Abstract: It is studied the acute toxic effects of ammonia nitrogen on two specifications of L. vannamei which mean body weights （MBW） were 0.356 g and 0.036 g, respectively. The concentrations of blood ammonia and activities of alkaline phosphatase（AKP） were analyzed. The results show that variation trend of concentrations of blood ammonia were consistent with the ammonia nitrogen, while the activities of alkaline phosphatase（AKP） were decreasing when the ammonia nitrogen raised. The median lethal concentrations （LC₅₀） of NH₃⁺ on shrimp and the safe concentration（Sc） were also investigated. The LC₅₀ of NH₃⁺ on shrimp with MBW 0.356 g were 2.068 mg/L at 24 h, 1.654 mg/L at 48 h and 1.395 mg/L at 96 h, while the LC₅₀ of NH₃⁺ on shrimp with MBW 0.036 g were 1.577 mg/L, 1.042 mg/L and 1.012 mg/L, respectively. The Sc was 0.396 9 mg/L and 0.206 5 mg/L to shrimp with MBW 0.356 g and 0.036 g, respectively. It is concluded that the changes of ammonia nitrogen have certain influence on L. vannamei, and the ammonia nitrogen was one of the important factors for the health and the survival of L. vannamei.

Key words: Litopenaeus vannamei, alkaline phosphatase, blood ammonia, free nitrogen, the median lethal concentration, the safe concentration