California Scientists Solve Puzzles About the Oceanic Twilight Zone’s Barreleye Fish

Marine biologists are slowly solving mysteries about the barreleye fish. The creature has a fluid-filled, clear head. The see-through head means its brain and green eyes are on full display. The barreleye fish is rarely observed. It lives in the ocean’s twilight zone. That's the layer of water just beyond sunlight’s reach. It’s 650 to 3,300 feet below the surface.  

Last December, the Monterey Bay Aquarium Research Institute (MBARI) in California got footage of a barreleye. It used special cameras on a remotely operated vehicle (ROV). The institute tweeted, “MBARI’s ROVs … have logged more than 5,600 dives and recorded more than 27,600 hours of video—yet we’ve only encountered this fish nine times!”

Scientists at MBARI solved one mystery about the fish in 2008. The barreleye is named for its tube-shaped, glowing eyes. They can see prey in the murky depths. But scientists didn't know how the fish actually sees to eat. The fish’s eyes appeared to only point upwards.  

“It always puzzled me that … the field of view did not include their mouths,” a scientist at MBARI told The Guardian.

The scientists were able to net-catch and watch a live barreleye. They confirmed something they suspected based on ROV footage. The fish's eyes rotate. That means the fish can track prey as it floats down through the water. Then, it can rotate its eyes forward and take a bite.  

On its website, MBARI states the barreleye’s adaptations “have puzzled oceanographers for generations.” It states the latest observations are only possible thanks to new technology. 

Photo by Kim Reisenbichler courtesy of Wikimedia Commons.