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

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. The results of numerous studies revealed the important roles of some steroid
hormones such as E2, T, 11-KT and C21 steroids in reproductive process of teleosts. In addition,
GnRH-a is effective for inducing maturation of some fish species.