The sea star, Pisaster ochraceus, has unique and dominant bacteria.

All animals live in close association with bacteria, but for most hosts little is known about the distribution and host-specificity of these bacteria. We used amplicon sequencing to survey the bacterial communities living on and in the sea star Pisaster ochraceus, as well as in their environment and on sympatric marine hosts, across three geographically distinct populations. Overall, the bacterial communities on Pisaster are distinct from their environment and differ by both body region and geography. We detected core bacteria that are more abundant on hosts compared to environmental bacterial communities across all sampled natural sites. The core taxa fall within Spirochaetes (genus Salinispira and two uncultured clades), Mollicutes (genus Hepatoplasma), Peregrinibacteria, and Bacteroidetes (genus Reichenbachiella).