What the Barreleye Fish Is and Where It Lives
The barreleye fish is a small, deep-sea creature with a completely transparent head and tubular eyes that point upward inside its skull. Its scientific name is Macropinna microstoma, and it lives in the oxygen minimum zones of the ocean between roughly 2,000 and 3,000 feet deep. You will never fish for one intentionally — the barreleye is too small, too deep, and too fragile to survive capture and handling.
The fish measures only about 6 inches long and weighs almost nothing. Its body is dark and unremarkable, but its head is the reason it has fascinated marine biologists since its discovery in 1939. The transparent skull allows the fish to see upward through its own head, hunting for the silhouettes of prey against the faint light filtering down from above. This adaptation is so specialized that the barreleye has almost no use for side vision.
Barreleyes live in waters off the coasts of California, Hawaii, and Japan, as well as in other deep ocean basins. They are not migratory in the way that many fish are — they stay in their depth zone year-round, moving only vertically to follow the daily migration of smaller organisms they hunt.
Key Takeaways
- The barreleye fish lives too deep (2,000–3,000 feet) and is too fragile to be caught by any recreational or commercial fishing method.
- Its transparent head and upward-pointing eyes are adaptations for hunting in the twilight zone of the ocean where almost no light reaches.
- The barreleye is so specialized for deep-sea life that it cannot survive in surface waters or in captivity.
- Most of what we know about barreleyes comes from deep-sea research submersibles and remotely operated vehicles, not from fishing or collection.
Why the Barreleye's Body Design Is So Unusual
The barreleye's transparent head serves one purpose: to let it see prey above without moving its body. In the deep sea, energy is scarce, and every movement costs calories. By keeping its body still and rotating only its eyes upward inside its transparent skull, the barreleye hunts efficiently in near-total darkness. The tubular shape of its eyes gives it extreme magnification in the upward direction, making even tiny silhouettes visible.
The rest of the barreleye's body is built for minimal energy use. It has a small mouth, a thin body, and weak muscles. It does not chase prey across open water — instead, it hangs nearly motionless and waits for small crustaceans and fish larvae to drift into range. This hunting style works only in the deep sea, where currents move slowly and prey is sparse but predictable.
The transparency of the barreleye's head is not perfect. The eyes themselves are opaque and dark green, which helps them absorb light without reflecting it back out. The rest of the skull is made of a clear, gelatinous material that lets light through while still protecting the brain and eyes underneath.
The Oxygen Minimum Zone: The Barreleye's Habitat
Barreleyes live in what oceanographers call the oxygen minimum zone, a layer of ocean where oxygen levels drop so low that most fish cannot survive. This zone sits between roughly 1,500 and 3,500 feet deep, depending on the location and season. In these waters, oxygen is scarce because the zone is too deep for photosynthesis to produce oxygen and too shallow for water circulation to bring oxygen down from above.
Most fish avoid this zone entirely. The barreleye, however, has evolved to thrive there. Its slow metabolism and minimal activity level mean it needs very little oxygen. By living where few competitors and predators can go, the barreleye has found a refuge. It shares this zone with other specialized deep-sea fish like hatchetfish, viperfish, and lanternfish, each with its own strange adaptations.
The oxygen minimum zone is also where the barreleye finds most of its food. Small organisms migrate into and out of this zone on a daily cycle, following the faint light and the availability of nutrients. The barreleye positions itself to intercept these migrants, hunting at the edges of the zone where prey is most abundant.
How Deep-Sea Research Revealed the Barreleye
The barreleye was first described from a single specimen caught in a deep-sea trawl net in 1939, but the specimen was so damaged that scientists could not see its most distinctive feature — the transparent head. For decades, the barreleye remained a mystery, known only from a few preserved specimens that looked nothing like the living fish.
In 2004, researchers using a remotely operated vehicle (ROV) filmed a living barreleye for the first time. The footage showed the transparent head, the tubular eyes, and the fish's actual behavior in its natural habitat. This discovery changed how scientists understood deep-sea vision and hunting. Since then, ROVs have recorded barreleyes multiple times, each observation adding details about how the fish moves, feeds, and interacts with its environment.
All of our knowledge about living barreleyes comes from these deep-sea research vehicles. No barreleye has ever been kept alive in captivity, and none ever will be — the pressure and temperature changes required to bring one to the surface would kill it instantly. The fish is known only through video, photographs, and the rare specimen preserved in museum collections.
Why You Cannot Fish for Barreleyes
The barreleye is protected by its habitat, not by law. It lives so deep that no fishing gear can reach it without destroying it. Commercial deep-sea trawlers do occasionally catch barreleyes as bycatch, but the fish are so fragile that they arrive at the surface already dead, crushed by the pressure change and the weight of the net.
Even if you wanted to target barreleyes, the depth and the specialized equipment required make it impossible for recreational fishing. The barreleye is not a food fish — it has almost no meat and no commercial value. It exists in the deep sea for the same reason many other strange creatures do: because that is where the conditions favor its particular adaptations, and where human fishing pressure cannot reach.
The barreleye's rarity in human experience is part of what makes it fascinating. For most of human history, we did not even know what a living barreleye looked like. Today, we know it only through the lens of deep-sea research technology. It remains one of the ocean's most alien-looking fish, a creature perfectly suited to a world of darkness and pressure that no recreational angler will ever visit.
Other Deep-Sea Fish With Unusual Adaptations
The barreleye is not alone in the deep sea. Other fish in the same habitat have evolved equally strange features. Viperfish have teeth so large they cannot close their mouths. Hatchetfish have bodies flattened to paper-thin profiles and eyes that point upward like the barreleye's. Lanternfish produce their own light through bioluminescent organs along their sides.
Each of these adaptations solves a specific problem posed by deep-sea life: how to hunt in darkness, how to avoid being seen by predators, how to find mates, how to conserve energy. The barreleye's transparent head is just one solution among many. Together, these fish show how evolution shapes creatures to fit extreme environments that seem impossible to us.
If you are interested in deep-sea fish, the barreleye is a good starting point for understanding how different the ocean becomes below the reach of sunlight. The fish's adaptations are so extreme that they seem almost fictional, yet they are perfectly real and perfectly suited to the world the barreleye inhabits.
Frequently Asked Questions
Can you see a barreleye fish in an aquarium?
No. Barreleyes cannot survive in captivity. The pressure and temperature changes required to bring one to the surface kill it. No aquarium in the world has ever kept a living barreleye, and none ever will. The only way to see a living barreleye is through video footage recorded by deep-sea research vehicles.
How deep do barreleyes live?
Barreleyes live between roughly 2,000 and 3,000 feet deep, in the oxygen minimum zone where few other fish can survive. They stay in this depth range year-round and do not migrate to shallower or deeper waters.
What do barreleye fish eat?
Barreleyes hunt small crustaceans and fish larvae that drift through the oxygen minimum zone. They hang nearly motionless and wait for prey to come within range, then strike upward. Their transparent head and upward-pointing eyes let them see prey silhouetted against the faint light above.
Is the barreleye fish endangered?
The barreleye is not listed as endangered because so little is known about its population that scientists cannot assess whether it is declining. It is too deep and too small to be fished commercially, so human fishing pressure does not threaten it directly.
Where was the barreleye fish first discovered?
The barreleye was first described from a specimen caught in a deep-sea trawl net in 1939, but the specimen was damaged and scientists did not understand its distinctive features. The first footage of a living barreleye was not recorded until 2004, using a remotely operated vehicle.