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. H(+) + hydrogencarbonate = CO2 + H2O Homooligomer; disulfide-linked. May also be disulfide-linked to insoluble organic matrix. Expressed at whole regions of the mantle epithelium tissue. Is found in the aragonitic nacreous and calcitic prismatic and foliated layers. N-glycosylated. Two hypotheses for calcium binding are proposed. Either the Gly-Xaa-Asn repeat domain bind calcium or sulfite and sialic acid provide the necessary negative charge in the N-glycan to promote calcium uptake. Belongs to the alpha-carbonic anhydrase family.