The rockhead poacher is a weird-looking fish. It has a large, bowl-shaped dip in the middle of its head. Scientists have long wondered why and a now they’ve come up with an interesting theory. Researchers at Louisiana State University (LSU) have suggested that the fish uses the bowl like a percussion instrument to create buzzing sounds for communication.
Rockhead poachers are notable for their strange head, which has a hole in it. “It looks like the fish has a large chunk of its skull missing – as though somebody took a little ice cream scoop and took a piece of it away,” says Daniel Geldof, a researcher at LSU. “This sort of feature is more-or-less unheard of among fishes – or any other vertebrate.”
Unsurprisingly, this unique feature has attracted scientists’ attention. “I am not the first ichthyologist [fish biologist] to notice that the rockhead is weird, or to wonder why it has a bizarre hole in its head,” says Geldof.
Theories around the purpose of this curious cup include being useful for camouflage, sensing the world around it and making noise.
To get to the bottom of the mystery, researchers had to find specimens to study, which isn’t an easy task. These fish live in shallow, rocky intertidal zones from California to Alaska and are very good at hiding.
They can sometimes be found at a place called Deadman Bay in Washington at low tide. “We’ll show up with a team of 5–25 people and walk down to a pair of very large permanent tidepools. Once the tide is too low to refill the pools, we form a bucket brigade and empty them, leaving a puddle full of confused fishes at the bottom,” Geldof explains. “When we’re lucky, a few of these fishes are rockheads.”
Once he’d found the fish, Geldof used a microCT scanner to create a detailed 3D model of its bowl-shaped head without having to dissect the preserved animal.
The model was so detailed that it showed individual nerves. “When you zoom in, it becomes clear that there are lots of tiny structures within the fish’s bizarre head-hole,” he says. “There are visually obvious rods (a few millimetres long) on the rear face of the pit, and extremely tiny spines of bone, all pointing inwards.”
These new images gave Geldof a better idea of how the rockhead poacher might use its pit to sense motion and create sound. He believes that the rockhead uses the tiny spines inside its head as water motion sensors because they would move as water flows through the pit.
He was also excited to discover more about how rockhead poachers make sound. “If you pick up a poacher underwater, it’ll generally get annoyed and start ‘talking’,” he says. “It feels like you’ve grabbed a cell phone on vibrate mode.”
The scans showed that the fish’s first set of ribs were very large. “They became huge and butted right up against the hard, bony dish at the base of the cranial pit,” he says. Powerful muscles allow these fish to use these ribs like drumsticks: vibrating them against the dip in the skull. “Essentially, this fish seems to have built a percussion instrument into its head.”
This is useful because it’s difficult for the rockhead to be heard among the crashing waves in its tidal habitat. “The rockhead is… an extremely small fish in a very, very noisy environment,” he says. “It is extremely difficult to localise sound underwater. The shallower you go, the noisier it gets.”
Geldof has many more questions he wants to answer, including what would happen if the fish’s bowl was not filled with water (something he suggests doing by putting a harmless hat on live individuals).
What they learn could help engineers understand how to produce and transmit sound more effectively underwater.
And further studies could have surprising results, he says, as this “is how spectacular, unexpected discoveries begin.”
Top image credit: Alex Bairstow
More amazing wildlife stories from around the world