Caves full of stars, trees covered with flashing lights, and boats that leave a trail of light through dark waters. These sound like science fiction or movie special effects. They're really products of amazing tiny creatures that can glow in the dark.
Maybe you've seen little spots of light flitting through the air at night. These fireflies are the best-known group of glowing creatures. Sometimes called lightning bugs, they are really beetles with light organs under their tails. More than 1,900 kinds of fireflies produce their own light. They flash their lights on and off. Each kind uses it own special code. One firefly can recognize another of its own kind by the number of flashes, the color of the light, and the time between flashes.
Some fireflies "gang up" to get attention. In Thailand, Malaysia, Burma, and the Philippines, millions of male fireflies gather in trees. They all start flashing their lights at the same time. The tree looks like a big neon sign. This attracts female fireflies.
Some creatures glow for other reasons. The glowworm isn't really a worm. It's a firefly in an early stage of development called the larval stage. Most adult fireflies never eat, because they did all their eating when they were larvae. The light from a larva's glowing body attracts tiny flies and mosquitoes for the larva to eat.
The Waitomo Caves in New Zealand house a memorable type of glowworm. Tourists entering the Glowworm Grotto in small boats see thousands of lights on the cave ceiling. The glowworms look like stars in a night sky. If you cough or use a camera flash, the lights instantly go off. But wait quietly for a few minutes and they flicker back on, until the cave-ceiling "sky" is again filled with "stars."
Many glowing creatures live in water. Anglerfish live deep in the ocean where sunlight never reaches. Some have a rod on their heads with a tip that glows in the dark. The anglerfish waves this "fishing rod" at its prey, then pulls the rod in to lure the prey closer. Like the glowworm, the anglerfish uses its light to catch its dinner.
Other fish use their lights for protection. Some squid that live in dark waters can eject a cloud of light to distract enemies. Lantern fish hide from enemies below them by producing light that matches the sunlight or moonlight coming through the water above.
Ancient sailors saw 'sea fire'
Have you ever heard of a red tide? This happens when tiny one-celled organisms reproduce by the millions. These organisms, called dinoflagellates (di-no-FLAJ-uh-lates), live in all the earth's oceans and produce light when the water is stirred. A boat passing through them at night will leave a glowing trail behind it. When millions of newborn organisms turn the water reddish-brown, it's called a red tide.
Ancient sailors sometimes saw the sea behind their ship aglow with bright blue lights. They called it "sea fire." But this light has nothing to do with fire. The light from these sea creatures has no heat at all.
Light produced by animals or fungi is not hot. It is called "cold light" or bioluminescence. "Bio" (BY-oh) means "living," and "luminescence" (LOO-muh-ness-uns) means "light." Creatures make the light by combining special substances in their bodies. In fireflies, a substance called luciferin (loo-SIF-uh-rin) reacts with oxygen when another substance, luciferase (loo-SIF-uh-raze), is present. Other creatures may use different substances, but the light is always made by a chemical reaction.
Glowing pizzas, too?
While many glowing creatures live deep in the oceans, fireflies and others help to make our nights a little brighter on dry land. Even some types of mushrooms produce light. The ones on your pizza won't glow in the dark. But others, like the orange milkcap, give off a steady glow. Many other types of fungus can create light, too. So can some millipedes, jellyfish, and bacteria.
We have a pretty good idea why some living things produce light. For others, we can only guess. Bioluminescence isn't as mysterious as it used to be, but there is still much to learn about it - and many wonders to enjoy.