Effects of dietary n-3 highly unsaturated fatty acids on growth, nonspecific immunity, expression of some immune related genes and disease resistance of large yellow croaker (Larmichthys crocea) following natural infestation of parasites (Cryptocaryon irritans).

The study was conducted to investigate the effects of dietary n-3 highly unsaturated fatty acid (n-3 HUFA) on growth, nonspecific immunity, expression of some immune related genes and disease resistance of juvenile large yellow croaker (Larmichthys crocea) following natural infestation of parasites (Cryptocaryon irritans). Six isoproteic and isolipidic diets were formulated with graded levels of n-3 HUFA ranging from 0.15% to 2.25% of the dry weight and the DHA/EPA was approximately fixed at 2.0. Each diet was randomly allocated to triplicate groups of fish in floating sea cages (1.0 × 1.0 × 1.5 m), and each cage was stocked with 60 fish (initial average weight 9.79 ± 0.6 g). Fish were fed twice daily (05:00 and 17:00) to apparent satiation for 58 days. Results showed that moderate n-3 HUFA level (0.98%) significantly enhanced growth compared with the control group (0.15% HUFA) (P < 0.05), while higher n-3 HUFA levels (1.37%, 1.79% and 2.25%) had detrimental effects on the growth though no significance was found (P > 0.05). Nitro blue tetrazolium (NBT) positive leucocytes percentage of head kidney and serum superoxide dismutase (SOD) activity increased with increasing n-3 HUFA from 0.15% to 0.60%, and decreased with further increase of n-3 HUFA from 0.60% to 2.25% (P < 0.05). Serum lysozyme activity increased significantly as n-3 HUFA increased from 0.15% to 1.37%, and then decreased with n-3 HUFA from 1.37% to 2.25% (P > 0.05). There were no significant differences in phagocytosis index (PI) of head kidney leucocytes among dietary treatments (P > 0.05). The hepatic mRNA expression of Toll-like receptor 22 (TLR22) and Myeloid differentiation factor 88 (MyD88) was significantly up-regulated in fish fed the diets with low or moderate levels, while in kidney this increment was only found at specific sampling time during the natural infestation of parasites. The 13 d cumulative mortality rate following natural infestation of parasites decreased with n-3 HUFA increased from 0.15% to 0.60% (P < 0.05), and significantly increased with n-3 HUFA from 0.60% to 2.25% (P < 0.05). Results of this study suggested that fish fed low or moderate dietary n-3 HUFA had higher growth, nonspecific immune responses, expression levels of some immune related genes and disease resistance of large yellow croaker following natural infestation of parasites and dietary n-3 HUFA may regulate fish immunity and disease resistance by altering the mRNA expression levels of TLR22 and MyD88.