March 31, 2009 | David F. Coppedge

Envying the Tooth of the Sea Urchin

Did you know the lowly sea urchin has a tooth?  It’s not just any tooth: it’s “a remarkable grinding tool,” according to a team of international scientists.  They even used the word “exquisite” in the title of their paper in PNAS.1  Humans might benefit from knowing more about this tool.  “The improved understanding of these structural features,” they said, “could lead to the design of better mechanical grinding and cutting tools.
    The sea urchin “tooth” is not really a tooth, but a hard rod with a serrated edge used for crushing the animal’s food (see description at Univ. of Wisconsin about the work of Pupa Gilbert, one of the co-authors).  The urchin tooth, which grinds down hard limestone, has the hardness of teeth in higher animals.  “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,” the abstract from the paper reads.  It continues—

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 [magnesium] 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 slight misalignment and interdigitation appears to provide a functional advantage, they found.  It provides a corrugated edge that fractures along its cleavage planes so as not to fracture the tooth but actually sharpen it as it cuts.  “We also note that in this model, the edges of the individual plates would remain anatomically sharp due to cleavage along the {104} planes, and the cleavage would probably not propagate through the whole tooth tip because of the small misalignment between neighboring plates.”  In other words, even the apparent misalignment has a function.  They said, in conclusion,

The mature sea urchin tooth possesses incredible structural and compositional complexity.  Here, we show the presence of crystalline blocks composed of 3 different coaligned elements: needles, plates, and polycrystalline matrix.  We also show that the tip, and presumably the whole tooth, is composed essentially of 2 such coaligned blocks that differ in their orientations by [less than] 6°.  The blocks are also interdigitated in the tip.  Furthermore, the Mg concentrations increase toward the center of the tooth tip.  We propose that all of these features contribute to the grinding capability of the tooth.  A deep understanding of the structural design features of the tooth tip sheds light on the manner in which one crystalline phase, calcite, can be tailored to fulfill grinding and self-sharpening functions that enable the tooth to be used to grind holes into a substrate that is also composed only of calcite.  Much can be learned from the sea urchin tooth that can be applied to the development of improved grinding and cutting tools.


1.  Ma, Aichmeyer, Paris, Fratzl, Meibom, Metzler, Politi, Addadi, Gilbert and Weiner, “The grinding tip of the sea urchin tooth exhibits exquisite control over calcite crystal orientation and Mg distribution,” Proceedings of the National Academy of Sciences USA, published online before print March 30, 2009, doi: 10.1073/pnas.0810300106.

All together, everyone: how much was said about evolution in this paper?  ZILCH!  Instead, they used the D word design: they wanted to gain a “deep understanding of the structural design features” of this “exquisite” grinding tool to learn how we might tailor our own bottom-up nanofabrication of crystals to fulfill functions useful to us.  This paper had intelligent design all over it – in the research, in the understanding, and in the application.  Show this to your biology teacher and tell him or her that “Nothing in biology makes sense except in the light of design.”

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