Structural features associated with viviparity and placentation in teleosts

Abstract

Teleosts show unique features of the female reproductive system compared with the rest of vertebrates. Teleosts lack oviducts and therefore the ovary of viviparous teleosts is responsible for not only oogenesis, but also the reception of spermatozoa, fertilization, and intraovarian gestation which is unique among the vertebrates. The gestation may be intrafollicular, when the embryo remains in the follicle throughout embryogenesis, as in poeciliids; or intraluminal, when the embryo moves during early development from the follicle into the lumen where embryogenesis continues, as in goodeids. In viviparous teleosts, the embryonic and maternal tissues form placentas, mediating all exchanges: trophic, excretory, respiratory, immunological, and metabolic waste. These adaptations occur according to lecithotrophy, when the eggs may store enough yolk for the nutrition of the embryo until birth, and matrotrophy, when the eggs store scarce yolk, insufficient for the nutrition of all embryogenesis, which has to be complemented by transfer of maternal nutrients during gestation. This paper focuses primarily on the ovarian structure and embryonic traits of poeciliids and goodeids to illustrate the morphological and physiological elements that characterize viviparity in teleosts.