Spermatogenesis and sperm structure of the hydrothermal vent prosobranch gastropod lepetodrilus fucensis (lepetodrilidae, mollusca)

Sperm and spermatogenic ultrastructure in a member of the gastropod superfamily Lepetodriloidea (Lepetodrilus fucensis, Lepetodrilidae), is described for the first time and the results compared with available information on other major prosobranch taxa. The head of the sperm of L. fucensis is comprised of an elongate nucleus (about 9 μm×0.4 μm mid-diameter) capped by a conical acrosome (2.3 μm) which sits on a centrally perforated basal plate. The acrosome is invaginated posteriorly, 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 centriolar complex, is surrounded anteriorly by a long (2.5 μm) cytoplasmic sheath which contains regularly spaced (periodicity 62 nm) electron-dense 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 structures associated with the cytoplasmic sheath (seen in Lepetodrilus and Addisonia) are strongly reminiscent of a similar, possibly homologous, feature observed in euspermatozoa of the Neritimorpha. Neritimorphs nevertheless show several sperm differences from L. fucensis and Addisonia sp. Spermatogenesis in L. fucensis follows a similar pattern to that described for many shallow water gastropods. Acrosome formation begins in spermatocytes with the production of small electron-dense vesicles, which, probably via fusion form the larger definitive acrosomal vesicle of spermatids. The pattern of chromatin condensation and nuclear elongation is typical of that of molluscs, but, as observed in other vetigastropods and the Neritimorpha, elongation of the nucleus occurs without the participation of a microtubular manchette