Naturally produced light-emitting chemicals offer undersea advantages to (clockwise from top left) a pelagic worm, squid, krill, scaleless black dragonfish, and deepwater jellyfish.
Though research on bioluminescence recently garnered a Nobel Prize, the phenomenon is still poorly understood, according to a new paper reviewing recent discoveries about bioluminescence's benefits, its evolution, and the surprising diversity of ways plants and animals generate glowing substances.
Eighty percent of all creatures known to produce their own light live in the ocean, according to the report, published today in the journal Science.
"There are no hiding places in the open ocean, so a lot of animals have evolved this trick of hiding in the dark depths during the day and coming up to eat at the surface water under the cover of darkness," said Edith Widder, a marine biologist at the Ocean Research and Conservation Association in Fort Pierce, Florida.
"This means they spend most of their lives in near darkness," she said. "And bioluminescence is very useful in that kind of environment"—be it for finding food and mates, thwarting predators, or simply lighting the way.
Scientists think the Periphylla periphylla jellyfish (pictured)—one of the deep sea's most common jellies—glows for a variety of reasons, Widder said.
"If you just kind of bump it, it can release bioluminescent particles from flaps on the bottom of its body, so it can use the luminescence as a distraction," she said. "But if you keep hassling it, then it produces a spectacular display of waves and waves of light along its bell."
The flashy show is designed to attract the attention of other nearby animals, one of which might come and prey on the creature troubling the jellyfish, she said.
In a flash photo, Periphylla periphylla appears rusty orange rather than electric blue.
Most bioluminescent marine organisms have evolved to glow blue, presumably because blue light travels farther than any other color in seawater. "If you've ever opened your eyes underwater, you'll know that seawater acts like a filter and screens out all the reds and oranges and greens," Widder said.
That means glowing blue can make a big difference when attempting to attract mates—or meals. "The anglerfish has a built-in bioluminscent lure, and some animals have built-in flashlights that help them see in the dark," Widder said.
The reason for this deep-sea bamboo coral's blueish glow is a mystery, Widder said. Keratoisis flexibilis's bioluminescence might be a warning to other marine organisms that the coral's prickly spines are coated in an unpleasant slime, which scientists think might be toxic.
Seen in a flash photo, the bamboo coral's sharp spines stand out. In her other, natural-light pictures for the new study, Widder was determined to show the glowing creatures as they would look to the human eye—no simple task in dark depths.
Most color cameras aren't sensitive enough to pick up the soft glow of bioluminescent creatures in the dark, she explained.
To capture the light of the bamboo coral and other organisms, Widder used a high-resolution and very light-sensitive video camera that shoots in black-and-white. She would then take a frame from the video and "colorize" it based on detailed color measurements taken with an instrument called an optical multichannel analyzer.
When the lights are off (bottom), the body of the marine tunicate Pyrosoma atlanticum glitters with glowing blue dots. Scientists estimate that bioluminescence has evolved independently at least 40 different times among animals and bacteria.
What's more, a wide variety of light-emitting chemicals are produced by bioluminescent species. "We talk about eyes evolving multiple times" in the evolutionary history of animals, Widder said. "But that's all based on the one chemical, rhodopsin."
The fact that marine animals have evolved bioluminescence in so many different ways underscores its importance to deep-sea survival. "The selective pressure to develop this ability must be very high for so many organisms to have it," she said.