Steroid Hormones in Reproduction and Roles of GnRH-a in Gonadal Maturation of Marine Fish: A Review

Reproduction in teleosts is regulated by a series of hormones including gonadotropin-releasing hormones (GnRHs), gonadotropins (GTHs) and steroid hormones. To contribute better understanding of steroid hormones in reproduction and GnRH-a in gonadal maturation, this literature review is concerned with the changes of steroid hormone levels in relation with sex inversion, reproductive behavior and gonadal development as well as the application of GnRH-a for inducing maturation of marine fish. The results revealed that in many species of teleost, steroid hormones E2, 11-11-KT and DHP are abundantly produced in gonadal tissues under the control of pituitary gonadotropins, and are essential for critical steps of gametogenesis. Plasma steroid levels have been used as indicators for both of the sex of the fish and its stage in the seasonal reproductive cycle, particularly with regard to induction of spawning. Determination of plasma steroid levels in relation with the sexual status of the gonads over several reproductive seasons might provide valuable information on the mechanisms of sex inversion in ambisexual fish species. In addition, changes of plasma steroid levels in correlation with gonadal development, number of spawning, fecundity, were described clearly in many marine species. The review also indicated that exogenous administration of GnRH-a triggered for final maturation of brood stock of some teleosts. In summary E2, T, 11-KT and C21 steroids are in relation with sex inversion, reproductive processes and GnRH-a is successful for inducing gonadal maturation in some fish species.