The evolutionary transition from water to land has occurred repeatedly throughout time, but we still have limited knowledge about how symbiotic microbes help their hosts adapt to challenges on land. Fiddler crabs represent a unique opportunity, as they live both on land and in the water. So, researchers examined the gill microbiome of fiddler crabs from mangrove forests spanning from the Red Sea to the Indian Ocean. Fiddler crabs rely on their gills to breathe and to perform other metabolic functions. Regardless of where the crabs lived, the crabs had a tightly associated layer of rod-shaped bacteria along their gill tissue. Their gill microbial communities were also low in diversity and heavily dominated by the actinobacteria group Ilumatobacter. Subsequent analysis suggested that actinobacteria members perform key metabolic functions for the crabs, like eliminating toxic sulfur and nitrogenous waste compounds. Though more research is needed, this study suggests that gill bacteria help fiddler crabs adapt to their dynamic intertidal ecosystems and provides new insights for researchers into the role of microbes in the water to land transition.