Transparent-head deep sea mysterious fish

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 olfactory organs called nares, which are analogous to human nostrils. Image: © 2004 MBARI

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 its 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 mm (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.

This face-on view of a barreleye shows its 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

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.

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.

Lobster can feel pain

www.lobsterlib.com - Lobster Liberation

A new report by Advocates for Animals, a Scottish animal protection organization, presents a wide array of scientific evidence that lobsters, like other animals, are capable of experiencing pain and suffering. The report arrays 15 years of available research showing that lobsters respond behaviorally as if they are in pain, have biological structures for mediating pain, and demonstrate mental cognitive processes such as learning, memory, association, and generalization—all of which would require that they have the brain capacity sufficient to feel pain. Some highlights of the report include the following:
• Lobsters can attain a high degree of associative learning and can make distinctions about their environment. They can learn to associate certain chemical signals with punishments and others with rewards.
• Lobsters have memories. They explore new environments but not ones that they’ve previously explored. They form stable social hierarchies, and they can remember and recognize individuals whom they have fought with in the past.
• Lobsters can make generalizations. They can associate an individual odor with a mixture that contains it, perhaps using higher-order processing such as “configural discrimination,” which means that they can discern that a combination is not the same thing as the sum of its parts.
• Measurements of brain activity in the closely related crayfish show that their brains respond differently to interesting or irrelevant objects. Their brains also show “expectation,” which in humans is considered to be a sign of higher mental processing, associated with conscious experiences.
• Lobsters subjected to harmful events, such as electric shocks, respond similarly to the way that mammals react to pain. Experiments on crustaceans commonly assume that electric shocks are painful and use them as aversive stimuli.
• The nervous systems of lobsters and crabs produce opioids, which in mammals are chemicals that mediate pain. They also possess opioid receptors, which appear to function the same way as in other animals. Studies in crabs show that their defensive reaction to electric shocks or to being struck is reduced by morphine, that this effect is dose-dependent, and that the effect can be counteracted by naloxone, an opioid antagonist, as is also the case in mammals. It is implausible that lobsters would have pain-mediating chemicals and receptors and respond to painkillers just as other animals do if they could not feel pain.

Advocates for Animals’ report notes that lobsters do not receive humane consideration. They are typically cooked alive by boiling. Unattended traps result in death by starvation, dehydration, heat, or fights with other lobsters. During transportation and storage, they suffer from dehydration, red-tail bacterial disease, shell disease, and “bumper car” disease as a result of extremely crowded conditions. Rough handing of lobsters—including throwing them—causes open wounds and lesions. They also suffer from an inability to breathe properly in air, which results in acidosis and toxin buildup; conflict with other lobsters; sudden temperature changes; low water quality; and physical injuries. A Canadian study found that 19 percent of lobsters had a missing claw when they arrived at factories and packing stations. In Australia’s Western rock lobster fishery, on average 20 percent of the lobsters arrive at the factory too weak to be considered fit for live export. Mortality rates often reach 10 to 15 percent.
With clear evidence that lobsters are sentient beings who suffer when they are mistreated, the only ethical conclusion is that every effort must be made to treat these animals humanely. Cruel practices that are common in the catching, handling, and killing of lobsters should be outlawed.