Image: University of Florida Nighttime seas off Washington’s coast hide a creature that looks like it belongs in science fiction: the spotted ratfish, also known as a “ghost shark.” Researchers have now confirmed that adult males of this species grow real teeth on their foreheads, not just inside their mouths. These sharp, retractable teeth sit on a rod-like structure called the tenaculum, which is used to grasp females during mating. The discovery, published in the Proceedings of the National Academy of Sciences, challenges the long-standing belief that teeth only grow inside mouths and opens new insights into how dental features evolved over millions of years.
What are ghost sharks
Ghost sharks, or chimaeras, are ancient cartilaginous fish closely related to sharks and rays. The spotted ratfish (Hydrolagus colliei) is one of the most abundant ghost sharks in Puget Sound, off the coast of Washington, where researchers collected specimens. These deep-sea fish grow to about two feet in length, with long slender tails making up nearly half their body size. Despite their nickname, ghost sharks are not true sharks but share a common ancestor dating back hundreds of millions of years.
Teeth where you least expect them
Only adult males develop the tenaculum, a peanut-shaped appendage that sprouts between the eyes. During courtship, this structure becomes hooked, barbed, and lined with rows of retractable teeth. Scientists found these forehead teeth form from the same tissue that creates oral teeth, express the same tooth-specific genes, and strongly resemble shark jaw teeth. Fossil evidence dating back 315 million years shows that this structure has existed in chimaeras’ ancestors, proving it is not a bizarre modern mutation but a deep evolutionary trait.
How ghost sharks use their forehead teeth
The tenaculum functions as a mating tool. Males use its retractable teeth to latch onto females underwater, ensuring successful reproduction. The grip can be so strong that it sometimes leaves scars on the fins of female ghost sharks. This adaptation highlights how evolution can repurpose existing biological programs — in this case, the genetic blueprint for tooth development — to serve entirely new functions outside the mouth.
Why this matters for science
For decades, scientists assumed teeth were strictly oral structures. This discovery overturns that assumption, showing that tooth-forming cells can operate in unexpected parts of the body. According to Dr. Gareth Fraser of the University of Florida, who led the study with co-authors Dr. Karly Cohen (University of Washington) and Dr. Michael Coates (University of Chicago), this finding illustrates evolutionary “bricolage,” where organisms re-use existing genetic programs in innovative ways.
A glimpse into evolutionary history
By combining fossil analysis, CT scans, and genetic testing, researchers pieced together the origins of the ghost shark’s tenaculum. The study suggests that early vertebrates may have experimented with placing teeth outside the mouth long before modern sharks and fish refined dental structures for feeding. As Dr. Cohen notes, “The more we look at spiky structures on vertebrates, the more teeth we are going to find outside the jaw.” This discovery not only reshapes our understanding of ghost sharks but also hints at hidden evolutionary secrets waiting to be uncovered in other species.