UTEROGESTATION AND PLACENTATION IN ELASMOBRANCHS

Elasmobranch fishes utilize a variety of means to provide nourishment for their developing young. All employ internal fertilization and sper m storage within the female genital tract. Some elasmobranchs, includi ng all the skates and some sharks, are oviparous. In these species, fe rtilized eggs are enclosed in a tough egg case that is secreted by the nidamental or shell gland. The female lays the egg cases and developm ent is entirely dependent on the yolk stores sequestered in the yolk s ac. Upon oviposition, the embryo weighs less than the fertilized egg. The majority of elasmobranchs are viviparous, however, and utilize a v ariety of strategies to provide nourishment and satisfy respiratory de mands of the developing young. Some sharks simply retain their young i n the dilated posterior segment of the oviduct. In its simplest form, the maternal uterus does not provide any additional nutrients to the e mbryos. Other elasmobranchs develop secretory uterine villi that produ ce nutrient histotroph to supplement oocyte yolk stores. Uterine secre tions find their zenith in the stingrays. Following yolk depletion, th e uterine lining hypertrophies into secretory appendages termed tropho nemata. The process by which the uterine secretions, also known as ute rine milk or histotroph, are elaborated resembles the production of br east milk in higher vertebrates, and the milk is rich in protein and l ipid. Concurrent with growth of the embryos, the vascular bed of the t rophonemata enlarges to form sinusoids that project out to the surface to form a functional respiratory membrane. In lamnoid sharks, followi ng yolk use, the embryos develop precocious dentition and feed on intr auterine eggs and siblings. There is generally one fetus per uterus an d it grows to enormous proportions of up to 4 feet in length. In place ntal sharks the yolk sac is not withdrawn to become incorporated into the abdominal wall. Instead, it lengthens to form an umbilical cord an d the yolk sac becomes modified into a functional epitheliochorial pla centa. (C) 1993 Wiley-Liss, Inc.