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). H(+) + hydrogencarbonate = CO2 + H2O Homooligomer; disulfide-linked. May also be disulfide-linked to insoluble organic matrix. Expressed in both the dorsal region of the mantle and the mantle edge. Is dispersed in calcium carbonate and also linked by disulfide bonds to the organic core of nacre. Sulfite and sialic acid may provide the necessary negative charge in the N-glycan to promote calcium uptake. Belongs to the alpha-carbonic anhydrase family.