Structure, composition and properties of naturally occurring non-calcified crustacean cuticle

Crustaceans are known for their hard, calcified exoskeleton; however some regions appear different in colour and opacity. These include leg and cheliped tips in the grapsid crab, Metopograpsus frontalis. The chelipeds and leg tips contain only trace levels of calcium but a significant mass of the halogens, chlorine (Cl) and bromine (Br). In contrast, the carapace is heavily calcified and contains only a trace mass of Cl and no Br. In transverse section across the non-calcified tip regions of cheliped and leg the mass percent of halogens varies with position. As such, the exoskeleton of M. frontalis provides a useful model to examine a possible correlation of halogen concentration with the physical properties of hardness (H) and reduced elastic modulus (Er), within a chitin-based matrix. Previously published work suggests a correlation exists between Cl concentration and hardness in similar tissues that contain a metal (e.g. zinc). However, in M. frontalis H and Er did not vary significantly across cheliped or leg tip despite differences in halogen concentration. The non-calcified regions of M. frontalis are less hard and less stiff than the carapace but equivalent to values found for insect cuticle lacking metals. Cheliped tips showed a complex morphological layering that differed from leg tips.