Hardness and toughness of exoskeleton material in the stone crab, Menippe mercenaria

Techniques from the field of materials science and engineering were utilized to examine the mechanical effects of dark pigment in exoskeleton material on the distal portion of stone crab, Menippe mercenaria, chelae. We made specimens from dark-and light-colored exoskeleton material to measure hardness and fracture toughness. The results from these tests showed the dark material to be much harder and tougher than light-colored material from the same crab chela. Scanning electron microscope photographs are presented to document the microstructure and level of porosity. We think that the structural difference in material properties is due to the lower level of porosity and phenolic tanning in dark material and that this tanning caused the dark color and filling of porosity. The exoskeleton structure is a laminated organic-inorganic structure which can serve as a model for self-healing structures.