Jun 16, 2019 | Updated: 11:54 AM EDT

Researchers Discover What Makes Deep-sea Dragonfish Teeth Transparent

Jun 12, 2019 06:16 PM EDT

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(Photo : Una Smith) This deep-sea fish, Photostomias guernei, has a built-in bioluminescent "flashlight" it uses to help it see in the dark.

A team of researchers from the University of California, San Diego have discovered why the teeth of the deep-sea dragonfish is transparent. This adaptation which is considered unique, helps camouflage the dragonfish from their prey, results from their teeth having an unusually crystalline nanostructure mixed with amorphous regions. This discovery could provide bioinspiration for researchers who are looking to develop ceramics that are transparent.

Deep-sea creatures have evolved fascinating adaptations like bioluminescence, eyes that can see in low light, and mouth that can take on much larger prey. Some species, like the deep-sea dragonfish, have transparent teeth.

"It's an adaptation that, to our knowledge, has not yet been explored in detail," said Audrey Velasco-Hogan, a materials science Ph.D. student at the UC San Diego Jacobs School of Engineering and first author of the study. "By studying why these teeth are transparent, we can better understand deep-sea organisms like the dragonfish and the adaptations they evolved to live in their environments."

A dark body along with transparent teeth, make the dragonfish invisible to their prey, explained Velasco-Hogan. Because of this camouflage, dragonfish is among the top predators of the deep sea even though it is small and slow.

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"They spend most of their time sitting around with their jaws open, waiting for something to come by. Their teeth are always exposed, so it's important that they are transparent so they don't reflect or scatter any bioluminescent light from the environment," Velasco-Hogan said.

Researchers imaged and analyzed the nanostructure of the teeth to solve the mysteries of the dragonfish's dental disguise, they used a combination of electron microscopy, focused ion beam and nanoindentation tests. They discovered that the teeth of the deep-sea creature have unique characteristics both in their outer layer and inner layer.

The outer layer consists of hydroxyapatite nanocrystals structured in a way that prevents light from scattering or reflecting off the surface of the teeth. The inner layer is also structured in its own way, it lacks microscopic channels called dentine tubules, which gives the teeth of humans and other animals color. This absence is the reason why the teeth of the dragonfish is transparent.

"Typically, teeth are not nanostructured. And they tend to have microscale features such as dentin tubules. From a materials perspective, it's really interesting to see that dragonfish teeth have architectures that we do not see in others," Velasco-Hogan said.

"I also find it fascinating how there are fundamental similarities between materials in the lab and in nature," she added. "Experimentally, we know that the way to make a material transparent is by reducing its grain size to make it nanostructured. So to see that is also how nature is accomplishing transparency is an interesting parallel."

Velasco-Hogan was part of an interdisciplinary team of researchers who were the first to study the dragonfish teeth in detail. She worked under the direction of Marc Meyers, a professor in the Departments of NanoEngineering and Mechanical and Aerospace Engineering at UC San Diego.

"My group is always looking for new materials in nature to study," said Meyers, whose research focuses on biomimicry. "And interdisciplinary collaborations are a key part of our work. When we bring scientists from different backgrounds together, we can advance the knowledge in our fields in ways that a single lab could not do alone."

They collaborated with Dimitri Deheyn, a marine biologist at the Scripps Institution of Oceanography at UC San Diego who researches bioluminescence and biomimicry.

"Taking advantage of the ultimate adaptation organisms show to specific environments has always been a driver for technological innovation, and the dragonfish is no exception to this," said Deheyn. "There is clearly still broad inspiration to gather from the dragonfish and nature in general, and this intercept between biology and engineering through biomimicry is clearly a lucrative path for sustainable innovations."

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