Photograph by Hiroya Minakuchi, Minden Pictures/Nat Geo Image Collection
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A comb jelly in the genus Mnemiopsis shimmers in the dark ocean.

Photograph by Hiroya Minakuchi, Minden Pictures/Nat Geo Image Collection
AnimalsExplainer

How bioluminescence works in nature

Most ocean animals produce their own light or host bacteria that do—a useful skill for communication, finding prey, camouflage, and more.

If someone says “You’re glowing!” you may be in love. Or, more likely, you’re a marine animal.

A whopping 76 percent of ocean animals are bioluminescent, which means they produce their own light through a series of chemical reactions or host bacteria that do. (Read more about luminous life in National Geographic magazine.)

It’s a separate process from biofluorescence, in which blue light hits the surface of an animal and is reemitted as a different color, usually orange, red, or green.

Marine creatures rely on bioluminescence for communication, finding prey, camouflage, and more. It’s so important, in fact, that the trait has evolved 27 times among ray-finned fishes, a huge group that makes up half of all vertebrate species alive today.

Why Deep-Sea Creatures Glow

The mysteries of why and how many animals in the deep sea emit light, known as bioluminescence, continue to puzzle scientists studying the evolution of this natural glow.

Some terrestrial critters also glow; some famously, such as fireflies, and others more under the radar, like mushrooms or beetles.

Who glows?

At least 1,500 species of fish are known to be bioluminescent, including sharks and dragonfish—and scientists regularly discover new ones.

Among the most iconic are deep-sea fishes like the anglerfish, whose females sport a lure of glowing flesh that acts as bait for any prey close enough to be snatched.

Hawaiian bobtail squid light up via bioluminescent bacteria living in one of their organs; the light camouflages them against moonlight on the surface and eliminates their shadow, obscuring them from predators. (Read about nature’s living fireworks—animals that bioluminesce.)

If you shine a light on a comb jelly, light refracted off its moving cilia might be mistaken for bioluminescence.

Their true bioluminescence cannot be seen in light, says marine biologist Edie Widder, founder of the Ocean Research and Conservation Association. Their real bioluminescence comes from light-producing chemicals which different species use in different ways, such as flashing the chemicals to deter predators.

Then there’s the world’s smallest shark, the six-inch lanternshark, which advertises its own goods via photophores (or light-producing organs) clustered around its reproductive organs.

An underwater glow. A fleeting gleam across a field. These lights seem mysterious, but organisms generate them for practical purposes. Bioluminescence fends off predators, lures prey, and attracts mates. Making light is such a useful trait that it has evolved independently at least 40 times. It occurs most commonly in the ocean, where bioluminescence is often the only source of light. Under the right conditions, a bioluminescent flash can be seen a hundred yards away.
(Read more about “Luminous Life” in this month’s issue of National Geographic.)

Click each illustration to see it animate

Defense


  • Surprise

    The prey produces a bright flash that startles a predator, making it easy to escape.

  • Smoke Screen

    The prey emits a glowing fluid or a cloud of sparks to misdirect the predator from its real location.

  • Decoy

    The prey jettisons one of its body parts. The luminescent limb distracts the predator, allowing escape.

  • Camouflage

    A shining underbelly matching the light from the surface conceals prey from predators below.

  • Alarm

    The prey’s bioluminescence makes its predator visible—alerting the predator’s predators.

  • Warning

    Gleaming prey signals to a predator that its next meal could taste terrible—or even be toxic.

Offense


  • Shock

    A burst of bright light from a bioluminescent predator stuns prey and leaves it open to attack.

  • Lure

    Like a moth to a flame, prey is drawn to the glow produced by a predator lurking all too close.

  • Beacon

    Predators seek out the glimmer that tells them that bioluminescent creatures are gathering.

  • Searchlight

    A predator turns on its natural spotlight to locate prey in a dark ocean.

Reproduction


  • Come-On

    Flickers of light signal that a bioluminescent insect is ready to meet new mates.

  • Invitation

    Mushrooms may spread their spores by using luminescence to entice insects to land on them.

Jason Treat and Daniela Santamarina, NGM Staff. Art: Eleanor Lutz. Animations: J. L. Wang. Source: Steven Haddock, Monterey Bay Aquarium Research Institute

Males and females are “strutting their stuff, showing where their stuff is,” says George Burgess, formerly of the Florida Museum of Natural History. Each species has a specific light pattern, “like a name tag,” so they can find mates in the dark ocean depths, he adds.

Why don’t freshwater animals glow?

The reason is simple: “Very few bioluminescent fish can tolerate low salinity,” says marine biologist Edie Widder, founder of the Ocean Research and Conservation Association.

The only instance of bioluminescence in a freshwater animal is Latia neritoides, a limpet-like snail native to New Zealand streams, where it emits a glowing slime when bothered.

Where to see glowing animals

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The bioluminescent bay on the Puerto Rican island of Vieques teems with microscopic life called dinoflagellates.

Bioluminescent marine plankton really put on a show.

Some species of dinoflagellates light up using a similar to chemical reaction to that of fireflies; both use a naturally occurring molecule called luciferin, named for Lucifer, the light-bearer. (Read how fireflies glow.)

Millions of these one-celled organisms create a beautiful shimmering effect, particularly when there’s little moonlight; in Puerto Rico, tourists can kayak through the undulating, electric blue water of three bioluminescent bays.

Movement stimulates the dinoflagellates, so if a fish happens to swim through the water, “you would expect to see [its] luminescent outline,” says Senjie Lin, a marine biologist at the University of Connecticut who specializes in these plankton. (Visit these eight incredible bioluminescent phenomena around the world.)

The plankton’s glow is usually blue, “but when it is intense, it can look like white to human eyes,” Lin adds.

And if you happen to be in Australia or New Zealand, you can visit caves of glowworms—actually the larvae of a small fly, Arachnocampa flava—that dangle sticky bioluminescent threads to lure and capture unlucky prey.