The spotted ratfish is a two-foot-long fish with a big head and a long, skinny tail that lives in the northeastern Pacific Ocean. It belongs to a group of fish called chimaeras that are closely related to sharks. (Chimaeras are sometimes called ghost sharks.) Like most vertebrate creatures, it has teeth in its mouth. Unlike other vertebrates it also has teeth in another location: its forehead. It uses these forehead teeth for sex.
Researchers have long wondered where the ratfish’s forehead teeth come from. New research, published on September 4 in the Proceedings of the National Academy of Sciences USA, has traced their origin.
The ratfish’s extra teeth line a cartilaginous appendage called a tenaculum that in males can be erected and deployed to grasp a female during mating. Many fish have clasping appendages near the pelvis that they use to hold their mate close during the deed. The spotted ratfish has the tenaculum in addition to pelvic claspers.
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Scientists have known about the ratfish’s toothy tenaculum for a while. But they weren’t sure exactly how the teeth originated or how they ended up in this weird location outside the mouth. Many of the ratfish’s close relatives—the sharks, rays and skates—have toothlike structures in their skin called denticles that are made of the same material as teeth but are not true teeth. Are the ratfish’s forehead teeth just modified skin denticles, or do they share their structure and origin with mouth teeth?
To find out, Karly Cohen of the University of Washington and her colleagues traced the development of the ratfish’s tenaculum using micro-CT scans and tissue samples, and they compared the modern fish with fossil ancestors. The researchers determined that the ratfish’s forehead teeth are true teeth—they develop from a structure called the dental lamina that is present in the jaw but not in skin denticles. The dental lamina has never been found outside of the mouth until now.
“This insane, absolutely spectacular feature flips the long-standing assumption in evolutionary biology that teeth are strictly oral structures,” Cohen said in a press release. Who knows where they’ll turn up next?