Weird fish from the deep: transparent head and tubular eyes

The Barreleye is known since 1939, but only from mangled specimens dragged to the surface by nets. Scientists had long been puzzled by its tubular eyes. While these are very good at collecting light, the eyes seemed to provide only a narrow view of whatever was directly above. A recent study of the first live, intact specimen of the fish shows that these unusual eyes can rotate within a transparent shield that covers the fish's head.

The fish is about 15cm long and found at around 600-800m deep in the sea.
The barreleye (Macropinna microstoma) has extremely light-sensitive eyes that can rotate within a transparent, fluid-filled shield on its head. The fish's tubular eyes are capped by bright green lenses. The eyes point upward (as shown here) when the fish is looking for food overhead. They point forward when the fish is feeding. The two spots above the fish's mouth are are olfactory organs called nares, which are analogous to human nostrils. Image: © 2004 MBARI

Researchers solve mystery of deep-sea fish with tubular eyes and transparent head
MBARI News Release 23 Feb 09

Researchers at the Monterey Bay Aquarium Research Institute recently solved the half-century-old mystery of a fish with tubular eyes and a transparent 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 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.

Deep-sea fish have adapted to their pitch-black environment in a variety of amazing ways. Several species of deep-water fishes in the family Opisthoproctidae are called "barreleyes" because their eyes are tubular in shape. 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. Furthermore, until now, most marine biologists believed that barreleye's eyes were fixed in their heads, which would allow them to only look upward. This would make it impossible for the fishes to see what was directly in front of them, and very difficult for them to capture prey with their small, pointed mouths.
In this image, you can see that, although the barreleye is facing downward, its eyes are still looking straight up. This close-up "frame grab" from video shows a barreleye that is about 140 cm (six inches) long. Image: © 2004 MBARI

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. The ROV video also revealed a previously undescribed feature of these fish--its eyes are surrounded by a transparent, fluid-filled shield that covers the top of the fish's head.

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.

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 (much like MBARI's ROVs). Their small mouths suggest that they can be very precise and selective in capturing small prey. On the other hand, their digestive systems are very large, which suggests that they can eat a variety of small drifting animals as well as jellies. In fact, the stomachs of the two net-caught fish contained fragments of jellies.

After documenting and studying the barreleye's unique adaptations, Robison and Reisenbichler developed a working hypothesis about how this animal makes a living. Most of the time, the fish hangs motionless in the water, with its body in a horizontal position and its eyes looking upward. The green pigments in its eyes may filter out sunlight coming directly from the sea surface, helping the barreleye spot the bioluminescent glow of jellies or other animals directly overhead. When it spots prey (such as a drifting jelly), the fish rotates its eyes forward and swims upward, in feeding mode.
This face-on view of a barreleye shows it's transparent shield lit up by the lights of MBARI's remotely operated vehicle Tiburon. As in the other photos, the two spots above the fish's mouth are are olfactory organs called nares, which are analogous to human nostrils. Image: © 2006 MBARI

Barreleyes share their deep-sea environment with many different types of jellies. Some of the most common are siphonophores (colonial jellies) in the genus Apolemia. These siphonophores grow to over 10 meters (33 feet) long.
Like living drift nets, they trail thousands of stinging tentacles, which capture copepods and other small animals. The researchers speculate that barreleyes may maneuver carefully among the siphonophore's tentacles, picking off the captured organisms. The fish's eyes would rotate to help the fish keep its "eyes on the prize," while its transparent shield would protect the fish's eyes from the siphonophore's stinging cells.

MBARI researchers speculate that Macropinna microstoma may eat animals that have been captured in the tentacles of jellies, such as this siphonophore in the genus Apolemia. The "head" of the siphonophore (at right) pulls the animal through the water, its stinging tentacles streaming out like a living drift net.
Image: © 2001 MBARI

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. The bizarre physiological adaptations of the barreleyes have puzzled oceanographers for generations. It is only with the advent of modern underwater robots that scientists have been able to observe such animals in their native environment, and thus to fully understand how these physical adaptations help them survive.

