From colorful displays to aggressive fights, mating rituals vary widely across the animal kingdom. But one of the strangest comes from male ghost sharks – also known as chimaeras – who use their teeth to grasp onto female mates. The strange part? The teeth in question happen to be located on their forehead.
Although many creatures have tooth-like growths elsewhere on their body, this new study in the Proceedings of the National Academy Sciences confirms the chimaera’s external teeth are the same as those in the mouth. Having teeth located outside the mouth is a biological rarity and the ghost shark’s exposed chompers are something scientists have never quite seen before.
“If these strange chimaeras are sticking teeth on the front of their head, it makes you think about the dynamism of tooth development more generally,” said Gareth Fraser, professor of biology at the University of Florida, in a press release. “If chimaeras can make a set of teeth outside the mouth, where else might we find teeth?”
Why Ghost Sharks Have Teeth on Their Forehead
CT (computed tomography) scan of the adult male Spotted Ratfish frontal clasper (Tenaculum) covered in rows of teeth (rainbow colors).
(Image Credit: Specimen scanned by Karly Cohen; rendering and image by Ella Nicklin)
This evolutionary mating marvel is called a tenaculum and is a fleshy rod jutting out from the ghost shark’s forehead. The tenaculum is present in both fossilized and modern species of chimaeras, and includes rows upon rows of sharp shark teeth. The teeth are also retractable so that the chimaera can expose them when necessary during mating.
Using teeth for mating practices is nothing new as shark species often use the teeth in their mouths for the same purpose. The teeth act like hands, gripping the mating partner so the two don’t drift apart during the act. But since these shark relatives don’t have teeth inside their mouths, they needed to develop them elsewhere, making them a curiosity of evolutionary biology.
“This insane, absolutely spectacular feature flips the long-standing assumption in evolutionary biology that teeth are strictly oral structures. The tenaculum is a developmental relic, not a bizarre one-off, and the first clear example of a toothed structure outside the jaw," said Karly Cohen, from the University of Washington’s Friday Harbor Labs, in a press release.
Read More: Shark Teeth May Start to Decay as Ocean Water Becomes Increasingly Acidic
Are Ghost Sharks’ Forehead Teeth Actually Teeth?
The chimaera tenaculum is not a new discovery, but there have always been questions about whether or not the tenaculum teeth are actual teeth or just teeth-like growths. For this study, researchers used both fossilized remains and modern ghost shark specimens to help answer this question.
Juvenile Spotted Ratfish (Hydrolagus colliei) sits in the palm of the hand.
(Image Credit: Gareth J. Fraser, University of Florida)
Scientists relied on two main methods to investigate the ghost sharks’ toothy appendage: genetic testing and CT scans. Starting with CT scans, the team got a first-of-its-kind look at the details of the tenaculum, both fossilized and modern. The scans revealed that the tenaculum teeth looked almost identical to the teeth inside sharks’ jaws.
Genetic evidence confirmed the CT findings, showing the ghost sharks’ forehead teeth to have genetic signatures only present in true oral teeth and not in teeth-like growths. The ability for ghost sharks to develop these true, biological teeth outside of their mouths is astounding scientists and is a testament to the creativity some species show during their evolutionary journeys.
“What I think is very neat about this project is that it provides a beautiful example of evolutionary tinkering or ‘bricolage.’ We have a combination of experimental data with paleontological evidence to show how these fishes co-opted a preexisting program for manufacturing teeth to make a new device that is essential for reproduction,” said Michael Coates, a professor of biology from the University of Chicago, in the press release.
Read More: Baby Shark Spotted: Scientists Film Their First Footage of a White Shark Newborn
Article Sources
Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:
- Proceedings of the National Academy Sciences. Teeth outside the jaw: Evolution and development of the toothed head clasper in chimaeras
