Living Light
by Alison Fromme

"There are two kinds of light—the glow that illuminates, and the glare that obscures."
—James Thurber

Myriad living things create their own light, called bioluminescence, to defend themselves against predators, find food, and attract mates. For these creatures, bioluminescence is a matter of life and death.

"Is that my dinner? Am I your dinner? Should we get together and mate? These are important distinctions to make," says Steve Webster, a retired senior biologist at the Monterey Bay Aquarium in California. Many organisms, from fireflies to deep-sea fish, communicate this crucial information with bioluminescence. "Bioluminescence cuts across all kinds of groups of critters," he says.

Living light has fascinated people for millennia. Fireflies received attention in the first major collection of Chinese poetry, the Book of Songs (1500-1000 BCE), and jellyfish slime rubbed on a walking stick lit a path for Roman scholar Pliny the Elder in the first century, when glowing clams were the highlight of some Roman banquets.

Once considered mysterious and magical, we now know that bioluminescence is created by a chemical reaction involving oxygen and two molecules called luciferin and luciferase, which are named for the Latin word lucifer, meaning "bearer of light." Bioluminescent organisms actually convert chemical energy into visible light, which is quite different from animals that fluoresce (in a process similar to the fluorescence of a white shirt under a black light) or phosphoresce (like a phosphorescent glow-in-the-dark toy that absorbs light and emits it later).

Defensive Maneuvering
The extent of bioluminescence in the natural world reaches far beyond the familiar fireflies of our backyards. In the oceans, bioluminescence is the rule, not the exception, according to Edith Widder, president and senior scientist at the Ocean Research and Conservation Association in Florida. More than 60 percent of the ocean's open-water fish species are bioluminescent, and more than 90 percent of individual fish are bioluminescent. Many of these fish, as well as marine invertebrates, use light to defend themselves against predators.

"Out in the open ocean, there are no trees or bushes for animals to hide behind and yet they have to play all the same games of hide-and-seek that animals do on land," says Widder. Predators are constantly searching for prey, while prey are desperately trying to escape notice.

Sunlight creates a unique environment as it filters down from the ocean's surface, progressively becoming dimmer and dimmer, until it completely disappears at a depth of about 1,000 meters. Wavelengths of red are the first to be absorbed by the water; blue light permeates the deepest.

Predators in the upper 1,000 meters of the ocean look up toward the sunlight to spot the silhouettes of potential victims swimming above. But prey with bioluminescent bellies can obscure their own silhouettes by matching the surrounding light, making themselves almost invisible to predators. Light-producing organs, called photophores, dot the undersides of many fish and marine invertebrates, including hatchetfish (Argyropelecus affinis), benttooth bristlemouth fish (Cyclothone pallida), swordfish squid (Chiroteuthis spp.), cockatoo squid (Galiteuthis spp.), and northern krill (Meganyctiphanes norvegica) to help them escape the notice of predators. Some bioluminescent squid and cuttlefish manipulate lenses, mirrors, irises, colored filters, and shutters on their bodies to control light emission for precision camouflage.

Other animals use light more actively to defend themselves. Living things such as the deep-sea jelly Atolla wyvillei and the scaly dragonfish (Stomias brevibarbatus) activate a "burglar alarm" by lighting up when bullied. The sudden burst of light causes a commotion to confuse the aggressor, and attracts larger predators that might want to feed on the instigator.

There are many variations on this theme. Some fireflies flash at predators, one deep-sea shrimp (Acanthephyra purpurea) spits bioluminescent goo at attackers, and a primitive relative of octopus and squid (Vampyroteuthis infernalis) releases a cloud of bioluminescence instead of ink to escape. The skin of a swimming sea cucumber (Enypniastes eximia) lights up—and sheds—when the animal is threatened.

Tiny microorganisms that live near the ocean's surface called dinoflagellates also light up with agitation when attacked. Their predators, small crustaceans known as copepods, stop feeding and flee when dinoflagellates flash. Fish can also disturb dinoflagellates and leave starry wakes, drawing the attention of bigger predators. Steve Webster, the retired Monterey Bay Aquarium biologist, witnessed this when he was awoken by the sound of loud splashing in the middle of the night near the Sea of Cortez. Pelicans dove for fish in the darkness, their way lit by dinoflagellates.

