Biomineralisations in crustaceans: storage strategies

Crustaceans are a remarkable group of animals because of their ability to elaborate cyclically two kinds of calcified biomineralisations: an unstretchable exoskeleton (or cuticle) and also, for many species, depending of the way of life of the considered animal, transitory calcium deposits. They are in consequence subjected to a cyclic life and notably, to a periodical balance between two sources of calcium, exogenous and/or endogenous. The storage structures, essentially composed of calcium carbonate precipitated within a proteinaceous organic matrix, are very diversified. The calcium carbonate of the cuticle of most of the species is under a crystalline state and/or an amorphous form, whereas calcium deposits are always in amorphous form. The organic matrix is responsible for the mineral polymorph and the morphology of the calcified structures. The knowledge of the features of the components of organic matrices is a necessary prerequisite to understand how these calcified mineralisations are elaborated. Nevertheless, few organic matrix proteins involved in calcification are well characterized in crustaceans to date. Another interest is that the storage structures are elaborated by calcifying/decalcifying epithelia, which mimic vertebrate epithelia. Diverse enzymatic activities have been registered at this level, such as Mg ²⁺ -ATPase, Na ⁺ /K ⁺ -ATPase, and carbonic anhydrase. Finally, crustaceans represent convenient models to study the hormonal control of mineralising systems because, besides the involvement of ecdysteroids, the vertebrate tripartite calcium hormonal system (calcitonin, parathyroid hormone, vitamin D) could also regulate the formation of the crustacean calcified structures.