Life in the ocean’s dark depths can be lonely, especially for a male chimaera. So when one of the creatures finds a mate, he needs to hold her close.
To do this, a male chimaera, also known as a ratfish or a ghost shark, uses a fleshy, clublike appendage sticking out of his forehead. It’s called a tenaculum, and it grips the female’s pectoral fin during mating.
If that sounds strange, it gets stranger: The bulbous end of this forehead grip is studded with structures that amount to teeth. How chimaeras evolved such a strange feature has long puzzled biologists. “We haven’t seen anything like this anywhere else in the animal kingdom, period,” said Gareth Fraser, an evolutionary biologist at the University of Florida.
In a paper published on Thursday in the journal Proceedings of the National Academy of Sciences, Dr. Fraser and his colleagues reveal that the structure is covered in rows of teeth similar to the jaws of a shark. That finding offers insights into how the animals ended up with their frightening forehead teeth.
Chimaeras are cartilaginous fish like sharks, but the two are only distantly related. The two groups diverged nearly 400 million years ago, and chimaeras largely lack other sharklike traits, including scales and sharp teeth. Instead, they have tooth plates used to crack shells and grind up prey.
Another distinguishing feature is the tenaculum, which houses those forehead teeth and is also called a head clasper. While the clasper looks somewhat similar to the lure of an anglerfish, another deep-sea organism with an odd sex life, it is not an extension of a dorsal fin as it is in that fish.
The clasper sits inside a pocket above the fish’s eyes when not in use. Beyond its basic function, little was known about how the structure developed or whether it was covered by teeth or skin features.
Dr. Fraser specializes in studying bizarre structures in sharks and other fishes and has long been interested in chimaeras. But the deep-sea fish are difficult to study.
However, during a recent stint at Friday Harbor Laboratories at the University of Washington, Dr. Fraser had a chance to observe chimaeras in the wild. His team collected spotted ratfish, a chimaera with large green eyes and a venomous spine, from an area in the San Juan Channel.
The team examined 40 ratfish specimens. Some were just under 10 inches, while others were two and a half feet long. The researchers used micro-CT scans to track how the shape of the tenaculum grew over time from a pimplelike projection in juveniles to the fully formed club on adult males.
The budding toothlike structures proved to be actual teeth with mineralized tips. Molecular tests identified tooth-forming genes in the tenaculum that are usually found in the mouths of fish and other vertebrates.
“I think it’s utterly amazing that ghost sharks have teeth growing out of their forehead,” said Dominique Didier, an ichthyologist at Millersville University in Pennsylvania who studies chimaeras and related fishes but was not involved in the new paper.
The structure of the teeth reminded Dr. Fraser of a shark jaw. “The rows of teeth are all organized in a very similar way in this conveyor belt of teeth that we see in sharks,” he said.
His team also examined fossils of prehistoric chimaeras and their relatives, including Helodus simplex, which lived 315 million years ago.
Helodus possessed the oldest known example of a tenaculum-like structure, which stretched from the top of the fish’s snout to the front of its upper jaw. The whorl of teeth covering Helodus’s tenaculum was indistinguishable from the teeth in its mouth.
Dr. Fraser posits that even as the tenaculum moved farther away from the fish’s jaws over the eons, chimaeras later retained their ability to form sharklike teeth, helping them dole out love bites to prospective mates.
However, Dr. Didier thinks it’s possible that the tenaculum originally evolved for other purposes. She notes that the females of one ghost shark lineage retain a remnant of a tenaculum, suggesting that the structure was once found in both sexes. This could be a hint that a club of forehead teeth originated as a defensive weapon or a warning signal.
Dr. Fraser believes that is plausible. During a night dive at Friday Harbor, he was approached by a male chimaera flexing its tenaculum. He still is not sure if the fish was warning him to back off or planning to show him affection.
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