Trophic upgrading of long-chain polyunsaturated fatty acids by polychaetes: a stable isotope approach using Alitta virens

Polychaete worms are rich sources of polyunsaturated fatty acids (PUFA) and are increasingly incorporated into aquaculture broodstock diets. Conventionally, the build-up of PUFA in polychaetes was considered passive, with direct accumulation along the food web, originating with microalgae and other primary producers. However, it has been argued that polychaetes (and other multicellular eukaryotes) are capable of PUFA biosynthesis through the elongation and desaturation of precursor lipids. We further test this hypothesis in the ecologically and economically important nereid polychaete Alitta virens by adopting a stable isotope labelling approach. Worms were fed a 13C-1-palmitic acid (C16:0) enriched diet with the resulting isotopically enriched lipid products identified over a 7-day period. The data showed strong evidence of lipid elongation and desaturation, but with a high rate of PUFA turnover. A putative biosynthetic pathway is proposed, terminating with docosahexaenoic acid (DHA) via arachidonic (AA) and eicosapentaenoic acids (EPA) and involving a Δ8 desaturase.