Karly Cohen, Biology PhD alum and postdoctoral researcher at UW's Friday Harbor Labs, was lead author on a paper about spotted ratfish and the rows of teeth on top of their heads.

Excerpt from UW News article: 

When it comes to teeth, vertebrates have a lot in common. No matter the shape, size or sharpness, teeth share genetic origins, physical characteristics and, of course, a home in the jaw.

New findings call into question one of these core assumptions. Spotted ratfish, a shark-like species native to the northeastern Pacific Ocean, have rows of teeth on top of their heads, lining a cartilaginous appendage called the tenaculum that loosely resembles Squidward’s nose.

Researchers have long speculated about the origins of teeth — structures so vital to survival and evolution that most of us rarely stop to think about them. However, the debate centers on the evolution of oral teeth, without considering the possibility that teeth could be elsewhere, too. With the discovery of teeth on the tenaculum, researchers wonder where else they might be growing, and how this could alter conceptions of dental history.

“This insane, absolutely 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.”

The findings were published in Proceedings of the National Academy of Sciences on Sept. 4.

Spotted ratfish are one of the most abundant fish species in Puget Sound. They belong to a category of cartilaginous fish called chimaeras that split from sharks on the evolutionary tree millions of years ago. Measuring about 2 feet long, spotted ratfish are named for the long slender tails that account for half of their length. Only adult males have a tenaculum adorning their foreheads. At rest, it looks like a small white peanut between their eyes. When erect, the tenaculum is hooked and barbed with teeth.

Males flare their tenaculum to intimidate competitors. While mating, they grip females by the pectoral fin to avoid drifting apart.

“Sharks don’t have arms, but they need to mate underwater,” Cohen said. “So, a lot of them have developed grasping structures to connect themselves to a mate during reproduction.”

Spotted ratfish also have pelvic claspers that they use for this purpose.

Read the full article in UW News.