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Why are dragonfish teeth transparent?

6 June 2019

Nanocrystals dispersed throughout the teeth prevent them from reflecting much ambient light in seawater.

Closeup of a dragonfish's mouth

Deep-sea dragonfish (Aristostomias scintillans) lurk more than 500 meters under the sea with their mouths open and a bioluminescent lure hanging from their chins. Sporting proportionally enormous jaws, they can ingest prey half their size, which helps sustain them for longer periods in the dark, food-scarce ocean depths.

Dragonfish are ambush predators, and key to their evolutionary success is their mouthful of long, transparent teeth, which are effectively invisible to prey that swim nearby in the dim bioluminescence. The jaw muscles of the dragonfish are weak, but those saber-like teeth, shown here, are razor sharp, the better to pierce prey when they snap shut. (Whereas piranha teeth boast tips with radii of 14 µm, those in dragonfish range from 2.5–5.0 µm.)

Materials scientist Marc Meyers and his graduate student Audrey Velasco-Hogan at the University of California, San Diego, have now collaborated with marine biologist Dimitri Deheyn and materials scientists Eduard Arzt, Marcus Koch, and Birgit Nothdurft to understand what’s behind the transparency. In some ways dragonfish teeth are like those of other animals—composed of an enamel layer covered by hard dentin. But in specimens collected off the coast of San Diego, the team used transmission electron microscopy and found details that set the dragonfish apart. Embedded in the amorphous matrix of their enamel layer are collagen and hydroxyapatite nanocrystals, each about 1 nm wide and up to 25 nm long. Similar nanocrystals coat the dentin layer. In the team’s optical spectroscopy experiments, the nanocrystals sharply reduced the Rayleigh scattering of light from the teeth. The reduction acts like camouflage. According to the team’s analysis, the smaller the nanocrystals’ radii, the less efficient they are as light scatterers. (A. Velasco-Hogan et al., Matter 1, 1, 2019.)

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