Male ghost sharks, deep-sea fish called chimaeras, have evolved a bizarre method for romance: they grow sharp teeth on rods jutting from their foreheads, which they use during mating.
New research published in the Proceedings of the National Academy of Sciences reveals these rods, known as tenacula, are true rows of teeth formed outside the mouth, the first such example recorded in vertebrates.
These tenacula are exclusive to males and function much like the mouths of sharks, gripping females during copulation. Scientists led by Dr. Gareth Fraser at the University of Florida, working with colleagues from the University of Washington and the University of Chicago, traced the origins of the tenaculum through both fossil and genetic analysis.
A 315-million-year-old fossil showed the tenaculum bearing teeth similar to those in the jaw. In living ghost sharks from Puget Sound, genetic tests confirmed that the forehead teeth express the same genes as oral teeth, ruling out resemblance to shark skin denticles.
CT scans of fossils and modern fish revealed stunning details: the tenaculum teeth are arranged in neat rows much like shark jaws, showing evolutionary “tinkering” where old genetic programs are repurposed for new uses.
“We have experimental and fossil evidence showing these fishes co-opted a preexisting program for manufacturing teeth to create a new device essential for reproduction,” said Dr. Michael Coates from the University of Chicago.
First author Dr. Karly Cohen noted, “The tenaculum is a developmental relic, not a bizarre one-off, and the first clear example of a toothed structure outside the jaw.”
This groundbreaking discovery demonstrates the impressive flexibility of evolution, revealing how nature repurposes structures for unexpected roles.
Dr. Fraser summed up the excitement: “There are still plenty of surprises down in the ocean depths that we have yet to uncover.”
The findings open new questions about the dynamism of tooth development and hint that similar evolutionary oddities could exist in other species.
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