This Fish Really Does Need a Hole in Its Head

Explore

IIn their watery ocean worlds, fish have strange characteristics that are used to do strange things, something we rarely witness. By “studying how strange fish do strange things,” biologist Daniel Geldof, then a master’s student at Louisiana State University, decided to investigate the rock-headed poacher (Bothragonus swanii), known for the large cavity of its skull. In his master’s thesis, Geldof described the unique anatomy of this cranial fossa that could explain its function.

Rockhead poachers are found along the Pacific coast from Alaska to central California, often seen in intertidal tide pools when tides are low. Early naturalists thought the head pit might improve camouflage in rock pools, where their gray, bony armor blends in well. Or the head cavities could be part of a system to produce their characteristic buzzing sounds. Geldof explained in a statement that “the goal of my entire thesis project was to understand why” rockhead poachers have unusual head anatomy.

Read more: “The unexpected music of a coral reef”

Using a 5,000-pound Heliscan MKII X-ray microscope at LSU, Geldof created 3D models of rockhead poacher heads from previously collected fish specimens. In the incredibly detailed models, he traced the nerves extending from the fish’s brain and discovered that a branch of the lateral line nerve, involved in detecting movement, enters the cranial fossa. And so, it probably plays a role in sensing water movement relative to the fish.

But Geldof also noticed that the fish’s first set of ribs were larger and flatter than normal, positioned close to the fossa and docked into muscles and tendons. He hypothesizes that poachers use their modified ribs as pestles to strike the base of the cranial fossa in rapid succession to create buzzing sounds. The drums likely propagate signals through the ground, where the fish typically rest, to communicate with other rockhead poachers in the area.

“The ocean, especially in the shallow, rocky areas where the rock-headed poacher lives, is incredibly noisy and acoustically complex,” Geldof said. “This fish uses its small body efficiently so that it can still be heard in unique conditions.”

Essentially, the “truth” is that the mystery pit appears to be multifunctional, serving both sensory reception and substrate-based communication.

Enjoy Nautilus? Subscribe for free to our newsletter.

Main image: Rhinopias / Wikimedia Commons

• Devin Reese

  Published on January 20, 2026

  Devin Reese is the editor-in-chief of Natural history and a science writer based in Alexandria, Virginia.