Geography, not food availability, reflects compositional differences in the bacterial communities associated with larval sea urchins

Determining the principles underlying the assembly, structure, and diversity of symbiont communities remains a focal point of animal-microbiome research. Much of these efforts focus on taxonomic variation within or between animal populations, but rarely test the proportional impacts of ecological components that may affect animal-associated microbiota. Using larvae from the sea urchin Strongylocentrotus droebachiensis from the Atlantic and Pacific Oceans, we test the hypothesis that inter-population differences in the composition of animal-associated bacterial communities are more pronounced than intra-population variation due to a heterogeneous feeding environment. Despite significant differences in bacterial community structure within each S. droebachiensis larval population (based on food availability, time, development, and phenotype), variation in OTU membership and community composition correlated more strongly with geography. Moreover, across these three locations, 20-30% of OTUs were specific to a single population while less than 10% were shared. Taken together, these results suggest that inter-populational variation in symbiont communities is more pronounced than intra-populational variation. This difference may suggest that ecological variables over broad geographic scales may mask smaller scale ecological variables; however, explicit testing of this potential principle requires common garden experiments as well as molecular and functional manipulations.