Deep in the ocean, there lives a strange group of fish called ghost sharks, or chimaeras, which are distant relatives of sharks and rays.
These eerie animals have puzzled scientists for centuries, and now new research has uncovered one of their strangest secrets yet: males grow rows of sharp teeth on their foreheads to help them mate.
The research, published in the journal Proceedings of the National Academy of Sciences, reveals that the male ghost sharks’ unusual forehead rod—called a tenaculum—is not just a spiky-looking structure.
It is covered in real, retractable teeth that grow outside the mouth.
These teeth are thought to act as grasping tools, allowing males to hold onto females during mating, much like sharks use their mouths to latch onto partners.
“This discovery makes you think about the flexibility of tooth development in animals,” said Gareth Fraser, a professor of biology at the University of Florida and senior author of the study. “If ghost sharks can grow teeth outside their mouths, where else in nature might we find them?”
To solve the mystery, scientists from the University of Florida, University of Washington, and University of Chicago examined both fossils and living specimens.
A fossil over 300 million years old showed a tenaculum attached to the upper jaw with tooth-like structures nearly identical to those inside the mouth.
When the team studied living species, such as the Spotted Ratfish from Puget Sound, they found the same process at work—teeth forming on the head in a way very similar to how they form in shark jaws.
High-resolution CT scans gave researchers detailed views of the teeth on the tenaculum, showing clear similarities to modern shark teeth.
The most convincing evidence came from genetic tests: the forehead teeth expressed the same genes used to form oral teeth, and not the genes linked to the tooth-like scales that cover the skin of sharks.
Over evolutionary time, the tenaculum appears to have shrunk, but it kept its ability to produce true teeth. This suggests that ghost sharks essentially repurposed their existing tooth-making machinery to build a new tool for reproduction.
“This is a beautiful example of evolutionary tinkering,” said Michael Coates, a professor of biology at the University of Chicago.
“Ghost sharks took an ancient program for growing teeth and reused it to create a forehead appendage essential for mating.”
Karly Cohen, a postdoctoral researcher at the University of Washington and first author of the study, added that this is the first confirmed example of a toothed structure outside the jaws in vertebrates.
The findings highlight just how inventive evolution can be, turning old structures into new solutions for survival and reproduction. And as Fraser points out, the deep sea still holds many more surprises waiting to be discovered.
