UNUSUAL SPERM MORPHOLOGY IN A DEEP-SEA HYDROTHERMAL-VENT POLYCHAETE, PARALVINELLA-PANDORAE (ALVINELLIDAE)

The ultrastructure of the sperm and some stages of spermiogenesis are described for Paralvinella pandorae, an alvinellid polychaete known only from deep-sea hydrothermal vents of the northeastern Pacific. In males of this species, groups of spermatocytes and spermatids attached to central cytophores develop in the coelom. As the nuclear material condenses, the nucleus and the spermatid itself elongate. Numerous electron-opaque bodies are formed in the cytoplasm, apparently as Golgi derivatives; these bodies amalgamate to form a large convoluted structure in the midpiece of the sperm. The elongate sperm (head length up to 17 mu m) are stored in the nephridia of males. The anterior pole of the sperm is a long, thin apical extension of the cell filled with glycogen-like granules, and the nucleus measures 6.0-7.5 mu m in length. In the midpiece, several mitochondria are found anterior and posterior to the large convoluted structure, which may represent the acrosome. The flagellum emerges at an acute angle posterior to the mitochondria and convoluted structure, indicating limited motility of the sperm. Based on the unusual sperm morphology of P. pandorae, sperm are probably transferred to females as bundles of mature ent-aquasperm, with fertilization taking place in the female's tube or in a gelatinous mass outside the tube.