Structural Changes and Mechanical Resistance of Claws and Denticles in Coconut Crabs of Different Sizes

SIMPLE SUMMARY: Understanding the diverse mechanisms by which organisms achieve exceptionally high mechanical properties may enable the development of unique, biologically inspired materials. We assessed the microstructure, composition, and mechanical resistance of the pinching side with denticles and of the outer side without denticles in robust claws of coconut crabs with body weight (BW) of 300 g to 1650 g. Surprisingly, they were independent of BW except for low hardness near the surface of the denticles of a small crab of 300 g. Although the microstructure of the denticles was clearly different from that of the exocuticle, their mechanical resistance indicated the same maximum value. The denticle can be regarded as a bulge of the cuticle without phosphorus. The design principles found in the exoskeleton provided promising opportunities for the research and development of novel structural materials. ABSTRACT: The exoskeleton of the pinching side of claws with denticles and of the outer side without them on the coconut crab, Birgus latro, which is a rare organism, were studied using a materials science approach. The mechanical resistance of three claws of different sizes was investigated along the exoskeleton thickness from the outer surface to the inner surface, and the results were compared, including the contribution of the microstructure and chemical compositions. Mechanical properties, hardness (H) and stiffness (E(r)), were probed through nanoindentation tests. The results showed the H, E(r), microstructures, and chemical components of the exocuticle and endocuticle layers were almost the same, in a BW range of 300 g to 1650 g. At the same time, the H and E(r) near the surface of the denticles of a small coconut crab of 300 g were lower than those of other large coconut crabs. The microstructure of the denticles was clearly different from that of the exocuticle, but the maximum mechanical properties near their surface indicated almost the same values, H(max) = 4 GPa and E(r)((max)) = 70 GPa, regardless of being on the pinching side or the outer side. A denticle can be regarded as a bulge of the cuticle without phosphorus and with high magnesium. The results provided novel information that expanded our knowledge about the claw microstructure of coconut crabs with different body sizes, and may be used in further studies