Salinity-induced changes in reproductive endocrinology, gonadal structure, and steroidogenic gene expression in pure...

Salinity can impair fish reproduction, but its integrated effects on endocrine function, gonadal structure, and reproductive gene expression remain unclear, particularly in pure and hybrid tilapia. This study evaluated salinity-induced reproductive changes in pure Oreochromis niloticus and O. niloticus × O. aureus hybrids.

Sexually differentiated fish of both sexes were exposed for eight weeks to 0, 16, 20, 24, 28, or 32‰ salinity, with three replicate tanks per treatment. Serum progesterone and testosterone were measured by competitive ELISA. Gonads were examined histologically using a semi-quantitative damage score, and ovarian cyp19a1a, esr2b, and pgr and testicular hsd17b, cyp19a1a, and gh expression were quantified by qPCR. Increasing salinity progressively reduced progesterone and testosterone concentrations in both stocks. Gonadal lesions, including seminiferous tubule disorganization, germ-cell loss, oocyte atresia, follicular degeneration, and vacuolation, became pronounced at 28-32‰ and were generally more severe in hybrids. Gene expression showed a biphasic pattern, increasing at 16-20‰ and declining at higher salinities. Hybrid tilapia showed higher baseline testosterone and higher expression of some reproductive transcripts but exhibited a greater proportional decline in testosterone and numerically greater gonadal damage at high salinity.

Expression of several reproductive genes increased at moderate salinity and declined at higher salinity, while hormone concentrations decreased and gonadal damage increased across the salinity gradient. The largest alterations were observed at 28-32‰. Hybrid tilapia were more vulnerable than pure O. niloticus under high-salinity conditions. These findings indicate that salinity exposure during the juvenile growth phase disrupts gonadal development and steroidogenic gene expression, with implications for reproductive competence later in life; direct extrapolation to spawning-age broodstock requires confirmation in reproductively mature fish.