Studies on sharks. I. Reproduction in the female sumitsuki shark Carcharhinus dussumieri

During the last 5 years, the T.S. Nagasaki Maru, Nagasaki University, Japan, carried out trawling operations in the waters off Kuching, northwest Borneo, for 1 week in July of each year. During this period, 119 specimens of the sumitsuki shark Carcharhinus dussumieri Valenciennes (Müller et Henle, 1841) were caught and their reproductive system investigated. In these waters, C. dussumieri has no definite reproductive seasons; it was, therefore, possible to obtain sufficient suitable material to examine all stages of the sexual cycle. The left ovary is rudimentary throughout the life cycle, and only the right ovary is functional. The ova in the ovary are already developing for the succeeding ovulation while the embryo already present in the uterus is growing; the 2 mature eggs from the right ovary are ovulated in the peritoneal cavity soon after parturition. The developing ova which remain in the ovary after ovulation are reabsorbed. The 2 mature eggs ovulated enter a common ostium and each oviduct receives one egg. In the nidamental gland, the fertilized egg is enclosed by an embryonic membrane and descends into the uterus where it grows into an embryo (simple pregnancy: 1 embryo in each uterus). During earlier embryonic development, nourishment is obtained from the yolk stored in the yolk sac. The placenta is established when the embryo attains a total length of about 150 mm; the embryo is thenceforth nourished by the placenta through the umbilical stalk until birth.