Biological remanie; gastropod fossils used by the living terrestrial hermit crab, Coenobita clypeatus, on Bermuda

Coenobita clypeatus, the purple-clawed (terrestrial) hermit crab, almost exclusively uses the extinct shells (fossil and subfossil) of the West Indian topshell, Cittarium pica in Bermuda. These coenobitid crustaceans were found to: (1) taphonomically modify the Cittarium shells, and (2) caused temporal reworking of the extinct shells within a modern community. Fossil shells inhabited by these crabs had extreme apertural modifications: missing columellar and umbilicular areas, and outer lip ridges. Additionally, almost all shells possessed a drag mark region. Modern Cittarium inhabited by hermits from the Bahamas have similar modifications, except lack the outer lip ridge. Living Cittarium have no such modifications. Using these taphonomic criteria, one-third (18%) of in situ (collected) Pleistocene shells from Bermuda were once-occupied by Pleistocene hermits. These findings elucidate, in part, the enigma of why a large marine shell (Cittarium) is present in back-beach, dune and paleosol deposits in the Pleistocene of Bermuda. A legend until now, the transport of marine shells by terrestrial hermits in the Pleistocene allowed the now marginal population (n = <150) of hermits to live on the mainland of Bermuda. Contrary to previous reports, the fossil shell supply is not dwindling, but the chief limiting factor that affects the quantity of shells is the periodicity of hurricanes. Importantly, if these taphonomic criteria are not used, misrepresentation of the paleoenvironment or anomalous age of deposit (i.e., using one shell) will result from this phenomenon of biological remanie.