The immune response of white shrimp Litopenaeus vannamei and its susceptibility to Vibrio alginolyticus at different salinity levels

White shrimp Litopenaeus vannamei held in 25‰ seawater were injected with TSB-grown Vibrio alginolyticus (1 × 104 cfu shrimp−1), and then transferred to 5, 15, 25 (control) and 35‰. Over 24–96 h, the mortality of V. alginolyticus-injected shrimp held in 5‰ and 15‰ was significantly higher than that of shrimp held in 25‰ and 35‰, and the mortality of V. alginolyticus-injected shrimp held in 5‰ was the highest. Shrimp held in 25‰ and then transferred to 5, 15, 25 (control) and 35‰ were examined for THC (total haemocyte count), phenoloxidase activity, respiratory burst, superoxide dismutase (SOD) activity, phagocytic activity and clearance efficiency to V. alginolyticus after 12–72 h. The THC, phenoloxidase activity, respiratory burst, SOD activity, phagocytic activity and clearance efficiency decreased significantly for the shrimp held in 5 and 15‰ after 12 h. It is concluded that the shrimp transferred from 25‰ to low salinity levels (5 and 15‰) had reduced immune ability and decreased resistance against V. alginolyticus infection.