Effects of dietary LNA/LA ratios on growth performance, fatty acid composition and expression levels of elovl5, Δ4 fad and Δ6/Δ5 fad in the marine teleost Siganus canaliculatus

Rabbitfish Siganus canaliculatus is a crucial cultured herbivorous marine teleost, and the first marine teleost demonstrated to have the ability of biosynthesizing long-chain polyunsaturated fatty acids (LC-PUFA) from linolenic (LNA) and linoleic acid (LA) precursors, suggesting the application potential of vegetable oils (VO), rich in LNA or LA but lack of LC-PUFA, as the dietary lipid resources. In the present study, seven diets (D1-D7) were prepared for the culture of rabbitfish juveniles lasting 12 weeks. D1 (control diet) contained fish oil (FO) as the sole lipid, while D2-D7 contained a blend of glycerol trioleate and perilla oil with an ALA/LA ratios ranging from 0.05 to 2.45. The results showed that fish fed D3 or D5-D7 displayed as good growth performance as that fed D1. The fatty acid (FA) composition of liver and fillet reflected dietary FA profiles, and relatively higher levels of n-3 LC-PUFA was observed in D5-D7 groups. While the hepatic mRNA levels of ∆ 4 and ∆ 6/∆ 5 fatty acyl desaturase (Fad) and elongation of very long-chain fatty acids (Elovl) genes were affected by dietary LNA/LA ratios, with the highest expression of all three genes in fish fed D6 with an LNA/LA ratio of 1.93. Take the above results together, it can be concluded that diet contained a blend of VO with around 1.93 LNA/LA ratio is suitable for rabbitfish culture concerning the growth performance and LC-PUFA biosynthesis.