Three-dimensional microstructure of robust claw of coconut crab, one of the largest terrestrial crustaceans

Abstract

Research that seeks to mimic the complex hierarchical structures of organisms is often conducted as a technique for breaking through the characteristic limits of materials. The coconut crab, Birgus latro, is a rare organism with the greatest force; it has a pinching force 90 times its body weight. The correlation between the tissue structure, hardness, and composition of the coconut crab claw was studied using a materials science approach. The exocuticle adjacent to the surface was approximately 0.25 mm thick, which accounts for about 10% of the exoskeleton’s thickness of 2.5 mm. It had a twisted plywood pattern structure rotated 180° around an axis normal to the surface; its stacking height was about 2.3 μm. No such plywood structure was observed in the endocuticle, which is 10 times thicker. The hardness of the exocuticle was about five times that of the endocuticle and this increase was due to the dense tissue structure and calcium carbonate calcification. In the intermediate layer, the hardness and composition also changed significantly. Three-dimensional analysis have been very helpful in understanding the complex tissue structure. The tissue structure of the coconut crab claw may provide ideas for developing stronger, tougher ultimate materials.