The Biomechanics of Climbing in Grapsid Crabs

Several crab species have evolved the ability to climb vertically and live in trees. An arboreal lifestyle provides resource advantages to crabs, but also presents biomechanical challenges as they adjust from horizontal crawling to climbing in the vertical plane. The Grapsidae family includes crabs with varying levels of climbing ability, from species that are restricted to rocky shores to others that fully inhabit tree canopies. Here we sought to determine characteristics that confer climbing ability in crabs by examining the morphology and climbing kinematics of three Grapsid species with different climbing abilities: mangrove crabs (Aratus pisonii), marsh crabs (Sesarma cinereum), and shore crabs (Pachygrapsus crassipes). Comparison of leg dimensions revealed that mangrove crabs, the most adept climbers, have significantly longer merus segments and shorter dactyls compared to the other two species, which would provide them with greater stride length and velocity as well as a stronger grip. All three species had optimal distal leg ratios, indicating that the Grapsid body plan is inherently suitable for maneuvering over irregular obstacles. Gait patterns were determined from high speed videos of crabs crawling a vertical (90 degree) surface covered in mesh and revealed that mangrove and marsh crabs both have longer duty factors that prolong contact with the surface. Shore crabs had greater difficulty gripping the surface and climbing. Our analysis suggests that the Grapsid crab body plan is inherently suitable for climbing and that the morphology of the gripping structures (dactyls) are key to their climbing ability. This study provides some of the first insights into crab climbing ability, which is important for understanding their evolutionary transition to the arboreal lifestyle as well as the breadth of climbing biomechanics among arboreal animal lineages.