Aquatic versus terrestrial crab skeletal support: morphology, mechanics, molting and scaling

The transition from aquatic to terrestrial environments places significant mechanical challenges on skeletal support systems. Crabs have made this transition multiple times and are the largest arthropods to inhabit both environments. Furthermore, they alternate between rigid and hydrostatic skeletons, making them an interesting system to examine mechanical adaptations in skeletal support systems. I hypothesized that terrestrial crabs have modified morphology to enhance mechanical stiffness and that rigid and hydrostatic skeletons scale differently from each other, with stronger allometric relationships on land. Using the aquatic blue crab, Callinectes sapidus, and the terrestrial blackback land crab, Gecarcinus lateralis, I measured and compared body mass, merus morphology (dimensions, cuticle thickness and the second moment of area I) and mechanics (flexural stiffness EI, elastic modulus E, critical stress and hydrostatic pressure) of rigid and hydrostatic stage crabs encompassing a range of sizes (C. sapidus: 1.5-133 g, N≤24; G. lateralis: 22-70 g, N≤15). The results revealed that rigid G. lateralis has similar morphology (limb length to diameter L/D and cuticle thickness to limb diameter T/D ratio) to C. sapidus, and the mechanics and most scaling relationships are the same. Hydrostatic land crabs differ from aquatic crabs by having different morphology (thinner cuticle), mechanics (greater internal pressures) and scaling relationship (cuticle thickness). These results suggest that the rigid crab body plan is inherently overbuilt and sufficient to deal with the greater gravitational loading that occurs on land, while mechanical adaptations are important for hydrostatically supported crabs. Compared with other arthropods and hydrostatic animals, crabs possess distinct strategies for adapting mechanically to life on land.