Micromechanics of Sea Urchin spines

Abstract

The endoskeletal structure of the Sea Urchin, Centrostephanus rodgersii, has numerous long spines whose known functions include locomotion, sensing, and protection against predators. These spines have a remarkable internal microstructure and are made of single-crystal calcite. A finite-element model of the spine's unique porous structure, based on micro-computed tomography (microCT) and incorporating anisotropic material properties, was developed to study its response to mechanical loading. Simulations show that high stress concentrations occur at certain points in the spine's architecture; brittle cracking would likely initiate in these regions. These analyses demonstrate that the organization of single-crystal calcite in the unique, intricate morphology of the sea urchin spine results in a strong, stiff and lightweight structure that enhances its strength despite the brittleness of its constituent material.

Figure 1. SEM micrographs of a Sea Urchin spine.

A: Cross-section of the spine showing its hollow center and porous wall architecture (scale bar = 1.0 mm). B: Outer surface of the spine. Barbs point toward the spine’s tip, shown here on left (scale bar = 200 micron). C: Fracture surface of a wedge of the spine (scale bar = 100 micron). The appearance is reminiscent of fracture morphology of glass. Top arrow points to the root of crack initiation. Bottom arrow points to a feature on the external surface of the wedge, also seen in B, which identifies the external surface, and confirms that crack initiation started on the outer surface.

Figure 2. MicroCT reconstruction of a portion of the spine showing details of its internal anatomy.

1 - inner wall, 2 - wedge, 3 - barb, 4 - bridge, 5 - porous zone.

Figure 3. Distribution of von Mises stress under 1% applied compressive strain.

The value of stress (MPa) is indicated in the insert; blue - low level, red - high level of stress. Top: outer surface of spine. Bottom: inner surface.

Figure 4. Vertical cuts through selected wedges showing internal stresses.

The stress distributions here are cross-sectional cuts through some of the wedges shown in Figure 3, showing inhomogeneous stress, from low in the barbs (dark blue), to high in between the barbs (orange). Width of each image is approximately 600 micron.

Figure 5. Distribution of von Mises stress under torsional loading.

The value of stress (MPa) is indicated in the insert; blue - low level, red - high level of stress. Top: outer surface of spine. Bottom: inner surface.

Figure 6. Contrast improvement of microCT image.

A: original microCT image, B: noise reduction, and C: anisotropic diffusion filtering.

Figure 7. SEM micrograph of spine transverse section.

White rectangle indicates single wedge area used for chemical investigation.

Figure 8. SEM micrograph of the area after chemical analysis.

Note the line of 10 analysis spots at a spacing of 38 micrometers along a line from the tip to the base of this wedge.

Figure 9. Maps of X-ray intensities.

Intensities of Mg (left) and S (right) detected across the area used for chemical analyses. In both maps, brightest regions are those with highest abundance.

Figure 10. Chemical analyses.

Representative results of analyses made along the line indicated in Figure 8. Some variation in composition was measured from the wedge tip (point 1 plotted at left) to the base (point 10 plotted at right), reinforcing the trends from X-ray-intensity maps of Figure 9.