Requirement of arachidonic acid in juvenile sea cucumber Apostichopus japonicus Selenka

Arachidonic acid (ARA) is a primary precursor of highly bioactive eicosanoids, and adequate ARA levels are essential for maintaining growth and biological processes in most aquaculture species. However, no studies have yet been conducted on the nutritional requirements of sea cucumbers for ARA. Therefore, the present study examined the requirement of arachidonic acid in the juvenile sea cucumber Apostichopus japonicus . Juvenile sea cucumbers (initial body weight: 10.78 g) were fed six isonitrogenous and isoenergetic diets containing graded levels of ARA as follows: 0.02% (Control), 0.17%, 0.36%, 0.51%, 0.59% and 0.98% for 56 days. Each diet was randomly assigned to triplicate groups of 40 sea cucumbers in a recirculating aquaculture system. The results showed that specific growth rate and feed efficiency initially increased with dietary ARA level up to 0.51%, after which they declined. Survival rate was not affected. The body wall crude lipid content decreased and then increased with increasing ARA, whereas crude protein, crude ash, and moisture contents showed no significant differences among groups. With increased dietary ARA levels, the contents of ARA and n-6 PUFA in the body wall increased, whereas those of EPA and DHA decreased. Meanwhile, the content of polyunsaturated fatty acids initially increased, then declined. The activities of antioxidant enzymes (superoxide dismutase, catalase, and total antioxidant capacity) in the intestine exhibited a unimodal pattern, peaking at 0.51% ARA. Conversely, the malondialdehyde content showed an inverse trend. Significant improvements were observed in alanine aminotransferase and aspartate aminotransferase activities in the 0.36% and 0.51% groups compared to the control group. Similarly, both acid phosphatase and alkaline phosphatase activities were significantly higher in the 0.36% and 0.51% groups than in the control and 0.98% groups. The appropriate ARA supplement increased Cyclin B mRNA expression; excessive ARA downregulated it. Meanwhile, the expression of Follistatin mRNA showed an inverse trend with that of the CyclinB gene. The optimal dietary ARA content for juvenile sea cucumber was 0.51% based on the second-order polynomial regression models of SGR. 0.36%- 0.51% dietary ARA could significantly enhance the growth performance and non-specific immunity of sea cucumbers. These results may also indicate that dietary ARA is an essential fatty acid in sea cucumbers.