Orange-spotted grouper Epinephelus coioides that have encountered low salinity stress have decreased cellular and humoral immune reactions and increased susceptibility to Vibrio alginolyticus

Orange-spotted grouper Epinephelus coioides reared at 34 parts per thousand and 27 degrees C were abruptly transferred to 6 parts per thousand, 20 parts per thousand and 34 parts per thousand (control) and examined for innate cellular and humoral parameters after 3-96 h. Total leucocyte count (TLC), respiratory burst (RB), phagocytic activity (PA), alternative complement pathway (ACP) and lysozyme activity were significantly decreased 3-6 h, 3-6 h, 3-96 h, 3-96 h and 3-96 h, respectively after transferal into 6 parts per thousand salinity. TLC, RB and PA significantly increased after 3-48 h, 3-96 h and 3-24 h, respectively, with recovery of TLC and PA after 96 h and 48-96 h, whereas ACP and lysozyme activity significantly decreased 3-96 h after being transferred to 20 parts per thousand. In another experiment, grouper reared at 34 parts per thousand and 27 degrees C were injected with Vibrio alginolyticus grown in tryptic soy broth (TSB) at 2.3 x 10(9) colony-forming units (cfu) fish(-1) and then transferred to 6 parts per thousand, 20 parts per thousand and 34 parts per thousand (control). The cumulative mortalities of V. alginolyticus-injected fish held in 6 parts per thousand were significantly higher than in injected fish held at 20 parts per thousand and 34 parts per thousand. It was concluded that grouper E. coioides encountering a 34 parts per thousand-6 parts per thousand salinity drop stress exhibited a depression in immunity as evidenced by decreased cellular and humoral parameters and increased susceptibility to V. alginolyticus. Grouper encountering a salinity stress drop from 34 parts per thousand to 20 parts per thousand, however, exhibited decreased humoral immune parameters but also increased TLC and cellular immune parameters, indicating immunomodulation.