SOLDIER CRAB (COENOBITA CLYPEATUS) SHELL USE AND WEAR ON SAN SALVADOR ISLAND, BAHAMAS

The Caribbean terrestrial soldier crab, Coenobita clypeatus (J.C. Fabricius, 1787), has coexisted with land snails of the genus Cerion (Roding, 1798) on San Salvador Island, Bahamas for years as evidenced by fossil trackways from the Holocene age Rice Bay Member (3200-5300ka). Soldier crabs rely on gastropod shells for protection, since they have an uncalcified abdomen, leaving them vulnerable for predation and water loss. This creates a strong pressure to obtain shells which fit the crab and provide adequate protection against desiccation. We were interested in the frequency of Cerion shell use based on several populations and associated assemblages of discarded shells. Second, we examine how the shell was modified by the crab. Unoccupied shells in C. clypeatus colonies were collected and measured to determine preference for shell type. Previous studies have shown preference the crabs have for Cerion via living populations on San Salvador Island. Dripping Rock Cave between Grotto Beach and Sandy Point hosted soldier crabs occupying 51% Cerion, 12% Cerithium, 12% Lombotus, and 7% Nerita shells. The occupancy frequencies at North Point were 54.5% Cerion, 9.1% Cenchritus muricatus, and 7.3% Nerita sp. shells. In contrast, the percentages of Cerion in the assemblages of discarded shells ranged from 75.5% to 94.7%. These discarded shells were also highly modified by the soldier crab for a better fit. In this contribution we looked at modification patterns from six colonies of C. clypeatus. Soldier crabs are known for making major changes to their shell morphology to reduce energy output since they have to drag them across land. Changes consist of thinning the shell walls via abrasion, hollowing out the columella, and clipping the aperture. Understanding the pattern of shell modification has implications for paleoecologists obtaining data on Cerion in the fossil record and/or use the shells for isotopic studies. It is significant to be able to identify shells only used by the mollusks, as opposed to those obtained by the crabs. Second, the use of death assemblages of shells in live/dead comparisons can be misleading because of the behavior of hermit crabs.