Crystal architecture of the tooth and jaw bone (pyramid) of the sea urchin Paracentrotus lividus

Sea urchins use their teeth to grind off algae from rocks. Since calcite is a soft and brittle material, it is intriguing how this material is structured to be suitable for a tooth. The crystallographic architecture of the tooth and the jaw bone (pyramid) of the sea urchin Paracentrotus lividus was determined by electron backscatter diffraction, microtomography, and scanning electron microscopy. Sea urchin teeth are low and high Mg-calcite hybrid composites that form a hierarchical multiplex composite crystal. The tooth architecture consists of the slightly (3°–5°) misoriented major structural units, each of which is assembled of subunits, which are again misoriented by 1°–2° relative to each other. The misorientation is systematic throughout the entire length of the tooth. The jaw bones (pyramids) are composed of large porous single-crystal units. The multiplex architecture of the tooth combines calcite components of different sizes, shapes, compositions and crystallographic orientation to provide a functional mechanism. The central “stone” part is hardest and forms the cutting tip. It is encased in the softer “plates” and “lamellar needle complex” units which interdigitate in the stone. Continuous self-sharpening of the tooth occurs by chipping off the “plates” and “lamellae/needles.” The single-crystal-like texture guarantees a constant mechanism of self-sharpening.