SPERMATOZOA AND SPERMATOGENESIS IN A MONOPLACOPHORAN MOLLUSK, LAEVIPILINA-ANTARCTICA - ULTRASTRUCTURE AND COMPARISON WITH OTHER MOLLUSCA

Ultrastructural features of spermatozoa and spermatogenesis are descri bed for the first time in a monoplacophoran and compared with data for other conchiferan molluscs. Spermatozoa of Laevipilina antarctica War en and Hain, 1992, are of the structurally simple, aqua-sperm type, fe aturing a conical acrosome, a compact nucleus with lacunae, a short mi dpiece and a single flagellum. The acrosomal vesicle shows an electron -dense inner zone, and a basal invagination (subacrosomal space) conta ins 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 t he five (rarely four) midpiece mitochondria. Two centrioles are presen t, the distal connected to the annulus by satellite fibres and acting as a basal body for the flagellum (axoneme probably 9+2 structure). Sp ermatogonia, characterized by an oblong nucleus and one or two nucleol i, line the basal membrane of the testis wall; spermatids of varying s tages of maturity occupy the remainder of the testis. Acrosome and fla gellum 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 chroma tin condenses in large tracts, leaving electron-lucent lacunae. Mitoch ondria collect posteriorly and form, presumably by fusion, the five (o r four) larger, spherical mitochondria which gather around the centrio les.