The ability to make light—bioluminescence—is both commonplace and magical. Magical, because of its glimmering, captivating beauty. Commonplace, because many life-forms can do it. On land the most familiar examples are fireflies, flashing to attract mates on a warm summer night. But there are other luminous landlubbers, including glowworms, a snail, some millipedes, and—you are not hallucinating—certain mushrooms.
bioluminescence (bī´ōlōō´mĬnĕs´əns), production of light by living organisms. Organisms that are bioluminescent include certain fungi and bacteria that emit light continuously. The dinoflagellates, a group of marine algae, produce light only when disturbed. Bioluminescent animals include such organisms as ctenophores, annelid worms, mollusks, insects such as fireflies, and fish. The production of light in bioluminescent organisms results from the conversion of chemical energy to light energy. In fireflies, one type of a group of substances known collectively as luciferin combines with oxygen to form an oxyluciferin in an excited state, which quickly decays, emitting light as it does. The reaction is mediated by an enzyme, luciferase, which is normally bound to ATP (see adenosine triphosphate) in an inactive form. When the signal for the specialized bioluminescent cells to flash is receive, the luciferase is liberated from the ATP, causes the luciferin to oxidize, and then somehow recombines with ATP. Different organisms produce different bioluminescent substances. Bioluminescent fish are common in ocean depths; the light probably aids in species recognition in the darkness. Other animals seem to use luminescence in courtship and mating and to divert predators or attract prey.

But the real light show takes place in the sea. Here an astonishing array of beings can make light. Such as ostracods—tiny animals that look like sesame seeds with legs—flashing to attract mates, like seafaring fireflies. Or dinoflagellates—speck-of-dust-size beings named for their two whiplike flagella and the whirling motion they make (dinos means “whirling” in Greek). Dinoflagellates light up whenever the water around them moves; they are the critters typically responsible for the sparks and trails of light you sometimes see when swimming or boating on a dark night.

Then there are lightmaking fish, squid, jellyfish, shrimp, the aforementioned ctenophores, several types of worms, and sea cucumbers. There are luminous siphonophores—sinister, stringlike predators with long, stinging tentacles that hang down like a curtain. And there are luminous radiolarians—amoeboid beings that typically live in colonies built on exquisite glass scaffolds. Not to mention glowing bacteria. Indeed, of all the groups of organisms known to make light, more than four-fifths live in the ocean.