Effects of photoperiod on pituitary levels of three forms of GnRH and reproductive hormones in the male European sea bass (Dicentrarchus labrax, L.) during testicular differentiation and first testicular recrudescence.

Photoperiod is one of the main external cues in the control of different stages of fish development. In the present work, the effect of several photoperiods, known to delay spawning in adult fish, was investigated in juvenile male sea bass (Dicentrarchus labrax) during testicular differentiation and first testicular recrudescence. Fish were exposed to constant long (LO), expanded (EX), constant short photoperiod with long photoperiod in October (SLoct) and constant short photoperiod with long photoperiod in December (SLdec) during two consecutive years. Pituitary levels of three forms of gonadotropin-releasing hormone (GnRH) [sea bream GnRH (sbGnRH), chicken GnRH-II (cGnRH-II), and salmon GnRH (sGnRH)] were analysed and plasma levels of gonadotropin (LH), testosterone (T), and 11-ketotestosterone (11-KT) were determined in the same periods. During both periods, pituitary levels of sbGnRH were higher than cGnRH-II and sGnRH in all groups. The three GnRHs had significantly lower values in November 1995 in groups exposed to artificial photoperiods during the testicular differentiation and growth period compared to the control group. In addition, a significant effect of LO, EX, and SLoct photoperiods was observed in the LH plasma profile, showing a significant delay in the peak of LH compared to the control group. However, no significant effect of the artificial photoperiods on the pituitary content of the three GnRHs has been observed during the first testicular recrudescence. Deeper studies are needed to understand the effect of artificial photoperiods on the endocrine events occurring during both histological testicular differentiation and growth and first testicular maturation in the sea bass.