Microstructural determinants of mechanical properties in exoskeletons: a comparison between hydrothermal vent crab and ghost crab

The exoskeleton of crabs serves functions in protection, support, and sensing. Among the microstructures that compose the exoskeleton, the Bouligand structure is known to contribute to its mechanical properties. Previous research on the influence of microstructures on the mechanical properties of the crustacean exoskeleton has primarily focused on stacking height (SH), yet it remained controversial whether SH is the dominant determining factor of the mechanical properties. In this study, we comprehensively analyzed the pitch angle, diameter of the chitin-protein fiber, and the interlamellar spacing in the Bouligand structure to compare their contribution to the mechanical properties. We found that in vent crabs, the carapace was harder than the claw, while the opposite was observed in ghost crabs. In vent crabs, SH was 1.95 times greater than in the claw, a difference likely attributable to the pitch angle-the only microstructural feature that varied. In contrast, no structural differences were detected between regions in ghost crabs, where SH was extremely small (< 1 μm) and thus mechanical properties appear to be governed by material characteristics rather than structure. These findings indicate that pitch angle influences the mechanical properties of the crab exoskeleton only when SH is sufficiently large.