Molluscs come in all shapes and sizes, and produce beautiful shells. But how do they make these shells?

Shells are made of calcium carbonate and are formed through a process known as biomineralisation. Biomineralisation takes place when a mollusc secretes fluid from its mantle organ into the extrapallial space, encapsulated by an already existing shell, mantle and periostracum (outer, protein-based, conchiolin layer of molluscan shell), at the growing edge of its shell. The fluid contains a large concentration of cations (Ca2+) and anions (HCO3-) that crystallise into calcium carbonate (CaCO3) while hydrogen anion (H−) is removed from the extrapallial space. The process is repeated as the mollusc grows, forming different patterns of colour and sculpture that are genetically determined (although some intraspecific variation often occurs).

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