A bioluminescent squid glows in deep Caribbean sea waters off Belize, Central America. Photo © Stephanie Benson

Living Light

What makes some living things glow?

December/January 2017

For two weeks each year at the beginning of summer, fireflies light up a portion of Great Smoky Mountains National Park. They glow in unison or in waves as males communicate with females in an elaborate mating dance that no one fully understands. There are at least 19 firefly species in the park, but Photinus carolinus is one of the only fireflies in America known to light in unison. Tourists arrive by the thousands to watch.

The weird, alien phenomenon of bioluminescence has always captivated us, especially before we understood what caused it. Aristotle wrote about glowing marine organisms. Huckleberry Finn and Tom Sawyer attempted to use a glowing fungus to light their way as they dug a tunnel. At about the same time, Jules Verne described the glowing “milky sea” effect in 20,000 Leagues Under the Sea.

Today scientists know quite a bit more about bioluminescence and the many forms it takes. For one thing, those fireflies we all know so well are actually one of the more uncommon forms of biological light. As much as 80 percent of all bioluminescent organisms live in the ocean. Those on land are typically insects, such as fireflies (also known as lightning bugs), or fungi, like the Brazilian “coconut flower” mushroom recently covered in Nature Conservancy magazine. (Read more stories from our December/January issue here.)

Panellus Stipticus displaying bioluminescence. Photo by Ylem / Wikimedia in the Public Domain
Panellus Stipticus displaying bioluminescence. Photo by Ylem / Wikimedia in the Public Domain

On land bioluminescence fascinates us and often works as an integral part of the organism’s reproduction. In the dark, fireflies recognize each other based on the pattern and type of light each emits. (Check out the “blue ghost” fireflies that emit a tinted light.) Fungi are believed to use their glow to attract nocturnal animals, which will then carry spores elsewhere. But, in the deep ocean, bioluminescence becomes even more critical for its organisms; there it’s the main source of light in a dark world.

“Understanding it is critical to understanding life in the ocean,” says biologist and self-described “bioluminescence junkie” Edith Widder in her popular TedTalk on the subject. Widder and her partners are responsible for some of the underwater technology that has helped scientists document and better understand the role of biological light in the ocean.

She explains that bioluminescence in the deep ocean can be used to attract mates and food, or can serve as a defense mechanism. Some organisms send light along the length of their bodies or even release clouds of light just as a squid releases ink. And that “milky sea” effect of Verne’s? Scientists aren’t positive what causes a portion of the ocean to sometimes light up so brightly it can be seen from satellites. But a common theory suggests that effect comes from enormous groupings of bacteria floating near the surface of the Indian Ocean.

Lupines and fireflies. Photo © Mike Lewinski / Flickr through a Creative Commons license
Lupines and fireflies. Photo © Mike Lewinski / Flickr through a Creative Commons license

The basic process behind much of this is a chemical reaction inside the animal. In fireflies, for example, an enzyme in the beetle’s abdomen reacts with oxygen and a chemical compound called luciferin. The resulting energy is released in the form of light.

Scientists like Widder are still eagerly studying bioluminescence throughout the world, and new examples are still being found today. That coconut flower mushroom, long thought to be extinct, was only recently rediscovered in Brazil. At 3 inches in diameter, it’s now thought to be the largest bioluminescent fungus in the world.

While scientists continue searching for more answers on these natural phenomena, you can encounter it for yourself. Here are some tips.

Look for places will little light pollution. The lights of cities or boats can make the soft glow of phytoplankton or fireflies difficult to spot. Consider the moonlight as well. A bright full moon could make the phenomenon harder to see.

Sea sparkle at the Yacht Port of Zeebrugge, Belgium. Photo © Hans Hillewaert / Wikimedia through a Creative Commons license
Sea sparkle at the Yacht Port of Zeebrugge, Belgium. Photo © Hans Hillewaert / Wikimedia through a Creative Commons license

Be kind to the fireflies. Shield any source of light, such as a cellphone, that might distract them while you watch. The National Park Service suggests covering flashlights with red or blue cellophane to avoid disturbing their mating rituals.

Head to the water. Puerto Rico’s lagoons are famous for their eerie glow. Other options? Sea kayak near Washington’s San Juan Islands or travel to Navarre Beach in Florida.

Preserve the light for the future. If you encounter a glowing mushroom, leave it where it is. And, in a broader sense, consider what may degrade the phenomenon over time. When one of Puerto Rico’s bays went temporarily dark, a team from the Department of Natural Resources suggested that storms, sewer runoff, or the removal of mangroves could be affecting the plankton’s typical ecosystem. — NCM

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  1. I cannot imagine a childhood without the magic of fireflies. A long time ago, I lived far enough out in the country that there was no light pollution. The stars were so plentiful and bright at night that you could easily walk through the fields without a candle and you could see meteors streak across the sky every night of the year. My favorite memory is of Firefly Season. There were so many of them that you couldn’t tell where the stars ended and the fireflies began. It was like walking in space among twinkling stars – just magical. More than half a century later, I still love fireflies. Now I share them in the garden with my cats, who think they’re pretty special, too!