SHELL USE BY MARINE AND LAND HERMIT CRABS: EXPLORING TAPHONOMIC BIAS

In comparing life and associated death assemblages in a tropical rocky shoreline, Sørensen and Surlyk (2013) noted that two species of Mancinella, an abundant genus of gastropods with large robust shells, should be present in high numbers in death assemblages. Their absence from the death assemblage was attributed to transport of shells by marine or land hermit crabs away from the shore and/or being preferentially destroyed by shell crushing predators. On San Salvador Island, Bahamas, the grazing gastropod species Turbo castaneus, shows this same pattern. We have not found it in near shore death assemblages, however, Kuhlmann and McCabe (2014) found that Turbo castaneus was the most common molluscan shell present in octopus middens (15.225%). In May 2016, we observed octopus middens and were able to locate one with 17 T castaneus shells present. Three were live individuals, one was occupied by a hermit crab. Unoccupied shells were undrilled, one was clearly drilled by an octopus, and a portion of the shells were drilled by muricids. Clearly the octopi predate both live individuals and hermit crabs occupying T. castaneus. Furthermore, we observed live hermit crabs in Cerithium shells visiting the midden and other hermit crabs in smaller shells. Therefore, the assemblage of octopi predated gastropod shells is modified by the vacancy chain of hermit crabs. Not only are T castaneus favored by aquatic marine hermit crabs, but they are also represented in populations of land hermit crabs. The present example illustrates how both patterns of behavior, predation, and resource-use can create biases in the fossil record.