1 The Maternal-Embryonic Relationship in Viviparous Fishes

This chapter discusses the maternal–embryonic relationship in viviparous fishes. Viviparity is a highly successful mode of reproduction that has evolved independently many times and with many variations in widely separated taxonomic groups. It occurs in all classes of vertebrates, except birds, and among many different groups of invertebrates. Initial steps in the evolution of viviparity involved a shift from external to internal fertilization and the retention of fertilized eggs in the female reproductive system. The osmoregulation of early embryos can be accomplished more efficiently and with less expenditure of embryonic energy in a maternally controlled uterine environment, but as development progresses to term, the embryos presumably acquire an increasing degree of osmoregulatory independence. Available evidence suggests that maternal regulation of the osmotic and chemical environment of the embryo also confers a selective advantage on viviparous teleosts. The uterine wall of most viviparous elasmobranchs and the coelocanth both delimits and defines the embryonic environment. The most spectacular maternal specializations for uterine gestation involve the uterine wall and involve (1) the amplification of the surface area in the form of folds, villi, or trophonemata; (2) the production of histotrophe or uterine milk' (3) the compartmentalization of embryos; and (4) the development of placental attachment sites.