Washington, Feb 24 : Researchers at the Monterey Bay Aquarium Research Institute recently solved the half-century-old mystery of a deep-sea fish with tubular eyes and a transparent head.

The object of study is the barreleye (Macropinna microstoma), which has extremely light-sensitive eyes that can rotate within a transparent, fluid-filled shield on its head.

Ever since the "barreleye" fish Macropinna microstoma was first described in 1939, marine biologists have known that it's tubular eyes are 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.

A new research paper, by Bruce Robison and Kim Reisenbichler, shows that these 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.

Robison and Reisenbichler used video from MBARI's remotely operated vehicles (ROVs) to study barreleyes in the deep waters just offshore of Central California.

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

Most existing descriptions and illustrations of this fish do not show its fluid-filled shield, probably because this fragile structure was destroyed when the fish were brought up from the deep in nets.

However, Robison and Reisenbichler were extremely fortunate. They were able to bring a net-caught barreleye to the surface alive, where it survived for several hours in a ship-board aquarium.

Within this controlled environment, the researchers were able to confirm what they had seen in the ROV video.

The fish rotated its tubular eyes as it turned its body from a horizontal to a vertical position.

Robison and Reisenbichler hope to do further research to find out if their discoveries about Macropinna microstoma also apply to other deep-sea fish with tubular eyes.

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, and to maneuver very precisely. Their small mouths suggest that they can be very precise and selective in capturing small prey. (ANI)