Mechanical Properties, Tissue Structure, and Elemental Composition of the Walking Leg Tips of Coconut Crabs

The coconut crab has black protrusions on the tops of its walking legs and claw fingers. In addition, there are regularly aligned small black protrusions on part of the exoskeleton surface of the claws and leg. The elemental composition, crystal structure, tissue structure, and mechanical properties of these protrusions were studied using a materials science approach, and the results were compared with those of mineralized cuticle. These leg tips were a non-calcified fibrous tissue of α-chitin connected to the mineralized cuticle. The tip of the second walking leg was elongated and had a pointed shape with an oval cavity at its center that was more than 1000 times larger than the pore tubes (100–350 nm) of the mineralized cuticle. It was very soft, with a hardness of 0.4 GPa, corresponding to 11–12% of the hard exocuticle and 55–57% of the soft endocuticle. The elastic modulus of 8.0 GPa obtained by nanoindentation testing was consistent with that of α-chitin fibers of shrimp shells obtained by tensile testing. These soft protrusions provide a secure grip on the surfaces of trees or rocks and protect the claw fingertips. It was concluded that the black protrusions are related to a unique ecological aspect of the coconut crab, the largest terrestrial crustacean.