| Summary: | The razor clam <i>Sinonovacula constricta</i> is the first marine mollusk demonstrated to possess the complete long-chain polyunsaturated fatty acids (LC-PUFA) biosynthetic pathway. This study explored the impact of different microalgae on growth, <i>Fad</i> and <i>Elovl</i> expressions, and fatty acid (FA) profiles in juvenile <i>S. constricta</i>. Results revealed that juveniles fed with <i>Isochrysis galbana</i> (rich in DHA) or <i>Chaetoceros calcitrans</i> (rich in EPA) consistently exhibited higher growth than those fed <i>Chlorella</i> sp. (rich in LA and ALA), underscoring the importance of dietary LC-PUFA in <i>S. constricta</i>’s development. Expression of most <i>Fad</i> and <i>Elovl</i> in <i>C. calcitrans</i> and <i>I. galbana</i>-fed juveniles were initially up-regulated, then down-regulated, suggesting LC-PUFA demand for faster growth. Although <i>Chlorella</i> sp.-fed juveniles exhibited decreased mRNA levels for most genes, levels were notably higher lately compared to those fed <i>C. calcitrans</i> or <i>I. galbana</i>, hinting at potential LC-PUFA biosynthesis induction. FA profiles in <i>S. constricta</i> generally mirrored those in ingested microalgae, implying direct FA accumulation from diets. Some microalgal FA were absent in farmed <i>S. constricta</i>, while others emerged, indicating <i>S. constricta</i>’s ability to selectively accumulate and synthesize FA. This study enhances the understanding of dietary FA metabolism in <i>S. constricta</i>, valuable for selecting appropriate microalgae in its farming practices.
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