If someone says â€śYouâ€™re glowing!â€ť you may be in love. Or, more likely, youâ€™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.)
Itâ€™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. Itâ€™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.
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 natureâ€™s living fireworksâ€”animals that bioluminesce.)
The Leidys comb jelly has photophores inside its bell, or main body, that refract light, producing a shimmering rainbow to startle predators.
Then thereâ€™s the worldâ€™s smallest shark, the six-inch lanternshark, which advertises its own goods via photophores (or light-producing organs) clustered around its reproductive organs.
Males and females are â€śstrutting their stuff, showing where their stuff is,â€ť says George Burgess, formerly of the Florida Museum of Natural History. Each species has a specific light pattern, â€ślike a name tag,â€ť so they can find mates in the dark ocean depths, he adds.
The reason is simple: â€śVery few bioluminescent fish can tolerate low salinity,â€ť says marine biologist Edie Widder, founder of the Ocean Research and Conservation Association.
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 thereâ€™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, â€śyou would expect to see [its] luminescent outline,â€ť 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 planktonâ€™s glow is usually blue, â€śbut when it is intense, it can look like white to human eyes,â€ť Lin adds.
And if you happen to be in Australia or New Zealand, you can visit caves of glowwormsâ€”actually the larvae of a small fly, Arachnocampa flavaâ€”that dangle sticky bioluminescent threads to lure and capture unlucky prey.