The Structure and Calcification of the Crustacean Cuticle

The integument of decapod crustaceans consists of an outer epicuticle, an exocuticle, an endocuticle and an inner membranous layer underlain by the hypodermis. The outer three layers of the cuticle are calcified. The mineral is in the form of calcite crystals and amorphous calcium carbonate. In the epicuticle, mineral is in the form of spherulitic calcite islands surrounded by the lipid-protein matrix. In the exo- and endocuticles the calcite crystal aggregates are interspersed with chitin-protein fibers which are organized in lamellae. In some species, the organization of the mineral mirrors that of the organic fibers, but such is not the case in certain cuticular regions in the xanthid crabs. Thus, control of crystal organization is a complex phenomenon unrelated to the gross morphology of the matrix. Since the cuticle is periodically molted to allow for growth, this necessitates a bidirectional movement of calcium into the cuticle during postmolt and out during premolt resorption of the cuticle. In two species of crabs studied to date, these movements are accomplished by active transport effected by a Ca-ATPase and Na/Ca exchange mechanism. The epi- and exocuticular layers of the new cuticle are elaborated during premolt but do not calcify until the old cuticle is shed. This phenomenon also occurs in vitro in cuticle devoid of living tissue and implies an alteration of the nucleating sites of the cuticle in the course of the molt.