Atomic force microscopy of the nacreous layer in mollusc shells

Abstract

We present atomic force microscopy (AFM) observations of the aragonite tablets of mature nacre in two types of mollusc, a bivalve (Atrina sp.) and a gastropod (Haliotis rufescens). By imaging in liquids it was possible to dissolve away the nacre layer by layer to reveal both the structure of a single tablet and its relation to vertically adjacent tablets. Atrina tablets (inner face) had a concave appearance; the central depression was surrounded by elongate rings that mimicked the orientation and aspect ratio of the unit cell viewed along the c axis. The tablet surfaces had a rough texture, and flat (001) planes of aragonite were rarely observed. Unit cell orientations were generally aligned both vertically and laterally between tablets of Atrina. Etching tablets with HCl initially removed the elongate rings and produced etch rows parallel to the a axis. Further etching of bleached Atrina nacre lifted off individual tablets to reveal underlying nacreous layers, showing no morphological registry between vertically adjacent tablets. The nacreous structure of Haliotis differed from Atrina in three ways: (i) the tablets were flatter and showed no elongate rings; (ii) the positions of the central depressions approximately repeated between nacreous layers, showing that the (presumed) nucleation sites line up along a given stack; and (iii) the unit cell orientations were not preserved between laterally adjacent tablets but were approximately aligned between vertically adjacent tablets.