Meeting Abstract
Vertical climbing is a biomechanically challenging behavior that many vertebrates and insects have mastered for their arboreal habitats. Less is known about arboreal crabs and their adaptations in overcoming the challenges of climbing. To gain insights into crab climbing, we compared the walking leg morphology and mechanics of two closely related crabs (family Grapsidae): the arboreal mangrove crab, Aratus pisonii, and the rocky intertidal shore crab, Pachygrapsus crassipes. A. pisonii lives and climbs in mangrove trees, whereas P. crassipes climbs rocks easily, but not trees. We hypothesized that mangrove crabs would require relatively shorter, stiffer legs and harder gripping structures (dactyls) to overcome gravity and maintain balance when scaling trees. Crab mass and carapace dimensions were measured (A. pisonii: N = 9; P. crassipes: N = 15), along with dimensions of the second pereopod. We also measured the flexural stiffness and failure strength of the meropodite, and the hardness of the dactyls. Results show that A. pisonii have relatively larger, more flexible meropodites than P. crassipes, which may aid them to cling to narrow branches. The meropodites of both species are similar in strength, but failed by buckling in A. pisonii, and cuticle fracture in P. crassipes, which likely reflects differences in cuticle morphology. There was no difference in dactyl hardness and stiffness between the species, suggesting that the dactyls are sufficient for gripping and supporting a crab on a variety of substrates. Overall, the walking leg morphology of grapsid crabs appears to require minimal modifications to enable vertical climbing, but a broader comparison among aboreal and non-arboreal crabs would yield greater insights into adaptations for tree climbing in crabs.
