Effects of acute low-salinity stress on osmoregulation, antioxidant capacity, and growth of the black sea bream (Acanthopagrus schlegelii).

The black sea bream (Acanthopagrus schlegelii) is an important marine economic fish found on the southeast coast of China. Because of the frequent climate change, the salinity of the waters inhabited by A. schlegelii often decreases, which interferes with the fish's physiological homeostasis. The isotonic salinity of teleosts are usually lower than that of seawater, so maximum economic benefits cannot be obtained from conventional mariculture. This study was performed to preliminarily clarify the osmotic regulation and antioxidant mechanism of juvenile A. schlegelii and find an appropriate culture salinity value. We selected 5 psu, 10 psu, 15 psu, and 25 psu (control) to conduct physiological experiments for 96 h and growth experiments for 60 days. We found that the juvenile A. schlegelii could adjust their osmotic pressure within 12 h. The growth hormone and cortisol were found to be seawater-acclimating hormones, whereas prolactin was freshwater-acclimating hormone. The activity and mRNA expression of Na/K-ATPase showed a U-shaped trend with the decrease of in salinity at 12-96 h. Serum ion concentration and osmotic pressure remained at a relatively stable level after being actively adjusted from 6 to 12 h. At 96 h, the osmotic pressure of the serum isotonic point of juvenile A. schlegelii was approximately equal to that of water with 14.94 salinity. The number and volume of Cl-secreting cells in the gills decreased. The glomeruli were more developed and structurally sound, with the renal tubules increasing in diameter and the medial brush border being more developed; this may indicate a decrease in salt secretion and an enhanced reabsorption function in the low salinity groups. The activities of superoxide dismutase and catalase and concentration of malondialdehyde were the lowest in the 15 psu group. In addition, the culture conditions of the 15 psu group improved the feed conversion rate without significant differences in weight gain when compared with the control group. Our results show that 15 psu salinity may be the best parameter for obtaining the maximum economic benefits.