This means that much of the night, unless there is a relatively bright moon out, they can’t school and instead must hide or rely on inefficient shoals or try to otherwise hide. However, schooling is usually only possible when there is enough light to see each other, says Gruber. Schooling provides several benefits, including reduction in predation-the old safety in numbers concept-easier movement (due to reduced drag) and a better chance at finding more food. They are also known to use the ability to distract predators in a behavior called “blink and run.”īut the idea of using bioluminescence to coordinate schooling is novel. It had been thought that the fish use their glowing ability like a flashlight to help them find prey. The fish are poorly studied, in large part because they’re small and elusive. It even has a film of black lines to prevent the light from blinding itself, akin to the black cheek marks of football players. The fish has a fold of black skin that it can use to cover the light, effectively switching it on and off at will. The glowing light is created by bacteria the flashlight fish hosts in special organs below the eyes, which keep the symbiotic organisms happy, including serving them with extra oxygen through a system of blood vessels. This species of flashlight fish, Anomalops katoptron, is about the size of an index finger and is found in the western and central Pacific Ocean. The discovery that bioluminescence can drive nighttime schooling in flashlight fish is being published in the journal PLOS ONE on August 14. The finding thus has potential implications for better understanding how fish behave in the mysterious deep sea. It’s believed that vision is the primary way fish coordinate schooling, Gruber says, and that requires light, which is why schooling at night or in the deep sea has rarely been observed. They observed that flashlight fish use their glowing light to coordinate their schooling together, even in light so dim they would otherwise not be able to see each other. And it was even more impressive than they’d realized-a whole new animal behavior, in fact. With more sophisticated camera equipment and the expertise of several National Geographic engineers, plus fish experts and other collaborators, the scientists got a better look at the phenomenon. Their cameras, however, weren’t sensitive enough to record the fish in that low level of light. I’ve seen a lot.”Ī subsequent review of the scientific literature suggests this was the largest group of flashlight fish anyone had ever seen, anywhere. “It’s the coolest thing I’ve ever seen underwater,” says Phillips, an assistant professor of ocean engineering at the University of Rhode Island. The fish schooled together in undulating patterns, like a flowing river, eventually descending over a drop-off and disappearing into the deepness below. Their illumination was bright enough to reveal the shape of the corals below. They streamed over the reef “like a carpet of lights,” says Phillips. Hundreds, if not thousands, of small flashlight fish-glowing blue-began emerging from the underwater cave. “It was like a scene out of Avatar,” says Gruber, a biologist at City University of New York’s Baruch College and the American Museum of Natural History. When they reached a cave-like cove along the jungle-covered island, they were stunned. Gruber swam after his colleague through the dark waters around Mborokua Island, which were known to be home to dangerous saltwater crocodiles. Turn your dive lights off and follow me, Phillips signaled. National Geographic Explorer David Gruber was studying coral while diving at dusk off an uninhabited volcanic island in the South Pacific in 2013 when fellow explorer Brennan Phillips swam up to him excitedly.
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