TAYLOR, J. R. A.*; KIER, W. M.; Univ. of North Carolina, Chapel Hill: Hydrostatic support in a land crab

As crabs evolved adaptations for life on land, they retained a molting process similar to that of aquatic crabs and become soft and vulnerable each time they shed their rigid exoskeleton. Aquatic crabs remain mobile during molting by switching to a hydrostatic skeletal support system (Taylor & Kier, 2003, Science, 301:209). But molting in air presents an additional challenge for the use of hydrostatic skeletal support since there is no buoyant support of water. This study was designed to test the hypothesis that newly molted land crabs remain mobile in the terrestrial environment by using a hydrostatic skeleton. In hydrostatic skeletons, support for movement is provided by an incompressible volume of fluid in a tension-resisting container. Transmission of force due to muscular contraction results in an increase in hydrostatic pressure. To test for hydrostatic skeletal support in the land crab, Gecarcinus lateralis, internal hydrostatic pressure and the force exerted by cheliped flexure were simultaneously measured in newly molted crabs. A strong correlation was observed between the force exerted by the cheliped and the hydrostatic pressure within the cheliped. Crabs measured after the new cuticle hardened showed large forces, but no increases in pressure. These results support the hypothesis that terrestrial crabs switch to a hydrostatic skeleton during molting. This research demonstrates that terrestrial crabs also alternate between two different skeletal support systems, like their aquatic counterparts, and that hydrostatic support is sufficient for animals of this size and body plan to locomote on land. Funded by Sigma Xi, NSF IBN-972707, and DARPA N66001-03-R-8043.