SPERM ULTRASTRUCTURE IN THE MARINE BIVALVE FAMILIES CARDITIDAE AND CRASSATELLIDAE AND ITS BEARING ON UNIFICATION OF THE CRASSATELLOIDEA WITH THE CARDITOIDEA

An investigation of sperm ultrastructure in representatives of the mar ine bivalve families Carditidae (Carditoidea) and Crassatellidae (Cras satelloidea) reveals features of taxonomic significance. Spermatozoa o f Cardita muricata (Carditidae) and Eucrassatella cumingii, E. kingico la, Talabrica aurora (Crassatellidae) differ from the classic aquasper m type in having an elongate acrosomal vesicle and elongate nucleus. I n addition, the midpiece region in these species is composed of a dist inctive, and here considered to be apomorphic, arrangement of 8 (rarel y 7 or 9), tightly abutted mitochondria grouped around a dense rod whi ch is continuous with the distal centriole (basal body). A recognizabl e (i.e. triplet-substructure) proximal centriole is therefore absent i n mature spermatozoa of crassatellids and carditids. This situation co ntrasts with the presence of an unmodified proximal centriole in the s permatozoa of all other investigated bivalves. Observations on crassat ellid and carditid spermatids indicate that the dense rod is derived t hrough metamorphosis of the proximal centriole. The shared and highly characteristic midpiece features of spermatozoa of the Crassatellidae and Carditidae clearly indicate a close relationship between these fam ilies, and support the unification of the Crassatelloidea and Carditoi dea into a single superfamily Carditoidea Fleming, 1820 (dare priority over Crassatelloidea Ferussac, 1822).