Changes in plasma levels of reproductive hormones during first sexual maturation in European male sea bass ( Dicentrarchus labrax L.) under artificial day lengths

Abstract

Photoperiod has been considered one of the most important factors triggering puberty, as well as reproduction, in several fish species, including sea bass ( Dicentrarchus labrax L.). In the present work, the effect of expanded (EX) and compressed (CO) photoperiods on plasma levels of reproductive hormones (gonadotropin-2 (GTH-2), testosterone (T) and 11-ketotestosterone (11-KT)) and gonadal maturation (spermiation time and gonadosomatic index (GSI)) were investigated in male sea bass during the first sexual maturation (October to April). Spermiation in controls was apparent from December to February–March. In EX and CO groups, spermiation was advanced by 2 months, although the CO group displayed a bimodal pattern, and testicular growth in both experimental groups was significantly reduced with respect to the controls. Plasma GTH-2 levels in controls showed the highest value (∼30 ng/ml) in the middle of the spermiation period, while EX group displayed the maximum level 4 months earlier (35±2.7 ng/ml) than controls. The CO group presented two peaks, the first of which (15.16±5.20 ng/ml) was advanced by 3 months with respect to the control peak. T and 11-KT levels in the control displayed the highest values during the spermiation period. The EX group showed lower T levels than controls, but both peaked at the same time. However, 11-KT levels remained low and unchanged. The CO group displayed two significant increases in T levels accordingly with the spermiation pattern, while the 11-KT profile only exhibited a significant increase 2 months earlier than in controls. Results obtained indicated an involvement of GTH-2 in gonadal maturation. In addition, T is suggested to be involved in the activation of the brain–pituitary–gonad (BPG) axis during pubertal development, while 11-KT may act by stimulating spermatogenesis and/or spermiation in juvenile male sea bass. Furthermore, the profiles of these reproductive hormones were altered by both expanded and compressed photoperiods, and first sexual maturation was advanced by at least 2 months.