Seasonality shapes gut microbiota composition in two sympatric sea urchins

Microbial communities in both echinoids were dominated by Bacillota, Bacteroidota, Desulfobacterota, and Pseudomonadota. Core microbiota analysis revealed stable taxa across hosts and seasons, most notably <i>Propionigenium</i>, supporting host-driven selection mechanisms that maintain functional stability despite seasonal turnover. Core microbiome analyses revealed Bacillota, Bacteroidota, Desulfobacterota_G_459543, and Pseudomonadota as persistent phyla across seasons, and genera such as <i>Desulfotalea</i>, <i>Photobacterium</i>, and <i>Propionigenium</i> consistently present in both species. Alpha diversity was significantly higher in winter for both <i>D. antillarum</i> and <i>E. lucunter</i>, while species-level differences were not significant. Beta diversity analyses showed significant seasonal clustering, with no consistent segregation between species within seasons. Our findings demonstrate that shared environmental conditions, particularly seasonality, are the primary modulators of gut microbiota in sympatric sea urchins, while host selection preserves a conserved core community. This dual influence of environment and host highlights the ecological plasticity and resilience of sea urchin microbiotas under fluctuating reef conditions.