the emission of light by an organism or by a test-tube biochemical system derived from an organism. It could be the ghostly glow of bacteria on decaying meat or fish, the shimmering phosphorescence of protozoans in tropical seas, or the flickering signals of fireflies. The phenomenon occurs sporadically in a wide range of protists and animals, from bacteria and fungi to insects, marine invertebrates, and fish; but it is not known to exist naturally in true plants or in amphibians, reptiles, birds, or mammals. Bioluminescence results from a chemical reaction (chemiluminescence) in which the conversion of chemical energy to radiant energy is direct and virtually 100 percent efficient; i.e., very little heat is given off in the process. For this reason the emission is called cold light, or luminescence (q.v.)

Light production appears to be associated with the protection and survival of a species. This is quite clear in certain squids, who secrete a luminous cloud to confuse an enemy and make an escape, and in many deep-sea fishes (deep-sea fish) who dangle luminous lures to attract prey or who show light organs to disguise their form from enemies, frighten predators, or simply light the way in the darkness of the ocean deeps. The survival value of bioluminescence is indisputable for many organisms who use their flashes as species-recognition and mating signals. In Photinus pyralis, a common North American firefly, the male flashes spontaneously while in flight, emitting on the average a 0.3-second flash every 5.5 seconds if the temperature is 25° C (77° F). The females watch from the ground and wait for a male to flash. Upon seeing a flash, a female flashes a response after an interval of about two seconds; it is this response that attracts the male. The female is unable to identify a male by his flashing; thus it is the male that recognizes the correct signal—i.e., interval between flashes—and seeks out the female. The interval between the male's signal and the female's response, therefore, is crucial. Similar specific recognition codes are used by many species of fireflies. Other fireflies possibly rely on colour differences in the light signals between sexes.

Lantern fishes (lantern fish) and hatchetfishes (hatchetfish), along with many other deep-sea organisms, possess distinct arrangements of light organs on the body that may serve as species- and sex-recognition patterns. The light organs, or photophores (photophore), of many deep-sea fishes (fish) are placed on the ventral and lateral surfaces of the body, and the light is emitted downward and outward. Such an arrangement is believed to allow the light of the photophores to be used to match the intensity of sunlight penetrating from above, thus concealing the fish's own shadow from a predator below. Some lantern fishes possess, in addition, a large nasal organ; others have a patch of luminous tissue in the tail region. In deep-sea angler fishes (anglerfish), the first dorsal spine is turned forward into an elongated rod, from the end of which dangles a luminous organ. When an unsuspecting prey approaches the luminous lure, it is engulfed in the fish's large jaw.

The functional role of bioluminescence in lower organisms such as bacteria, dinoflagellates, and fungi is difficult to discern. Partly because the glow of luminous bacteria is extinguished when oxygen is removed, it has been suggested that the bioluminescent reaction was originally used to remove oxygen toxic to primitive types of bacteria that developed during a time when oxygen was absent or very rare in the Earth's atmosphere. The metabolic reaction that combines the oxygen with a reducing substance ( luciferin) liberates sufficient energy to excite a molecule in the organism to emit visible radiation. Most of those luminous primitive organisms have subsequently developed systems of utilizing oxygen, but they have retained the luminescent capability as parts of related metabolic pathways or for some survival value that luminescence may confer on the organism.

Luminous species are widely scattered taxonomically, with no clear-cut pattern discernible. Many luminous shrimps (shrimp) are known but no luminous crab. Many luminous squids (squid) are known but only a single luminous octopus (Callistoctopus arakawai of Japan). Again, luminous centipedes and millipedes are not uncommon, but luminous scorpions and spiders are apparently nonexistent. Many plantlike protists (protist) (bacteria and fungi) exhibit bioluminescence, but no luminous true plant is known.

Almost half the animal phyla contain luminous forms, but the number of representatives is very small compared with the total number of known animal species. The protists are not so rich in luminous species but are greatest in sheer abundance, especially in tropical seas. In fact, the majority of luminous organisms are marine.

The ocean surface in many parts of the tropics is dense with single-celled luminous planktonic (plankton) organisms, primarily dinoflagellates (dinoflagellate), that glow when stimulated mechanically, as by the churning of the waves, or, when washed ashore, by the pressure of a foot. Some organisms exhibit a 24-hour rhythm of light intensity, highest at night and lowest during the day.

Among crustaceans, luminous species are especially remarkable in the copepods (copepod), shrimps, and ostracods. Luminous copepods are widely distributed throughout the world's waters. Some are surface dwellers, while others live in the deep sea. Two famous groups of luminous copepods are Pleuromma and Metridia. Some shrimps (Hoplophorus) emit a luminous secretion from luminous organs, while others possess true light organs (photophores), which consist of a lens, reflector, and light-emitting photogenic cells. Of the three or four species of the ostracod (mussel shrimp) genus Cypridina known to be luminous, the most famous is Cypridina hilgendorfii, found in the coastal waters and sands of Japan. This tiny, shelled organism, which ejects a blue luminous secretion into the water when disturbed, may be collected and dried for the light-emitting components, which are active indefinitely.

Other organisms responsible for large patches of light in the ocean are jellyfish and other coelenterates and comb jellies (ctenophores (ctenophore)). A large proportion of the floating, transparent siphonophores and the feathery, bottom-dwelling sea pens are luminous. Many of the hydroids and jellyfish are also luminous. Sea pens (Pennatula), sea cactus (Cavernularia), and sea pansy (Renilla) are colonies, which upon stimulation generate a wave of luminous light that travels down the organism. The luminescence in these organisms appears to be under nervous control.

