The calcitic columnar prisms of pteriomorphian bivalves have the crystallographic c-axis oriented perpendicular to the shell surface and the a-axes rotated without any preferential orientation. In oysters, SEM, XRD and EBSD analyses show that individual prisms initially have their a-axes randomly oriented but are able to progressively orient them parallel to those of their neighbors. This ability is apparently confined to groups, such as oysters and scallops, in which prisms are internally constituted by smaller lath-like crystal units. We have developed a competition model - not between prisms, but between the lath-like secondary units of prisms - which is based on differences in the inclination of laths relative to the shell growth surface. Units having a growth component which coincides with the growth direction protrude faster from the growth surface and out-compete those which are not favorably oriented, which reduces the overall dispersion of the a-axes of the prismatic lamella. The extent of re-alignment increases with the relative inclination of the growth surface and the length attained by the prisms. Oysters are the only group in which these two characters are pronounced enough to provide a measurable re-alignment. The proposed competition model is unprecedented in biomaterials and reveals how important crystal growth processes are in microstructure organization.