In this study, the bacterial communities of the cœlomic microbiota were characterized in three Echinoderms: the deposit feeder sea Cucumber Holothuria forskali, the herbivorous sea Urchin Sphaerechinus granularis, and the carnivorous sea Star Marthasterias glacialis. Samples were collected from the same habitat in the Glénan Archipelago (Brittany, France) at different times for 2 years. The cœlomic microbiota were analyzed by targeted metagenomic with V4-16S metabarcoding and by a culturable approach with the isolation of strains and antimicrobial activity assays. Most of the OTUs of the cœlomic microbiota were affiliated with the phylum Proteobacteria and, notably, five orders: Burkholderiales, Flavobacteriales, Alteromonadales, Vibrionales and Pseudomonadales. Significant differences were observed regarding richness, biodiversity and composition between species and sampling dates. They could be explained by sub-abundant taxa that represented the global diversity. Cœlomic microbiota also revealed shared and unshared bacterial communities, validating a potential “specific” microbiota among the three Echinoderm species. Moreover, significant variations of the microbiota occurred among the sampling dates, suggesting a plasticity and, thus, a potential selection of these microbiota. Finally, out of the 831 bacterial strains isolated from culturable microbiota, 20 strains exhibited antibacterial activities, most of them assigned to the genera Shewanella, Pseudoalteromonas and Vibrio.