Observations on size composition and breeding of Northumberland populations of Zirphaea crispata (Pholadidae: Bivalvia)

Along the Northumberland coast, Zirphaea crispata (Linné) occurs in soft shales from approximately 1 ft (0.2 m) above datum to at least 20 fathoms (37 m). It also occurs sublittorally in dumped power station fly ash wastes. The populations are polymodal. Growth rate is estimated at approximately 8 mm a year with a life span of approximately 5 years in the case of those found intertidally and 6 to 7 years in the case of the sublittoral populations. Although breeding occurs from March to October, there is evidence that there are two main breeding peaks, one from March to May and the other from August to October. Differences in shell shape between different populations can probably be explained by the differences in effort required to drill into the substratum.