| Summary: | Driven by the concept of sustainable aquaculture, natural feed additives with growth-promoting, antioxidant, and immune-enhancing properties have become a key research focus. This study assessed the effects of dietary red pepper powder (<i>Capsicum annuum</i>) supplementation at 0%, 0.5%, 1.0%, and 2.0% over 50 days on the growth, digestive function, immune and antioxidant capacities, intestinal microbiota, and gene expression in <i>Strongylocentrotus intermedius</i> (<i>S. intermedius</i>). The results indicated that red pepper powder significantly promoted growth and decreased the feed conversion ratio (FCR) (<i>p</i> < 0.05), with the 1.0% group showing the highest growth rate. Additionally, supplementation improved gonadal coloration and increased crude protein and lipid contents in the gonads, particularly in the 1.0% and 2.0% groups (<i>p</i> < 0.05). Supplementation with 1.0% and 2.0% red pepper powder enhanced digestive, immune, and antioxidant enzyme activities, while reducing malondialdehyde (MDA) levels, indicating lower lipid peroxidation. α-diversity analysis revealed the highest ACE, Chao, and Shannon indices and the lowest Simpson index in the 1.0% group, indicating greater microbial diversity. Community analysis revealed that in the red pepper powder treatment groups, beneficial bacteria, such as <i>Firmicutes</i> and <i>Unclassified</i><i>_f__Rhodobacteraceae</i>, increased in relative abundance, while potential pathogens like <i>Arcobacter</i>, and <i>Epsilonbacteraeota</i> were less abundant. Red pepper powder supplementation upregulated key immune- and antioxidant-related genes while downregulating pro-inflammatory and stress-associated genes. Overall, optimal dietary supplementation of red pepper powder, particularly at 1.0%, enhanced antioxidant and immune functions, optimized intestinal microbiota, mitigated oxidative stress, and consequently promoted growth, improved gonadal quality, and strengthened overall health in <i>S. intermedius</i>.
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