Ontogenetic and spatial variation in the feeding habits of Tripneustes gratilla

As a keystone species in coral reef ecosystems, the collector sea urchin Tripneustes gratilla maintains its ecological balance by regulating algal populations; however, a systematic understanding of its role as a primary consumer in tropical marine environments remains limited. In this study, we investigated T. gratilla populations in coastal waters near Wuzhizhou Island, Sanya city, Hainan Province, China. Using 18S rDNA and 16S rDNA high-throughput sequencing, we analyzed differences in the T. gratilla diet in autumn across different habitats and body size classes. The analysis revealed that Tripneustes gratilla has a diverse diet spanning multiple phyla, including Arthropoda, Rhodophyta, Apicomplexa, and Cyanobacteriota. Distinct spatial dietary patterns were observed across different benthic habitats. In offshore rocky substrates, the gut contents primarily consisted of Rhodophyta (red algae) and Arthropoda (crustaceans), collectively accounting for more than 40% of the ingested material. Conversely, nearshore sandy-gravel substrates showed remarkable dietary specialization, with Arthropoda and Rhodophyta constituting more than 70% of the total dietary composition. Body size influenced the T. gratilla diet: large individuals (500-600 g) consumed specialized diets dominated by Eukaryota and Rhodophyta (>80%), whereas medium and small individuals presented greater dietary diversity. The gut microbiota demonstrated regional universality, with Proteobacteria and Bacteroidetes constituting the core microbiome. Environmental factors (e.g., pH) significantly affect T. gratilla feeding behavior. In this study, we identified habitat heterogeneity and ontogenetic shifts as critical drivers of T. gratilla trophic ecology, providing novel insights into its functional role in ecosystem dynamics.