“There’s a whole netherworld of the deep sea that we don’t see when we have our lights on,” says Kevin Raskoff, a scientist at California State University, Monterey Bay. “And that’s the natural light of the deep sea: bioluminescence.”
“It’s such a bizarre and exciting phenomenon to see,” agrees Steven Haddock of the Monterey Bay Aquarium Research Institute. “The first time you see one of these animals just glow, it’s pretty amazing.”
Bioluminescence is light produced by a chemical process within a living organism. The glow occurs when a substance called luciferin reacts with oxygen. This releases energy, and light is emitted. An enzyme called luciferase facilitates the reaction. Sometimes luciferin and luciferase are bound together with oxygen into a single molecule, or photoprotein. When an ion such as calcium is present, an ensuing reaction emits light. To glow on a regular basis, an organism must continually bring fresh luciferin into its system. Some acquire it through their diet; others produce their own.
Bioluminescence is relatively rare on land. It is most commonly seen among certain insect species like fireflies and glowworms (a form of insect larvae); some mushrooms and fungi also glow in the dark. In the deep sea, however, bioluminescence is found in virtually every type of animal: squids, octopuses, fishes, shrimps, single-celled organisms, and jellies of all kinds. Two thousand years ago, the Roman scholar Pliny the Elder noted that if he rubbed the slime of Pulmo marinus, a jellyfish from the Bay of Naples, on his walking stick, it “will light the way like a torch.” Raskoff estimates that 90 percent of all the animals in the deep sea are in fact bioluminescent. “In the deep sea, it’s the norm. You’re odd man out if you don’t bioluminesce.”
Underwater, bioluminescence finds all manner of purpose. Some animals use it to attract mates. A male sea-firefly (Vargula hilgendorfii) will squirt out a bright dot of light, zip upward, and then squirt another and another, essentially drawing an arrow that points out his whereabouts. Other creatures use bioluminescence to detect or lure prey. The viperfish ( Chauliodus sloani) dangles a luminescent lure in front of its mouth and then snaps up any creature that dares to investigate.
Other organisms use their bioluminescence to fend off or dupe predators. The deep-sea shrimp (Acanthephyra purpurea) vomit bioluminescent goop into the face of threatening diners, presumably either as a scare tactic or to create a distraction while the shrimp escapes. Other organisms seem to employ their bioluminescence as a kind of defensive burglar alarm: they light up to attract a second predator that will eat the first one (or to make the first predator think that a second one is coming, and so prompt it to leave).
For still other animals, bioluminescence provides camouflage. Certain species of squid bioluminesce only on the underside of their bodies, so they match the background light shining down from above; this hides their silhouettes from any predators or prey below. Many shrimp and fish emit a constant, dim glow to match the ambient light around them. In short, there is no single answer for why organisms bioluminesce, and no shortage of scientific debate around the subject. “We have little tantalizing indications of how it may benefit them,” Haddock says. “But we don’t have really a good explanation that applies across the board.”
A larger mystery is how bioluminescence evolved in the first place. In recent lab experiments, Haddock has found that many jellyfish don’t produce their own luciferin; rather, they acquire it from their diet, which consists of small, bioluminescent crustaceans. This suggests to Haddock that, although jellyfish first emerged hundreds of millions of years ago, they gained their bioluminescent abilities much later, after consuming luciferin proved to be advantageous.
Scientists themselves have had to adapt in order to study bioluminescence. The collection of live jelly specimens, made possible by the development of submersible vehicles, has made it easier for researchers like Haddock to study bioluminescence up close in the lab. Edith Widder, a marine scientist at the Harbor Branch Oceanographic Institution in Florida, is developing “Eye in the Sea,” a supersensitive camera that will sit on the seafloor and watch bioluminescent organisms light up in their natural environment.
And much like jellies, many scientists have even incorporated bioluminescence into their own work lives, often unaware of its original origin. Photoproteins, first isolated from jellyfish several decades ago, are now an integral part of laboratory biology and help researchers do things like mark and identify crucial gene sequences in medical studies.
“Jellies are important for humans,” Haddock says. “They have provided us with a lot of the tools that we use now in molecular biology. You can have a biomedical researcher who is using a photoprotein that came from a jellyfish. He has no idea where it came from. He just knows that it’s one of the most useful tools that he has in his lab.”
Therein lies the importance of doing of basic research in the natural world, he adds. One never knows where a discovery might lead, or when the study of a weird, or cool, or seemingly unimportant phenomenon might shed light on everyday human matters. “You can be asking a question just based on your curiosity,” says Haddock, “trying to figure out how these organisms make this light, how did it come about, without thinking all the way ahead to all the ways that it might be used to cure cancer someday. Yet the tools that come out of this phenomenon can then be applied to a lot of things that really impact everyone.”
Information on bioluminescence from Harbor Branch Oceanographic Institution