A Head of Glass
Deep in the twilight zone of Monterey Bay, between 600 and 800 meters (about 2,000 to 2,600 feet) below the surface, lives a fish with an anatomy that defies belief. The barreleye fish, Macropinna microstoma, measures only about 15 centimeters (6 inches) long and possesses a completely transparent, fluid-filled dome that covers the top of its head. Inside this clear shield are its eyes: two large, tubular, bright green orbs. These eyes are incredibly light-sensitive and are typically directed upwards, allowing the barreleye to spot the faint silhouettes of prey against the dim light filtering from the ocean surface.
The two dark spots on the front of the fish's face, where eyes would normally be, are actually its olfactory organs, equivalent to nostrils. For decades, this creature puzzled scientists. It was first described in 1939, but the transparent head was unknown because this fragile dome would always collapse when specimens were brought up from the depths in nets. It wasn't until 2004 that researchers at the Monterey Bay Aquarium Research Institute (MBARI) observed and photographed a live barreleye with its dome intact, using a remotely operated vehicle (ROV).
A subsequent MBARI study in 2009 solved another mystery. Scientists had believed the fish's eyes were fixed in an upward gaze, creating a severe case of tunnel vision. Live observation revealed the eyes can rotate. While the barreleye hovers, its eyes look up. When it moves in to feed, the eyes rotate forward, allowing it to see what it is eating. The green pigment in the lenses helps filter out surface sunlight, making it easier to spot the faint bioluminescence of prey like jellyfish.
A Life of Stealth and Theft
The barreleye's unusual body is perfectly adapted for its deep-sea habitat. Its large, flat fins allow it to remain almost completely motionless in the water, giving it extreme precision and stability while it waits for a meal. This stillness is an important part of its hunting strategy.
Its diet consists mainly of small zooplankton and crustaceans. The barreleye has developed a clever, if risky, feeding behavior: it steals food from siphonophores. Siphonophores are long, gelatinous animals that capture prey in their stinging tentacles. Researchers theorize that the barreleye carefully maneuvers among these dangerous tentacles to pluck out the trapped organisms. The transparent shield covering its head serves an important purpose here, protecting its sensitive eyes from the siphonophore's stinging cells.
Studying the barreleye remains a challenge. Encounters are exceptionally rare. As of 2022, after more than 5,600 deep-sea dives spanning over three decades, MBARI's ROVs have only come across this fish nine times. Each observation provides a small piece of the puzzle, slowly showing how this extraordinary fish survives in one of Earth's most demanding environments.