An Ultrastructural Investigation of Spermatogenesis in the Holopelagic Polychaetes Vanadis formosa and Krohnia lepidota (Polychaeta: Alciopidae)

¹ Division of Science and Mathematics, University of Tampa, Tampa, Florida 33606
² Reproductive Biology Program, Harbor Branch Oceanographic Institution, 5600 Old Dixie Highway, Fort Pierce, Florida 34946

The earliest stages encountered were clusters of spermatogonial cells floating free in the coelom. Neither germinal epithelium nor definitive gonad was observed. Mitotic divisions of spermatogonial cells resulted in large coelomic clusters containing up to 800 cells each. Cells within these clusters were connected to neighboring cells via cytoplasmic bridges that were characterized by a dense collar around the cell membrane and by microtubules traversing the common cytoplasm between cells. Peripheral cells detached from large cell clusters and entered meiotic divisions in the coelom. Spermatid nuclei initially contained granular chromatin that became fibrillar and eventually electron opaque as nuclear elongation and condensation proceeded. The acrosome first appeared in the posterior cytoplasm as a proacrosomal vesicle with an associated Golgi complex. The plate-like proacrosome migrated toward the anterior end of the sperm and reached the apex prior to completion of nuclear condensation. The middlepiece contained 5-7 mitochondria in Vanadis formosa and 8-10 mitochondria in Krohnia lepidota. Two centrioles, perpendicular to each other, were located in the middlepiece with the proximal centriole residing in a shallow fossa at the base of the nucleus. The distal centriole was associated with a nine-spoke, branched anchoring apparatus at the posterior edge of the middlepiece. Sperm development and mature sperm ultrastructure were very similar in V. formosa and K. lepidota. The mature coelomic sperm of K. lepidota were longer than those of V. formosa (8.7 µm vs. 11.0 µm, nucleus + middlepiece). The posterior nuclear region of V. formosa sperm was irregular in shape while that of K. lepidota was symmetrical. A series of dense accessory "membranes" was observed between the nucleus and the plasmalemma of K. lepidota sperm but not in V. formosa. Stored sperm in female worms were observed in V. formosa but no female K. lepidota were available in this study. Sperm storage and elongated sperm structure in alciopid polychaetes may be related to the pelagic environment in which they live and the suspected infrequent encounters between individuals.