Structure, composition and mechanical relations to function in sea urchin spine.

Sea urchins have characteristic spines that fulfil critical functions. Several studies revealed marked spine internal heterogeneities at different structural levels despite the single-crystal character of the spines. Most of these studies did not speculate about the functional meaning of these heterogeneities. Spine heterogeneities were investigated in the sea urchin Paracentrotuslividus and their possible functional implications discussed. Spines mainly show two morphological parts: the base, made of a meshwork stereom, and the shaft, with longitudinal plain septa and a central core of meshwork stereom. Electron Backscatter Diffraction showed no difference in crystallite orientation between the two structures. Atomic Absorption Spectrometry and Energy dispersive X-ray analysis revealed that Mg was not uniformly distributed in the spine. Mg concentration is higher in the inner part of the septa than in the septum outer part. Furthermore, a cyclic pattern of Mg concentration in septa was observed. This is suggested to be linked to the spine ontogeny. Nano- and microindentation analyses revealed that the septa have higher stiffness and hardness than the meshwork stereom and that septum stiffness and hardness present different trends in longitudinal and transverse section. These mechanical heterogeneities may have an adaptive functional value.