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The Pacific saury Cololabis saira is an important fishery resource in the Far East, where it is appreciated for its high lipid content, which includes long-chain polyunsaturated fatty acids (LC-PUFAs) such as docosahexaenoic acid (DHA). Although it has been hypothesized that these fatty acids are derived from their prey items, this study focused on evaluating the capacity of saury for endogenous production of LC-PUFAs. To describe its LC-PUFA biosynthetic pathway, we characterized fatty acid desaturase 2 (Fads2) and elongation of very long-chain fatty acid protein 5 (Elovl5) in Pacific saury. Two paralogous fads2 genes, namely fads2a and fads2b, were isolated from the saury and their recombinant protein showed ∆6∆8 and ∆4∆5 desaturase activities, respectively. Meanwhile, saury Elovl5 had elongase activity toward C18 and C20 PUFA. These three enzyme genes were expressed in the brain and liver, although fads2a was absent in the latter. Our results suggest that the saury has sufficient enzymatic functions, particularly in its brain, for DHA biosynthesis through the ∆4 pathway even from α-linolenic acid. This information provides novel insights into the origin of LC-PUFAs in Pacific saury and the future perspective for its potential as a source of such vital fatty acids for human consumption. [ABSTRACT FROM AUTHOR]