Bacterial symbionts associated with two species of Xestospongia giant barrel sponges

The giant barrel sponges Xestospongia muta and Xestospongia testudinaria are ubiquitous in coral reefs in the Atlantic and Pacific Oceans, respectively. They are key species in their respective environments, and are hosts to diverse assemblages of bacteria. A detailed analysis of the bacterial communities associated with these two sponges, consisting of over 600 nearly full-length 16S rRNA gene sequences obtained from six different individuals, showed remarkable similarity between the bacterial communities associated with the two sponges. Even though these two sponges have been geographically separated for over three million years, the dominant bacterial groups in the two sponges, Acidobacteria, Actinobacteria, and Chloroflexi, have been shared and maintained. Analysis of these three dominant bacterial groups suggests that the bacteria associated with the two Xestospongia spp. have been symbionts since the time of a common Xestospongia ancestor, and have been maintained since the geographic separation and subsequent speciation of the two giant barrel sponges.