Biological light-weight materials: The endoskeletons of cephalopod mollusks

Structural biological hard tissues fulfill diverse tasks: protection, defence, locomotion, structural support, reinforcement, buoyancy. The cephalopod mollusk Spirula spirula has a planspiral, endogastrically coiled, chambered, endoskeleton consisting of the main elements: shell-wall, septum, adapical-ridge, siphuncular-tube. The cephalopod mollusk Sepia officinalis has an oval, flattened, layered-cellular endoskeleton, formed of the main elements: dorsal-shield, wall/pillar, septum, siphuncular-zone. Both endoskeletons are light-weight buoyancy devices that enable transit through marine environments: vertical (S. spirula), horizontal (S. officinalis). Each skeletal element of the phragmocones has a specific morphology, component structure and organization. The conjunction of the different structural and compositional characteristics renders the evolved nature of the endoskeletons and facilitates for Spirula frequent migration from deep to shallow water and for Sepia coverage over large horizontal distances, without damage of the buoyancy device. Based on Electron-Backscatter-Diffraction (EBSD) measurements and TEM, FE-SEM, laser-confocal-microscopy imaging we highlight for each skeletal element of the endoskeleton its specific mineral/biopolymer hybrid nature and constituent arrangement. We demonstrate that a variety of crystal morphologies and biopolymer assemblies are needed for enabling the endoskeleton to act as a buoyancy device. We show that all organic components of the endoskeletons have the structure of cholesteric-liquid-crystals and indicate which feature of the skeletal element yields the necessary mechanical property to enable the endoskeleton to fulfill its function. We juxtapose structural, microstructural, texture characteristics and benefits of coiled and planar endoskeletons and discuss how morphometry tunes structural biomaterial function. Both mollusks use their endoskeleton for buoyancy regulation, live and move, however, in distinct marine environments.