Ghost Sharks Grow Teeth Out of Their Foreheads for Sex, Groundbreaking Study Reveals

NEED TO KNOW

• A new study discovered male ghost sharks grow teeth out of their foreheads on an appendage called a tenaculum

• The Washington-based study “flips the long-standing assumption in evolutionary biology that teeth are strictly oral structures,” says one of the study’s researchers

• The male ghost sharks use these teeth to grip a female mate during sex

Ghost sharks are lurking off the coast of Washington.

Also known as ratfish or chimaeras, ghost sharks are deep-sea fish characterized by their green, glowing eyes and shimmering bodies, typically measuring around two feet in length.

A new study has revealed another odd feature of these cartilaginous fish, which have been found swimming in the waters near San Juan Island in Puget Sound: forehead teeth.

According to The New York Times and CBC, the ratfish’s male population all have a peculiar appendage called the tenaculum on their foreheads. Even stranger, the appendage is covered in hooked protusions at its tip. Scientists thought these spiky bits might be spiny scales, like those found on the bodies of rays and sharks.

But a new study recently published in the Proceedings of the National Academy of Sciences concluded that the protusions aren’t spines, but teeth, which help the males grasp onto females during sex.

“This insane, absolutely spectacular feature flips the long-standing assumption in evolutionary biology that teeth are strictly oral structures,” Karly Cohen, a researcher at the University of Washington’s Friday Harbor Labs who took part in the study’s research, told BBC.

“The tenaculum is a developmental relic, not a bizarre one-off, and the first clear example of a toothed structure outside the jaw,” she added.

Dr. Gareth Fraser and a group of scientists studied numerous ratfish from an area in the San Juan Channel in Washington to confirm the discovery.

“The rows of teeth are all organized in a very similar way in this conveyor belt of teeth that we see in sharks,” Fraser, an evolutionary biologist at the University of Florida and one of the study’s authors, said, per the Times.

The mystery of how a marine animal would have evolved to grow such structures on its forehead baffled the global scientific community for years.

And while the study’s discovery doesn’t answer all of scientists’ questions about the tenaculum, it does show vertebrates can grow teeth outside of the mouth, challenging a longstanding idea.

“We haven’t seen anything like this anywhere else in the animal kingdom, period,” Fraser said.

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Ghost shark, also known as a ratfish

The study conducted molecular tests of the ghost sharks’ tenaculum protusions and the tissue around them to help determine what the spikes were. The tests showed that the protusions’ material bears a close genetic resemblance to the tooth-growing genes that are typically present in fish mouths. The tissue in which the teeth were embedded, called the dental lamina, was similarly only recorded inside a vertebrate’s jaw before the study, per CBC.

Ghost sharks use their forehead teeth to grip a female mate’s pectoral fins while reproducing, according to video records examined in the study. And when the male is done using the tenaculum, it gets tucked away in a small rivet also on the forehead.

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