Nutrition During Gestation in 2 Species of Viviparous Fishes (Poeciliidae): Poecilia latipinna (Lecithotrophic) and Heterandria formosa (Matrotrophic)

In viviparous teleosts, the lack of oviducts defines intraovarian gestation, with the ovary being the site for oogenesis but also the site for insemination, fertilization, and gestation. Consequently, intraovarian gestation is a complex and exceptional type of reproduction among vertebrates. The analysis of the morphological and physiological components of intraovarian gestation documents the evolutionary process of nutrition in viviparous species. Two types of embryonic nutrition may occur during gestation: (a) lecithotrophy, when most nutrients for the embryo come from the abundant yolk stored during oogenesis, and (b) matrotrophy, when nutrients for the embryo with scarce yolk must be obtained during gestation by additional maternal provisioning, developing a placenta. Then, investment of maternal nutrients for the embryo is greater during oogenesis in lecithotrophic species, and investment of maternal resources for the nutrition of the embryo is greater during gestation in matrotrophic species. Microscopic techniques allow for proper observation of maternal and embryonic structures involved in both types of nutrition during the development of embryos. Specifically, we focused on the morphology of placental structures associated with embryonic nutrition at different stages of development, which are the yolk sac and the pericardial sac. The oocytes of Poecilia latipinna contain a large amount of yolk (an average oocyte diameter of 1.9 mm); in contrast, the oocytes of Heterandria formosa contain extremely reduced amounts of yolk (an average oocyte diameter of 0.4 mm). Therefore, these species represent appropriate models for studying the strategy of two different types of embryonic nutrition, lecithotrophy and matrotrophy, in viviparous teleosts.