Ghost sharks grow forehead teeth to help them have sex
University of Florida
image:
CT (computed tomography) scan of the adult male Spotted Ratfish frontal clasper (Tenaculum) covered in rows of teeth (rainbow colors).
view moreCredit: Specimen scanned by Karly Cohen; rendering and image by Ella Nicklin
Male “ghost sharks” — eerie deep-sea fish known as chimaeras that are related to sharks and rays — have a strange rod jutting from their foreheads, studded with sharp, retractable teeth. New research reveals these are not merely lookalikes, but real rows of teeth that grow outside the mouth.
What’s more, the toothy appendage is likely used for mating. Found only in males, the forehead rod — called a tenaculum — is the ghost sharks’ only source of distinct teeth, and it seems to be used to grasp females in much the same way sharks use their toothy mouths in mating.
“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, Ph.D., a professor of biology at the University of Florida and senior author of the study. “If chimaeras can make a set of teeth outside the mouth, where else might we find teeth?”
The team, including scientists from the University of Washington and the University of Chicago, studied both fossils and living specimens to solve the mystery. A 315-million-year-old fossil showed the tenaculum attached to the upper jaw, bearing teeth incredibly similar to those in the mouth. Modern chimaeras collected from Puget Sound revealed the same tooth-growing process on the head, seen in modern-day shark jaws. And genetic testing confirmed they expressed the same tooth-specific genes as oral teeth.
“What we found is that the teeth on this strange appendage look very much like rows of shark teeth. The ability to make teeth transferred onto that appendage, likely from the mouth,” Fraser said. “Over time, the tenaculum shortened but retained the ability to make oral teeth on this forehead appendage.”
Fraser collaborated with Washington’s Karly Cohen, Ph.D., and Michael Coates, Ph.D., from Chicago on the study, which was published this week in the Proceedings of the National Academy of Sciences.
As experts in shark evolution and anatomy, the scientists were intrigued by these tooth-filled rods sprouting from the ghost shark foreheads. The central mystery: Is the tenaculum covered in true teeth related to oral teeth or more similar to the tooth-like scales plastering the skin of sharks and some ghost sharks?
CT scans of the fossils and modern chimaeras gave the scientists unprecedented, detailed insights into the development of the tenaculum teeth, which looked remarkably similar to the teeth of today’s sharks. The nail in the coffin came from genetic evidence. The tenaculum teeth express genes found only in true teeth, never in shark skin denticles.
"What I think is very neat about this project is that it provides a beautiful example of evolutionary tinkering or ‘bricolage,’” said Coates, a professor of biology at the University of Chicago. “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."
Cohen, a postdoctoral researcher at the University of Washington’s Friday Harbor Labs and first author of the paper, said scientists had never spotted teeth outside the mouth in this way before.
“The tenaculum is a developmental relic, not a bizarre one-off, and the first clear example of a toothed structure outside the jaw,” she said.
The bizarre path from a mouth full of teeth to forehead teeth used for mating demonstrates the impressive flexibility of evolution, the researchers say, always ready to repurpose structures for strange and unexpected new uses.
“There are still plenty of surprises down in the ocean depths that we have yet to uncover,” Fraser said.
Juvenile Spotted Ratfish (Hydrolagus colliei) sits in the palm of the hand.
Caught male Spotted Ratfish (Hydrolagus colliei) showing the [extended/erect] frontal clasper (tenaculum) on the front of the head.
CreditGareth J. Fraser, University of Florida.
Journal
Proceedings of the National Academy of Sciences
Method of Research
Observational study
Subject of Research
Animals
Article Title
Teeth outside the jaw: Evolution and development of the toothed head clasper in chimaeras
Article Publication Date
4-Sep-2025
This common fish has an uncommon feature: Forehead teeth, used for mating
image:
The researchers identified teeth on the tenaculum of ancient relatives to the modern adult male spotted ratfish. This fossil record helped them establish the historical significance of this structure, brought to life by local artist Ray Troll.
view moreCredit: Ray Troll
When it comes to teeth, vertebrates have a lot in common. No matter the shape, size or sharpness, teeth share genetic origins, physical characteristics and, of course, a home in the jaw.
New findings call into question one of these core assumptions. Spotted ratfish, a shark-like species native to the northeastern Pacific Ocean, have rows of teeth on top of their heads, lining a cartilaginous appendage called the tenaculum that loosely resembles Squidward’s nose.
Researchers have long speculated about the origins of teeth — structures so vital to survival and evolution that most of us rarely stop to think about them. However, the debate centers on the evolution of oral teeth, without considering the possibility that teeth could be elsewhere, too. With the discovery of teeth on the tenaculum, researchers wonder where else they might be growing, and how this could alter conceptions of dental history.
“This insane, absolutely spectacular feature flips the long-standing assumption in evolutionary biology that teeth are strictly oral structures,” said Karly Cohen, a UW postdoctoral researcher at the University of Washington’s Friday Harbor Labs. “The tenaculum is a developmental relic, not a bizarre one-off, and the first clear example of a toothed structure outside the jaw.”
The findings were published in Proceedings of the National Academy of Sciences on Sept. 4.
