A Friday Harbor researcher recently identified the first example of ‘true teeth’ outside of the jaw in a vertebrate.
Teeth are the “ultimate tool,” if you ask Karly Cohen, a postdoctoral researcher at the University of Washington’s Friday Harbor Labs. She recently led a study of the Pacific Ocean’s spotted ratfish — Hydrolagus colliei — and found, throughout evolution, the fish have repurposed their teeth in an unusual way. The researchers’ discovery of forehead teeth on the ratfish, which the adult males use to latch onto females during mating, was the first finding of “true teeth” outside of the jaw in a vertebrate.
“It has never been found before, true teeth outside of the head,” said Cohen, who was surprised by the finding. “I mean, it’s a mouth outside of a mouth.”
The finding shows scientists how evolutionary tinkering can repurpose core traits like teeth, she added. While sharks, the spotted ratfish’s close cousin, have tooth-like structures on their skin for protection and hydrodynamics, Cohen’s team found these ratfish are using actual teeth, outside of their jaw, to help them reproduce.
“I think that the more we look at the diversity of vertebrates, the more we’re going to find true teeth in places we just didn’t think about before,” Cohen said.
Her findings were published in the scientific journal Proceedings of the National Academy of Sciences in September. To make the discovery, researchers collected dozens of spotted ratfish, abundant in the waters near Friday Harbor, in different stages of development. Using x-rays, tissue staining and fossil records, Cohen and her team were able to trace the dental origins of the cat claw-like retractable prongs on the adult male ratfish’s forehead.
Yara Haridy, a postdoctoral researcher and evolutionary biologist at the University of Chicago who was not involved in the study, said she wasn’t necessarily surprised to see true teeth outside of the jaw.
“It’s been discussed that this could happen … there’s hints of it in other fish,” Haridy said. “But it’s so nice to see really good imagery and proper data that shows it.”
An ancient fish
The spotted ratfish is a deep-ocean cartilaginous fish, part of the Chimaera genus that diverged from sharks about 400 million years ago. They’re commonly caught accidentally by fishers in the Puget Sound.
“They are one of the most abundant fishes we have here in Washington,” Cohen said. “NOAA pulls them up by the tens of thousands when they’ve done their annual trawls.”
Spotted ratfish have a club-shaped forehead appendage called a tenaculum Cohen described as “bonkers, totally bizarre.” The adult males extend their tenaculum, the size of a headphone earbud, covered in teeth, to mate.
“It’s a little bit of a love bite,” she said. “They need to grab on to the female in order to mate, so it latches onto her pectoral fin.”
To determine if the tenaculum teeth were, well, actually teeth, Cohen and her team compared them to skin denticles on sharks, which are tooth-like structures used for protection and reducing drag as they swim. On the other hand, so-called “true teeth” have origins in the mouth.
True teeth come from tissue called the dental lamina. It’s something humans have too, early in life, until we have our permanent teeth. It helps us generate teeth, before it breaks apart and disappears. In other animals including snakes, alligators and sharks, the dental lamina is used to generate additional sets of teeth over time.
The research
After collecting spotted ratfish of all ages in Friday Harbor’s San Juan Channel, Cohen and her team were able to show the ratfish’s tenaculum tissue more resembled a dental lamina than denticles, by examining CT scans and genetic markers throughout development.
“The dental lamina was the big feature that helped us distinguish that these were true teeth,” Cohen said.
They also used fossil data to suggest the tenaculum likely had origins in the jaw.
“Through evolutionary time, it kind of just crawled up the forehead and kept its little toolbox of knowing how to make true teeth,” Haridy said.
Uniquely Friday Harbor research
To understand how the tenaculum developed, the researchers needed to examine fish of all ages. Friday Harbor turned out to be the ideal place for this study.
“There’s an abundance of ratfish that move through here, and of all sizes, itty bitty babies to really big adults,” Cohen said. “It’s a really rare opportunity that we have here.”
Researchers have speculated the islands nearby may create the perfect environment for nursery sites, she added. Through research funded by the Save Our Seas Foundation, Cohen has surveyed the region for eggs and is helping map nursery sites along the islands.
The PNAS study also illustrates how scientists need to study different types of organisms to paint a better, fuller, picture of evolution, she said.
“Most studies are on mice, zebrafish and flies,” Haridy said. “There is so much biodiversity that might actually tell us more about our own origins.”
— By Anna Marie Yanny
Also read in Salish Current:
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• “Friday Harbor scientists research eelgrass superpowers,” Aug. 5, 2025