MECHANISM OF INTERNAL FERTILIZATION IN PEGEA-SOCIA (TUNICATA, THALIACEA), A SALP WITH A SOLID OVIDUCT

The ovary of the salp Pegea socia (Bose, 1802) is located at the end o f an atrial diverticulum. The ovary consists of a single oocyte encase d in a layer of follicle cells and is connected to the atrial epitheli um by an oviduct. Transmission electron microscopy shows that the oocy te lacks a vitelline layer, cortical granules, and yolk granules and t hat the oviduct lacks a continuous lumen. What previous authors though t was a lumen is a line of dense intercellular junctions running down the center of the oviduct. The sperm nucleus in this species, as in ot her salps, is elongate. The tubular mitochondrion spirals about the sp erm nucleus giving it a corkscrew-shape appearance. Sperm reach the ov ary when the oocyte is still at the germinal vesicle stage. Many sperm swim up the atrial diverticulum and burrow through the cells of the a trial epithelium, oviduct, and follicular epithelium. Thus oviduct sho rtening, which occurs when the oocyte is in the meiotic divisions, is evidently unrelated to sperm moving up the oviduct. All previous autho rs, who argued either that a continuous lumen is necessary for sperm t o move up the oviduct or that sperm bypass the oviduct, were incorrect . (C) 1994 Wiley-Liss, Inc.