Compact climbing configuration and dactyl grip underlie vertical climbing in crabs

Vertical climbing imposes mechanical challenges that differ from horizontal locomotion, yet the adaptations that enable crabs to climb remain poorly understood despite repeated transitions to terrestrial and arboreal habitats. This study integrates morphology, mechanical properties, and locomotor kinematics to examine climbing in the highly arboreal mangrove tree crab Aratus pisonii and the rocky intertidal lined shore crab Pachygrapsus crassipes. The arboreal species was predicted to possess traits that reduce toppling and improve grip during vertical locomotion. Aratus pisonii had relatively longer and slenderer pereopods, a more flexible merus, shorter and sharper dactyls, and higher dactyl friction than P. crassipes. During climbing, both species reduced speed, lowered abdominal clearance and body angle, and increased duty factor, indicating a shared shift toward slower, more supported locomotion. Contrary to expectation, static and dynamic toppling moments did not differ between species. Instead, the species differed in how they achieved similar toppling outcomes: A. pisonii adopted a more compact climbing configuration, with lower body angle, shorter leg span, and limbs oriented closer to the substrate, whereas P. crassipes used a broader limb posture and greater dactyl rotation. These results show that crab climbing expresses general principles of vertical locomotion through a distinctive brachyuran body plan, integrating compact climbing configuration and dactyl grip to maintain attachment and limit toppling. By linking morphology, mechanics, and kinematics, this study establishes a biomechanical framework for crab climbing and identifies trait combinations likely to contribute to repeated arboreal evolution in brachyurans.