The immune response of white shrimp Penaeus vannamei and its susceptibility to Vibrio alginolyticus under sulfide stress

White shrimp Penaeus vannamei held in 34‰ seawater were injected with tryptic soy broth (TSB)-grown Vibrio alginolyticus (6.2 × 105 cfu shrimp− 1), and then placed in water containing different concentrations of sulfide at 0 (control), 48, 111, 492 and 1026 μg l− 1, respectively. After 48–144 h, mortality of V. alginolyticus-injected shrimp exposed to = or > ∼ 490 μg l− 1 sulfide was significantly higher than that in the control solution. In another experiment, P. vannamei which had been exposed to 0, 49, 105, 488 and 967 μg l− 1 sulfide for 6, 12, 24 and 48 h were examined for immune parameters, phagocytic activity and clearance efficiency of V. alginolyticus. Sulfide concentrations = or > ∼ 490 μg l− 1 for 24 h resulted in decreased hyaline cell count, total haemocyte count, phenoloxidase activity, phagocytic activity and bacterial clearance efficiency, whereas a sulfide concentration at = > ∼ 490 μg l− 1 for 24 h caused a significant increase in respiratory burst and superoxide dismutase activity of P. vannamei. It is concluded that concentrations of sulfide = or > ∼ 490 μg l− 1 increased the susceptibility of P. vannamei against V. alginolyticus infection by a depression in immune ability.