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 marine bivalve families Carditidae (Carditoidea) and Crassatellidae (Crassatelloidea) reveals features o f taxonomic significance. Spermatozoa of Cardita muricata (Carditidae) and Eucrassatella cumingii, E. kingicola, Talabrica aurora (Crassatellidae) differ from the classic aquasperm type in having an elongate acrosomal vesicle and elongate nucleus. In addition, the midpiece region in these species is composed of a distinctive, and here considered t o be apomorphic. arrangement of 8 (rarely 7 or 9), tightly abutted mitochondria grouped around ii dense rod which is continuous with the distal centriole (basal body). A recognizable (i.e. triplet-substructure) proximal centriole is therefore absent in mature spermatozoa of crassatellids and carditids. This situation contrasts with the presence of an unmodified proximal centriole in the spermatozoa o f all other investigated bivalves. Observations on crassatellid and carditid spermatids indicate that the dense r o d is derived through metamorphosis of the proximal centriole. The shared and highly characteristic midpiece features of spermatozoa of the Crassatellidae and Carditidae clearly indicate ii close relationship between these families and support the unification of the Crassatelloidea and Carditoidea into a single superfamily Carditoidea Fleming. 1820 (date priority over Crassatelloidea Férussac. 1822).