Microstructures of crab chela: A biological composite for pinching

We have investigated the microstructures and mechanical properties of four crab species Paralithodes brevipes, Eriocheir japonicas, Geothelphusa dehaani, and Telmessus acutidens and analyzed the molecular and elemental data of their chela. In the visible brown region (BR) at the distal end of the dactylus tip in Paralithodes brevipes and Eriocheir japonicas, the elastic modulus and calcium (Ca) content were lower than in the white region (WR). Near the interface between BR and WR, Ca, and carbon (C) composition changed continuously. Molecular analysis shows that the dactylus tips of the chela in P. brevipes and E. japonicas were composed of chitin and calcium carbonate. In G. dehaani and T. acutidens, the Ca concentration was homogeneous in the top portion (TP) of the dactylus dentitions. The lowest value of Ca concentration was found near the surface of the bottom portion (BP) on the dactylus dentitions. In G. dehaani, the elastic modulus distribution in the TP was maximum near the outermost surface, and gradually decreased in the inner layers; the lowest elastic modulus in the BP was near the outermost surface, and the distribution increased in the inner layer. These results show that the biological composite was as a continuous structure near the interface of the dactylus tip. Understanding the microstructures of the dactyla of crabs might help the study and development of bio-inspired composites for pinching.