Spotted ratfish are one of the most abundant fish species in Puget Sound. They belong to a category of cartilaginous fish called chimaeras that split from sharks on the evolutionary tree millions of years ago. Measuring about 2 feet long, spotted ratfish are named for the long slender tails that account for half of their length. Only adult males have a tenaculum adorning their foreheads. At rest, it looks like a small white peanut between their eyes. When erect, the tenaculum is hooked and barbed with teeth.
Males flare their tenaculum to intimidate competitors. While mating, they grip females by the pectoral fin to avoid drifting apart.
“Sharks don’t have arms, but they need to mate underwater,” Cohen said. “So, a lot of them have developed grasping structures to connect themselves to a mate during reproduction.”
Spotted ratfish also have pelvic claspers that they use for this purpose.
Many common sharks, rays and skates are covered in tooth-like structures called denticles. Aside from the denticles on their pelvic claspers, spotted ratfish are “pretty naked,” Cohen said, leading the researchers to wonder: Where did all their denticles go?
Before this study, they had two theories. One suggested that the “teeth” on their tenaculum were denticles, a vestige of the past. The other proposed that they were true teeth, like those present in the oral cavity.
“Ratfish have really weird faces,” Cohen said. “When they are small, they kind of look like an elephant squished into a little yolk sack.”
The cells that form the oral region are spread farther afield, making it plausible that at some point, a clump of tooth-forming cells might have migrated onto the head and stuck.
To test these theories, the researchers caught and analyzed hundreds of fish, using micro-CT scans and tissue samples to document tenaculum development. While sharks can be quite hard to study, spotted ratfish abound in Puget Sound. They frequent the shallows surrounding Friday Harbor Labs, the UW research facility located on San Juan Island. They also compared the modern ratfish to ancestral fossils.
The scans showed that both male and female ratfish begin making a tenaculum early on. In males, it grows from a small cluster of cells into a little white pimple that elongates between the eyes. It attaches to muscles controlling the jaw and finally, erupts through the surface of the skin and sprouts teeth. In females it never materializes — or mineralizes — but evidence of an early structure remains.
The new teeth are rooted in a band of tissue called the dental lamina that is present in the jaw but has never been documented elsewhere. “When we saw the dental lamina for the first time, our eyes popped,” Cohen said. “It was so exciting to see this crucial structure outside the jaw.”
In humans, the dental lamina disintegrates after we grow our adult teeth, but many vertebrates retain the ability to replace their teeth. Sharks, for example, have “a constant conveyor belt” of new teeth, Cohen said. Dermal denticles, including the ones on the spotted ratfish’s pelvic claspers, do not have a dental lamina. Identifying this structure was compelling evidence that the teeth on the tenaculum really are teeth and not leftover denticles. Genetic evidence also backed this conclusion.
“Vertebrate teeth are extremely well united by a genetic toolbox,” Cohen said.
Tissue samples revealed that the genes associated with teeth across vertebrates were expressed in the tenaculum, but not the denticles. In the fossil record, they also observed evidence of teeth on the tenaculum of related species.
“We have a combination of experimental data with paleontological evidence to show how these fishes coopted a preexisting program for manufacturing teeth to make a new device that is essential for reproduction,” said Michael Coates, a professor and the chair of organismal biology and anatomy at the University of Chicago and a co-author of the paper.
The modern adult male spotted ratfish can grow seven or eight rows of hooked teeth on its tenaculum. These teeth retract and flex more than the average canine, enabling the fish to latch onto a mate while swimming. The size of the tenaculum also appears to be unrelated to the length of the fish. Its development aligns instead with the pelvic claspers, suggesting that the migrant tissue is now regulated by other networks.
“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, a professor of biology at the University of Florida and the study’s senior author.
Sharks often serve as the model for studying teeth and development because they have so many oral teeth and are covered in denticles. But, Cohen added, sharks possess just a sliver of the dental diversity captured by history. “Chimeras offer a rare glimpse into the past,” she said “I think the more we look at spiky structures on vertebrates, the more teeth we are going to find outside the jaw.”
This research was funded by National Science Foundation, the Save Our Seas Foundation, and internal endowments at Friday Harbor Labs supporting innovative early-career research.
For more information, contact Karly Cohen at kecohen@uw.edu.
Journal
Proceedings of the National Academy of Sciences
Method of Research
Experimental study
Subject of Research
Animals
Article Title
Teeth outside the jaw: Evolution and development of the toothed head clasper in chimaeras
Article Publication Date
4-Sep-2025
In this other illustration by Ray Troll, the tenaculum is lowered, revealing how close the tip sits to the mouth and highlighting the possibility that tissue could have migrated up. This is one of the theories the researchers propose to explain the presence of teeth outside the jaw.
Credit
Ray Troll
The adult male spotted ratfish, a relative of modern sharks and one of the most abundant fish species in Puget Sound. The tooth-lined tenaculum appears as a small white forehead hump above the mouth and to the left of the eyeball.
Credit
Tiare Boyes
The shape and structure of an adult male spotted ratfish, captured by micro-CT scan. This technology depicts morphological features, including the tenaculum, in vivid detail.
Credit
University of Washington
