At first, John Sparks thought the photographer was pulling his leg when he showed him a picture of a glowing green eel swimming dangerously close to an octopus with a long reach.
“I thought it was a joke,” says Sparks, an ichthyology curator at the American Museum of Natural History. Along with his colleague David Gruber and a National Geographic photographer, he had just been scuba diving after dark, exploring a biofluorescent reef in the Cayman Islands for an exhibit. The photographer had been insisting that more than just coral was throwing off light that night—then he showed the two scientists the first evidence ever of a glowing eel.
Scientists have known for decades that coral, jellyfish, and other cnidarians glow. But fluorescent fish? “I didn’t believe it at first,” says Sparks. “But it was real. And it got us thinking, wondering how widespread it is in fish.”
Very widespread, the aptly named Sparks has discovered after three years and numerous diving expeditions around the globe. More than 180 species of fish glow in a dizzying array of colors and patterns. They absorb the blue light that dominates their watery environs and reemit it in neon greens, reds, and oranges. (Squid and many other marine species have long been known to bioluminesce, a similar-sounding term for a different biological process in which light is produced by a chemical reaction within an organism.) Waters off the Solomon Islands, Bahamas, United States, and elsewhere host a spectacular kaleidoscope of glowing gobies, many types of eels and flatfishes, and even some sharks and rays. Some light up only around the eyes; others get a full-body glow on.
The research also hints that biodiversity on reefs might be greater than previously thought. Take lizard fish. “They’re mottled and difficult to tell apart under natural light,” says Sparks. “We’ve thought that we were looking at one species, but then you look at fluorescence and see that each species has a dramatically different pattern.” The increased, unknown diversity is a key point in the argument for more protection for coral reefs, which are threatened around the globe by warming waters and ocean acidification. For those motivated by financial arguments, rather than environmental ones, there are also economic incentives to saving these habitats: Reef fish might provide a treasure trove of novel fluorescent proteins that could be used for medical research, such as cancer treatments, brain activity studies, and tests for liver disease.
So why was such a common (and bright) phenomenon only recently discovered? “There aren’t a lot of people out diving at night,” says Sparks, who says it’s a rush to be out in the ocean then—though also a bit unnerving knowing that unseen sharks could be nearby. To see the glowing creatures better, the team has developed a yellow-green filter for diving masks and custom-built underwater cameras that block out the ambient blue light, leaving only the neon wavelengths the fish emit.
While damselfish and angelfish might seem like the flashiest on the reef, Sparks has found that the most fluorescent fish aren’t brightly colored at all—under natural light, that is. But under the yellow-green filter, those drab-seeming swimmers light up. And they very likely see a much more vibrant world than the human eye can detect, says Sparks. Many fluorescent fish have yellow filters in their eyes, which could allow them to see the light show.
As for why the fish glow, Sparks says it might be important for communication and mating, camouflage (hiding against coral that gives off the same red hue, for instance), or luring prey. “Lizard fish bury themselves in sand and leave a little fluorescent speck sticking out,” says Sparks. “Something goes to eat it and gets eaten itself, because lizard fish have a huge mouth.”
Sparks is developing new technology, such as extremely bright blue lights, to use on future expeditions, and he’d like to dip into some waters he hasn’t explored before, such as Australia’s Great Barrier Reef.
And this summer, he’s taking part in a project closer to home that could one day take biofluorescence research to new depths. He’ll don the Exosuit—an astronaut-style getup that allows its wearer to descend to 1,000 feet and spend several hours underwater—and explore the waters off Rhode Island (glowing fish are not just the stuff of the tropics). It’s an unprecedented opportunity to study bioluminescent creatures (the ones that can turn the lights on and off) in their natural deep-sea habitat. “It’ll be so fun,” he says. Once he’s out at sea, that is: The suit weighs 530 pounds.
Sparks trained in the aluminum alloy outfit last summer. “On land it’s extremely heavy and hot, but as soon as you get in water it cools down right away,” he says Foot power thrusters help him maneuver through the water, and he’ll be tethered to an ROV (remotely operated underwater vehicle) that will take video and photographs. The setup will allow Sparks to stay in the deep water for hours, documenting various species’ flashing patterns and behavior, and collecting new specimens. “The Exosuit could certainly be adapted for biofluorescence research down the road,” he says.
“One thing I really like about science is that there is always something new,” he says, “it’s never boring.” Especially when you get to sit—er, float—back and watch a light show nobody has seen before.
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