Effects of replacing Laminaria japonica with mixed seaweed meal on growth performance, digestive and metabolic enzyme activities, and immune function of the sea cucumber Apostichopus japonicus (Selenka)

Laminaria japonica is a widely cultivated and economically significant seaweed, commonly used as a primary feed ingredient in sea cucumber aquaculture due to its cost-effectiveness and high nutritional value. However, declining production and rising costs have prompted the exploration of alternative feed sources to support sustainable aquaculture practices. This study assessed the feasibility of substituting L. japonica with a mixed seaweed meal to enhance feed sustainability while sustaining the health and growth performance of Apostichopus japonicus . The control diet (P0) contained 70 % L. japonica meal, whereas experimental diets replaced it with a mixed seaweed blend (2:1:1:1 ratio of Sargassum polycystum , degummed L. japonica , Ulva lactuca , and Enteromorpha prolifera ) at substitution levels of 25 %, 50 %, 75 %, and 100 % (P25-P100). Over an 84-day feeding trial, specific growth rate and protein efficiency ratio decreased significantly at substitution levels exceeding 25 %, with parallel reductions observed in intestinal trypsin and lipase activities. Hexokinase and succinate dehydrogenase activities peaked at 25 % substitution, and malate and pyruvate dehydrogenase reached maximum levels at 50 %. Acid and alkaline phosphatase activities increased at 25 % and 50 % substitution levels, whereas superoxide dismutase activity decreased, accompanied by elevated malondialdehyde content. Additionally, catalase and lysozyme activities, along with their corresponding mRNA expression, decreased at higher substitution levels. Notably, NF-κB (p105) mRNA expression declined at 75 % and 100 % substitution, while Toll-like receptor mRNA showed slight upregulation. The integrated biomarker response model revealed that 25 % substitution level yielded the highest physiological adaptability, and substitution levels exceeding 50 % induced significant stress responses. Cubic regression analysis identified an optimal substitution level of 17.28 % for maximizing growth performance. These findings provide a valuable reference for incorporating mixed seaweed meal into sea cucumber diets effectively. • Replacing 25 % of Laminaria japonica with mixed seaweed enhances growth performance and immune function. • Moderate mixed seaweed inclusion improves digestive enzyme activity and metabolic efficiency. • High substitution (>50 %) reduces intestinal integrity and immune-related gene expression. • Regression analysis suggests 17.28 % as the optimal substitution level.