This post was originally published on here
Evolution has created some pretty crazy things, and the head of this fish is definitely one of them.
The rockhead poacher (Bothragonus swanii) is small, armored, and shaped remarkably like a stone. Sitting atop its head is a deep, smooth bowl. It looks as if someone scooped out a piece of bone and simply walked away. For more than a century, biologists have stared at that hollow and wondered why it exists.
Now, a new study suggests that the fish may be using its head as a kind of drum.
“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,” said Daniel Geldof, a researcher at Louisiana State University, as per Discover Wildlife. “This sort of feature is more-or-less unheard of among fishes—or any other vertebrate.”
The Bucket Brigade
The species was first described in 1876. It lives in shallow, rocky intertidal zones from Alaska to California, rarely deeper than about 18 meters (60 feet). It hides well, which helps explain why its strange anatomy remained unresolved for so long.
For years, scientists proposed several explanations for its unusual head. The pit might help the fish blend in, improve hearing, or detect movement in the water. Until recently, none of these ideas could be tested.
Finding them is difficult work. Geldof and his colleagues had to scour places like Deadman Bay in Washington, forming a “bucket brigade” to manually drain tidepools at low tide just to find a specimen.
Once they had the fish, they didn’t dissect the fish. Instead of cutting them open, Geldof turned to high-resolution micro-CT scanning. This allowed him to build detailed 3D models of preserved specimens, revealing the orientation of bones, muscles, and nerves without damaging the delicate structures.
Built-in Percussion
The scans revealed the smoking gun. The rockhead poacher’s first set of ribs were enormous compared to the rest of its body. They were flattened, mobile, and positioned right next to the cranial pit—but importantly, they weren’t attached to it. Strong muscles and tendons anchored the ribs at their bases.
When you put the anatomy together, it looks less like armor and more like an instrument.
“When you zoom in, it becomes clear that there are lots of tiny structures within the fish’s bizarre head-hole,” Geldof added. “There are visually obvious rods… and extremely tiny spines of bone, all pointing inwards.”
“Essentially, this fish seems to have built a percussion instrument into its head,” Geldof said.
The idea is that when the fish vibrates those ribs, they can strike the hardened bowl of bone, generating a low buzz or pulse sound. Geldof has felt it himself. “If you pick up a poacher underwater, it’ll generally get annoyed and start ‘talking’,” he said. “It feels like you’ve grabbed a cell phone on vibrate mode.”
Fish Language
Why would a fish need a drum in its head? The answer likely lies in its environment.
Shallow coastal waters are incredibly loud. Waves crash, rocks tumble, and sound ricochets unpredictably off the seabed. In this acoustic chaos, a tiny fish trying to broadcast a vocal call through the water is like trying to whisper in a nightclub.
However, the rockhead poacher might be bypassing the water entirely. Sound travels faster and more efficiently through solids. By tapping its head against the seafloor, the fish could be sending vibrations through the ground. In a rocky habitat, a seismic signal could carry much farther than a shout in the surf.
The fish swims close to the bottom, making this strategy plausible. Many members of its family, the Agonidae, also grunt, buzz, or growl. The cranial pit may make the rockhead poacher especially effective at it.
The scans also hinted that the pit is a multitasker. A branch of the lateral line nerve—the system fish use to sense pressure and motion—runs directly into the cavity. The tiny inward-pointing spines likely move as water flows through the bowl, helping the fish detect nearby predators or prey.
So far, no one has recorded a rockhead poacher drumming in the wild. That will require underwater microphones and patient fieldwork. Geldof has even floated the idea of temporarily blocking the pit, to see how the fish behaves without it.
Understanding how these animals communicate in such noisy, turbulent environments could help engineers design better underwater communication systems for us, too.
