Shell morphology and identification of early life history stages of congeneric species of Crassostrea and Ostrea

Series of sequences of SEM micrographs are presented to elucidate species-specific shell features in larval and post-larval stages of four Ostreidae (Bivalvia) species ( Crassostrea gigas (Thunberg), C. virginica (Gmelin), Ostrea edulis L., and O. equestris Say). The diagnostic characters, including hinge structure, shell shape, and the dimensions of the prodissoconch, are identified and summarized in a key at both generic and species levels. In larval Crassostrea the skewed, backwards-pointed umboned shells with two hinge teeth on each side of the provinculum are significantly different from the round, dorsally-directed umboned shell with fewer, remarkably asymmetric teeth of larval Ostrea .In C. gigas the dimensions of the provinculum are longer and narrower (56x10 µrn) than those of C. virginica (50x14 µm); the lengths of prodissoconch I and the provinculum are greater in O. edulis (168 µm and 86 µm, respectively) than in O. equestris (120 µm and 74 µm, respectively). The present study suggests that precise quantitative measurements are needed for differentiation of the studied congeneric oyster species.