Self Righting by Tridacnid Clams

Abstract

TRIDACNID clams can be divided into two distinct groups on the basis of whether they are physically fastened to the substrate. Of the six species comprising the family Tridacnidae¹, Tridacna crocea, T. maxima and T. squamosa live byssally attached to dead coral, coral rubble or limestone during the whole of their adult lives. The other species, T. gigas, T. derasa and Hippopus hippopus, live byssally anchored as juveniles, but when approaching maturity their byssus glands atrophy, and the adult clams take up unattached existence on coral rubble or hard packed sand. This habitat difference is also expressed in the weight and morphology of their shells. The byssally attached species possess shell valves which are in most respects larger editions of those found in other bivalves (Fig. 1 A). The unattached clams, on the other hand, possess bizzare shells which are unusually massive resulting from a unique deposition of solid shell material within the cavities of the umbones (Fig. 1 B).