The mineralized exoskeletons of crustaceans

The crustaceans constitute one of the oldest arthropod taxa, from which insects later evolved (Giribet et al., Nature 413:157-161, 2001; Regier et al., Nature 463:1079-U1098, 2010; Giribet and Edgecombe, Annu Rev Entomol 57:167-186, 2012). A typical feature that characterizes the Crustacea is their mineralized chitinous exoskeleton. The reinforcement of the chitinous exoskeleton with calcium salts and the formation of inorganic-organic composite materials by the crustaceans represent one of the oldest biomineralization mechanisms to have evolved in animals. The basic function of mineralization is to enhance the mechanical strength of the skeleton. When compared to other animals with mineralized skeletons, crustaceans face two distinct challenges inherent in the fact that their skeleton is external: first, the animal's locomotion abilities must not be compromised by its mineralized exoskeleton, and second, the growth mode by periodic molting requires intensive mobilization of minerals during the resorption of the old cuticle and the rapid recalcification of the new cuticle. These two demands are among the prime determinants that govern the various calcification patterns in Crustacea. This review focuses on the mineralogical aspects of the crustacean exoskeleton with emphasis on the controllable parameters of the mineral phase properties, namely, the degree of mineralization, the degree of crystallization, the phosphate/carbonate ratio, and the involvement of proteins. It also explores potential biomimetic applications inspired by the crustacean exoskeleton against the background of similarities between crustaceans and vertebrates, namely, both groups are the only groups in the animal kingdom that combine advanced locomotion with jointed mineralized skeletons. In addition, many crustaceans have the ability of calcium phosphate mineralization, like vertebrates. These similarities provide unique opportunities to compare different evolutional solutions to similar functional challenges that, in turn, can inspire biomimetic approaches to the development of synthetic bio-composites for various skeleton-related medical applications.