THE CHALLENGES OF PISCINE VIVIPARITY

ABSTRACT

Viviparity in vertebrates first appeared among fishes where it had independently evolved at least 42 times in five of the nine major groups. It is the dominant mode of reproduction among sharks and rays, but less prevalent among bony fishes. The evolution of viviparity from oviparity involves: (1) a shift from external to internal fertilization, (2) retention of embryos in the female reproductive system, (3) utilization of the ovary or oviduct as sites of gestation, (4) structural and functional modification of the embryo and the female reproductive system, and (5) modification of extant endocrine systems controlling reproduction. Retention of developing young to term in the maternal reproductive system poses a series of physiological challenges. The challenges are reflected in the new maternal-embryonic relationships that evolved in viviparous fishes, namely: (1) trophic, (2) osmoregulatory and excretory, (3) respiratory, (4) endocrinological, and (5) immunological. In sharks, rays, and the coelacanth, gestation takes place in the oviduct, but in teleosts, gestation occurs either in the ovarian follicle or ovarian lumen. The cystovarian teleostean ovary is hypothesized to function both as ovary and oviduct. Oviductal, ovarian lumenal, and follicular epithelial cells are the maternal sites of metabolic exchange. Exchange in embryos takes place across the epithelia of the general body surface and its derivatives or across the gut epithelium and its derivatives. Four patterns of placentation have evolved, viz.: (1) yolk sac, (2) follicular, (3) branchial, and (4) trophotenial placentae.