A fish with a completely transparent head filled with fluid, allowing its tubular eyes to rotate inside its skull to look upward through its own forehead. It hovers motionless in the deep sea, watching for prey silhouettes against the faint surface light.
Unknown authorUnknown author, Public domain, via Wikimedia Commons
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.
💡Fun Facts
The two dark spots on the barreleye's face that resemble eyes are actually its organs of smell (nares).
The fish's transparent head shield is so delicate that it was unknown to science until 2004, as it would always be destroyed when specimens were brought to the surface.
Large, flat fins enable the barreleye to hover with such stability that it can remain almost perfectly motionless in the water.
The green pigment in its eyes is a biological filter, thought to help it distinguish the bioluminescent glow of prey from the faint sunlight above.
The barreleye fish lives in the wild at inaccessible depths and is not on public display. It can only be seen through footage from research expeditions.
Admission
Not applicable for wild viewing. The Monterey Bay Aquarium, MBARI's partner, has an "Into the Deep" exhibit showcasing deep-sea life.
Accessibility
The location is the deep Monterey Submarine Canyon. This environment is only accessible via specialized deep-sea research submersibles.
