Origin and fate of long-chain polyunsaturated fatty acid (LC-PUFA) in reproduction of female Hediste diversicolor (OF Müller, 1776) (Annelida: Nereididae)

Meeting the aquatic protein demands of a growing global population necessitates increased aquaculture production. Further expansion of sustainable aquaculture depends on identifying alternative sources of marine proteins and lipids that support animal welfare. Polychaetes have emerged as promising substitutes for fish meal and fish oil in aquafeeds. Notably, Hediste diversicolor has previously been shown to possess the enzymatic machinery required for long-chain (C 20–24 ) polyunsaturated fatty acids (LC-PUFA) biosynthesis, making it a promising candidate for converting low-value sidestreams into high-quality biomass within circular value chains. This study investigated the incorporation of dietary carbon into key LC-PUFAs during female maturation, including arachidonic acid (ARA, 20:4n-6), eicosapentaenoic acid (EPA, 20:5n-3), and docosahexaenoic acid (DHA, 22:6n-3), which are essential for successful oogenesis and reproduction in teleost fish. Over a 203-day experiment, immature specimens were fed a PUFA-deficient diet spiked with 13 C palmitic acid (C16:0). Growth, survival, gamete development, fatty acid composition, and compound-specific stable isotope analysis (CSIA) were assessed in somatic and reproductive tissues. 13 C enrichment was detected in several fatty acids, including ARA, EPA and DHA, in both somatic and reproductive tissues of female polychaetes. DHA showed stronger enrichment in oocytes than in somatic tissue, indicating preferential allocation of dietary carbon to reproductive LC-PUFA pools during maturation. These results demonstrate that dietary carbon contributes to the accumulation of reproductively important LC-PUFAs during female maturation. Although the data do not conclusively demonstrate endogenous DHA biosynthesis, they provide the first evidence that dietary 13 C-palmitic acid contributes to the DHA pool in female H. diversicolor during maturation. These findings improve our understanding of LC-PUFA metabolism and support the development of nutritionally optimized broodstock strategies for sustainable polychaete production.