Why Deep-Sea Dragonfish Has Transparent Teeth

A species of dragonfish called Aristostomias scintillans is a voracious predator of the deep sea with an arsenal of tools to hunt prey. It has proportionately enormous jaws capable of a special mechanism of opening and closure referred to as loosejaw. In contrast to its dark pigmented skin, the species is equipped with transparent teeth. A new study has revealed that the teeth of Aristostomias scintillans evolved to reduce light scatter, allowing the fish’s wide-open mouth to effectively disappear right before its jaws snap onto its prey.

Deep-sea fishes living in darkness have evolved unique predation and communication mechanisms involving bioluminescence and other strategies. Another adaptation is the transparency of the teeth of several species, which is hypothesized to provide stealth for predation. Velasco-Hogan et al investigated the transparent teeth of Aristostomias scintillans and discovered that it is due to the mixture of nanocrystalline and amorphous phases in both its enamel-like and dentin layers and the absence of tubules. This structure significantly decreases the Rayleigh scattering of the hydroxyapatite-collagen mixture. Image credit: Velasco-Hogan et al, doi: 10.1016/j.matt.2019.05.010.

Deep-sea fishes living in darkness have evolved unique predation and communication mechanisms involving bioluminescence and other strategies. Another adaptation is the transparency of the teeth of several species, which is hypothesized to provide stealth for predation. Velasco-Hogan et al investigated the transparent teeth of Aristostomias scintillans and discovered that it is due to the mixture of nanocrystalline and amorphous phases in both its enamel-like and dentin layers and the absence of tubules. This structure significantly decreases the Rayleigh scattering of the hydroxyapatite-collagen mixture. Image credit: Velasco-Hogan et al, doi: 10.1016/j.matt.2019.05.010.

Deep-sea creatures have evolved some fascinating adaptations such as bioluminescence, eyes that can see in low light, and mouths that can engulf much larger prey. Some species, such as Aristostomias scintillans, have transparent teeth.

“Most deep-sea fauna have unique adaptations, but the fact that dragonfish have transparent teeth puzzled us, since the trait is usually found in larger species,” said senior author Dr. Marc Meyers, a researcher at the University of California, San Diego.

“We thought that the nanostructure would be different, and when we looked at this, we found grain-sized nanocrystals embedded throughout the teeth are responsible for this uncanny optical property.”

“It’s an adaptation that, to our knowledge, has not yet been explored in detail,” said first author Audrey Velasco-Hogan, a PhD student at the University of California, San Diego.

“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.”

To solve the mysteries of the dragonfish’s dental disguise, the team imaged and analyzed the nanostructure of the teeth using a combination of electron microscopy, focused ion beam and nanoindentation tests.

The scientists discovered that transparency of the teeth is different from how other organisms have evolved this adaptation.

First, they saw that dragonfish teeth, like human teeth, are comprised of an outer enamel-like layer and an inner dentin layer.

The nanocrystals, about 20 nm in size, are dispersed throughout the amorphous matrix of the enamel, preventing any light that is in the environment from reflecting or scattering off the surface of the teeth.

The teeth are also relatively thin compared to other predatory fish, adding to this light 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,” Dr. Meyers said.

“But the dragonfish’s teeth are huge in proportion to its mouth 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.”

The findings appear in the journal Matter.

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Audrey Velasco-Hogan et al. On the Nature of the Transparent Teeth of the Deep-Sea Dragonfish, Aristostomias scintillans. Matter, published online June 5, 2019; doi: 10.1016/j.matt.2019.05.010

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