The biology and functional morphology of the predatory septibranch Cardiomya costellata (Deshayes, 1833) (Bivalvia: Anomalodesmata: Cuspidariidae) from the Acores: survival at the edge

Abstract

This is the first comprehensive anatomical study of a representative of the septibranch Cuspidariidae. Particular interest in Cardiomya costellata is related to the fact that only two species of such predatory septibranchs have been recorded from the remote Açorean Archipelago and, here, individuals of both taxa are half the shell length of conspecifics throughout the species’ North-eastern Atlantic range. The shell of C. costellata is thin, fragile and rostrate. This latter attribute allows the inhalant siphon (as in other cuspidarioids) to be extended towards potential prey to effect their capture. How this extension is effected has been described but, herein, the hydrodynamic forces needed to achieve this are put into a firmer anatomical context. Uniquely amongst the Anomalesmata, cuspidarioids have, previously, been regarded as dioecious. This is not the case for C. costellata, which is a protandric consecutive hermaphrodite. The gonads and reproductive strategy of this species are compared with those of the spheniopsid Grippina coronata that is representative of a second cuspidarioid family of deeper water predators and which is a simultaneous hermaphrodite brooding self-fertilized embryos in the gonadial follicles with their release being post mortem. Some evidence suggests that in the Açores, the possible crustacean prey of C. costellata are also smaller than their mainland conspecifics, which, when viewed in the overall context of the predator's biology and anatomy, might explain its poor success in the oligotrophic waters of these mid-Atlantic islands.