Amorphous calcium carbonate in the shell material of the brachiopod Megerlia truncata

Abstract

Terebratulide brachiopod shells have a thin, hard outer nanocrystallinecalcite layer and a hybrid fibre composite inner layer with CaCO(3)fibres embedded in an organic matrix. Here we report our observation bytransmission electron microscopy (TEM) of a large compartment filledwith amorphous calcium carbonate (ACC) in the shell of the modernbrachiopod Megerlia truncata. The compartment has the typical shape andsize of fibrous calcite crystals composing the inorganic component ofthe hybrid fibre composite secondary shell layer. The ACC compartment isadjacent to an inclusion of foreign material that is entirelyincorporated into the shell. It has most probably been produced in thecourse of shell reparation. Under TEM imaging conditions the amorphouscarbonate crystallized in situ to vaterite and calcite. The distributionpattern of the organic component of the shell material is spatiallydifferentiated. While in the outer, nanocrystalline primary shell layerwe do not observe any organic material between the crystallites by TEM,the CaCO(3) fibres of the secondary layer are surrounded by an organicsheath. In the innermost segment of the secondary layer, in addition tothe organic sheaths, thick organic membranes are present. Thecompartment containing ACC is located between two, 1-2 mu m thick,organic membranes. Our observations indicate that brachiopod shellformation may occur via an ACC precursor that is produced in an initialStage prior to the crystallization of calcite.