Barreleye Fish Aren't Something You Can See While Snorkeling

The barreleye is a small deep-sea fish, measuring about 6 inches (15 cm) long.

Its most striking feature is its transparent head, which allows you to see the fish's internal structures, including its brain and tubular eyes. This fish's eyes are capable of rotating to look upward, forward or even backward.

The fish's body is torpedo-shaped, with a dark, silvery coloration that helps it blend into the shadows of the deep sea. It also has small, paddle-like and flat fins that allow it to remain motionless in the water as it searches for prey.

They move so fluidly that they have more in common with remotely operated vehicles than other fish in some cases.

Macropinna microstoma is the only known species of barreleye fish, making it a standout among deep-sea creatures. While there aren't subspecies, related fish in the Opisthoproctidae family share some similarities, such as upward-facing, unusual eyes and translucent features.

These relatives, however, lack the fully transparent head that makes the barreleye so unique. This specialized adaptation sets it apart as a true marvel of the deep.

The barreleye is a solitary creature, spending its life drifting quietly in the ocean's twilight zone. It doesn't form schools or interact with others of its kind, except perhaps during reproduction, which scientists still know little about.

The fish often hovers motionless in the water, using its fins to stabilize itself while keeping its tubular eyes trained on potential prey above. This patient, solitary hunting strategy makes the barreleye a true specialist of deep sea life.

The barreleye is a predator, feeding primarily on small, gelatinous creatures like jellyfish and zooplankton. Its tubular eyes, located at the top of its transparent head, see faint silhouettes of prey in the dim light filtering down from the surface.

Barreleyes use their excellent vision to locate prey and their small, sharp teeth to capture and consume it. Their ability to maneuver slowly and hover motionless in the water helps them carefully target and catch their food.

Not much is known about the life cycle of the barreleye, but scientists believe it follows a typical deep-sea fish pattern. The fish likely spawns by releasing eggs and sperm into the water column, where fertilization occurs externally.

Once these fertilized eggs become larvae, they drift near the surface before gradually descending into deeper waters as they mature. As adults, their unique adaptations make them perfectly suited to the twilight zone, where they live out their solitary lives.

The barreleye is typically found at depths of 2,000 to 2,600 feet (600 to 800 meters), in a region known as the mesopelagic or twilight zone. This part of the ocean is dimly lit and home to some of the most bizarre creatures in the animal kingdom.

You're most likely to see barreleye fish in the Pacific Ocean, particularly off the coasts of California and Hawaii. Their transparent heads and upward-facing eyes are perfect for spotting prey in this challenging environment.

The barreleye is not currently endangered, but its natural habitat makes it difficult to study and understand its population trends. Deep-sea fishing and habitat disturbances, such as deep-sea mining, could pose threats to this and other twilight zone species in the future.

We created this article in conjunction with AI technology, then made sure it was fact-checked and edited by a HowStuffWorks editor.