Ultrastructure of Euspermatozoa of cerithiacean gastropods (Prosobranchia: Mesogastropoda).

Euspermatozoa of selected cerithiacean gastropods have been studied using transmission electron microscopy and the results compared (primarily) with previous studies of mesogastropod and neogastropod euspermatozoa. Cerithiacean euspermatozoa each possess a well-defined acrosome (extremely varied in shape), a short (2.25-3 μm), very electron-dense nucleus, an elongate midpiece, and an elongate glycogen piece. A dense ring structure associated with the plasma membrane occurs at the junction of the midpiece and glycogen piece. While features such as the dense ring structure and the systematic periaxonemal arrangement of "glycogen" granules can be understood from a purely functional standpoint, it is suggested that euspermatozoon features also provide information of taxonomic and phylogenetic relevance. On the basis of euspermatozoon midpiece structure, true cerithiaceans can be easily distinguished from other mesogastropods and from neogastropods and are divided tentatively into two groups: Group 1 (Turritellidae, Cerithiidae, Australaba (family uncertain), Planaxidae, Potamididae (subfamily Batillariinae)), and Group 2 (Potamididae (subfamily Potamidinae), Modulidae, Obtortio (family uncertain)). Using midpiece and acrosomal features, group 1 can be further subdivided into two subgroups: Subgroup 1(i) (Turritellidae, Cerithiidae, Australaba) and Subgroup 1(ii) (Planaxidae, Potamididae (subfamily Batillariinae)). It is suggested that the pronounced differences existing between the two subfamilies of the Potamididae may indicate the necessity for a separate family for the Batillariinae.