It’s a common belief that teeth develop exclusively within the mouths of animals. However, recent research has uncovered a fascinating exception to this rule. The spotted ratfish (*Hydrolagus colliei*), often referred to as 'ghost sharks', has been found to grow its teeth in an unexpected location. Adult male spotted ratfish possess unique hooked protrusions located on a head structure known as the tenaculum. This structure serves a critical role during mating, enabling the fish to grasp their partners. The findings challenge long-held assumptions in evolutionary biology regarding the origins of teeth.
According to Karly Cohen, a researcher from the University of Washington’s Friday Harbor Labs, this discovery is “insane” and “absolutely spectacular.” Cohen emphasizes that the tenaculum is not just a bizarre anomaly but represents the first clear example of a toothed structure existing outside of the jaw. Published in the Proceedings of the National Academy of Sciences, the study indicates that these protrusions are indeed true teeth, developed from the same tissue that forms oral teeth.
This revelation suggests that tooth-forming cells may have migrated outside the mouth early in evolutionary history, offering new insights into how dental structures have evolved over time. In addition to this groundbreaking discovery, researchers are also investigating other species to learn more about tooth development across the animal kingdom.
The spotted ratfish belongs to an ancient group of cartilaginous fish known as chimaeras, which diverged from sharks millions of years ago. These fish typically measure about two feet in length and are distinguished by their long, slender tails, which account for nearly half of their body length. Only adult males develop the tenaculum, which takes on a distinctive white, peanut-shaped form between their eyes and becomes hooked and barbed during courtship rituals.
Cohen notes that while sharks lack arms, they require a means to mate underwater. As a result, many species have evolved specialized structures for grasping their mates during reproduction, exemplified by the tenaculum in the spotted ratfish.
To explore the origins of the tenaculum, researchers collected and analyzed hundreds of ratfish from the waters surrounding San Juan Island in Puget Sound, a significant ocean inlet off the coast of Washington, USA. Utilizing micro-CT scans and tissue samples, they tracked the development of this unique structure and compared their findings to fossils of related species.
The scans revealed a surprising fact: both male and female ratfish begin to grow a tenaculum early in life; however, only males fully develop this structure. Cohen expressed her excitement upon discovering the dental lamina—a crucial tissue layer responsible for tooth production—present in the tenaculum. This finding confirmed that the protrusions are indeed true teeth, rather than the hardened skin denticles commonly found in many sharks and rays.
Michael Coates, a professor at the University of Chicago and co-author of the study, shared insights on how these fish have adapted preexisting biological processes for tooth production to create a new reproductive tool. Senior author Gareth Fraser, a professor at the University of Florida, highlighted the significance of this discovery in understanding the diversity of dental structures throughout evolutionary history.
Cohen adds that the study of chimaeras may provide a rare glimpse into evolutionary pasts. She suggests that as researchers continue to examine spiky structures on vertebrates, they may uncover even more instances of teeth located outside the jaw.
The discovery of the spotted ratfish's unique tooth development not only expands our understanding of fish biology but also reshapes our perspective on the evolution of dental structures across the animal kingdom.
Top image: spotted ratfish. Credit: Seattle Aquarium
