A ghoulish-looking predator of the deep sea has evolved a unique adaptation to make up for their relatively small size: light-dodging transparency.
Measuring just 15 centimeters (6 inches) in length, the dragonfish (Aristostomias scintillans) has an “enormous” jaw relative to their size capable of extending and opening to beyond that of a conventional jaw. It’s also lined with dozens of fang-like teeth sharper than those found in a piranha. To keep their prey in the dark, the teeth of dragonfish have evolved a transparent structure that essentially makes their fearsome mouth invisible.
"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 author Marc Meyers, from the University of California, San Diego, in a statement. "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."
Researchers collected specimens off the coast of California at a depth of some 500 meters (1,640 feet) under the surface. Using a specialized electron microscope, they observed the morphometry, nanoscale structure, and composition of the thin teeth and found that dragonfish have an enamel-like outer layer and inner dentin layer much like humans. Nanocrystals dispersed throughout these layers prevent any light from reflecting or scattering off the surface. Sharper than even the sharpest of knives, the transparent teeth act as a camouflage to conceal their power against the darkness. The extreme environment characterized by lack of ambient light, low temperatures, and high pressure have resulted in the evolution of “fascinating adaptations” among many deep-sea fish.
"Down at great depths there's almost no light, and the little light there is coming from fish, such as the dragonfish, that have small photophores that generate light, attracting prey," said 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."
The teeth of several deep-sea fishes are somewhat transparent but have not been studied because of the difficulty in obtaining such specimens. Before now, the researchers note that knowledge of their structure and physical properties were poorly understood, even though they likely make a difference in their ability to survive.
“The dragonfish is indeed a top predator of the deep sea with an arsenal of tools to hunt prey and remain concealed,” wrote the authors in the journal Matter, adding that its “loosejaw” adaptation allows it to eat fish up to half their size, which could be crucial to ingesting large prey for sustained energy at depths where food is scarce.