The grinding tip of the sea urchin tooth exhibits exquisite control over calcite crystal orientation and Mg distribution

Abstract

The sea urchin tooth is a remarkable grinding tool. Even though the tooth is composed almost entirely of calcite, it is used to grind holes into a rocky substrate itself often composed of calcite. Here, we use 3 complementary high-resolution tools to probe aspects of the structure of the grinding tip: X-ray photoelectron emission spectromicroscopy (X-PEEM), X-ray microdiffraction, and NanoSIMS. We confirm that the needles and plates are aligned and show here that even the high Mg polycrystalline matrix constituents are aligned with the other 2 structural elements when imaged at 20-nm resolution. Furthermore, we show that the entire tooth is composed of 2 cooriented polycrystalline blocks that differ in their orientations by only a few degrees. A unique feature of the grinding tip is that the structural elements from each coaligned block interdigitate. This interdigitation may influence the fracture process by creating a corrugated grinding surface. We also show that the overall Mg content of the tooth structural elements increases toward the grinding tip. This probably contributes to the increasing hardness of the tooth from the periphery to the tip. Clearly the formation of the tooth, and the tooth tip in particular, is amazingly well controlled. The improved understanding of these structural features could lead to the design of better mechanical grinding and cutting tools. echinoderms ͉ grinding tool ͉ self-sharpening ͉ spectromicroscopy ͉ X-ray microdiffraction

Testing the cation-hydration effect on the crystallization of Ca–Mg–CO 3 systems

“…Both experimental and computational investigations in the literature provided evidence indicating that calcite crystals with low concentrations of Mg are stable in structure (19,(69)(70)(71); these studies, however, predicted that high concentrations of Mg (≥ ∼50 atom %) may prevent crystal formation because of the increasing structural stiffness and distortion resulting from the random Mg substitution of Ca (72,73). Recent studies (73) using computational methods reported that substituting Ca by Mg in calcite crystals can alter the cation-C and the cation-cation interatomic distances significantly and cause local tilt of the CO 3 2− groups while maintaining the C-O (within the CO 3 2− ) and the cation-O bond lengths constant.…”

Structure and mechanical properties of selected protective systems in marine organisms

“…These teeth, despite being composed primarily of calcite, are able to grind through rocks of a similar composition [174][175][176]. This is achieved by highly aligned and continuous growth of Mg-reinforced calcite that results in self-sharpening grinding tips [174,176].…”

Mechanism of Calcite Co-Orientation in the Sea Urchin Tooth

“…8 All echinoderms align the calcite crystals of their mineralized structures in a highly cooriented fashion, 10 making this interesting physical aspect a shared trait, a synapomorphy, of this phylum. 11 Recently, Ma et al 12 reported that the teeth from the species Paracentrotus liVidus have two highly cooriented crystalline blocks oriented differently by a few degrees and that these crystalline blocks interdigitate near the grinding tip. They also reported that the calcite of all components in each block, the plates, the fibers, and the polycrystalline matrix, is highly cooriented from the nanometer to the centimeter scale.…”