Spermiogenesis and spermatozoon ultrastructure of Polydora neocaeca (Polychaeta : Spionidae) from Rhode Island

The ultrastructure of spermatozoa and spermiogenesis in Polydora neocaeca i s described. Most morphological features and measurements of cellular compo nents of the spermatozoa are similar to previously studied members of the g enus Polydora. However, early spermatids contain finely granular chromatin which becomes finely fibrillar in late spermatids and uniformly electron-de nse in mature spermatozoa. Fibrillar chromatin morphology has not been repo rted within the Spionidae and may represent a species-specific trait or a p reviously unrecognized transient developmental process. Spermatozoa of Poly dora neoceaca exhibit an elongate, cylindrical nucleus and a bullet-shaped acrosome that contains channels of electron transparent material. No microt ubules are associated with nuclear elongation. The basal body of the axonem e occupies a short implantation fossa in the nucleus. The middlepiece conta ins 6-10 mitochondria closely aligned with the axoneme. Platelets, presumab ly carbohydrate storage bodies, are found along the nucleus and middlepiece . Additional electron-dense granules occur in the cytoplasm of the middlepi ece.