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

Area of Science:

• Marine Biology
• Biomineralization
• Skeletal Biology

Background:

• Sea urchin spines possess critical functions and exhibit internal structural heterogeneities.
• Previous studies noted these heterogeneities but did not fully explore their functional significance.

Purpose of the Study:

• To investigate spine heterogeneities in *Paracentrotus lividus*.
• To discuss the potential functional implications of these observed variations.

Main Methods:

• Morphological analysis of spine structures (base and shaft).
• Electron Backscatter Diffraction (EBSD) for crystallite orientation.
• Atomic Absorption Spectrometry (AAS) and Energy Dispersive X-ray (EDX) analysis for elemental composition (Mg).
• Nano- and microindentation for mechanical properties (stiffness and hardness).

Main Results:

• Spines consist of meshwork stereom and shaft with longitudinal septa and a central core.
• No significant difference in crystallite orientation was found between structures.
• Magnesium concentration varied, being higher in the inner septa and showing a cyclic pattern potentially linked to ontogeny.
• Septa demonstrated higher stiffness and hardness than meshwork stereom, with anisotropic mechanical properties.

Conclusions:

• Observed heterogeneities in Mg distribution and mechanical properties within sea urchin spines.
• These variations, particularly in stiffness and hardness, likely confer adaptive functional advantages.
• The findings suggest a link between spine ontogeny, chemical composition, and mechanical performance.