THE CHALLENGES OF PISCINE VIVIPARITY

Viviparity in Vertebrates first appeared among fishes where it had ind ependently evolved at least 42 times in five of the nine major groups. It is the dominant mode of reproduction among sharks and rays, but le ss prevalent among bony fishes. The evolution of viviparity from ovipa rity involves: (1) a shift from external to internal fertilization, (2 ) retention of embryos in the female reproductive system, (3) utilizat ion of the ovary or oviduct as sites of gestation, (4) structural and functional modification of the embryo and the female reproductive syst em, and (5) modification of extant endocrine systems controlling repro duction. Retention of developing young to term in the maternal reprodu ctive system poses a series of physiological challenges. The challenge s are reflected in the new maternal-embryonic relationships that evolv ed in viviparous fishes, namely: (1) trophic, (2) osmoregulatory and e xcretory, (3) respiratory, (4) endocrinological, and (5) immunological . In sharks, rays, and the coelacanth, gestation takes place in the ov iduct, but in teleosts, gestation occurs either in the ovarian follicl e or ovarian lumen. The cystovarian teleostean ovary is hypothesized t o function both as ovary and oviduct. Oviductal, ovarian lumenal, and follicular epithelial cells are the maternal sites of metabolic exchan ge. Exchange in embryos takes place across the epithelia of the genera l body surface and its derivatives or across the gut epithelium and it s derivatives. Four patterns of placentation have evolved, viz.: (1) y olk sac, (2) follicular, (3) branchial, and (4) trophotenial placentae .