Living World / Unusual Organisms

In 1984, encased in a huge yellow metal diving suit equipped with foot-controlled thrusters, deep-sea biologist Edith Widder sank into the Pacific Ocean on her first deep dive. With her suit’s lights turned on, she watched shrimp, fish, and jellyfish rising past a clear, bubblelike dome that covered her head. Beyond the reach of the artificial lights, the sea was pitch black. Then at 800 feet, she switched them off.

As her eyes adjusted, she says, “little dots like fairy dust, splats like puffs of liquid, sparks like embers thrown up from a campfire” emerged from the dark. “Except all these lights were blue. It was absolutely mesmerizing, and I couldn’t believe how much there was. It was everywhere!”

What Widder observed was bioluminescence, the lighting that sea creatures use to lure prey, attract sexual partners, and deter predators. Some of the flashes had no particular shape. Others outlined a creature or disguised it in an almost psychedelic array of dots, flashes, pulses, gleams, expanding rings, steady glows, and bursts of light. Some of the lights were tiny: The flash of a dinoflagellate is just a tenth of a millionth of a watt and lasts but a tenth of a second. Certain jellyfish, by contrast, can generate expanding rings of glowing blue light for half a minute or more. It is an extraordinary phenomenon that can’t be put easily into words. Widder simply says, “You have to see it.” Once she did, “there was no turning back.”

Fireflies, some earthworms, fungi, and bacteria can do it, and a field full of fireflies with light pulses in sync can be fascinating to watch. But nothing above the sea’s surface compares with the display of light below. Bioluminescence is found in every ocean, in every sea, from surface to seafloor. In the upper 3,200 feet of the ocean, as many as 90 percent of the creatures are bioluminescent, says Laurence Madin, a pioneering marine ecologist at Woods Hole Oceanographic Institution. Earth’s most abundant vertebrate, an inch-long fish called the bent-tooth bristlemouth, is bioluminescent. Another, the viperfish, has lighting that looks as if it came from fiber optics at the base of its protruding teeth and at its fins. There is an illuminated lure that arcs out from the top of its head, a light under each eye to help it see, lights on its belly for camouflage, and tinted photophores in a mucous layer along its belly and back. So when startled, the viperfish presents an outline of itself, like a neon sign hung outside a bait shop.

Although reported for centuries, bioluminescence is one of the great mysteries of the sea, and scientists are only now asking important questions about it. “What organisms are bioluminescent, and where are they found?” asks Peter Herring, a marine scientist at the Southampton Oceanography Centre in Britain and author of The Biology of the Deep Ocean. What are the characteristics of these emissions, how are they achieved—and, most important to the ecologist, when and why is bioluminescence produced? Herring says we know a good deal about which creatures glow and where they live. But the question of when and why bioluminescence is produced is complicated and can be answered only with more field observations. “That is ultimately the only way we are going to understand what bioluminescence is really for and how important it is to the success of those organisms that have it,” Herring says. “Part of its intrinsic fascination is, of course, that we humans can’t do it.”

All light comes from the same physical process: An electron circling the nucleus of an atom in its customary orbit is energized—often by heat—and moves into a higher orbit. As it drops back to its normal, lower orbit, it releases some energy in the form of light—a photon. In incandescent light, the trigger to push electrons into a higher orbit is heat provided by electricity passing through a carbon filament in a vacuum; in bioluminescence, the electrons are pushed up by two chemicals working together. One, known as a luciferase, stimulates another, called a luciferin, causing it to oxidize, moving electrons up into higher orbits that decay and produce a glow. The resulting light is usually at the blue-green range of visible light. At the other end of the range, red light is quickly absorbed in seawater.

Although most organisms have evolved to produce only blue-green light, there are exceptions—some animals can see red. The dragonfish not only sees red but can also create, from an organ just below its eye, a red beam to illuminate prey or communicate with its own kind without being detected. “It’s like a flashlight and a sniperscope,” says Sonke Johnsen, an assistant professor of biology at Duke University who worked with Widder as a postdoctoral student. “We don’t know how far the beam goes, but it doesn’t reach very far. Everything down there is slow moving, it’s cold, there’s not much food—it’s kind of a sad little life, really. You don’t want to see things too far away, to be distracted and waste time and energy.”

Some jellyfish also seem to exploit red pigment’s ability to absorb blue light: Their stomachs are colored red, so when they swallow their glowing blue-green prey, the meal seems to vanish, hidden behind a red wall. That “keeps their lunch from giving them away,” says Madin.