Comparative study on growth, hepatopancreas and gill histological structure, and enzyme activities of Litopenaeus vannamei under SO₄ ²⁻/Cl⁻ stress in low saline water

The proportion of sulfate ions in saline-alkali waters and fresh water in northwest China is usually high. In order to investigate the adaptability of Litopenaeus vannamei to high sulfate content saline water, taking juvenile shrimps [Average body mass of (0.9±0.2) g] as objects, we conducted a 5-week aquaculture experiment in a controlled indoor environment. The experimental water was prepared with customized artificial sea salts and the salinity was 5‰. The sulfate ion concentrations were 0.385 (Control), 1.175, 1.735 and 2.300 g·L⁻¹, corresponding to ${\mathrm{SO}}_4^{2{\text{−}}} /{\rm{Cl}}^{\text{−}}$ (mg/mg) ratios of 0.140, 0.577, 1.127 and 2.225 (Labeled as Group A, B, C and D). The results show that the survival rates of L. vannamei in all the groups were above 70%, and there were no significant differences in the survival rate, wet mass, body mass gain rate and specific growth rate among all the groups (p>0.05). The SOD activity of hepatopancreas and gills of L. vannamei increased first and then decreased as the water sulfate concentration increased. The SOD activity of hepatopancreas in Group D was significantly lower than that in the other groups (p<0.05), while the SOD activity of gills in Group A and D was significantly lower than that in the other groups (p<0.05). There were no significant differences in the GSH-Px, sodium-potassium ATPase activity and MDA content among all the groups (p>0.05). There were no significant differences in the hepatopancreas ACP and AKP and tyrosinase activity among all the groups (p>0.05), but the ACP activity of gills in all the groups decreased as the sulfate concentration increased. The ACP activity of gills in Group D was the lowest, and its AKP and TYR activities were significantly lower than those in Group A (p<0.05). No obvious changes were observed in the structure of gill tissues under HE staining in any group. In Group D, the lumen of hepatopancreas was deformed and enlarged, and some B cells were broken. In summary, L. vannamei has tolerance to certain sulfate content in low salinity water, but high sulfate stress may cause changes in the immune and antioxidant enzyme activity of gills and hepatopancreas, as well as damage to hepatopancreas tissue. The suitable range of ${\mathrm{SO}}_4^{2{\text{−}}} /{\rm{Cl}}^{\text{−}}$ for L. vannamei is 0.140 to 1.127 under salinity 5‰.