Male ghost sharks, also known as chimaeras, are eerie deep-sea fish that share a fascinating evolutionary link to sharks and rays. Recent research published in the prestigious journal Proceedings of the National Academy of Sciences has shed light on an intriguing feature of these creatures: a strange rod protruding from their foreheads, adorned with sharp, retractable teeth. This study reveals that these are not mere lookalikes but are, in fact, real rows of teeth that grow externally from the mouth, serving a crucial role in the mating process.
The forehead rod, scientifically referred to as the tenaculum, is a unique characteristic found exclusively in male ghost sharks. This structure serves as the sole source of distinct teeth, which are likely employed to grasp females during mating, mirroring the toothy mouths used by sharks for the same purpose. Gareth Fraser, Ph.D., a biology professor at the University of Florida and the senior author of the study, highlights this evolutionary adaptation: "If chimaeras can make a set of teeth outside the mouth, where else might we find teeth?"
The research team, which included experts from the University of Washington and the University of Chicago, meticulously examined both fossils and living ghost sharks to unravel this mystery. One significant discovery came from a 315-million-year-old fossil showcasing the tenaculum attached to the upper jaw, bearing teeth that closely resembled those found in the mouth. Additionally, modern chimaeras collected from Puget Sound exhibited a similar tooth-growing process on their heads, akin to the teeth found in contemporary shark jaws.
Genetic testing further revealed that the tenaculum teeth express the same tooth-specific genes as oral teeth, confirming their biological significance. Fraser elaborates, "What we found is that the teeth on this strange appendage look very much like rows of shark teeth. The ability to make teeth transferred onto that appendage, likely from the mouth." Over time, the tenaculum has shortened, yet it retains the capability to produce oral teeth on this distinctive forehead appendage.
Fraser collaborated with Karly Cohen, Ph.D., from the University of Washington, and Michael Coates, Ph.D., from the University of Chicago, both of whom have expertise in shark evolution and anatomy. They were captivated by the tooth-filled rods emerging from ghost shark foreheads. The central question they sought to answer was whether the tenaculum's teeth are true teeth related to oral teeth or more akin to the tooth-like scales found on the skin of sharks and some ghost sharks.
CT scans of the fossils and modern chimaeras provided the scientists with unprecedented insights into the development of tenaculum teeth, revealing remarkable similarities to the teeth of today's sharks. The definitive evidence came from genetic analysis, which demonstrated that the tenaculum teeth express genes exclusive to true teeth, unlike the denticles found on shark skin. Coates remarked, "What I think is very neat about this project is that it provides a beautiful example of evolutionary tinkering or 'bricolage.'" This research combines experimental data with paleontological evidence, illustrating how these fishes adapted a preexisting program for manufacturing teeth into a new reproductive device.
Cohen, the first author of the paper and a postdoctoral researcher at the University of Washington's Friday Harbor Labs, emphasized the significance of this finding: "Scientists had never spotted teeth outside the mouth in this way before." The tenaculum represents a developmental relic, marking the first clear example of a toothed structure outside the jaw. This peculiar evolution from oral teeth to forehead teeth utilized for mating underscores the remarkable adaptability of evolution, always ready to repurpose structures for novel and unexpected functions.
As researchers continue to explore the mysteries of the deep sea, Fraser notes, "There are still plenty of surprises down in the ocean depths that we have yet to uncover." The study of male ghost sharks and their unique tenaculum not only enriches our understanding of these enigmatic creatures but also highlights the endless possibilities of evolution in the animal kingdom.
