For decades, biologists have puzzled over those fishes' eyes, because apparently they could look in only one direction - upward - and have wondered at the role of the mysterious transparent shield that covers their heads much like the cockpits of jet fighter planes.
Now two marine biologists in Monterey Bay, trolling with remote-controlled submarines from their surface ship, have tracked and captured the 6-inch living barreleyes on film and studied them in a lab to solve the puzzle: Those tunnels that hold their eyes, the scientists discovered, can actually pivot up and down - a rarity for any animal.
If they could only peer upward, some biologists had long reasoned, the eyes probably evolved that way over millions of years to allow the fish to keep on the lookout for predators bearing down on them from above so they could swim swiftly to safety. Others theorized that the strange eyes allow the barreleyes to scan directly above for signs of food already caught in the long stinging tentacles of passing jellyfish and available for stealing.
"Looking straight up is helpful when predators are around, but how in the world did those fish manage to eat enough to survive if they couldn't chase their prey straight ahead?" wondered Bruce Robison, a senior scientist at the Monterey Bay Aquarium Research Institute, who has pioneered the use of underwater ROVs, or remotely operated vehicles, for deep-sea research.
Landing a barreleye
Robison and his colleague Kim Reisenbichler at the institute in Moss Landing have spent days at sea for more than two years using the remotely operated vehicles to track the barreleyes where they live 2,000 to 2,500 feet deep in near-total darkness. The scientists filmed the fish in action at close range, while the specialized ROV pilots aboard the institute's surface ship, the Western Flyer, snared a couple of them by placing open canisters around them, snapping the traps shut and bringing the fish back alive for study in saltwater tanks aboard the ship.
The scientists recently published an account of their findings in the major journal of ichthyology, Copeia.
Robison told The Chronicle that videos of the fish hanging motionless in the deep sea and swimming swiftly with their noses pointed forward revealed aspects of their structure and behavior that have long been mysterious.
The fish's tubular green eyes, Robison said, are extremely sensitive even to the dimmest of light, and the thin transparent shield of tissue around their heads protects them from the stinging tentacles of jellyfish known as siphonophores.
Those tentacles can extend ropelike in the water for up to roughly 120 feet, but they also provide a valuable source of some food for the barreleyes, because the long strands snare bits of tiny shrimplike plankton and other living tidbits. When the tentacles draw near, the barreleyes can "steal" the scraps of food by moving up until their upward-pointing eyes see the morsels. When that happens, the fish turn abruptly, nose first, and grab the quick snack.
But for a real meal, the barreleyes can do much better, despite the darkness, Robison said.
"They'll hold a position in the water and scan above for more food drifting downward - like small crustaceans, fragments of the jellyfish, and even smaller fish," he said.
A pivotal moment
And then came the real surprise: It turns out that the barreleyes can pivot their eyes directly forward so they can chase a target prey and grab it in their small, pointed mouths with a single snap of their jaws. "And they can swim real fast.", Robison said.
"Those tubular eyes are just amazing," he said. "They have really good depth perception, and they're wired together by an unusual nervous system that helps each eye see well in the dimmest light, while together they coordinate to see even better than that. What a weird fish!"
Sometime also known as spookfish, the barreleye species that Robison and Reisenbichler studied is known scientifically as Macropinna microstoma, a member of a wider family named the Opisthoproctidae.
John McCosker, chair of aquatic biology at the California Academy of Sciences, called the report from the scientists fascinating.
"We humans are so sunlight-centric that we can hardly imagine how animals have evolved to live in near or total darkness," he said. "The mere fact that half of Earth is in the dark at all times; that most of our planet is covered with water, and that most of it is beneath the depth where light penetrates, should convince us that we human animals are in the minority.
"This fascinating study using the technology of ROVs and lots of patience is shedding light on how the majority of life on Earth lives."
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