Freshwater-adapted polychaetes exhibit a complete enzymatic machinery for synthesizing long-chain polyunsaturated fatty acids.

The sustainability of aquaculture is challenged by limited fishmeal and fish oil supplies, key sources of long-chain polyunsaturated fatty acids (LC-PUFA) such as eicosapentaenoic acid (EPA, 20:5 n-3), docosahexaenoic acid (DHA, 22:6 n-3) and arachidonic acid (ARA, 20:4 n-6), essential for fish health and product quality. Polychaetes represent a promising alternative. While marine polychaetes show complete LC-PUFA biosynthetic pathways involving elongases (Elovl), front-end desaturases (Fed), and methyl-end desaturases (ω des), freshwater species remain poorly studied. We hypothesize that freshwater-adapted polychaetes exhibit enhanced LC-PUFA biosynthesis to compensate for limited dietary sources in freshwater environments. This study focuses on Namalycastis rhodochorde, a freshwater nereid polychaete found in Southeast Asia. We isolated and characterized elongase and desaturase genes from N. rhodochorde using a yeast-based heterologous expression system. Our results revealed three Elovl (Elovl2/5, Elovl4, Elovl1/7) that elongate PUFA substrates from C₁₈ to C₂₂, two Fed (Fed1 with Δ5 and Fed2 with dual Δ6/Δ8 activities), and two ω des: a Δ12 desaturase enabling linoleic acid (18:2 n-6) synthesis, and an ω3 desaturase converting n-6 into n-3 PUFA. These findings indicate that N. rhodochorde has the enzymatic capacity to synthesize LC-PUFA like ARA and EPA, supporting its potential for sustainable biomass production using low-nutrient substrates.