Effects of dietary beta-1, 3 glucan on innate immune response of large yellow croaker, Pseudosciaena crocea.

The present study was conducted to investigate the effects of dietary beta-1, 3 glucan on the innate immune response and protection against Vibrio harveyi infection in large yellow croaker, Pseudosciaena crocea. A basal diet was supplemented with 0% (control), 0.09% (low) and 0.18% (high) beta-1, 3 glucan to formulate three experimental diets. Each diet was randomly allocated to triplicate groups of fish in floating sea cages (1.5 x 1.5 x 2.0m), and each cage was stocked with 100 fish (initial average weight 9.75+/-0.35 g). Fish were fed twice daily (05:00 and 17:00) to apparent satiation for 8 weeks. The results of 8 weeks feeding trial showed that low glucan supplementation (0.09%) significantly enhanced fish growth, whereas high supplementation (0.18%) did not. The serum lysozyme activity was significantly increased with the increase of dietary glucan (P < 0.05), and fish fed the diet with high glucan had significantly higher lysozyme activity compared with low glucan. There were no significant differences in alternative complement pathway (ACP) activity between fish fed diets with and without supplementation of glucan. The phagocytosis percentage (PP) and respiratory burst activity in fish fed the diet with 0.09% glucan were significantly higher than those in fish fed with the control diet (P < 0.05), but both immunological parameters significantly decreased in fish fed the diet with high supplementation compared with low supplementation and no significant difference was observed between the control and high supplementation groups. The challenge experiment showed that fish fed the diet with low glucan had significantly lower cumulative mortality compared with the control and high glucan groups (P < 0.05), but no significant difference was observed between the control and high supplementation groups. These results suggested that low glucan could enhance growth and innate immunity of large yellow croaker with an 8-week oral administration, but higher supplementation did not influence growth, or further improve immunity of large yellow croaker.