A recent study published in Proceedings of the National Academy of Sciences has revealed an extraordinary feature in the male spotted ratfish, a shark-like species inhabiting the sea off the northeastern Pacific coast. Researchers discovered that this fish possesses true teeth on a forehead appendage used in mating, overturning decades of assumptions about the exclusive oral location of vertebrate teeth. This finding opens new perspectives on dental evolution and developmental biology.
The Tooth-Lined Forehead Of Male Spotted Ratfish
The male spotted ratfish develops a small forehead structure called the tenaculum, which emerges as a tiny white protrusion between the eyes. Over time, it grows into a hooked appendage lined with rows of teeth, which the fish uses during mating to grasp females by the pectoral fin and prevent separation.
“This insane, absolutely spectacular feature flips the long-standing assumption in evolutionary biology that teeth are strictly oral structures,” said Karly Cohen, a 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 structure is not only functional but also displays remarkable flexibility. When raised, the teeth on the tenaculum retract and flex more than typical canines, making them highly effective for underwater mating. Interestingly, only adult males develop this feature, while females retain evidence of the early-stage structure without mineralization. This sexual dimorphism points to a highly specialized evolutionary adaptation.
Evolutionary Context And Functional Significance
Spotted ratfish belong to the chimaeras, a group of cartilaginous fishes that diverged from sharks millions of years ago. Unlike sharks, which have extensive denticle coverage and many oral teeth, ratfish are relatively naked, except for small denticles on their pelvic claspers. The development of the tenaculum may reflect a rare evolutionary trajectory where tooth-forming cells migrated from the jaw to the forehead.
“Sharks don’t have arms, but they need to mate underwater,” Cohen explained. “So, a lot of them have developed grasping structures to connect themselves to a mate during reproduction.”
Fossil evidence suggests that this adaptation is not entirely new. Teeth on the tenaculum were observed in ancestral relatives of modern spotted ratfish, indicating a deep evolutionary history. The appendage provides a window into the diversity of vertebrate dental structures beyond traditional oral contexts, suggesting that similar hidden adaptations may exist in other species.
Genetic And Developmental Mechanisms
Researchers combined micro-CT scans, tissue sampling, and genetic analysis to confirm that the forehead teeth were true teeth rather than leftover denticles. A critical discovery was the presence of the dental lamina, a tissue responsible for generating teeth in vertebrates, outside the jaw.
“When we saw the dental lamina for the first time, our eyes popped,” Cohen said. “It was so exciting to see this crucial structure outside the jaw.”
Genetic markers associated with tooth development across vertebrates were active in the tenaculum but absent in surrounding denticles, providing strong evidence that these structures are genuine teeth. This remarkable co-option of preexisting tooth-forming pathways underscores the versatility of developmental biology in producing functional innovations for reproduction.
Morphological Oddities And Species Characteristics
Beyond the dental surprise, the ratfish’s overall facial morphology is unusual. “Ratfish have really weird faces,” Cohen noted. “When they are small, they kind of look like an elephant squished into a little yolk sack.”
The species grows to roughly two feet, with long tails accounting for half of the body length. The size and shape of the tenaculum are largely independent of body size but align with other reproductive structures such as pelvic claspers, suggesting coordinated developmental regulation.
This discovery highlights the diversity of adaptations in chimaeras and invites further exploration into other vertebrates. The study implies that vertebrate dentition is far more dynamic than previously thought, with the potential for teeth to appear in unexpected locations beyond the oral cavity.
Source link
