A recent study suggests that male ghost sharks, also known as chimaeras, use an unusual organ for mating – sharp teeth that grow out on their foreheads.
Unlike other sharks and rays, ghost sharks do not have distinct oral teeth. These ‘forehead rods’, known as tenacula, have retractable teeth studded on them that play a crucial role in mating.
The research was based on a 315-million-year-old fossil with a tenaculum attached to the upper jaw, lined with teeth nearly identical to those found inside a mouth.
About the tenaculum
Researchers say the tenaculum helps males grasp females during reproduction, acting like the mouth full of teeth that other sharks rely on. Found only in males, the forehead appendage is the only source of true teeth in ghost sharks.
“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 senior author of the study.
“If chimaeras can make a set of teeth outside the mouth, where else might we find teeth?” he stated.
Juvenile Spotted Ratfish (Hydrolagus colliei) sits in the palm of the hand. Image credit: Gareth J. Fraser, University of Florida
Genes confirm the mystery.
The researchers collected modern ghost sharks from Puget Sound with the same signs: teeth forming on the forehead appendage, much like those inside shark jaws. Genetic tests of the sharks revealed that the tenaculum teeth switch on the same tooth-specific genes used in oral teeth.
Skin denticles are tiny tooth-like scales that cover the shark’s skin. However, CT scans of the sharks gave scientists a detailed view and insight into the structures, confirming they were not just skin denticles, but actual teeth.
“What I think is very neat about this project is that it provides a beautiful example of evolutionary tinkering or ‘bricolage,’” said Michael 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 essential for reproduction,” he continued.
Evolution’s strange solution
“This insane, 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,” she stated.
Ghost sharks, which live in deep waters and are rarely seen, have long been a fascinating subject for biologists, thanks to their strange features. This discovery of forehead teeth highlights how little we know about life in the deep sea — and how evolution continues to surprise.
“There are still plenty of surprises down in the ocean depths that we have yet to uncover,” Fraser said.
The findings were published in Proceedings of the National Academy of Sciences.