The calcitic spines of the sea urchins Heterocentrotus mamillatus and H. trigonarius are promising role models for lightweight applications, bone tissue scaffolds and energy dissipating processes due to their highly porous and organized structure. Therefore, mechanical properties including Young's Modulus, strength, failure behaviour and energy dissipation efficiency have been investigated in depth with uniaxial compression experiments, 3-point bending tests and resonance frequency damping analysis. It was found that despite a very similar structure, H. trigonarius has a significantly lower porosity than H. mamillatus leading to a higher strength and Young's Moduli, but limited ability to dissipate energy. In order to show reliable energy dissipation during failure in uniaxial compression, a transition porosity of 0.55-0.6 needs to be exceeded. The most effective structure for this purpose is a homogeneous, foam-like structure confined by a thin and dense shell that increases initial strength and was found in numerous spines of H. mamillatus. Sharp porosity changes induced by dense growth layers or prominent wedges of the spines' radiating building principle act as structural weaknesses, along which large flakes can be spalled, reducing the energy dissipation efficiency considerably.The high strength and Young's Modulus at the biologically necessary high porosity levels of the spines is useful for Heterocentrotus and their construction therefore remains to be a good example of biomimetics. However, the energy dissipative failure behaviour may be regarded as a mere side effect of the structure.

Amorphous calcium carbonate (ACC) plays a crucial role in the formation of biogenic carbonates. It is widely accepted that ACC and organic macromolecules alter the fracture properties of Echinoderm calcite from the well-defined cleavage planes of the raw material to conchoidal. However, the influence of ACC on the outstanding macromechanical properties of Echinoderm calcite is unknown. To address this question, full-grown spines of the slate pencil urchin are shortly heated to 250 C. Differential scanning calorimetry indicates that all ACC is crystallized at this temperature. Heated spines are compared with an untreated control group and no significant differences in compressive strength, bending strength, damage tolerance, and Young's modulus are detected. This highlights the weak influence of %6 wt% ACC on the macromechanical properties of Echinoderm calcite, which are likely established by its intricate and damage tolerant microstructure. When heating Echinoderm calcite, organics decompose, Mg calcite transforms, water is lost, and cracks and micropores form. All these processes are analyzed to exclude their influence on the mechanical properties, and it is imperative to consider them all. Only this way meaningful results can be achieved as these processes are temperature and dwelling time dependent and may even occur below 250 C.

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