Crowding of white shrimp Litopenaeus vananmei depresses their immunity to and resistance against Vibrio alginolyticus and white spot syndrome virus

Immunity parameters and the expression levels of several immune-related proteins, including lipopolysaccharide and β-glucan binding protein (LGBP), peroxinectin (PX), intergin β (IB), prophenoloxidase (proPO) I, proPO II, α2-macroglobulin (α2-M), cytosolic mangangese superoxide dismutase (cytMnSOD), mitochondria manganese superoxide dismutase (mtMnSOD), catalase, glutathione peroxidase (GPx), lysozyme, and penaeidin 3a were examined in white shrimp Litopenaeus vannamei reared at stocking densities of 2, 10, 20, 30, and 40 shrimp L−1 after 3, 6, and 12 h. All immune parameters including haemocyte count, phenoloxidase (PO) activity, respiratory burst (RB), superoxide dismutase (SOD) activity, lysozyme activity, and haemolymph protein were negatively related to density and time. The PO activity, SOD activity, and lysozyme activity of shrimp reared at 10 shrimp L−1 after 12 h significantly decreased. The transcript levels of these immune-related proteins were down-regulated in shrimp reared at 20, 30, and 40 shrimp L−1 after 12 h. Phagocytic activity and clearance efficiency to Vibrio alginolyticus were significantly lower in shrimp reared at 30 and 40 shrimp L−1 after 12 h. The mortality rates of shrimp reared at 20 and 40 shrimp L−1 were significantly higher than shrimp reared at 2 shrimp L−1 over 12-144 h and 12-48 h, respectively. Shrimp reared at high densities (>10 shrimp L−1) exhibited decreased resistance against pathogens as evidenced by reductions in immune parameters together with decreased expression levels of immune-related proteins, indicating perturbations of the immune system.