Effects of Na⁺/K⁺ ratio on growth, body composition, hepatopancreas and gill microsturucture of Litopenaeus vannamei reared in low-salinity environment

Potassium deficiency often occurs in inland low-salinity saline waters, which is a major challenge to fishery utilization of such resources. In order to explore the effects of potassium deficiency in low-salinity water on the growth, survival, body composition, gill and hepatopancreas tissue structure of Litopenaeus vannamei, a 60-day growth experiment was set up using diluted artificial seawater at salinity of 2. Juvenile shrimps with body weight of (1.04±0.23) g were cultured under the conditions of Na⁺/K⁺ (mg/mg) ratio of 27, 47, 67, 87 and 107 (group A, B, C, D and E, respectively). The results showed that the survival rate of group E was 44.64%±20.95%, which was significantly lower than that of groups A, B and D; body weight of group E was (4.86±0.66) g, which was significantly lower than that of the other four groups. There were no significant differences in wet weight, weight gain and specific growth rate between groups A and D, but they were all higher than those in group E. The feed coefficient in group E was the highest and significantly different from that in groups A and D. In terms of body composition, there were no significant differences in potassium and ash content of whole body between all groups, and water content in group E was higher than that in other groups, and significantly different from that in groups A and B. The crude protein content in group E was the lowest, and was significantly different from that in group B. In group E, the shrimp gill cuticle was significantly damaged, the number of red blood cells decreased, vacuoles increased, the volume of hepatopancreas B cells and their internal transport vesicles increased, and the structure of hepatic tubules was damaged. The results indicated that severe potassium deficiency under low salinity conditions causes damage to the gills and hepatopancreas of the shrimp, which also reduces its survival and growth. Potassium deficiency in water body has a significant effect on the growth of L. vannamei even in the early stage, while its influence on the survival increasesh along the culture period, which means supplementation of potassium is necessarily at the beginning of farming. These findings would help to provide theoretical and practical support for the aquaculture exploitation of inland low salinity waters.