THREE SPECIMENS OF HERMIT CRABS FOUND ASSOCIATED WITH THEIR HOST GASTROPOD SHELL FROM THE PLIOCENE SAN DIEGO FORMATION, CALIFORNIA AND REASSESSMENT OF THE PAUCITY OF FOSSIL HERMIT CRABS ASSOCIATED WITH GASTROPOD SHELLS IN THE FOSSIL RECORD

Hermit crabs are very infrequently found associated with their host gastropod shells in the fossil record. However, isolated claw parts and empty gastropod shells with signs of hermit crab inhabitation are quite common in the fossil record. It has been speculated that the paucity of fossil hermit crabs associated with gastropod shells is due to hermit crabs abandoning their host gastropod shells and climbing upward through the sediment during times of stress, such as burial. Preliminary experiments with shelled Pagurus samuelis buried in medium size sand has demonstrated that hermit crabs that are buried rapidly do not abandon their gastropod shell, dying where they are buried. At death these hermit crabs relax their hold on the gastropod shells and are thereafter easily disassociated by very little movement. If buried slowly P. samuelis will dig up through the sediment, almost always still carrying the host gastropod shell. Only in cases where sediment burial is slow and hermit crabs cannot quite unbury themselves will the hermit crabs abandon their shells to dig up through the sediment. Therefore, it appears that hermit crabs, in most cases, should be associated with their host gastropod shell in the fossil record. One explanation for this disassociation of hermit crabs from their host gastropod shells maybe that hermit crabs occupy high energy environments, becoming disarticulated and therefore separated from their shells after death. Another factor maybe that sediment cleaned out of gastropod shells during museum accession may indeed contain hermit crab body parts that are overlooked by museum staff. The discovery of three hermit crabs tentatively assigned to the genus Isocheles sp. collected from the Pliocene San Diego Formation of California doubles the documented occurrences of fossil hermit crabs associated with their host gastropod shells. Further experiments on the behavior of hermit crabs during burial and the search for hermit crabs, both as isolated fossil occurrences and associated with their host gastropod shell in museum collections, may answer some of the questions related to the disassociation of hermit crabs from their shells in the fossil record.