In 1833, while sailing around the southern tip of South America, Charles Darwin described a "wonderful and beautiful spectacle" that was likely caused by dinoflagellates. "There was a fresh breeze, and every part of the surface, which during the day is seen as foam, now glowed with a pale light. The vessel drove before her bows two billows of liquid phosphorus, and in her wake she was followed by a milky train. As far as the eye reached, the crest of every wave was bright, and the sky above the horizon, from the reflected glare of these livid flames, was not so utterly obscure as over the vault of the heavens," he wrote in The Voyage of the Beagle.

Finding Food
The small cookiecutter shark (Isistius brasiliensis), which feeds on big, speedy tunas and swordfish by biting off chunks of their flesh, capitalizes on its camouflaged belly in an unusual way. The shark's underside glows with densely packed photophores to camouflage its silhouette, but does not have any photophores on a small band at its neck. Edith Widder, of the Ocean Research and Conservation Association, hypothesizes that the band mimics the silhouette of a smaller fish and entices large fish to come closer—only to be bitten by the shark.

While the cookiecutter shark uses the absence of light to lure prey, other marine organisms use light itself. Female deep-sea anglerfish (Cryptopsaras couesi) hang a fishing lure over their heads that has a glowing glob of bacteria dangling from its end. Prey are attracted to the glow because their usual food often includes fecal waste material that bioluminescent bacteria have latched onto as they drift through the ocean. "The advantage to the bacteria is that by making the fecal pellet conspicuous, they get gobbled up by some deep dweller and are returned to a food-rich environment instead of settling to the bottom where they would die for lack of nourishment. The esca, or lure, of an anglerfish looks like one of these glowing morsels," explains Widder.

Lures are not always formed with bioluminescent bacteria. A recently discovered siphonophore, a relative of Cnidarian jellies, swims through the ocean depths as a pulsating colony, dangling tentacles laced with much smaller stinging filaments called tentilla. Glowing red lures dance among the tentilla, inviting midwater fishes to their deaths by imitating the movement and shape of their food, copepods. Most jellies were thought to use bioluminescence as a defense, not a lure, so this finding surprised researchers.

"It is really exciting because we're seeing these things alive, swimming around, and no one's ever seen them before," says Steve Haddock, one of the Monterey Bay Research Institute scientists who discovered the animal. His research group has collected dozens of new bioluminescent species that are waiting to be described and named.

Light can also help animals find their prey simply by lighting the way. Four-inch-long flashlight fish (family Anomalopidae) sport big crescent-shaped bioluminescent patches under their eyes that act as flashlights to help them search for food. The pale green glow is created by tiny bioluminescent bacteria that can survive only inside the fish. In return for this cozy, nutrient-rich habitat, the bacteria provide the fish with light that illuminates small prey.

Attracting Mates
The flashes of fireflies—which are actually beetles (family Lampyridae, meaning "shining ones" in Greek) and not flies—might appear mysterious or haphazard, but they serve as precise mating calls. Many of the 2,000 species of firefly have evolved species-specific flash sequences used to advertise availability or convey a "come hither" message.

Males spend their adult lives flying through the night, searching for a suitable match, while females wait on low vegetation or the ground. If a female spots an attractive sequence of flashes, she'll flash in response to summon the male over. During mating, males of several firefly species give females a package, which scientists call a "nuptial gift," that contains sperm and a high-protein nutritional substance. A special gland in the female's body absorbs this nutrition, and may pass it on to her developing eggs.

Using computer-generated flashes to mimic males, Tufts University researcher Sara Lewis discovered that females of the firefly species Photinus ignitus prefer long, bright flashes. This isn't just a superficial preference, either. She also discovered that live males with attractive flashes leave females with nuptial gifts full of nutrition, while less attractive males' nuptial gifts are less nutritious.

The "fire" in fireflies is localized to the sixth and seventh segments of the abdomen and varies from green to yellow-orange. Different species create different colors based on the particular version of the luciferase molecule they possess.

In the ocean, males and females of several fish species have different bioluminescent patterns, suggesting to researchers that light plays a role in their mating rituals. Lanternfish (family Myctophidae), small fish of the open ocean, are speckled with photophores arranged in species-specific patterns. Males of some of the 246 lanternfish species have photophores on their backs near their tails, whereas females' light organs are under their tails. Deep-dwelling loosejaw fish (Photostomias guernei) have bioluminescent headlights that likely help them find food, and males have much larger headlights than females, which may help them find mates. The headlights can flick on and off and retract back into the head, allowing the fish to disappear into the darkness or send out flashes of communication that have not yet been decoded by researchers.

