Ultrastructure of spermatids and spermatozoa inRamex californiensisandNicolea zostericola(Terebellidae; Polychaeta)

The ultrastructure of the spermatozoa and some stages of spermiogenesis in Ramex californiensis Hartman, 1944 and Nicolea zostericola (Nicolea zostericolarsted, 1844) is described. Both species brood direct developing larvae, N. zostericola outside the tube in a jelly mass, and R. californiensis inside the tube in a cocoon. In both species, spermatids were seen in large groups of synchronously developing cells. Each spermatid was connected via a cytoplasmic bridge to a central cytophore. The acrosome initially developed at the posterior end of the spermatid near the centrioles. It then migrated to the anterior end of the sperm at the tip of the nucleus; in N. zostertcola the migration was much later than in R. califomimsis. No microtubular activity was involved in spermiogenesis. The mature sperm nuclei of R. californiensis and N. zostericola were basically cylindrical and elongate, measuring 9 µm and 10/Lm in length, respectively. In both species the acrosome was bullet-shaped, although in N. zostericola the subacrosomal space was proportionally much larger, and there were two regions of differing electron density. There was no sperm midpiece in either species. Instead the mitochondria lay in grooves along the posterior region of the nucleus; 2 mitochondria in R. californiensis sperm and 4 mitochondria in N. zostericola. The anchoring apparatus for the sperm of each species consisted of both the proximal and distal centrioles, and a complex satellite apparatus arising from the distal centriole. The morphology of the sperm is compared with other polychaetes; functional aspects and systematic implications are discussed. The close similarity of the sperm between N. zostencola and R. californiensis does suggest a similar fertilization mechanism is used by the two species. Until a phylogenetic hypothesis for the Terebellidae is developed the evolutionary change in sperm morphology and functional correlates with other factors in reproduction cannot be determined.