Tissue structure and mechanical properties of the exoskeleton of the huge claws of the mud crab, Scylla serrata

Abstract

AbstractThe mud crab, Scylla serrata, has huge claws in comparison with its body size. The color of the claw top’s finger surface changes from white to deep-mottled blue, and this discoloration was strongly associated with the change in hardness inside the finger cross section. With special attention to the discoloration points, the tissue structure of the exoskeleton was investigated via scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction (XRD), and the mechanical properties were examined using Vickers hardness and nanoindentation tests. The exocuticle in the deep blue surface exoskeleton was as thin as that in other crustaceans, and the exoskeleton was occupied by the endocuticle with a twisted plywood structure. On the other hand, in the white surface exoskeleton, the thickness of the hard and dense exocuticle accounted for 52–59% of the exoskeleton thickness. This percentage increased at the claw tip. The hardness of the exocuticle was 2.5 times that of the endocuticle, and the microstructures and mechanical properties gradually varied at the boundary between the exo- and endocuticle. The mechanical properties were almost constant in the exocuticle, but calcium (Ca) concentrations decreased from the outer surface toward the boundary in that region and magnesium (Mg) concentrations increased. The change in the unit cell volume obtained via XRD suggested that some of the Ca atoms in the calcite crystal structure in that region were replaced with Mg atoms. Changes in crustacean coloration may help us to understand the tissue structure and mechanical properties within the exoskeleton. Graphical Abstract