Titus, B. M.; Froehlich, C. Y. M.; Vondriska, C.; Baker, R.; Caves, E. M.

Ecologists have long sought general explanations for the co-occurrence of ecologically similar taxa. Niche theory explains co-occurrence via functional differences among taxa that reduce competition and promote resource partitioning. Alternatively, the unified neutral theory of biodiversity and biogeography suggests that co-occurrence can be attributed to stochastic processes, and thus, presupposes that ecologically similar species that occur in sympatry are functionally analogous. We test these alternative hypotheses in multiple dimensions using the most diverse crustacean-sea anemone symbiosis from coral reefs in the Tropical Western Atlantic. {delta}13C and {delta}15N stable isotope analyses of six crustacean symbionts that co-occur around the host anemone Bartholomea annulata exhibit highly differentiated isotopic niche space spanning three trophic levels. As multiple crustacean species within the symbiosis have been documented as cleaners that remove parasites from reef fishes, we extended our investigation into the broader cleaner community. Our stable isotope analyses of cleaners shows that only Pederson\'s cleaner shrimp Ancylomenes pedersoni exhibits {delta}15N isotopic signatures that are consistent with a dedicated cleaning lifestyle. Co-occurring species that have been previously described to clean reef fishes such as Periclimenes yucatanicus, Stenopus hispidus, and Stenorhynchus seticornis all occupy trophic levels well below An. pedersoni. Taken together, our data are consistent with the expectations of niche theory: co-occurring symbiotic crustaceans have highly partitioned niche space with low levels of functional redundancy. Finally, our findings reinforce and extend the ecological importance of An. pedersoni as likely the only dedicated cleaner shrimp on coral reefs in the Tropical Western Atlantic.