Mechanical properties of the rigid and hydrostatic skeletons of molting blue crabs, Callinectes sapidus Rathbun

Molting in crustaceans involves significant changes in the structure and function of the exoskeleton as the old cuticle is shed and a new one is secreted. The flimsy new cuticle takes several days to harden and during this time crabs rely on a hydrostatic skeletal support system for support and movement. This change from a rigid to a hydrostatic skeletal support mechanism implies correlated changes in the function, and thus mechanical properties, of the cuticle. In particular, it must change from primarily resisting compression, bending and torsional forces to resisting tension. This study was designed to explore the changes in the mechanical properties of the crustacean cuticle as the animals switch between two distinct skeletal support mechanisms. Samples of cuticle were removed from blue crabs, Callinectes sapidus, at 1 h (soft-shell stage), 12 h (paper-shell stage), and 7 days (hard-shell stage) following molting. We measured and compared the flexural stiffness, Young's modulus of elasticity (in tension), and tensile strength for each postmolt stage. We found that the hard-shell cuticle has a flexural stiffness fully four orders of magnitude greater than the soft-shell and paper-shell cuticle. Although the soft-shell cuticle has a Young's modulus significantly lower than that of the paper-shell and hard-shell cuticle, it has the same tensile strength. Thus, the soft-shell and paper-shell cuticles are unable to resist the significant bending forces associated with a rigid skeletal support system, but can resist the tensile forces that characterize hydrostatic support systems. The mechanical properties of the cuticle thus change dramatically during molting in association with the change in function of the cuticle. These results emphasize the significant role that mechanics plays in the evolution of the molting process in arthropods, and possibly other ecdysozoans.