ULTRASTRUCTURE OF THE MATURE SPERM AND SPERMIOGENESIS IN CALLISTA-CHIONE (MOLLUSCA, BIVALVIA)

The ultrastructure of the mature spermatozoon and spermiogenesis of the bivalve Callista chione (Heterodonta, Veneridae) are described. As our TEM and SEM observations show the spermatozoon of C. chione may be considered of primitive type. The head contains a slightly curved nucleus bearing at the apex a short cone-shaped acrosome. The structure of the acrosome is typical of heterodont bivalves and, as in other venerid bivalves, it comprises a distinct axial rod. The mid-piece is an annular band of five mitochondria which surround the proximal and the distal centrioles. Occasionally spermatozoa with one or more misplaced flagella may be observed. The structural changes that occur during spermiogenesis are similar to those described for other bivalves.