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

White shrimp Litopenaeus vannamei held in 25parts per thousand seawater were injected with TSB-grown Vibrio alginolyticus (1 x 10(4) cfu shrimp(-1)), and then transferred to 5, 15, 25 (control) and 35parts per thousand. Over 24-96h, the mortality of V. alginolyticus-injected shrimp held in 5parts per thousand and 15parts per thousand was significantly higher than that of shrimp held in 25parts per thousand and 35parts per thousand and the mortality of V. alginolyticus-injected shrimp held in 5parts per thousand was the highest. Shrimp held in 25parts per thousand and then transferred to 5, 15, 25 (control) and 35parts per thousand 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 15parts per thousand after 12 h. It is concluded that the shrimp transferred from 25parts per thousand to low salinity levels (5 and 15parts per thousand) had reduced immune ability and decreased resistance against V. alginolyticus infection. (C) 2004 Elsevier Ltd. All rights reserved.