Observations on the shell morphology of two species of cockle Cerastoderma edule and C. glaucum

Aspects of the shell morphology of the cockles Cerastoderma edule (L.) and C. glaucum (Poiret) collected from the R. Crouch and R. Roach (Essex, England) are considered with reference to interpretation of the environment of sub-fossil assemblages. Discrimination between the two cockle species is best achieved by assessment of qualitative shell characters, quantitative measurements being of rather limited value. Factors influencing shell rib-number differ in the two species of Cerastoderma; within a common environment C. glaucum has fewer ribs than C. edule . Mean rib-number of C. edule is directly related to salinity over the colonized salinity range 20–36.5%₀ whilst in C. glaucum rib-number is lowest in both low (<10%₀) and high (>35%₀) salinity environments, being maximal where salinities between 20–30%₀ occur. Internal shell pigmentation may be absent in C. glaucum from low salinity environments. Magnesium content of whole shells is similar in the two cockle species from a mixed population. In both cockles, however, the prismatic layer contained more magnesium than the nacreous shell layer. The most useful molluscan indicators of brackish water lagoon environments are C. glaucum and Hydrobia ventrosa; these species being absent from areas of only limited wave action. C. edule may be associated with marine or estuarine molluscan indicators.