Comparative studies on Cerastoderma edule (L.) and Cerastoderma glaucum (Poiret)
Comparative investigations of many aspects of the biology of the cockles Cerastoderma edule (L.) and Cerastoderma glaucum (Poiret) have been conducted. The nomenclature and systematic status of the two cockles has been reviewed. The The distribution of C.edule and C.glaucum in the British Isles has been investigated. Field studies were conducted to determine: (1) validity of the morphologica1 characters previously used to separate the two cockle types. (2) the relationship between adjacent populations of the two cockles and (3) habitat preferences of the cockles. A mixed population of C.edule and C.glaucum from the estuary of the River Crouch, Essex, has been examined from a comparative standpoint. The demographic properties, shore distribution and she11-ribbing of the Crouch Cerastoderma populations have been investigated. A study of the reproductive cycles of the Crouch cockles has established that in two seasons 1968 and 1969, C.edule spawned seven weeks' earlier than C.glauca. This factor explains how the two genetic types are maintained within a common environment. An investigation into the symbiont fauna of the cockles also lends support to the specific separation between C.edule and C.glaucum. A difference has been shown in the behaviour of the two cockles in air; C.edule breaths air during aerial exposure. The lower survival of C.glaucum in air compared with C.edule is suggested to be one factor accounting for the absence of this cockle from high shore levels. It is observed that the digestive gland colouration differed in the two cockles; being black in C.glaucum but variably coloured in C.edule. Spectrophotometric analysis of acetone extracts of the digestive gland was conducted in an attempt to establish the cause of this difference. A polar carotenoid has been discovered to be abundant in the digestive gland of C.glaucum. The lipids of the digestive gland and magnesium content of the shell have also been investigated. The relationship between the two cockles is concluded to be at a specific level.