Observations on the Oman Shark, Iago omanensis (Triakidae), with emphasis on the morphological and cytological changes of the oviduct and yolk sac during gestation.

The shark Iago omanensis (Triakidae, Selachia) is encountered in large populations in the Gulf of Aqaba, Red Sea, at depths of 150-1,500 m. It is a placental viviparous species, reproductive all year round and giving birth to four (occasionally five) young of 170- to 180-mm total length (TL). Its distribution and morphometrics, as well as histological and cytological changes in the oviducts, were studied. The ratio of weight of the female genital organs to body weight changes from 0.7% in nongravid females to 19.8% in the final stages of pregnancy. The ripe, liberated eggs, which are 11-12 mm long and 5 mm wide, pass through the nidamental gland and settle in the uterus. The embryo attains 9- to 11-mm TL and settles on a protruding ridge of the submucosa, covered with a microvillar endometrium. At this site of attachment, a placenta is formed and the participating uterine endometrium and wall of the yolk sac undergo profound histocytological changes, forming two parts of this organ. Three forms of food provisioning occur in the growing embryos: (1) lecithotrophic, based on yolk transported from the egg to the embryonic gut via the umbilical cord; (2) mixed food provision, during which, in addition to nourishment provided via the umbilicus, food is transported across the placenta through transfer from the female blood vascular system to the embryonic yolk sac via the trophic villi of the yolk sac; and (3) histotrophic, when all yolk reserves have been used and nutrition is provided from the so-called "milk" within the yolk sac, metabolized by the trophic structures of the sac and transported by blood vessels. Despite the gradual utilization of yolk, the yolk sac mass initially increases from 0.5-1.0 cc to 2.0-2.2 cc with the addition of primary and secondary trophic villi until, during the final stages of embryogenesis, it decreases again to 1.4-1.6 cc. Neonate juveniles are 35-40 times heavier than the original eggs. J. Morphol. 236:151-165, 1998. © 1998 Wiley-Liss, Inc.