A specific mix of generalists: bacterial symbionts in M editerranean I rcinia spp.

Microbial symbionts form abundant and diverse components of marine sponge holobionts, yet the ecological and evolutionary factors that dictate their community structure are unresolved. Here, we characterized the bacterial symbiont communities of three sympatric host species in the genus I rcinia from the NW M editerranean S ea, using electron microscopy and replicated 16 S r RNA gene sequence clone libraries. All I rcinia host species harbored abundant and phylogenetically diverse symbiont consortia, comprised primarily of sequences related to other sponge‐derived microorganisms. Community‐level analyses of bacterial symbionts revealed host species‐specific genetic differentiation and structuring of I rcinia ‐associated microbiota. Phylogenetic analyses of host sponges showed a close evolutionary relationship between I rcinia fasciculata and I rcinia variabilis , the two host species exhibiting more similar symbiont communities. In addition, several bacterial operational taxonomic units were shared between I . variabilis and I rcinia oros , the two host species inhabiting semi‐sciophilous communities in more cryptic benthic habitats, and absent in I . fasciculata , which occurs in exposed, high‐irradiance habitats. The generalist nature of individual symbionts and host‐specific structure of entire communities suggest that: (1) a ‘specific mix of generalists’ framework applies to bacterial symbionts in I rcinia hosts and (2) factors specific to each host species contribute to the distinct symbiont mix observed in I rcinia hosts.