SPERMATOGENESIS AND SPERM STRUCTURE OF THE HYDROTHERMAL VENT PROSOBRANCH GASTROPOD LEPETODRILUS-FUCENSIS (LEPETODRILIDAE, MOLLUSCA)

Sperm and spermatogenic ultrastructure in a member of the gastropod su perfamily Lepetodriloidea (Lepetodrilus fucensis, Lepetodrilidae), is described for the first time and the results compared with available i nformation on other major prosobranch taxa. The head of the sperm of L . fucensis is comprised of an elongate nucleus (about 9 mu mx0.4 mu m mid-diameter) capped by a conical acrosome (2.3 mu m) which sits on a centrally perforated basal plate. The acrosome is invaginated posterio rly, the invagination housing an axial rod. Posterior to the nucleus, the mid-piece consists of a complex centriolar apparatus and laterally positioned mitochondrion. The axoneme, which emerges from the centrio lar complex, is surrounded anteriorly by a long (2.5 mu m) cytoplasmic sheath which contains regularly spaced (periodicity 62 nm) electron-d ense microtubules and filaments. Spermatozoa of L. fucensis resembles most closely those of the lepetelloidean Addisonia sp. and to a lesser degree those of some skeneimorph vetigastropods. The electron-dense s tructures associated with the cytoplasmic sheath (seen in Lepetodrilus and Addisonia) are strongly reminiscent of a similar, possibly homolo gous, feature observed in euspermatozoa of the Neritimorpha. Neritimor phs nevertheless show several sperm differences from L, fucensis and A ddisonia sp. Spermatogenesis in L. fucensis follows a similar pattern to that described for many shallow water gastropods. Acrosome formatio n begins in spermatocytes with the production of small electron-dense vesicles, which, probably via fusion form the larger definitive acroso mal vesicle of spermatids. The pattern of chromatin condensation and n uclear elongation is typical of that of molluscs, but, as observed in other vetigastropods and the Neritimorpha, elongation of the nucleus o ccurs without the participation of a microtubular manchette.