The male spotted ratfish – a shark-like fish native to the northeastern Pacific Ocean – sports rows of teeth outside of its mouth.
The hooked and barbed appendage, called a ‘tenaculum’, protrudes from its forehead like an antenna when erect.
This strange, toothed club isn’t used for beating up food prior to a meal. Instead, researchers think males probably use it to keep hold of females when mating.
When it’s all over, the male retracts the tenaculum back into a recess between its eyes
“This insane, absolutely spectacular feature flips the long-standing assumption in evolutionary biology that teeth are strictly oral structures,” says Karly Cohen, a researcher at the University of Washington in the US.
“The tenaculum is a developmental relic, not a bizarre one-off, and the first clear example of a toothed structure outside the jaw.”
Spotted ratfish (Hydrolagus colliei) are a type of cartilaginous fish called chimaeras, which diverged from sharks more than 385 million years ago.
While all male chimaeras sport the distinctive facial appendage, Cohen and collaborators studied tenaculum development in spotted ratfish from the nearby Puget Sound using micro-computed tomography (CT) scans. They also compared the today’s ratfish to ancestral fossils.
“The modern adult male spotted ratfish can grow 7 or 8 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 researchers write in a paper presenting the findings in PNAS.
They found that evidence of an early structure remains in females, though the tenaculum never properly develops.
Further tissue analysis revealed genes associated with the teeth of vertebrates (animals with a backbone) were expressed in the tenaculum. The teeth were also rooted in a band of tissue called the ‘dental lamina’ which contributes to tooth development in the jaw.
“When we saw the dental lamina for the first time, our eyes popped,” Cohen says. “It was so exciting to see this crucial structure outside the jaw.”