Dietary arachidonic acid promotes growth, improves immunity, and regulates the expression of immune-related signaling molecules in Macrobrachium nipponense (De Haan)

Arachidonic acid (ARA) is involved in the growth and regulation of immunity in fish. Little research has focused on ARA in crustaceans. This study was an 8-week feeding experiment investigating the effects of dietary ARA on growth, antioxidative indices, and immune characteristics of the juvenile oriental river prawn (Macrobrachium nipponense). An ARA-enriched oil was supplemented into the basal diet to formulate six isonitrogenous and isoenergetic diets containing 0.02% (the control group), 0.28%, 0.54%, 0.86%, 1.22%, and 1.54% ARA dry weight. Diets were fed to juvenile prawns (mean weight: 0.104 ± 0.002 g) twice daily to apparent satiation in three replicates. Weight gain and specific growth rate of M. nipponense significantly increased with increasing dietary levels of ARA from 0.02% to 0.86% and thereafter declined. Hepatopancreas super oxide dismutase and catalase activities significantly increased with supplementation with ARA, particularly in prawns fed 1.22% and 1.54% ARA. The malondialdehyde level in prawns fed 1.54% ARA also significantly increased, and was higher than that of prawns fed ARA from 0.02% to 0.86%. Serum lysozyme activity was significantly greater in prawns fed 0.86% ARA compared with the other groups. mRNA expression of toll-like receptor signaling pathway-related genes was promoted by a dietary ARA level from 0.54% to 0.86% but inhibited with the higher ARA levels (1.22-1.54%). The mortality percentage increased after challenge with live Aeromonas hydrophila. The mortality percentage of prawns fed 1.54% ARA was significantly higher than that of prawns fed 0.28-1.22% ARA. The results suggest that the optimal dietary ARA requirement for juvenile M. nipponense is 0.82% (dry weight) based on second-order regression analysis of weight gain vs. dietary ARA level, and that 0.86% ARA could significantly enhance non-specific immunity of M. nipponense; however, too high a level of ARA will affect the health status of this prawn.