Figure 5.3: Periostracum
The figure shows the periostracum (≈ 7μm), composed of a double proteic sclera layer (internal and external), that covers the outside of nearly all mullusc shells. This layer has two principle functions: 1. it is support for where the first calcium carbonate crystals will nucleate and 2. act to completely isolate the extrapalial space, stopping the entreance of water coming in from the external environment. Isolating the extrapalial space is essential to shell generation: in this isolated space the ions responsible for creating the amorphous calcium carbonate (ACC) precursors accumulate, creating a supersaturated solution of calcium carbonate, without which biomineralization would not occur. The isolation also allows the animal control the local pH, that alters the solubility of the structure of β-chitin (nanostruture where the ions are deposited during heterogeneous nucleation CaCO3) activating the nucleation mechanism.
