HOW GASTROPOD ASSEMBLAGES IN THE INTERTIDAL ZONE DO NO REFLECT THE REAL COMPOSITION OF THE ORGANISMS PRESENT: THE CASE OF THE HERMIT CRAB

Gastropod communities of rocky shore habitats in the Bahamas have been shown to be homogenous. Wittmer (2011) suggested that a small area within a rocky intertidal community would preserve a representative fraction of the biodiversity and community structure. Only a few rocky intertidal taxa are represented in time-averaged deposits on San Salvador Island. Unbiased samples from strand lines associated with rocky shorelines on Abaco and San Salvador Island are characterized by an abundance of Cerithium species and a distinct low abundance of intertidal gastropods. The observed agents of these discrepancies are hermit crabs. Assemblages of hermit crabs from tidal pools on San Salvador Island predominately use shells of Cerithium thus increasing potential of this gastropod species of being included as part of the rocky intertidal death assemblage. Land hermit crabs also impact the composition of rocky intertidal taxa in the recent death assemblage. Densities of land hermit crabs have been measured as high as 46 m² for vegetated areas on small Bahamian islands (Morrison and Spiller, 2006). The most frequently used shells from vegetated areas associated with rocky shores were Tectarius maricatus, Nodillorina tuberculata, Nerita sp., and Cittarium pica. However, frequencies of shell usage changed in relation to distance from shoreline. Species of Cerion were used more frequently than intertidal gastropods in aggregations of hermit crabs found distally from the rocky shoreline. The role of hermit crabs in modifying faunal assemblages has long been documented. However, the densities of hermit crabs are high enough to modify gastropod assemblages so that species of gastropods occupying the upper regions of the high tide range can be effectively removed by crabs so they are not detected in intertidal rocky shoreline deposits.