Effect of salinity on the immune response of tiger shrimp Penaeus monodon and its susceptibility to Photobacterium damselae subsp. damselae

Addition of NaCl at 2.5% to tryptic soy broth (TSB) significantly increased the growth of Photobacterium damselae subsp. damselae. Tiger shrimp Penaeus monodon held in 25‰ seawater were injected with P. damsela subsp. damselae grown in TSB containing NaCl at 0.5%, 1.5%, 2.5% and 3.5% at a dose of 8.48 × 104 colony-forming units (cfu) shrimp−1. Over 24–96 h, the cumulative mortality was significantly higher for the shrimp challenged with P. damselae subsp. damselae grown in 2.5% NaCl than those grown in 0.5%, 1.5% and 3.5% NaCl. In another experiment, P. monodon held in 25‰ were injected with TSB-grown P. damselae subsp. damselae (8.48 × 104 cfu shrimp−1), and then transferred to 5‰, 15‰, 25‰ (control) and 35‰. After 96 h, the mortality was highest for the P. damselae subsp. damselae-injected shrimp held in 5‰, and the lowest for the P. damselae subsp. damselae-injected shrimp held in 25‰. In a separate experiment, P. monodon held in 25‰ and then transferred to 5‰, 15‰, 25‰ (control) and 35‰ were examined for immune parameters, and phagocytic activity and clearance efficiency of P. damselae subsp. damselae after 12–96 h. The THC, hyaline cell, phenoloxidase activity, respiratory burst, superoxide dismutase (SOD) activity, phagocytic activity and clearance efficiency decreased significantly for the shrimp held in 5‰, 15‰ and 35‰ after 12 h. It is concluded that tiger shrimp P. monodon transferred from 25‰ to low salinity levels (5‰ and 15‰) and high salinity (35‰) had reduced immune ability and decreased resistance against P. damselae subsp. damselae infection.