There's also a video of Macropinna microstoma narrated by Bruce Robison on the MBARI site.

Fish With Transparent Head, "Barrel" Eyes
Richard A. Lovett, National Geographic 23 Feb 09;

February 23, 2009--With a head like a fighter-plane cockpit, a Pacific barreleye fish shows off its highly sensitive, barrel-like eyes—topped by green, orblike structures—in a picture released today but taken in 2004.

The fish, discovered alive in the deep water off California's central coast by the Monterey Bay Aquarium Research Institute (MBARI), is the first specimen of its kind to be found with its soft transparent dome intact.

The 6-inch (15-centimeter) barreleye (Macropinna microstoma) had been known since 1939—but only from mangled specimens dragged to the surface by nets.

The beady bits on the front of the Pacific barreleye fish in this picture released February 23, 2009, aren't eyes but smell organs.

The grayish, barrel-like eyes are beneath the green domes, which may filter light. In this picture the eyes are pointing upward—the better to see prey above in the darkness of the barreleye's deep-sea home.

Since the eyes are upright tubes, "it just looked like [they only] looked straight up," MBARI marine technician Kim Reisenbichler said. But by watching live fish from a remotely operated vehicle (ROV) and by bringing a barrelfish to an aquarium for study, the scientists discovered that the eyes can pivot, like a birdwatcher pointing binoculars.

The transparent-headed Pacific barreleyes may steal fish from siphonophores (such as this one photographed in 2001)—jellies that can grow to more than 33 feet (10 meters) long, according to researchers who released new barreleye findings and pictures on February 23, 2009.

The barreleye's flat, horizontal fins may allow it to swim very precisely among the siphonophore's stinging tentacles—and if the fish fumbles, the clear, helmet-like shield may protect its eyes, according to MBARI scientists.

The barreleye lives more than 2,000 feet (600 meters) beneath the ocean's surface, where the water is almost inky.

The transparent-headed fish spends much of its time motionless, eyes upward, MBARI scientists discovered while watching the barreleye fish from a remotely operated vehicle.

The green cap atop each of the fish's eyes filters out what little sunlight makes it down from the surface, allowing the fish to focus on the bioluminescence of small jellies or other prey passing overhead.

Then the eyes rotate forward to follow the prey, allowing the fish to home in on its meal.

Strange Fish Has See-Through Head
LiveScience, Yahoo News 23 Feb 09;

A bizarre deep-water fish called the barreleye has a transparent head and tubular eyes. Since the fish's discovery in 1939, biologists have known the eyes were very good at collecting light. But their shape seemed to leave the fish with tunnel vision.

Now scientists say the eyes rotate, allowing the barreleye to see directly forward or look upward through its transparent head.

The barreleye (Macropinna microstoma) is adapted for life in a pitch-black environment of the deep sea, where sunlight does not reach. They use their ultra-sensitive tubular eyes to search for the faint silhouettes of prey overhead.

Scientists had thought the eyes were fixed in an upward gaze, however. This would make it impossible for the fish to see what was directly in front of them, and very difficult for them to capture prey with their small, pointed mouths.

Bruce Robison and Kim Reisenbichler of the Monterey Bay Aquarium Research Institute use 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), the ROV cameras typically showed these fish hanging motionless in the water, their eyes glowing a vivid green in the ROV's bright lights. The video also revealed a previously undescribed feature of these fish - its eyes are surrounded by a transparent, fluid-filled shield that covers the top of the fish's head.

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.

Robison and Reisenbichler were fortunate to bring a net-caught barreleye to the surface alive. Over several hours in an aquarium on the ship, they were able to confirm that the fish rotated its tubular eyes as it turned its body from a horizontal to a vertical position.

Barreleyes, just a few inches long, are thought to eat small fishes and jellyfish. The green pigments in their eyes may filter out sunlight coming directly from the sea surface, helping the barreleye spot the bioluminescent glow of jellies or other animals directly overhead. When it spots prey (such as a drifting jelly), a barreleye rotates its eyes forward and swims upward, in feeding mode.

The findings were detailed recently in the journal Copeia.