Spermatogenesis and sperm ultrastructure in three species of polydora and in streblospio benedicti (Polychaeta: Spionidae)

Summary Spermatogenesis was studied at the ultrastructural level in Polydora ligni, P. websteri, P. socialis and Streblospio benedicti. Spermatogonia, spermatocytes, spermatids and mature sperm are described. In all four species, meiosis occurs in the coelom following release of spermatogonia from the gonad. In Polydora spp., chromatin condensation is lamellar with no microtubules present during nuclear elongation. In S. benedicti, chromatin condensation is fibrous with a manchette of microtubules present around the nucleus. In all four species, the acrosome forms from a Golgi-derived vesicle situated at the base of spermatids. The acrosome in Polydora spp. is conical with a distinctive substructure whereas the S. benedicti acrosome is long and spiral. The implantation fossa is short in all species except P. ligni. All four species have elongated sperm heads. The middlepiece as well as the nucleus is elongated in Polydora spp. whereas S. benedicti has a long nucleus but a short middlepiece. Platelet-shaped electron-dense bodies are present throughout the nuclear region and middlepiece of Polydora spp. and the nuclear region of S. benedicti. These membrane-bounded bodies may be energy storage organelles. The use of ultrastructural data in analysis of sibling species complexes is discussed.Contribution Number 203 from Harbor Branch Foundation, Inc.