Sperm ultrastructure in two species of the polychaete genus Harmothoe (Polynoidae)

Abstract

The structure of spermatozoa is described for two species of polynoid polychaete, Harmothoe imbricata and Harmothoe impar, from material fixed and examined by both scanning and transmission electron microscopy. The two species undergo spermiogenesis within discrete testes. The testis of H. imbricata is shown to have a layer of epithelial cells which possess an outer cuticular layer and a microvillous inner surface. Spermatocytes of both species are spherical but there are marked differences in the shape and size of the spermatozoa of the two species. H. impar has a classical primitive spermatozoon with a rounded head (2 μm long) and a button-shaped acrosome. Fully differentiated spermatozoa of H. imbricata are modified from the primitive form by having a long head (10 μm length) with a pointed acrosome about 6 μm in length. Spermatozoa of H. imbricata have a ring of up to fourteen mitochondria around a centrally inserted flagellum at the posterior whereas H. impar has a ring of four or five spherical mitochondria. Spermiogenesis is well synchronised in H. imbricata but all developmental stages can be found simultaneously in the testis of H. impar. The differences in sperm structure of the two species may be related to differences in breeding biology which are hitherto unknown. © 1992 Biologische Anstalt Helgoland.