We finally know why flashlight fish glow

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. Turn your dive lights off and follow me, Phillips signaled.

Gruber swam after his colleague through the dark waters around Mborokua Island, which were known to be home to dangerous saltwater crocodiles. When they reached a cave-like cove along the jungle-covered island, they were stunned.

“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.

Hundreds, if not thousands, of small flashlight fish—glowing blue—began emerging from the underwater cave. They streamed over the reef “like a carpet of lights,” says Phillips. Their illumination was bright enough to reveal the shape of the corals below. The fish schooled together in undulating patterns, like a flowing river, eventually descending over a drop-off and disappearing into the deepness below.

“It’s the coolest thing I’ve ever seen underwater,” says Phillips, an assistant professor of ocean engineering at the University of Rhode Island. “And I’ve got a lot of hours underwater. I’ve seen a lot.”

A subsequent review of the scientific literature suggests this was the largest group of flashlight fish anyone had ever seen, anywhere. Their cameras, however, weren’t sensitive enough to record the fish in that low level of light.

So in 2016, a new expedition returned. 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. And it was even more impressive than they’d realized—a whole new animal behavior, in fact.

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. This discovery is a first in the ocean. 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. The finding thus has potential implications for better understanding how fish behave in the mysterious deep sea.

The discovery that bioluminescence can drive nighttime schooling in flashlight fish is being published in the journal PLOS ONE on August 14.

School is in session

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 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. The fish has a fold of black skin that it can use to cover the light, effectively switching it on and off at will. 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 are poorly studied, in large part because they’re small and elusive. It had been thought that the fish use their glowing ability like a flashlight to help them find prey. They are also known to use the ability to distract predators in a behavior called “blink and run.”

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But the idea of using bioluminescence to coordinate schooling is novel.

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. However, schooling is usually only possible when there is enough light to see each other, says Gruber. 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. (Shoaling is when fish group closely together but aren’t all facing the same direction, moving in a more haphazard fashion.)

By observing wild fish with highly sensitive cameras and by testing on fish they caught, Gruber, Phillips, and the team demonstrated that flashlight fish can use their natural glow to see each other well enough to keep their schools going even during the darkest, moon-free nights. They used SCUBA diving equipment, a three-person submarine, and remotely operated vehicles to monitor the animals, as well as a custom-built, high-speed, high-resolution camera, based on a metal-oxide semiconductor (sCMOS) camera.

“That’s an interesting finding,” says biologist Ann Money, who was not involved in the research. “It’s unusual for a smaller fish to hunt at night, because that’s when the big predators are out. It raises the question: Does the benefit of the luminescence as a means for schooling outweigh the attraction of predators?” says Money, who studies biofluorescence in corals as the director of education and research at the Oklahoma Aquarium. “It seems like if you have a big school of glowing fish that would really be a beacon.”

Cheater fish?

Gruber says more research is needed to better understand how individual fish coordinate their glowing in this group behavior. The new paper shows that night schooling is possible even with only a few of the individuals turning on their lights. But what isn’t known is who decides to glow, and when. Do they take turns, to spread out the risk of attracting predators?

“It opens the question of cheater fish,” says Gruber. “Those who decide they’re not going to flash to keep their predation risk even lower. We don’t think they are doing that, but we don’t know.”

Beyond this one species, this study raises the possibility that schooling via bioluminescence may be common among deep-sea animals, says Gruber. Deep-sea creatures are difficult to study and poorly understood, though scientists have noted that many of them seem to use some kind of bioluminescence to navigate their dark world.

“We know a lot more about land animals and how they use bioluminescence as communication,” says Money. Take fireflies, for instance. We know that different species flash in different patterns at different times, in part to attract mates. Some species even trick other fireflies—they lure males and then eat them.

“I’m wondering if there’s any communication between different species of flashlight fish, or between flashlight fish and anything else,” says Money.

It’s even possible that engineers could one day model the flashlight fish behavior and train robots to swarm together based on blinking lights, says Gruber. A school of swimming robots could monitor pollution, for instance, or study other fish.

Adding intrigue to these questions, when Gruber and Phillips returned to Mborokua Island this summer, the flashlight fish were nowhere to be found. “It’s bizarre,” says Phillips. The reef had experienced some bleaching and coral die off, and a recent storm had knocked over some of the coral. It’s unclear if the fish left as a result, or if they had migrated somewhere else.

“The idea that the deep sea could be filled with billions of fish that are schooling is kind of exciting,” Gruber says.