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The Microstructure of a Parrotfish Tooth Contributes to Its Toughness

During a 2012 visit to the Great Barrier Reef off the coast of Australia, ALS staff scientist Matthew Marcus became intrigued with parrotfish. “I was reminded that this is a fish that crunches up coral all day and is responsible for much of the white sand on beaches,” Marcus said. “But how can this fish eat coral and not lose its teeth?” So Marcus teamed up with Pupa Gilbert, a biophysicist at the University of Wisconsin–Madison, and an international team of researchers she assembled, to understand how parrotfish teeth work.

Because conventional microscopes can overlook the unique orientation of crystals in tooth enamel, the team used the technique called polarization-dependent imaging contrast (PIC) mapping that Gilbert invented, which uses the photoemission electron microscopy (PEEM) Beamline 11.0.1.1 at the ALS. The PIC maps allowed them to visualize the orientation of individual crystals of fluorapatite, the main mineral component of parrotfish teeth.

Separate experiments used tomography (Beamline 8.3.2) and microdiffraction (Beamline 12.3.2) to further analyze the crystal orientations and strains in the teeth.

The results revealed that bundles of crystals interwoven like chain mail give parrotfish teeth their remarkable bite and resilience. Mechanical measurements found that the hardness and stiffness increase gradually, from the back to the tip of each tooth. Interestingly, PIC mapping revealed that in the same direction, the diameter of the crystal bundles narrows.

The structure provides a blueprint for creating ultra-durable synthetic materials that could be useful for mechanical components in electronics and in other devices that undergo repetitive contact, movement, and abrasion.

PIC maps acquired at the tips of four different parrotfish teeth show that they consist of 100-nm-wide, microns-long crystals, bundled into “fibers” interwoven like warp and weft fibers in fabric. These fibers gradually decrease in average diameter from 5 μm at the back of a tooth to 2 μm at the tip. Intriguingly, this decrease in size is spatially correlated with an increase in hardness and stiffness. The orientation angle of the crystals is color-coded (chart at bottom).

M.A. Marcus, S. Amini, C.A. Stifler, C.-Y. Sun, N. Tamura, H.A. Bechtel, D.Y. Parkinson, H.S. Barnard, X.X.X. Zhang, J.Q.I.Chua, A. Miserez, and P.U.P.A. Gilbert, “Parrotfish Teeth: Stiff Biominerals Whose Microstructure Makes Them Tough and Abrasion-Resistant to Bite Stony Corals,” ACS Nano 11, 11856 (2017), doi: 10.1021/acsnano.7b05044.

Adapted from the Berkeley Lab press release, “X-Rays Reveal the Biting Truth About Parrotfish Teeth.”