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Mutualisms between chemosynthetic microbes and invertebrates form the basis of foodwebs in dark, extreme habitats (hydrothermal vents, cold seeps) and have likely facilitated the invasion of extreme underwater caves, as well. Anchialine caves often include distinct water layers of varying concentrations of dissolved oxygen and sulfide and provide an ideal system for the discovery of chemosynthetically based systems and novel symbioses. These caves can be harsh environments for eukaryotes, but they contain gradients favorable for chemosynthetic symbiotic microbes. Here, we provide evidence of chemosynthetic ectosymbiosis in a member of the Remipedia, Xibalbanus tulumensis (Yager, 1987), through electron microscopy and stable isotope analysis. Remipedes are considered to be top predators due to anecdotal observations of feeding in the lab and field, and description of venomous apparati; but they may supplement their diet with microbes. This finding sheds light on opportunistic feeding behaviors that may have evolved to combat resource-limited environments, such as dark, underwater caves.