Mineral and Matrix Components of the Operculum and Shell of the Barnacle Balanus amphitrite: Calcite Crystal Growth in a Hydrogel

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 ar...