Multiple colonist pools shape fiddler crab-associated bacterial communities.

Colonization is a key component of community assembly because it continuously contributes new species that can potentially establish and adds individuals to established populations in local communities. Colonization is determined by the regional species pool, which is typically viewed as stable at ecological time scales. Yet, many natural communities including plants, birds and microbes, are exposed to several distinct and dynamic sources of colonists and how multiple colonist pools interact to shape local communities remains unclear. Using a 16S rRNA amplicon survey, we profiled bacteria within surface, subsurface and burrow sediments and assessed their role as colonist pools for fiddler crab-associated bacteria. We found significant differences in composition among sediment types, driven by halophilic taxa in the surface, and different Desulfobacteraceae taxa in the subsurface and burrow. Bacteria from burrow sediment colonized the crab carapace whereas gut bacterial communities were colonized by burrow and surface sediment bacteria. Despite distinct colonist pools influencing gut bacteria, variation in composition across gut samples did not lead to significant clusters. In contrast, carapace bacterial communities clustered in six distinct groups loosely associated with crab species. Our findings suggest that multiple colonist pools can influence local communities but factors explaining variation in community composition depend on local habitats. Recognizing multiple colonist pools expands our understanding of the interaction between regional and local processes driving community structure and diversity.