Scientists in California have filmed a fish with a transparent head.

And the encounter has helped researchers solve the 50-year-old mystery of how the fish uses its extraordinary eyes to see in the gloomy ocean depths.

Ever since the Barreleye Fish was discovered in 1939, marine biologists have known that its tubular eyes were very good at collecting light.

However, the eyes were believed to be fixed in place and seemed to provide only a tunnel-vision view of whatever was directly above the fish's head.

New research shows that the unusual eyes can rotate within a transparent shield that covers the fish's head.

This allows the Barreleye to peer up at potential prey or focus forward to see what it is eating.

Barreleyes typically live near the depth where sunlight from the surface fades to complete blackness.

They use their ultra-sensitive tubular eyes to search for the faint silhouettes of prey overhead.

Bruce Robison and Kim Reisenbichler of the Monterey Bay Aquarium Research Institute used videos from the institute's remotely operated vehicles (ROVs) to study Barreleyes off Central California.

At depths of 2,000 to 2,600 feet, (600 to 800 meters) below the surface, the cameras typically showed the fish hanging motionless in the water, its eyes glowing a vivid green in the bright lights.

In addition to their amazing "headgear," Barreleyes have a variety of other interesting adaptations to deep-sea life. Their large, flat fins allow them to remain nearly motionless in the water.