Crystal formation and growth in bivalve nacre

THERE are two theoretical explanations of the initiation and growth of the tabular aragonite crystals forming the nacreous layer in molluscan shells. The ‘template’ or ‘matrix’ hypothesis assumes that crystal nucleation and growth occur on the open surface of conchiolin membranes and may depend on active sites of this organic substratum 2 . This implies a process comparable to epitaxy. In contrast, the ‘compartment’ hypothesis envisages that nucleation and growth occur within, and are controlled by, pre-existing hollow compartments of the organic deposits 3 . Two recent scanning electron microscope (SEM) studies 4 indicate that in gastropods there are no such compartments at the growth front of the nacreous layer. These studies are, however, not completely conclusive because of insufficient fixation of the samples. We have therefore studied the formation and growth of initial nacreous crystals in several species of bivalves.