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As an unusual economically important aquaculture species, Sinonovacula constricta possesses high levels of long-chain polyunsaturated fatty acids (LC-PUFA). Previously, our group identified fatty acyl desaturases (Fad) with Δ5 and Δ6 activities in S. constricta , which was the first report of Δ6 Fad in a marine mollusc. Here, we further successfully characterize elongases of very long-chain fatty acids (Elovl) in this important bivalve species, including one Elovl2/5, two Elovl4 isoforms (a and b) and a novel Elovl (c) with Elovl4 activity. In addition, we also determined the desaturation activity of S. constricta Δ6 Fad toward 24:5n-3 to give 24:6n-3, a key intermediate in docosahexaenoic acid (DHA) biosynthesis. Therefore, S. constricta is the first marine mollusc reported to possess all Fad and Elovl activities required for LC-PUFA biosynthesis via the 'Sprecher pathway'. This finding greatly increases our understanding of LC-PUFA biosynthesis in marine molluscs. Phylogenetic analysis by interrogating six marine molluscan genomes, and previously functionally characterized Elovl and Fad from marine molluscs, suggested that DHA biosynthetic ability was limited to a few species, due to the general lack of Δ4 or Δ6 Fad in most molluscs. • The S. constricta Elovl involved in LC-PUFA synthesis were investigated. • The desaturation ability of S. constricta Δ6 Fad toward 24:5n-3 was determined. • S. constricta is the first mollusc with capacity to operate the Sprecher pathway. • DHA biosynthetic ability was limited to certain marine molluscs. [ABSTRACT FROM AUTHOR]

Sinonovacula constricta is the first marine mollusc demonstrated to possess all fatty acyl desaturase (Fad) and elongase of very long chain fatty acids (Elovl) activities required for long-chain polyunsaturated fatty acids (LC-PUFA) biosynthesis via the Sprecher pathway, providing a good opportunity to investigate the essential fatty acids (FA) for its larval development. In the present study, we analyzed the Fad and Elovl expressions, FA compositions, and the feed effects of three representative microalgae (Chlorella sp. , Chaetoceros calcitrans and Isochrysis galbana) in S. constricta at early developmental stages. The results showed that S. constricta Fad and Elovl were all highly expressed in zygotes, which might occur from maternal transfer. Compared to transcripts of Elovl4 activity, Elovl2/5 was only expressed relatively high in zygotes, indicating that C ≥ 22 LC-PUFA were more important than C ≤ 20 LC-PUFA for S. constricta larval development. The zygotes accumulated abundant FA from parents, especially eicosapentaenoic acid (EPA), but with small amount of docosahexaenoic acid (DHA). Meanwhile, the trochophore larva fed with single or mixed microalgae with I. galbana (rich in DHA) exhibited significantly higher growth and survival rate compared to those fed with single Chlorella sp. (rich in 18:2n-6 and 18:3n-3) and C. calcitrans (rich in EPA) or both of them. Those findings suggested that the highly expressed Fad and Elovl might be largely responsible for DHA biosynthesis, but this capability was limited to meet the developmental requirement and the dietary DHA was necessary. Taken together, our results compellingly demonstrated that DHA was essential for S. constricta larval development, which is important to guide its breeding. • Fad and Elovl expressions were examined in developing S. constricta. • Fatty acid compositions were analyzed in developing S. constricta. • Trochophore larval growth and survival were evaluated by first-feeding diet. • DHA still appeared to be essential for S. constricta larval development. • Microalgae rich in EPA and DHA were critical for S. constricta breeding. [ABSTRACT FROM AUTHOR]