  Cuttlebone, the sophisticated buoyancy device of cuttlefish, is made of
extensive superposed chambers that have a complex internal arrangement of
calcified pillars and organic membranes. It has not been clear how this
structure is assembled. We find that the membranes result from a myriad of
minor membranes initially filling the whole chamber, made of nanofibres evenly
oriented within each membrane and slightly rotated with respect to those of
adjacent membranes, producing a helical arrangement. We propose that the
organism secretes a chitin-protein complex, which self-organizes layer-by-layer
as a cholesteric liquid crystal, whereas the pillars are made by viscous
fingering. The liquid crystallization mechanism permits us to homologize the
elements of the cuttlebone with those of other coleoids and with the nacreous
septa and the shells of nautiloids. These results challenge our view of this
ultra-light natural material possessing desirable mechanical, structural and
biological properties, suggesting that two self-organizing physical principles
suffice to understand its formation.
