Superior mechanical resistance in the exoskeleton of the coconut crab, Birgus latro

The hierarchical tissue structure that can balance the lightweight and strength of organisms gives hints on the development of biologically inspired materials. The exoskeleton of the coconut crab, Birgus latro, which is the largest terrestrial crustacean, was systematically analyzed using a materials science approach. The tissue structures, chemical compositions, and mechanical properties of the claw, walking legs, cephalothorax, and abdomen were compared. The local mechanical properties, hardness(H) and stiffness(E), were examined by nanoindentation testing. The stacking height, Sh, of the twisted plywood structure observed only in the exocuticle, the exoskeleton thickness, and the thickness and compositions at each layer differed significantly by body part. The exocuticle is strongly mineralized regardless of body parts. The claw and walking legs were thicker than the cephalothorax and abdomen, and their endocuticle was mineralized as compared to the endocuticle in the cephalothorax and abdomen. The H and Sh had a correlation in the exocuticle layer, and the H increased with decreasing the Sh. On the H−E map for abrasion resistance of materials, the results showed that the exocuticle layer of the coconut crab was superior to that of other arthropods and all engineering polymers and competitive with the hardest metallic alloys.