Biometric and Ecological Relationships in Two Sympatric Caribbean Gecarcinidae (Crustacea: Decapoda)

The terrestrial crabs Gecarcinus ruricola and G. lateralis are sympatrically distributed in many areas of the Caribbean. Biometric analyses were conducted using four carapace dimensions and branchial volumes for both species throughout the Caribbean. Mean size of adult G. ruricola does not differ significantly among areas of the Caribbean. G. lateralis shows an increase in mean size from eastern to western localities. The carapace of G. ruricola lateral to the orbits is proportionally larger than the corresponding area of G. lateralis, but growth in both species is allometric. Thus, one should employ caution in using carapace ratios for taxonomic discrimination between the species. The branchial chambers of both G. lateralis and G. ruricola increase in size approximately as the cube of the carapace length. An expansion in the lateral carapace dimension of G. ruricola provides a small increase in branchial volume and a relatively larger increase in surface area which presumably facilitates gaseous exchange with the atmosphere. G. lateralis occurs most frequently in the presence of easily burrowed sandy substrate with at least moderate protection by vegetation. G. ruricola occurs almost exclusively under a forest canopy where it seeks protection in crevices, holes, or other retreats with minimal need for burrowing.