Rockhead Poacher’s Mysterious Head Pit May Be Instrument Rising Above Noisy Habitat
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For the rockhead poacher, the noises are all in his head.
The fish is a modest and unassuming inhabitant of near-shore shallows, but it has a conspicuous divot at the top of its skull that appears to function like a drum. New research suggests that the movable, flattened ribs may rap against the underside of the fossa like drumsticks, perhaps so the fish can communicate with other members of its species.
“No fish has anything like it,” says functional morphologist Daniel Geldof, who defended his work in December for his master’s thesis at Louisiana State University in Baton Rouge.
Rockhead Poachers (Bothragonus swanii) are armored, teardrop-shaped fish found from Alaska to California, where they spend much of their time in shallow water, perched on the sea floor and camouflaged to look like rocks or sponges. Scientists had long noticed a deep pit – about as big as the fish’s brain – dug into the top of its head. But its function remained mysterious. Did it create sound or collect it like a satellite dish? Or has it been used in other senses?
To find out, Geldof and his colleagues scanned a preserved specimen with X-rays. Compiling thousands of individual images gave the team a detailed 3D model of the poacher’s strange head and everything inside it.
The underlying rib bones at the bottom of the head hole are unusually dense, large and flattened, says Geldof. They are also very mobile and attached to powerful muscles. Geldof believes these ribs are adapted to strike the bottom of the pit, creating noise.
“This fish basically has a little drum kit or maraca in its head,” he says. “I have dealt with many other annoyed poachers [species]and you can feel them vocalizing. It’s very similar to if you had a cell phone in your hand that is on vibrate mode.
The phenomenon of banging or scratching things together to produce noise is called stridulation. While other fish are known to stridulate, the rock-headed poacher “seems to be a rather extreme example,” Geldof says.
It’s possible that all this drumming and buzzing is an adaptation for surprising predators. But Geldof thinks it’s more likely to call and court other poachers in a harsh acoustic environment. The wave-battered intertidal shallows that poachers inhabit are turbulent and noisy. Rockhead poachers may be sending their buzzing vibrations into the rocks they rest on.
“They have to work around all these crazy challenges if they want to hear and be heard in this din,” says Geldof.
Audrey Looby, a fish ecologist at the University of Victoria in British Columbia who was not involved in the research, notes that there is growing evidence that fish may use sounds transmitted by the surfaces they touch. For example, marbled sculpins (Cottus bairdi) bang their heads against rocks and gravel to send vibrations through the substrate. “Just as we would study the sounds of birds to better understand their communication,” she says, we can do the same to understand the communication of fish.
Ecomorphologist Eric Parmentier of the University of Liège in Belgium isn’t convinced the fish were stridulating. The pit can amplify the sound, he says, but the ribs may not touch the underside of the pit to create that sound. The sounds made by bone hitting bone would mostly be at a much higher frequency than the 20 or so Hertz that Geldof and his colleagues predict – above 1,000 Hertz, he says.
“That would not match the types of sounds suggested in the report,” he says.
So far, the proposed drum mechanism has not been seen in action and the fish has not been recorded underwater making its sounds. Laboratory experiments and observations would help confirm how this percussion pit may work, Geldof says, and why such a strange quirk evolved in the first place.
