COENOBITA CLYPEATUS (THE TERRESTRIAL HERMIT CRAB) AND ITS PREFERENCE IN INHABITING CITTARIUM PICA (WEST INDIAN TOP) SHELLS: A TAPHONOMIC SIGNATURE FROM A COHORT STUDY FROM SAN SALVADOR ISLAND, BAHAMAS

Coenobita clypeatus is the only species of terrestrial hermit crab in the Bahamas and it prefers to inhabit Cittarium pica shells. The shell size range for Cittarium shells (<10mm to >100mm) permits crabs to easily obtain consecutively larger shells. The durability of the shells, ability to contain and store freshwater, stability in high energy environments for breeding female crabs depositing larvae, all appear to be factors for crabs favoring Cittarium shells. Walker (1994) documented that the modification of apertures, removal of columellae, umbilicular regions, and outer lip ridges regions of shells, as well as abrasion marks from dragging, were present on both living and large fossil shells used by Coenobita in Bermuda. At some point, the wear of shells requires new larger Cittarium shells to enter the shell exchange. After surveying crab populations, we found discrete cohorts based on size and areas linked with sources of appropriate shells. We often found just a single, or pairs of large individuals in the vicinity. The largest sized individuals range several 100-meters inland and are solitary, often traveling at night to scavenge debris along strandlines. Breeding females return and deposit larvae at new moons. During a new moon in May 2017, we observed a cohort dominated by larger Cittarium shells. This observation, and the range of shells documented (diameters of 9mm to 40mm), provide an interesting insight to Coenobita as a taphonomic agent. In general, the largest number of shells inhabited by Coenobita are within the cohort utilizing Cenchritis muricatus and Cerion. One would expect the crabs would concentrate these species within a particular nearshore, terrestrial habitat and remove them from wave deposited units. Large individuals obtain transported shells often predated by muricids or octopi (but middens often contain undrilled Cittarium). Shell history can be assessed using not only the modification of the shells by the crab itself, but also by looking at encrusting and boring fauna on the shell surface. Clearly, the relationships involved with shell exchange, and sources of shells, elucidate a complex web of interactions representing more than just the crab and the mollusk. These interactions, as well as the behavior, no doubt would impact these organism’s taphonomic signatures.