Mysteries Remain
The exact uses of light in many species are unknown, and many mysteries remain. We can describe the bioluminescence of many animals, but what information does it convey? Why would an octopus have suckers that glow instead of stick? Why is bioluminescence so common? How did it evolve? What undiscovered animals rely on it for survival? "I would make the argument that bioluminescence may be the most common form of communication on our planet, and it is virtually unknown," Widder says.

Even the relatively well-known firefly luminescent behavior is not fully understood, especially when fireflies in Asia, the Philippines, and Tennessee congregate by the thousands and coordinate their flickering in near-perfect synchrony. Perhaps the momentary darkness between synchronous flashes allows males to spot faint female responses, or maybe the males are working together to attract females from afar. It's also possible that the males are blinking as fast as possible, trying to be the first to attract females. Despite the conjectures, scientists remain baffled.

For scientists studying marine bioluminescence, the oceans provide unique challenges. Oceans are vast habitats, research expeditions are costly, and observing marine life is technically difficult. As technology improves, marine bioluminescence will be better understood.

Throughout history, sailors have described glowing seas in their logbooks, but until recently, scientists could study the phenomenon only by reading sailors' accounts. In 1996, a British ship in the Indian Ocean reported that "it appeared as though the ship was sailing over a field of snow or gliding over the clouds" after steaming into a milky white sea stretching from horizon to horizon. Many mariners before them had seen similar sights, but this particular ephemeral glow was caught on satellite images and analyzed by scientists, including Steve Haddock, of the Monterey Bay Research Institute.

Glowing bioluminescent bacteria likely generated the steady eerie white light, which covered an area of the ocean the size of Connecticut and differed visually from the brief flashes produced by dinoflagellates. According to Haddock, the phenomenon was probably caused by a brief bloom of densely packed bacteria, similar to population explosions of algae that create red tides. After three days, the spectacle vanished. Scientists are still uncertain as to why the bacterial population would swell in this way.

Besides satellite imagery, other new technology offers insight into bioluminescent marine creatures. Edith Widder recalls the magic of being surrounded by bioluminescence for the first time in 1984, when she descended in a submersible 800 feet below the ocean's surface off the coast of California. "I turned out the lights, and it was just like a fireworks display. I couldn't believe it." With the lights back on, the fireworks and the organisms creating them became invisible.

It was Widder's first clue that improving the way scientists observe the sea is crucial to understanding the bioluminescent creatures within. "We haven't seen a lot [of the life] down there because we've been scaring it away with our big nets and bright lights and big noisy submersibles," she says. The bait ordinarily used by researchers also tends to attract mostly scavengers.

So Widder tried a different approach. She developed an ultrasensitive camera called the Eye in the Sea, which uses wavelengths of light invisible to bottom-dwellers and can be left on the ocean floor for up to 48 hours. Unconventional bait was another innovation: An electronic jellyfish, which is essentially a ring of light-emitting diodes, can flash in programmable patterns to mimic real jellyfish communication. With these tools, Widder's team set out to observe bioluminescent creatures and the animals that interact with them.

"It worked beyond my wildest dreams, she says. Within a minute of deploying the decoy for the first time, in the Gulf of Mexico, it was attacked by a six-foot squid, previously unknown to science and still unnamed.

Widder expects the next big breakthrough will surface when she moors the Eye in the Sea off the coast of California in collaboration with the Monterey Bay Research Institute. It will stay offshore for months, constantly streaming data back to the laboratory and giving researchers a long-term window into the ocean.

"I think it's crucial for people to know that we've explored only about five percent of our oceans and right now we're destroying them faster than we're learning what's in them," says Widder, who dreams of someday having 100 underwater cameras around the world so that we can better understand our oceans. "It's like we've been living next to the Garden of Eden and throwing our trash over the wall, without even recognizing what was there."

—Alison Fromme is a freelance science writer living in Berkeley, California, who witnessed the beautiful flashes of dinoflagellates during a sailing expedition through the Caribbean Sea.

ZooGoer 35(1) 2006. Copyright 2006 Friends of the National Zoo. All rights reserved.



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