ENDOCRINE REGULATION OF THE ELASMOBRANCH REPRODUCTIVE-TRACT

Although the rich variety of reproductive tract adaptations in elasmobranchs has been well documented and involves many variants associated with oviparity and extensive evolution of viviparity in the group, little is known of the hormone dependence of reproductive tract development, differentiation, and function in the adult. In this paper, we present a brief review of some of our observations on the subject made over the past 15 years in the context of earlier observations. In total, despite the paucity of information, the observations suggest that hormonal mechanisms of reproductive tract regulation, considered until recently to be the priority of mammals, have been in place since the era of very early vertebrate evolution, 400 million years before present. These reinforce the view that the endocrine regulatory mechanisms of reproduction in elasmobranchs may serve as models for more highly evolved mammalian species including the human. Indeed, the enormous variety of reproductive tract adaptations in elasmobranchs foreshadows those subsequently evolved in mammals, and because of transitional stages provide needed insight into process which is not possible in the study of mammals alone. (C) 1993 Wiley-Liss, Inc.