The underwater creatures that stick with the blue-green glow have evolved useful techniques too. For example, some sea creatures, like the dragonfish, use bioluminescent appendages as lures to troll for food. Others may use the light to troll for mates. The male sea firefly, a crustacean the size of a sesame seed, squirts out light that hangs as a bright dot in the water, then zips upward and squirts out another and another, leaving a string of hanging dots spaced out like smoke signals. The spacing is species specific; mate-ready females can go to the head of the dot string and find an appropriate male. “That’s a real mindblower,” Widder says. “It’s a pretty complex behavior for these little guys.”
Some sea life uses light as a weapon. One fish, the shining tubeshoulder, literally squirts light out of a tube on its shoulder into the face of an enemy, much as a squid shoots ink. One specimen, brought up from the deep and into a fluorescent-filled shipboard laboratory, flashed so brightly that researchers gasped. “That would definitely blind a predator,” Widder says, laughing.
Other ocean creatures use bioluminescence as a warning: I’m toxic; leave me alone. Widder has recorded unusual displays by bamboo coral off the coast of the Carolinas. When touched, the coral lights up like a Christmas tree and disgorges sheets of thick, gill-clogging slime. The sea pen, long and slender, also had a surprise for Widder. When pinched with tweezers in a lab test, it shot green light down its thin stalk. When the light reached the plume at the end of the stalk, it turned blue. Widder thought the intent might be to draw a predator’s attention to a less vulnerable area. But when she tested the idea by pinching the stalk’s other end, the light changed direction and shot back the other way. When she pinched the middle, the light moved outward in both directions at once. She thinks it’s like a burglar alarm—to lure whatever creature is in the sea pen’s vicinity into attacking its attacker. Tiny dinoflagellates seem to use their bioluminescence to the same effect, “like a scream,” Widder says. When predators try to eat them, the dinoflagellates flash, lighting up the water. Creatures grazing on the dinoflagellates “don’t want to be lit up and munched on” by other predators, she says.
Still others use light as camouflage or “counterillumination.” The cookie-cutter shark employs a complicated lighting strategy to attract a meal. Seen from below, its belly glows with blue light, making it difficult to distinguish against the backdrop of sunlight coming from the ocean’s surface—except for a small dark band near its mouth. To hungry tuna attacking from below, the band might resemble a small fish.
Such camouflage techniques are not flawless. Sonke Johnsen has found that in clear water some creatures can see with high sensitivity. “They can detect lower light levels, a hundred times as low as we can see. But the evolutionary trade-off is that they sacrifice clarity,” he says. “We were looking at how it balances out, and we suspected that the water blurs their vision. But it turns out that in the clear water they can really pick patterns out. Very few of them have even illumination. There’s nothing to hide behind in the open ocean,” Johnsen continues, “but they still have to hide, so they use every trick in the book to stay hidden. But their predators are using every trick in the book to find them.”
Widder marvels at all the cleverness. “We live in a lighted world,” she says. “We have burglar alarms on cars to make noise to attract attention and scare off the burglar or bring the police. But the bioluminescent world is a minefield. Any movement will reveal you to predators.”
That’s why the U.S. Navy has funded some of Widder’s research. Motion in the water—a diver, a landing craft, a submarine—stimulates plankton to glow. In November 1918, the last German U-boat sunk during World War I was spotted by the bioluminescence it stirred up in the Mediterranean. Carrier-based aviators in World War II sometimes found their ship after a mission by following its bioluminescent wake. And during the Gulf War, Navy SEALs had to change a landing site to avoid bioluminescence that might have revealed their presence. Now the Navy is hoping scientists can chart the location, amount, and seasonal fluctuations of bioluminescence worldwide. In a world of submarines and nighttime operations, it would be exceptional intelligence to have.
As head of the bioluminescence department at Harbor Branch Oceanographic Institution in Fort Pierce, Florida, Widder, 52, is the leading investigator of this mysterious light and the animals that display it. She believes it is one of the most important processes in the ocean. “There’s so much bioluminescent energy being created and used,” she marvels, “and we don’t know anything about it. In studying the ecology of the ocean, ignoring the most important form of communication is like studying the ecology of land while ignoring animal vision.”
The oceans’ midwater zone, a vast area between 650 and 3,200 feet deep, is Earth’s greatest ecosystem. “There are more animals there than in all the other habitats,” Widder says. Scientists have explored less than 5 percent of it, and thus far, Widder says, they’ve done so with “embarrassingly primitive” tools. “I defy you to name another science that depends on 2,000-year-old technology: dragnets. Nets catch only the slow, the stupid, the greedy, and the indestructible—which then deteriorate on shelves in jars. They destroy the gelatinous animals, and they miss huge players—the big stuff that can outrun you. So you get a one-dimensional view of ocean life and no idea of distribution. It’s . . . primitive!”
Widder became determined to develop more sophisticated instruments. She started in graduate school. In 1984 the Navy asked her adviser, biologist James Case, to find a way of measuring oceanic illumination, and he recruited Widder as coinvestigator. It was a good match. “The Navy wanted to know how much light there was, and I wanted to know who’s making it and figure out the ecology,” Widder says. “I wanted to go to a chunk of ocean and say, ‘Oh, it’s all copepods here,’ or whatever.”
But it wasn’t easy to do that. Scientists had already tried lowering sensors and cameras to measure bioluminescence, but as the measurements bobbed up and down with the mother ship’s motion, they stimulated more bioluminescence. And creatures were known to change their natural behavior when they detected measuring devices nearby. So Widder covered a hoop with some netting from a research ship’s supply store and mounted it on a Deep Rover submersible with a video camera focused on the plane of the net. She called it the splat-cam—for spatial plankton analysis technique. When animals hit the net as the sub moved through the water, they were stimulated to glow and the camera recorded the events. Widder became the first scientist to count the numbers of bioluminescent creatures per cubic meter in a particular part of the ocean.