Effects of algal diet on the growth, body composition, and metabolism of juvenile sea cucumber Holothuria leucospilota

To develop formulated feed for juvenile sea cucumber Holothuria leucospilota and address key bottlenecks in their large-scale cultivation, this study investigated the effects of three algal diets— Ulva sp., Sargassum sp., and Laminaria japonica —used individually or in 1:1 mixtures on the growth performance, fatty acid profile, intestinal immunity, metabolic and digestive enzyme activities, as well as body wall metabolism-related gene expression in juvenile H. leucospilota . Over the 56-day feeding trial, the Sargassum sp. (S) group exhibited the highest growth performance among single-ingredient diets, with the wet weight increasing from 3.02 ± 0.58 g to 20.98 ± 2.35 g, whereas L. japonica (L) yielded the poorest results, reaching a final wet weight of only 8.08 ± 3.50 g. Although L. japonica contained higher levels of polyunsaturated fatty acids (PUFAs), at 39.32 ± 0.17 %, it may also harbor components detrimental to sea cucumber feeding and growth. The combination of Sargassum sp. and Ulva sp. (S+U) achieved the best growth efficiency, with final wet weight reaching 30.15 ± 1.98 g. Gene expression analysis revealed upregulation of glycolysis-related genes ( HK1 , PKM ) and fatty acid synthesis genes ( ACC , SCD5 ) in both the S+U and L groups, correlating with increased PUFA deposition. Intestinal enzyme activities and antioxidant responses further indicated diet-specific metabolic adaptations. Therefore, we recommend feeding a mixture of Sargassum sp. and Ulva sp. to maximize growth efficiency, and supplementing with L. japonica to enhance PUFA content in H. leucospilota . This study provides a foundational framework for the efficient aquaculture of tropical sea cucumbers. • Juvenile Holothuria leucospilota were fed with different algal diets. • Sargassum sp. had the best attractant effect; Laminaria japonica the worst. • Sargassum sp. + Ulva sp. diet maximizes growth of juvenile H. leucospilota. • L. japonica suppresses growth but enriches body wall PUFAs via SCD5 upregulation. • Intestinal enzyme and antioxidant profiles reflect diet-specific metabolic adaptation.