Acts as a negative regulator for calcification in the shells of mollusks. May function both as a calcium concentrator and as a carbonic anhydrase required for production of carbonate ions, which are assembled to CaCO(3) at mineralization sites. Is important for shell formation in both the calcitic prismatic layer and the aragonitic nacreous layer (By similarity). Shows inhibitory activity of crystal formation when present in free state but, when attached to the insoluble matrix, may regulate the form and size of aragonite crystal. H(+) + hydrogencarbonate = CO2 + H2O Homooligomer; disulfide-linked. May also be disulfide-linked to insoluble organic matrix (By similarity). Expressed in the mantle. The Gly-Xaa-Asn repeat domain binds calcium. Belongs to the alpha-carbonic anhydrase family.