Why deadly dragonfish have transparent teeth


Rebecca Pool

Thursday, June 6, 2019 - 11:00
Image: Nanocrystals render dragonfish teeth invisible to prey.[Velasco-Hogan et al./Matter]
Using electron microscopy, researchers from the University of California, San Diego, have discovered why deep-sea dragon fish have transparent teeth.
Investigations into this unique adaptation of the Aristostomias scintillans, reveal that its teeth evolved to reduce light scatter, allowing the fish's wide-open mouth to effectively disappear right before its jaws snap onto its prey.
"Most deep-sea fauna have unique adapations, but the fact that dragonfish have transparent teeth puzzled us, since the trait is usually found in larger species," says Professor Marc Meyers from Materials Science and Engineering at UCSD. "We thought that the nanostructure would be different, and when we looked at this, we found grain-sized nanocrystals embedded throughout the the teeth are responsible for this uncanny optical property."
Despite measuring only 15 centimetres in length, deep-sea dragonfish are apex predators in their part of the ocean, feeding on smaller fish up to 50% of their size.
This image shows the transparent teeth of the deep-sea dragonfish. [Audrey Velasco]
The dragonfish's most distinct feature is its extraordinarily large head full of fang-like teeth attached to a dark-skinned, eel-like body. 
To investigate the dragonfish's teeth, Meyers and graduate student Audrey Velasco partnered with marine biologist, Dimitri Deheyn, from the Scripps Institution of Oceanography and the Eduard Arzt Lab at the Leibniz Institute for New Materials, Germany.
They first imaged the fish and teeth using a Nikon SMZ 1500 stereoscope equipped with a Retiga 2000R colour digital camera.
The head was then sectioned off the main body using a clean razor blade with the teeth being imaged for transparency using a PARISS hyperspectral imaging system.
Analyses were performed with a Nikon 80i microscope outfitted with a monochrome Retiga 2000DC, CCD camera. 
(A) Head of a freshly collected specimen with arrow points to the barbel. Image taken in filtered seawater and captured using a polarizing filter. (B) Close-up, showing optical evidence of concentric layers and hollowness of the tooth. Image taken in filtered seawater with an immersive lens. Arrow points to the striations seen on the concave side. (C) Tooth imaged in seawater with colour line behind to demonstrate transparency. Image captured with a polarizing filter. (D) Tooth under fluorescence excited at a broadband excitation 440–490 nm and collected with a long-pass filter (>515 nm). This image shows little fluorescence. Image taken of a dry specimen in air. [Audrey Velasco-Hogan et al, June 05, 2019, Matter]
To characterise microstructure, forty teeth from ten different specimens were imaged using scanning electron microscopy, with fresh samples imaged using environmental scanning electron microscopy under a Quanta 400 field-emission gun in low vacuum mode.
Teeth were then prepared for TEM analysis using an FEI Versa3D Focused Ion Beam or by scratching a single tooth on sandpaper and investigating the enamel-like layer. The researchers used a JEOL JEM-2100 LaB6 for analyses.
In this way, the researchers discovered that dragonfish teeth, like human teeth, are comprised of an outer enamel-like layer and an inner dentin layer.
Nanocrystals, about 20 nanometers in size, are embedded in the amorphous matrix of the enamel, while in the dentin layer these nanocrystals coat nanoscale collagen fibrils, forming nanorods.
(A) TEM showing the dentin-enamel-like junction (DEJ). (B) TEM showing organized structuring of the enamel-like layer. (C) Diffraction pattern of the enamel-like layer showing discrete spot characteristics of nanocrystalline structure and an amorphous structure (indicative of the diffuse background). (D) TEM showing dentin consisting of a woven pattern of nanometer rods (∼5 nm diameter). (E) Diffraction pattern of dentin which shows greater amorphous characteristics than the enamel-like layer. [Audrey Velasco-Hogan et al, June 05, 2019, Matter]
This nanoscale structure reduces Rayleigh light scattering off the surface of the teeth.
What's more, the dragonfish teeth were also found to be relatively thin compared to other predatory fish, adding to this scattering effect.
"Down at great depths there's almost no light, and the little light there is comes from fish, such as the dragonfish, that have small photophores that generate light, attracting prey," says Meyers. "But the dragonfish's teeth are huge in proportion to its mouth; it's like a monster from the movie Alien and if those teeth should become visible, prey will immediately shy away. But we speculate that the teeth are transparent because it helps the predator."
Based on this study, the researchers are now raising funds to create transparent materials inspired by dragonfish teeth, using a combination of nanocrystals and ceramics.
Research is published in Cell Matter.
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