A 30-day feeding trial was designed to evaluate the effect of supplemental
fulvic acid (FA) on survival, growth performance, digestive ability and immunity of large yellow croaker (Larimichthys crocea) larvae (initial
body weight 11.33 ± 0.57 mg). Four isonitrogenous and isolipids diets containing 0.00%, 0.01%, 0.02% and 0.04% FA were formulated, respectively. Results showed that the supplementation of 0.04% FA significantly improved survival rate of large yellow croaker larvae. Meanwhile, supplemental FA significantly increased final
body weight and specific growth rate. Based on the specific growth rate, the optimal supplementation was 0.0135% FA. Larvae fed the diet with 0.01% FA had significantly higher villus height than the control. The supplementation of 0.01%-0.02% FA significantly increased the muscular thickness of intestine. Moreover, supplementation of FA significantly increased
mRNA expression of intestinal epithelial proliferation and barrier genes (
pcna, zo-1 and zo-2). Diets supplemented with 0.02%-0.04% FA significantly increased the activity of
trypsin in the intestinal segment, while 0.01%-0.02% FA significantly increased the activity of
trypsin in the pancreatic segment. Compared with the control, supplementation of FA remarkably increased activities of
alkaline phosphatase and
leucine aminopeptidase in the brush border membrane of intestine. Larvae fed the diet with 0.01% FA significantly increased activities of
lysozyme and total
nitric oxide synthase. Furthermore, the supplementation of 0.01% to 0.02% FA significantly decreased the
mRNA expression of pro-inflammatory
cytokines (tnf-α and il-6). Concurrently, supplemental FA significantly increased anti-inflammatory
cytokine (il-10)
mRNA expression level. In conclusion, this study indicated that the supplementation of FA could improve the survival rate and growth performance of larvae by promoting intestinal development, digestive
enzymes activities and innate immunity.