Nanostructures of the aragonitic otolith of cod (Gadus morhua)

Topographical relationships between the mineral and organic components are important to understand the process involved in biomineralisation. We examined nanostructural features of cod (Gadus morhua) otoliths using atomic force microscope (AFM). Acicular aragonitic fibres that form primary increments observed in scanning electron or light microscopy were composed of lamellae, themselves being composite arrangements of round granules (diameter <50 nm) surrounded by an organic (?) cortex. The lamellae presented a banding pattern, which might correspond to a growth period of about 15 min. Cod otolith granules were similar in size, shape and arrangement to that previously reported for another fish species. The existence of smallest otolith organization units is suggested since granules are themselves composite. Finally, the intra-daily growth rhythm makes it possible to consider very high-resolution biological and environmental reconstructions in the future.