Reproductive endocrinology of female elasmobranchs: Lessons from the little skate (Raja erinacea) and spiny dogfish (Squalus acanthias)

Conventional classification of reproductive modes in female elasmobranchs f ails to account for the diversity in ovarian dynamics that operate during o viparous and viviparous cycles. Delineating this diversity is crucial for u nderstanding the endocrine regulation of the manifold physiological mechani sms utilized to retain and protect eggs and developing embryos, to fuel emb ryogenesis, and to manage the intrauterine milieu. Oocyte development and f ollicular steroidogenesis overlap with egg retention and pregnancy in some species, whereas in others the follicular phase of the cycle is temporally separated from the gravid period. A luteal phase predominates the post-ovul atory period in viviparous species. In oviparous species, the luteal phase overlaps with the follicular cycle. This heterogeneity in ovulatory cycles suggests that the endocrine system evolved a transmutable system for regula ting steroidogenesis and the control of the reproductive events. The reprod uctive biology and endocrinology of the oviparous little skate and lecithot rophic viviparous spiny dogfish are reviewed in order to derive a working h ypothesis that explains the complex nature of endocrine patterns observed i n species utilizing disparate reproductive modes. An understanding of the a daptations in ovarian dynamics to particular ovulatory cycles is key to dev eloping theories about the evolution of reproductive strategies in female e lasmobranchs. (C) 1999 Wiley-Liss, Inc.