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Sabellariid (Polychaeta: Annelida) reefs as nursery ground for the hermit crab Clibanarius symmetricus (Randall, 1840) on the Amazonian coast of Brazil

ABSTRACT The present study investigated the structure of the crab Clibanarius symmetricus (Randall, 1840) population found in a biogenic reef formed by the polychaete Sabellaria wilsoni Lana & Gruet, 1989 on the Amazonian coast. The population was sampled over a one-year cycle on a reef located in the mesolittoral zone of a sandy beach. The mean and maximum size of the crabs collected on the reef were smaller than those recorded in other tropical and subtropical coastal habitats, given that most of the specimens collected were immature (undetermined sex juveniles and sexually immature adults). Thaisella coronata (Lamarck, 1816) was the gastropod species whose shells were most occupied by the hermit crabs (70% of the specimens), although the smallest juveniles have used more the shells of other species. The results of the study suggest that C. symmetricus uses the reef as a nursery habitat, with bimonthly peaks in density that reflect recruitment periods. The sexually mature adults appear to use the reef only as a foraging area and, occasionally, as a refuge. The results demonstrated the importance these reefs on the population biology of this species and supports the need for more detailed monitoring studies and conservation actions for these habitats.