Isolation and identification of bacterial endosymbionts in the brooding brittle star Amphipholis squamata

Symbiotic associations with subcuticular bacteria (SCB) have been identified and studied in numerous echinoderms, including the SCB of the brooding brittle star, Amphipholis squamata. These SCB, however, have not been studied using current next generation sequencing technologies. Previous studies on the SCB of A. squamata placed these bacteria in the genus Vibrio (γ-Proteobacteria), but subsequent studies suggested that the SCB are primarily composed of α-Proteobacteria. The present study examines the taxonomic composition of SCB associated with A. squamata from the Northwest Atlantic. DNA was extracted using a CTAB protocol and 16S rRNA sequences were amplified using gene-targeted PCR on an Illumina HiSeq at the UNH Genomics Center. Results show the presence of a single dominant bacterial type, within the family Rhodobacteraceae, which composes 70-80% of the A. squamata microbiome. The majority of sequences recovered from A. squamata were identified as members of the genus Octadecabacter (97% similarity). By comparison, adjacent seawater and sediment bacterial communities were significantly more diverse, hosting bacteria in the phyla Proteobacteria, Bacteroidetes, Cyanobacteria, Verrucomicrobia, and Actinobacteria. Thus, a distinct SCB community is clearly evident in A. squamata. Here, we hypothesize the potential functions of this symbiotic community, in addition to what may be driving the unique dominance of a member of the family Rhodobacteraceae. Although metatranscriptome studies are needed to characterize the functional attributes of the SCB community, we have identified a specific and potentially beneficial symbiont that may support metabolic requirements and nutrient uptake vital to the reproduction of A. squamata