Algal diet and habitat structure jointly shape growth, behavior, and gut microbiota in Tripneustes gratilla

Differences in food sources significantly affect the behavior, gut microbiota, and physiology of marine organisms. This study aimed to examine the effects of different algal diets and habitat conditions (presence/absence of coral reef structures) on the growth, gonadal development, digestive enzyme activity, feeding behavior, and gut microbiota of Tripneustes gratilla . Sea urchins fed mixed brown algae showed the highest weight gain rates and test diameter growth, with SL-c and SL groups significantly outperforming single-species brown algae treatments ( P < 0.05). In contrast, green algae treatments resulted in the lowest growth performance. Gonad index was significantly higher in red algae treatments than in green algae treatments ( P < 0.05), and coral structure further enhanced gonad index in specific groups (e.g., E-c and K-c). Wet gut index varied with diet, with brown algae promoting higher gut index under non-structure conditions. A positive correlation between gonad weight and wet gut weight was observed across all diet groups, with the strongest relationship in the brown algae group (Spearman r = 0.4501, P < 0.01), and weaker correlations in the red ( r = 0.3746, P < 0.05) and green ( r = 0.2531, P < 0.05) groups. Digestive enzyme activity differed significantly among diets: brown algae increased pepsin and lipase activities ( P < 0.05), whereas red and green algae resulted in higher amylase activity ( P < 0.05). Feeding and foraging exhibited pronounced diel rhythms, peaking at midnight and declining at noon, with no significant effects of habitat structure ( P > 0.05), whereas habitat use varied among diet groups and increased significantly at night under structured conditions. Gut microbial diversity and composition were strongly associated with diet. Brown algae treatments exhibited higher alpha diversity, and PCoA revealed clear clustering by diet. At the phylum level, Bacteroidota and Proteobacteria dominated across treatments, while relative abundances of Spirochaetota, Firmicutes, and Fusobacteriota varied among diets. Functional prediction indicated significant differences in KEGG pathways ( P < 0.05), with enhanced carbohydrate and amino acid metabolism in brown and green algae groups, and enrichment of glycan metabolism and antimicrobial resistance pathways in the red algae group. This study demonstrated that algal diet and habitat structure jointly influence the physiological performance, behavior, and gut microecology of T. gratilla , offering theoretical support for optimized aquaculture practices and reef-based ecological management.