The rockhead poacher’s mysterious head pit may be an instrument rising above a noisy habitat
The Rockhead Poacher: A Fish That Might Play Drums
The rockhead poacher (Bothragonus swanii) is one of the strangest fish in the northeastern Pacific. Covered in heavy bony armor, it hides among rocks in the shallow intertidal zone. But the fish’s most unusual feature isn’t its armor—it’s the deep, bowl-shaped cavity in its skull, roughly the size of its brain.
New research suggests the fish may use this cranial pit as a percussive instrument, with its first ribs acting like drumsticks.
A Tiny Fish with Big Surprises
- The fish is tiny—about the size of an index finger—but its skull cavity could hide a pea.
- Many of its relatives make grunting or buzzing sounds, though humans usually feel these vibrations more than hear them.
Investigating the Skull
Daniel Geldof, a recent LSU master’s graduate, studied preserved specimens using a micro-CT scanner capable of sub-micron resolution. His 3D models revealed:
- The first set of ribs is unusually large, flattened, and free-moving, attached to strong muscles rather than directly to the spine.
- These ribs can strike the hard bottom of the cranial pit, creating vibrations through the substrate.
Why Drum?
Rockhead poachers move along the sea floor using their pectoral fins rather than swimming, so communicating through ground-borne vibrations avoids the noisy open water of tidal pools. The cranial pit may amplify these signals, allowing the fish to warn or locate others.
The Next Steps
No one has yet observed a rockhead poacher pounding its own skull. Experiments using underwater microphones are needed to confirm the theory and study its behavior further.
As Adam Summers, a UW biomechanist, says, it’s a “stunning revelation”—a tiny fish with a built-in percussion instrument hidden in plain sight.