Many organisms rely on bioluminescence to breed, feed and evade predators

By Ben Harder
Monday, October 11, 2010; 5:00 PM

For a moment, I cradled the Milky Way in my hands. There, cupped in my open palms, lay dozens upon dozens of glittering stars. One by one, they faded to black. Yet all I had to do was scoop a hand into the water surrounding us, and I held another fistful of starlight.

My wife and I were bobbing in a kayak in one of Puerto Rico's famed bioluminescent bays. It was night, and while there were true stars above us, the ones in my grasp were really dinoflagellates - single-celled organisms that emit light, or bioluminesce, whenever they're disturbed.

Bioluminescence is as widespread as it is wild and mysterious. Jack-o'-lantern mushrooms, flashlight fish and fireflies are among the multitude of organisms that bioluminesce. Scientists are still finding previously unknown examples of the phenomenon, especially at sea, where bioluminescent species are particularly varied and abundant. In parts of the ocean, 80 to 90 percent of sea creatures make light or harbor microbes that do so.

Experts are studying not only why marine life-forms developed the capacity to flash or glow, but also how and when they do it.

The "why" is worth pondering. If you're a defenseless bit of biota whose major preoccupation is hiding from predators, shining a light on your location doesn't immediately stand out as a winning strategy. And yet, bioluminescent organisms can protect themselves by making light. One way, believe it or not, is camouflage.

Not far beneath the waves lies a realm that's in almost perpetual twilight. Only a little sunshine or moonlight filters down, and everything is bathed in a faint blue. In this world, an easy way to catch prey is simply to look up - and attack any appetizing morsel silhouetted against the light above.

That's where bioluminescence can provide a chameleonesque disguise. Nearly all krill, the tiny crustaceans that are food for everything from small fish to massive baleen whales, have eyelike structures called photophores on their undersides. The photophores give off light of similar color and intensity to that shining down from above, making the krill semi-invisible from below.

Bioluminescence can also help fend off an attack. Some species glow as a warning that they're poisonous; others confuse or blind attackers by releasing clouds or jets of light-emitting chemicals while trying to escape.

Some marine organisms, if attacked, produce what scientists call a burglar alarm - a visual 911 call designed to attract a larger fish that's only too happy to gobble up the flashy prey's assailant.

This is thought to be the case with the dinoflagellates I encountered. By lighting up, the dinoflagellates, which are sought after as food by small crustaceans known as copepods, may be trying to draw a copepod-eating fish to the scene. In lab experiments, copepods tend to scoot away when dinoflagellates luminesce, suggesting the light display does work as a deterrent.

In general, any flash of light in the deep ocean is a signal that action's afoot. Like men in a bar fight, some organisms will flee a fracas and others will charge toward it. In a lecture posted on in April, marine bioluminologist and MacArthur "genius grant" awardee Edith Widder showed footage of a squid previously unknown to science; she had lured it to her camera using a flashing light that resembled a jellyfish in distress.

Bioluminescence is used not only to avoid getting eaten but also, sometimes for the opposite purpose. In the open ocean, a steady rain of organic debris - mostly dead marine organisms and their fecal matter - drifts down through the water. To survive, the bacteria aboard this sinking stuff desperately need to find their way into the gut of a new host organism. Glowing is a great way to get the attention of something that's willing to stomach the waste heap you call home.

Certain predators, including some anglerfish, have what is essentially a glowing lure that dangles near their mouths. Mistake the lure for a yummy fecal ball and you're probably a goner.

The cookie-cutter shark may take the cake for the cleverest bioluminescent costume. This small shark feeds on larger prey, including tuna, with the help of a light display that simultaneously camouflages it and acts as a lure. Photophores cover its underbelly except for a patch under its chin that resembles a smaller fish. Thinking it has spotted an easy meal, the shark's mark swims up from below. Then the cookie-cutter twists and strikes first, carving out a cookie-shaped plug of flesh.

Another key reason for bioluminescence is to find a mate. In the sea, the lanternlike lures of the female anglerfish, for example, may double as beacons for males, which latch onto females for life (and, in a profound form of nuptial commitment, atrophy into little more than sacks of sperm). On land, lightning bugs use one another's flash patterns to pick a suitable mate.

The chemistry of bioluminescence offers insight into its evolution. Each light-generating organism employs two primary molecules - one a substrate or fuel known as a luciferin, the other an enzyme called a luciferase, which helps the luciferin react with oxygen to produce light.

Some closely related species share the same luciferin and luciferase, suggesting their ability to bioluminesce comes from a common ancestor. But other kindred species have totally different chemical machinery for making light. According to scientists, the evidence suggests that bioluminescence independently evolved on at least 40 occasions.

No wonder the bioluminescent club is so diverse. The menagerie comprises not only all those marine organisms but also a variety of insects and worms, at least one snail and the jack-o'-lantern mushroom: It looks like a tasty chanterelle, but at night it glows to warn hungry foragers it's toxic.

Lest you shrug off bioluminescence as a mere spooky trick, consider this: When the 2008 Nobel Prizes were given out, the winners in the physiology-or-medicine category were cited for creating an invaluable tool for medical research, the green fluorescent protein. Their raw material? One of the genes that a jellyfish species uses to light up the deep.

Harder is general manager of health and science at U.S. News & World Report.

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