Stable isotopes disentangle niche partitioning and co‐occurrence in a multi‐species marine mutualism

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 hypotheses using the most diverse crustacean‐sea anemone symbiosis from coral reefs in the Tropical Western Atlantic. δ 13 C and δ 15 N stable isotope analyses of six crustacean symbionts that co‐occur around the host anemone Bartholomea annulata exhibit highly differentiated isotopic niche space spanning several 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 Pederson's cleaner shrimp Ancylomenes pedersoni exhibits the highest δ 15 N isotopic values‐ significantly higher than all other putative cleaner species and consistent with expectations of a dedicated cleaning lifestyle. However, for other species previously described or observed to clean reef fishes, including Periclimenes yucatanicus, Stenopus hispidus and Stenorhynchus seticornis, δ 15 N isotopic values were substantially lower, raising questions about the degree to which these species rely on cleaning interactions to meet their nutritional requirements. 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.