Sessile barnacles produce two types of mineralized exoskeletons: a cone-shaped shell and an operculum that is used to seal the shell opening. The mineral of both types is calcite. We show that the calcite crystals of the shell and the operculum (specifically the scutum) of the sessile barnacle Balanus amphitrite both fracture with conchoidal cleavage, have surfaces decorated with small rhombohedral shaped calcite crystals, and are poorly oriented. The scutum calcite is significantly more disordered at the atomic level than the shell calcite. We also show that a major component of the intercrystalline organic matrix of the shell and scutum is a nonproteinaceous sulfate-rich polymer that behaves as a hydrogel, and that the intracrystalline matrix contains highly acidic proteins. The crystal properties and microstructure are consistent with the calcite crystals forming in a hydrogel-like environment. The barnacle shell and operculum have many unique properties indicating that the crystal growth conditions are well controlled and possibly adapted to fulfill mechanical functions, which enable the barnacle to survive in the high energy environment of the intertidal zone.

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Calcium transport from the environment to the final site of mineral deposition involves uptake from the water or the food into cells. Within the cells calcium ions are translocated to various organelles and vesicles where they accumulate, in such a way as to not raise the very low calcium concentrations in the cytosol. In various biomineralizing systems, the calcium is stored in vesicles as a highly disordered hence relatively soluble solid phase. The concentrated calcium phase is then translocated out of the cell to the site of mineralization. Additional pathways may involve transport through the vasculature as ions and possibly mineral from distant sites. Understanding calcium pathways is the foundation for not only better understanding biomineralization processes but also for better understanding calcium and its fundamental role in cell signaling.