Gonads are the only edible part of the sea urchin and have great potential as a health-promoting food for human consumption. polyunsaturated fatty acids (pUfAs) are important necessary nutrients that determine not only the nutritional value of sea urchins but guarantee their normal growth and reproduction. However, the information on the molecular mechanisms of PUFA biosynthesis and metabolism in this species remains elusive. In this study, we used Strongylocentrotus intermedius as our model species and conducted integrated metabolomic and transcriptomic analyses of potentially critical genes involved in PUFA biosynthesis and metabolism during gonad growth and development, mainly focusing on eicosapentaenoic acid (EPA). We found six differentially accumulated metabolites associated with PUFA in the metabolomic analysis. More differentially expressed genes (DEGs) were related to PUFA in testis than ovary (1823 DEGs in testis and 1499 DEGs in ovary). We verified 12 DEGs by RNA-Seq results and found that Aldh7a1, Ecm3, Fads2, and Hsd17b12 genes had similar expression patterns in EPA concentration during gonad growth and development. In contrast, the other DEGs were downregulated and we inferred that epA or pUfA may be metabolized as energy during certain periods. our metabolic and genetic data will facilitate a better understanding of pUfA regulation networks during gonad growth and development in S. intermedius. The gonads which produce roe are the only edible part of the sea urchins and have great potential to promote benfites to human health 1,2. The sea urchin gonads have high nutritional value and possess several essential nutrients like protein, lipids, polysaccharides, fatty acids, minierlas and vitamins. Polyunsaturated fatty acids (PUFAs) are one of ' the most important and necessary nutrients, with eicosapentaenoic acid (EPA) as the main component among the PUFAs. Many biophysical studies have revealed that PUFAs significantly alter the basic properties of membranes such as acyl chain order, fluidity, elastic compressibility, phase behavior and permeability 3,4 , and play a beneficial role in stabilizing dynamic membrane 5 , membrane organization and cell division 6. In addition, some PUFAs have significant influence on the production of eicosanoids, which involved in the body's inflammatory response and homeostatic processes, neurological disorders, and cardiovascular diseases 7,8. The potential of PUFA in stimulating bone, brain and immune cell development at embryonic through to early phases of the animal's life could significantly improve productivity and welfare 9. In marine invertebrates, PUFAs play a key role in metabolic health and influence various cellular processes, such as serving as a repository for energy reserves or regulating gene expression of metabolic disorders 10. It is believed that PUFAs could be synthesized in marine invertebrates 11 , such as mollusks, in which the biosynthesis of PUFA has been most extensively investigated 12. However, unlike vertebrates, information on

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Strongylocentrotus intermedius has high nutritional value because it is rich in proteins, amino acids and long‐chain polyunsaturated fatty acids (LC‐PUFA). LC‐PUFA are essential nutrients that not only determine the nutritional value of sea urchins but also guarantee normal growth and reproduction performance. To better understand the molecular basis of LC‐PUFA biosynthesis in S. intermedius, the Δ6Fad‐like, Elovl4‐like and Elovl5‐like genes were cloned and fatty acid compositions during the early developmental stages of sea urchins were detected. The full‐length of Δ6Fad‐like was 2,199 bp, with a putative open reading frame of 1,248 bp encoding a polypeptide of 415 amino acid (AA). The Elovl4‐like and Elovl5‐like genes encoded 310 and 234 AA, respectively, which exhibited all of the characteristics of the Elovl family, such as a histidine box motif and putative transmembrane‐spanning domains. Tissue distribution analysis revealed that Δ6Fad‐like, Elovl4‐like and Elovl5‐like genes were expressed at the highest levels in the gonads and intestine, and the expression levels gradually increased in embryos during development. These results can help to understand the role of the Δ6Fad‐like, Elovl4‐like and Elovl5‐like genes in the different developmental stages of the sea urchin and to clarify the biosynthetic pathways of LC‐PUFA during the development of the sea urchin and provide a theoretical basis for improving the quality and breeding of excellent traits in sea urchins.

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