An Alien-Like Fish With A See-Through Head And Green Eyes Lurks In The Ocean's Dark Depths

Of all the weird and wonderful adaptations that have evolved in the world’s oceans, the barreleye fish's might be the most impressive. These deep-sea dwellers are equipped with a transparent head, allowing their tubular eyes to look upwards, like gazing out of a car sunroof, as they hunt for prey above in pitch-black waters. 

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The barreleye fish typically sits motionless in deep, dark waters with its eyes looking upward, using its sensitive vision to detect silhouettes of prey above. It’s believed the green pigments in its eyes help to block any remaining sunlight from the surface, allowing the barreleye to better detect the faint bioluminescent glow of jellies and other creatures above it.

Some of its diet comes from the tiny crustaceans and other small animals that have been swept up by the trailing tentacles of jellies. It’s been suggested that the fish's unusual visual system helps it keep an eye on its prey, while its fluid-filled transparent head protects it from the tentacles' stinging cells.

The barreleye fish (Macropinna microstoma) was first described in 1939. While scientists aren’t sure exactly how widely it ranges through Earth's oceans, most sightings have been in the North Pacific, from the Bering Sea down to Japan and Baja California.

Because the barreleye’s fluid-filled shield usually collapses when the fish is hauled up in nets to the low-pressure surface, it rarely appears in scientific descriptions or drawings. 

However, in 2009, MBARI researchers had a stroke of luck when they captured a live specimen that survived for several hours in an onboard aquarium. This rare opportunity allowed them to confirm that the fish can rotate its tubular eyes forward to focus on food in front of it, overturning the long-held belief that its gaze was fixed permanently upward to the sky.

This behaviour was also spotted in their incredible footage gathered by remotely operated subs off the coast of Central California at a depth of 600 to 800 meters (2,000 to 2,600 feet). 

Barreleyes aren’t the only animals that have evolved transparent body parts. Glass squids are almost totally transparent, a relatively common trick in the deep sea used to camouflage themselves from prey and predators alike.

Up on Earth’s surface, glass frogs in tropical Central and South America have also developed translucent bellies, allowing onlookers to see their hearts, livers, and other innards. Once again, it serves as camouflage, blending the frogs seamlessly into their leafy surroundings and making them harder for predators to spot, especially when their silhouettes are viewed from above as they rest on green leaves.