The barreleye fish might look like a submarine, but its transparent head and tubular eyes are necessary to navigate the 2,500-feet-deep waters in which it dwells. And scientists have now solved the half-century-old mystery of how it uses them.

At first glance, the barreleye fish (Macropinna microstoma) might look more like an alien creature than anything earthly. But the “spookfish” (as its more aptly known) is very much a real creature that lives deep in our oceans. It is considered by biologists to be one of the “most peculiar and unknown fish groups in the deep-sea pelagic realm.”

The name of the barreleye comes from the pair of tubular eyes embedded in its transparent head which looks like a deep ocean submarine with a large glass window. This unique physical setup is actually one that illuminates the abyss in which the barreleye lurks, making it an astute hunting tool.

Marine biologists used to know very little about this creature – until now. One thing they have known ever since the “barreleye” fish (Macropinna microstoma) was first described in 1939 is that its tubular eyes are very good at collecting light. However, they believed the eyes were fixed in place and provided only a “tunnel-vision” view of whatever was directly above the fish’s head.

Then, in 2009, researchers at the Monterey Bay Aquarium Research Institute (MBARI) were able to observe a live barreleye fish that was successfully brought to the surface for several hours. During their observation of the fish, biologists uncovered some shocking revelations.

The new paper by Bruce Robison and Kim Reisenbichler shows that these unusual eyes can rotate within a transparent shield that covers the fish’s head, allowing it to peer up at potential prey or focus forward to see what it is eating.

Deep-sea fish have adapted to their pitch-black environment in a variety of amazing ways. For example, researchers noticed that the eyes of the barreleye weren’t just strange-looking, they were also green. They believe the green coloring helps the fish filter out sunlight from the surface far above it. It also likely helps it spot the bioluminescent glow of its prey above.

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.

Although such tubular eyes are very good at collecting light, they have a very narrow field of view. As noted above, until now scientists believed that the barreleye’s eyes were fixed in their heads in a stare straight above them. This, however, would make it impossible for the fish to see what’s directly in front, and very difficult to capture prey with its small, pointed mouth.

And indeed: as it turns out, those eyes actually rotate. This led marine biologists to believe that the barreleye peers upwards to spot the faint shadows of its potential prey and then fixes its eyes forward as it slowly rises to meet its victim.

Regarding the barelleye’s strange, see-through head, researchers found that it was actually filled with a fluid forming a transparent shield. Earlier descriptions of the barreleye failed to mention its bubble head, most probably because it was destroyed when the fish was brought to the surface from the deep in nets.

However, Robison and Reisenbichler were able to bring a net-caught barreleye to the surface alive, where it survived for several hours in a ship-board aquarium. This controlled environment allowed scientists to confirm that the fish rotated its tubular eyes as it turned its body from a horizontal to a vertical position.

The green tubes pictured here are the eyes of the barelleye, while the dark circles above them are the mouth and nares, or fish nostrils. In this image, you can see that, although the barreleye is facing downward, its eyes are still looking straight up. Credit: MBARI

Besides their amazing headgear, barreleyes also adapt to deep-sea life in a variety of other interesting ways. Their large, flat fins allow them to remain almost motionless in the water, and to maneuver very precisely. Their small mouths suggest that they can be very precise and selective in capturing small-sized prey. At the same time, their digestive systems are big, suggesting they can eat a variety of small drifting animals as well as jellies.

With all this information obtained about the barreleye’s unique adaptations, Robison and Reisenbichler developed a working hypothesis about how these animals subsist. For the most part, the fish hangs motionless in the water, with its body in a horizontal position and the eyes looking upward. But when it spots prey (such as a drifting jelly), it quickly rotates its eyes forward and swims upward, in feeding mode.

A drifting jelly? Yes, barreleyes share their deep-sea environment with many different types of jellies, some of which grow to over 10 meters (33 feet) long. Like living drift nets, those jellies trail thousands of stinging tentacles, which capture copepods and other small animals. The researchers speculate that barreleyes may maneuver carefully among the jellies’ tentacles, picking off the captured organisms. And their tubular eyes would rotate to help the fish keep its eyes on the prey, with its transparent shield protecting those eyes from the jelly’s stinging cells.

The bizarre physiological adaptations of the barreleyes have puzzled oceanographers for generations. Robison and Reisenbichler hope to do further research to find out whether their discoveries also apply to other deep-sea fish with tubular eyes.

Opisthoproctus soleatus. This image from the end of the 1800’s is drawn from a specimen taken to the surface; in a live specimen, the membrane over the top of the head forms a transparent dome.

Scientists are only beginning to understand this unique fish. For millions of years, it swam hidden in black oceanic depths. Now, scientists have finally brought it into the light of discovery – and the timing is by no means a surprise. Only with the advent of modern underwater robots have scientists been able to observe such animals in their native environment, and thus to fully understand how these physical adaptations help them survive.

And who knows what other strange, unimaginable inhabitants still lurk undetected in the dark down there, waiting to be seen?

UPDATE: Here’s a brand new sighting of the barreleye fish from MBARI!

Truly an amazing animal!