Fiddler crab sex and phylogenetic relatedness drive bacterial community composition and its similarity with microbial species pools

Background/Question/Methods Inferring local ecological processes that shape diversity and species composition is often challenging due to influences of neighboring communities. Along geographical gradients, the pool of species available for colonization may vary in composition and size resulting in variations in diversity at the local scale. Yet conceptual and experimental evidence relating to the variation of species pools and its effect on local communities is scarce. Microbial species are assumed to have higher dispersal than larger organisms but evidence shows there are limits to their species ranges. Animal hosts represent defined patches for microbial communities and colonization can occur at different stages in the animal development and life history. Similarities in bacterial community composition are often attributed to evolutionary mechanisms driving physiological, immunological, behavioral and ecological aspects of the host species. However, variations in host’s intraspecific traits are common among conspecifics and the resulting influence on associated bacterial communities is unclear. This study focuses on the microbial communities associated with two species of fiddler crab. First, I aim at determining whether crab species or sex drives community composition. Second, I assessed whether these differences in microbial communities are driven by differential contribution from different species pools in the fiddler crab habitat.

Results/Conclusions : Results suggest that phylogenetic relatedness interacts with sex-specific traits in driving bacterial communities associated with fiddler crabs. While one of the species shows consistent community composition, we found higher variation amongst the other species, mostly driven by sex. Crab communities are significantly different from communities in the surface, subsurface and burrow sediment (MANOVA, F=104, p<0.001). We found no significant differences suggesting gut microbial communities have differential contribution from species pools in the sediment. In contrast, we found females from both species showed communities with stronger contributions from surface and subsurface bacterial communities. Our results suggest microbial communities on fiddler crabs are strongly determined by crab species. However, sex associated effects in terms of microbial communities are species-dependent. Finally, the effect of species pool may be sex dependent and associated with differences in behavior or morphology between males and females driven by host’s behavior and habitat characteristics. Our understanding of local ecological processes will be enriched by considering the effect of different species pools.