Researchers Capture Footage of a Rare Deep-Sea Fish with Transparent Head

Earlier this month, the Monterey Bay Aquarium Research Institute (MBARI) shared footage of the barreleye fish, a deep sea creature that has a transparent forehead within which one can see its glowing green eyes.

Macropinna microstoma is a deep-sea ray-finned fish in the barreleye family Opisthoproctidae.

The species was discovered and described in 1939 by the U.S. marine biologist Wilbert McLeod Chapman.

Macropinna microstoma measures around 15 cm (6 inches) in length. It has a tiny mouth, most of its body is covered with large scales, and its eyes are capped with bright green lenses.

It also has a highly unusual transparent, fluid-filled shield on its head. This protects its sensitive eyes from the nematocysts (stinging cells) of the siphonophores, one of the apparent sources of its food.

The barreleye fish’s habitat ranges from the Bering Sea to Japan and Baja California. The fish live in the ocean twilight zone, which lies about 650 to 3,300 feet (200 to 1,000 m) underwater; specifically, barreleyes live about 2,000 to 2,600 feet (600 to 800 m) beneath the ocean surface, near the depth where the water plunges into complete darkness, according to MBARI.

“Its eyes look upwards to spot its favorite prey—usually small crustaceans trapped in the tentacles of siphonophores—from the shadows they cast in the faint shimmer of sunlight from above,” MBARI said in a statement, according to Newsweek.

“But how does this fish eat when its eyes point upward and its mouth points forward?” they added. “MBARI researchers learned the barreleye can rotate its eyes beneath that dome of transparent tissue.”

Macropinna microstoma normally hangs nearly motionless in the water using its large fins for stability.

Dr. Tommy Knowles and his team from the Monterey Bay Aquarium were aboard MBARI’s R/V Rachel Carson with ROV Ventana to collect jellies and comb jellies for the Aquarium’s upcoming Into the Deep exhibition when they spotted this fascinating fish.

“We stopped to marvel at Macropinna microstoma before it swam away,” the researchers said.