Cross-section of a Cherax quadricarinatus gastrolith showing an irregular layered structure. A–D: Light microscopy, without any treatment. A: cross-section of a whole gastrolith. The central arrow indicates the growing direction, the lateral arrows indicate the lateral covering process occurring at the end of the storage in late premolt. B–D: magnification of different areas of a cross-section. Dense calcium carbonate layers of different widths alternate with less dense layers. E–H: (E–F: Light microscopy, G–H: SEM) the presence and direction of organic matrix layers are clearly shown after partial surface decalcification (grey areas; the bright areas correspond to the native structure).