Morphogenesis of nacreous aragonite tablets: lessons from domed sheet nacre

It has been generally accepted that the nacreous tablets are transformed from the amorphous calcium carbonate (ACC) nanoparticles. However, how these nanoparticles are organized and transformed in three-dimensional (3D) space has not been well elucidated. Here, for the first time, we discovered a new type of immature tablets called dendritic tablets in domed sheet nacre of green mussels, which exhibits a dendritic surface texture in top views. By comparing their growth morphology with that of the granular tablets, we found that the ACC nanoparticles are always self-assembled into vertical clusters and then transformed into vertical crystallites of aragonite, each of which exhibits an equilibrium shape of abiotic aragonite. Most importantly, we found that the apparent contact angle between the tablet and substrate (i.e. the interlamellar membrane (ILM)) is unusually high (990 - 1260), which indicates that the ILM is non-wetting (or partial wetting) to the ACC nanoparticles. Therefore, we concluded that the non-wettability of the ILM may have great effects on the morphogenesis of the nacreous tablets.