Spermatozoa and spermatogenesis in a monoplacophoran mollusc, Laevipilina antarctica: ultrastructure and comparison with other Mollusca

Ultrastructural features of spermatozoa and spermatogenesis are described for the first time in a monoplacophoran and compared with data for other conchiferan molluses. Spermatozoa of Laevipilina antarctica Warén and Hain, 1992, are of the structurally simple, aquasperm type, featuring a conical acrosome, a compact nucleus with lacunae, a short midpiece and a single flagellum. The acrosomal vesicle shows an electron-dense inner zone, and a basal invagination (subacrosomal space) contains granular material but no axial rod. The nucleus exhibits a shallow indentation apically which contains subacrosomal material, and five (sometimes four) indentations posteriorly which partially accommodate the five (rarely four) midpiece mitochondria. Two centrioles are present, the distal connected to the annulus by satellite fibres and acting as a basal body for the flagellum (axoneme probably 9+2 structure). Spermatogonia, characterized by an oblong nucleus and one or two nucleoli, line the basal membrane of the testis wall; spermatids of varying stages of maturity occupy the remainder of the testis. Acrosome and flagellum production is already well advanced in spermatids and probably commences at the spermatocyte stage. Cytoplasmic bridges occur in all developmental stages, most visibly in spermatids. The spermatid chromatin condenses in large tracts, leaving electron-lucent lacunae. Mitochondria collect posteriorly and form, presumably by fusion, the five (or four) larger, spherical mitochondria which gather around the centrioles