Spermatozoa and spermatogenesis in Genostoma kozloffi (Plathelminthes, Rhabdocoela)

Abstract

Genostoma kozloffi Hyra, 1993, is a symbiotic plathelminth living beneath the carapace of Nebalia pugettensis (Clark, 1932) (Leptostraca Crustacea). Because of similarities in the structure of its epidermis with that of the major group of parasitic flatworms, the Neodermata, and because of similarity in body form to a member of the Revertospermata, which includes the Neodermata and taxa that may be the sister group to the Neodermata, Genostoma was recently classified in the Revertospermata. By electron microscopy we found, however, that spermiogenesis in G. kozloffi does not occur in the manner characteristic of the Revertospermata. Instead, positioning of the axoneme and nucleus takes place as in free-living turbellarians in which the axoneme is fully incorporated, that is, in a distal-proximal fashion. Mature spermatozoa of G. kozloffi are filiform, possess an elongate rod-like nucleus, and one short single, fully incorporated axoneme. A rod of multiple, fused mitochondria accompanies the nucleus and axoneme, and an array of cortical microtubules with thickened walls runs the length of the sperm. Neither dense bodies nor acrosomal vesicles could be found. These features of its spermatozoa as well as the presence of a tunica surrounding its testes are reminiscent of the free-living group Kalyptorhynchia, specifically the Schizorhynchia. It is unlike the Kalyptorhynchia in other respects, however, and its systematic position remains uncertain, albeit removed from the Revertospermata.