Stress-correlated stereom organization in the echinoid tubercle reveals hierarchical mechanical integration

Echinoid tests consist of a highly porous skeletal microstructure (stereom) optimized in terms of functional performance. One of the most mechanically demanding components of the test is the tubercle, which forms the mechanical interface between the test plates and spines and is subjected to complex loads generated by motion of the spine. However, the mechanical behaviour of tubercles under realistic loading conditions remains poorly explored. Here, we investigate the tubercle of Prionocidaris baculosa using high-resolution X-ray microscopy and finite-element analysis to simulate physiologically relevant loading, identify the main stress directions, and assess mechanical efficiency through topology optimization. By correlating experimental and numerical results, we quantify the relationship between stress distributions and stereom organization, providing new insights into the mechanical complexity of echinoid skeletons. We found a strong correlation between experimentally determined density and different stress parameters (von Mises stress, principal stress trajectories and mean normal stress), supporting the hypothesis of a functionally graded and mechanically optimized stereom architecture. These findings highlight the echinoid skeleton as a naturally optimized, load-adaptive structure with potential relevance for bioinspired design. In particular, stereom-inspired architectures could inform additive manufacturing approaches, enabling lightweight and material-efficient components with tailored mechanical performance derived from a single material.