Mollusc larval shell formation: amorphous calcium carbonate is a precursor phase for aragonite.

The larval shells of the marine bivalves Mercenaria mercenaria and Crassostrea gigas are investigated by polarized light microscopy, infrared spectroscopy, Raman imaging spectroscopy, and scanning electron microscopy. Both species contain similar shell ultrastructures. We show that larval shells contain amorphous calcium carbonate (ACC), in addition to aragonite. The aragonite is much less crystalline than non-biogenic aragonite. We further show that the initially deposited mineral phase is predominantly ACC that subsequently partially transforms into aragonite. The postset juvenile shell, as well as the adult shell of Mercenaria also contains aragonite that is less crystalline than non-biogenic aragonite. We conclude that ACC fulfills an important function in mollusc larval shell formation. It is conceivable that ACC may also be involved in adult shell formation.