also be known as the 'false angel wing'.
Method of introduction and spread
Petricola pholadiformis is a non-native, boring piddock that was unintentionally introduced from America with the American oyster, Crassostrea virginica, not later than 1890 (Naylor, 1957). Rosenthal (1980) suggested that from the British Isles, the species has colonized several northern European countries by means of its pelagic larva and may also spread via driftwood, although it usually bores into clay, peat or soft rock shores.
In Belguim and The Netherlands Petricola pholadiformis has almost completely displaced the native piddock, Barnea candida (ICES, 1972). However, there is no documentary evidence to suggest that native piddocks have been displaced in the British Isles (J.Light & I.Kileen pers. comm. to Eno et al., 1997).

This review can be cited as follows:

Georgina Budd 2005. Petricola pholadiformis. American piddock. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 28/11/2015]. Available from: <>