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Jellyfish glow with the flow in the Gulf of Maine and the Weddell Sea. (David Shale / NPL/ Minden Pictures / Ingo Arndt / Minden Pictures)

Bioluminescence: Light Is Much Better, Down Where It’s Wetter

From tracking a giant squid to decoding jellyfish alarms in the Gulf, a depth-defying scientist plunges under the sea

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(Continued from page 3)

Widder leads me into a light-tight closet at the back of her lab, then rummages in the fridge for a flask of seawater. It looks clear and still and not too promising. Then she turns off the light and gives the water a little swirl. A trillion sapphires ignite.

This glittering concoction, the color of mouthwash, is full of dinoflag­ellates, the same planktonic animals that enchant Puerto Rico’s bioluminescent bays and bathe speeding dolphins in otherworldly blue light. The chemistry behind the glow, shared by many bioluminescent creatures, involves an enzyme called luciferase, which adds oxygen to a compound called luciferin, shedding a photon of visible light—a bit like what happens when you snap a glow stick. Stimulated by Widder’s swirl, the dinoflag­elletes sparkle to discourage whatever has nudged them—be it a predatory copepod or a kayak paddle—in the hopes that it will forfeit its meal.

Larger animals exhibit the same startle response: Lit up along their light grooves, gulper eels look like cartoon electrocutions. Widder eventually realized that the Vegas-like displays she saw from the WASP were mostly examples of startle responses stimulated by contact with her diving suit.

Only a tiny percentage of terrestrial life is bioluminescent—fireflies, most famously, but also some millipedes, click beetles, fungus gnats, jack-o’-lantern mushrooms and a few others. The one known luminous freshwater dweller is a lonely New Zealand limpet. Most lake and river residents don’t need to manufacture light; they exist in sunlit worlds with plenty of places to meet mates, encounter prey and hide from predators. Sea animals, on the other hand, must make their way in the obsidian void of the ocean, where sunlight decreases tenfold every 225 feet, and disappears by 3,000: It’s pitch-black even at high noon, which is why so many sea creatures express themselves with light instead of color. The trait has evolved independently at least 40 times, and perhaps more than 50, in the sea, spanning the food chain from flaring zooplankton to colossal squid with large light organs on the backside of their eyeballs. Mollusks alone have seven distinct ways of making light, and new incandescent beings are being spotted all the time.

Scientists today believe that bioluminescence is always a means of influencing other animals—a signal fire in the deep. The message must be important enough to outweigh the risks of revealing one’s location in the blackness. “It’s the basic stuff of survival,” Widder says. “There’s incredible selective pressure on the visual environment, where you have to worry about what’s above you if you’re a predator and what’s below you if you’re prey. Often, you’re both.”

In addition to activating their startle responses, hunted animals also use light as camouflage. Many midwater predators have permanently upward-pointed eyes, scanning overhead for prey silhouetted against the downwelling sunlight. Viewed thus, even the frailest shrimp becomes an eclipse. So prey animals dapple their bellies with light organs called photophores. Activating these bright mantles, they can blend in with the ambient light, becoming effectively invisible. Fish can snuff out their stomachs at will, or dim them if a cloud passes overhead. The Abralia squid can match the color of moonlight.

Luring food is the second bioluminescent motive. The aptly named flashlight fish sweeps the darkness with its intense cheek lights, looking for tasty neighbors. In front of its cruel jaws, the viperfish dangles a glowing lure on the end of a mutated fin ray that resembles, to hungry passersby, a resplendent piece of fish poop—a favored deep-sea snack. (Rather than kindling their own light, some of these predators enjoy symbiotic relationships with bioluminescent bacteria, which they culture inside light-bulb-like cavities that they can snuff with sliding flaps of skin or by rolling the light organs up into their heads, “exactly like the headlights of a Lamborghini,” Widder says.)

Finally, light is used to recruit mates. “We think they flash specific patterns, or have species-specific-shaped light organs,” Widder says. Female octopods sometimes set their mouths ablaze with glowing lipstick; Bermuda fireworms enliven the shallows with ravelike green orgies. Most romantic of all is the love light of the anglerfish, one of Widder’s favorite animals. The female, a fearsome gal with a toothy underbite, brandishes a lantern of glowing bacteria above her head. The male of her species, tiny and lanternless but with sharp eyes, swims toward her and smooches her side; his lips become fused to her body until she absorbs everything but his testes. (You might say that she will always carry a torch for him.)

Some sea creatures’ use of light mystifies Widder. Why does the shining tube-shoulder fish shrug out light? Why does the smalltooth dragonfish have two headlights instead of one, in slightly different shades of red? How does the colossal squid use its light organ?

These questions aren’t just theoretical. Much of Widder’s early funding came from the U.S. Navy. Tiny creatures that could highlight the shape of a hidden submarine are a national security concern, so Widder invented a tool to measure light levels. Called a HIDEX, it sucks large amounts of seawater, and any bioluminescent animals within, into a light-tight chamber and reads their glow. “It tells you about the distribution of organisms in the water column,” she says.

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About Abigail Tucker

A frequent contributor to Smithsonian, Abigail Tucker is writing a book about the house cat.

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