Among annelids (annelid), marine worms and earthworms both contain luminous forms. Odontosyllis, the fire worm (fireworm) of Bermuda, swarms in great numbers a few days after the full moon. Female worms, about 2 cm (almost 1 inch) in length, rise to the surface shortly after sunset and swim in circles while ejecting a luminous secretion. Smaller male worms swim to where the females are circling and mate. The male is also luminous, but the light is intermittent and of intracellular character. It is not certain whether luminescence has any relationship to mating, since nonluminous Odontosyllis exhibits similar courtship behaviour. Chaetopterus (parchment worm) spends its life in a tube of parchment membrane, with openings at both ends. It luminesces when disturbed, but it is doubtful whether the luminescence has any special purpose. Polynoe and Polycirrus are luminous annelids that usually live in sand or rock. Luminous mollusks (mollusk) include Pholas (a bivalve), Phyllirrhoe (a floating nudibranch), Planaxis (a marine gastropod), Latia (a freshwater limpet), and squids (cephalopods).

The luminous squids and deep-sea fishes possess the most complicated light organs; they consist of photogenic cells, reflector, lens body, and, in certain cases, colour filters. Of the open-ocean squids (oegopsids) such as Lycoteuthis, Histioteuthis, and Enoploteuthis, as many as 75 percent are self-luminous; i.e., light results not from symbiotic luminous bacteria but from an internal biochemical reaction. In deep-sea squids, the light organ is often found on the eyelid or on the eyeball itself. In others—e.g., Watasenia scintillans—light organs are present also at the end of the tentacles and over other surfaces of the body.

A few genera of deep-sea fishes and several families of shallow-water fishes produce light by virtue of harbouring symbiotic luminous bacteria within light organs. This type of organ is endowed with a rich blood supply that nourishes and maintains the luminous bacteria. It appears that each fish species becomes infected with a specific bacterial type. The bacteria-filled organ is continuously luminous, but the light can be controlled either by melanophores scattered over the surface of the organ or by a black membrane that may be mechanically drawn over the organ. Control is brought about by the contraction and expansion of melanophores, or pigment granules. Expansion of the melanophores cuts off the light, whereas contraction allows light to pass through. The well-known flashlight fishes (flashlight fish) (Photoblepharon) of Indonesia possess large light organs beneath the eyes. The light is extinguished when a fold of black skin is drawn upward over the organ.

An indirect-emission type of luminous organ is present in some fish. The luminous organ is connected to the gut via a short duct and is often embedded in tissue. The light passes to the outside through the translucent keel and ventral muscles, as in Leiognathus, Acropoma (lanternbellies), and Archamia.

Among other higher animals, the chordate subphylum Tunicata (tunicate) contains luminous forms. The genus Pyrosoma includes several species that account for the brilliant luminescence among macroplanktons of the seas, giving rise to the name “fire body.” Pyrosoma is a floating colonial form, pelagic and translucent. The colonies usually reach a length of 3 to 10 cm (about 1 to 4 inches); and each individual is about 5 mm (0.2 inch) long.

Luminous plantlike protists are represented by only two groups, bacteria and fungi. Luminous bacteria are all marine forms, requiring salt for growth and luminescence, and are widely distributed throughout the oceans of the world. The most common are Vibrio and Photobacterium species. While luminous bacteria come in various shapes, they do not form clusters or chains, as do many other bacteria. The light of an individual bacterium, of course, cannot be seen with the naked eye, but the light from a liquid or agar culture containing billions of bacteria is readily visible. The light is bluish and continuous. Many luminous bacteria live in the light organs of fish and squids, without adversely affecting their hosts.

Small, whitish, luminous fungi (fungus) (“foxfire”) commonly grow on dead wood of forests, particularly where the ground is moist and wet; these forms predominate in the tropics. The light of fungi ranges from blue to green and yellow, depending on the species. Among the large luminous forms are Pleurotus lampas of Australia and the jack-o'-lantern (Clitocybe illudens) of the United States, which reach approximately 13 cm (about 5 inches) in diameter.

Luminosity among land animals is not associated with any particular habitat, but almost all these forms are nocturnal. The centipede Orphaneus, widely distributed in tropical Asia, gives off luminous secretions from each segment. The entire body of Luminodesmus sequoiae, a millipede found in the Sierra Nevada (mountains) of California, glows with a diffuse light. Luminous insects include some true flies (order Diptera (dipteran)), notably Arachnocampa luminosa, the larva of which luminesces a greenish blue from a knob at the end of its body. The larvae dangle at the ends of filaments that hang from the ceilings of caves in New Zealand. Luminous beetles include the fireflies and the elaterid Pyrophorus (the click beetle, or cucujo in South America). The luminescent larvae of fireflies and some luminescent wingless adults are known as glowworms. The female Diplocladon hasseltii, called starworm, or diamond worm, gives off a continuous greenish blue luminescence from three spots on each segment of the body, forming three longitudinal rows of light, the appearance of which inspired the common name night train. Phrixothrix, the railroad worm, possesses two longitudinal rows, with a red luminous spot on the head.

The limpet Latia neritoides, found in streams around Auckland, N.Z., is the only strictly freshwater luminous form known. The so-called firefly shrimp (hotaru ebi) is found in Lake Suwa, Japan, but the light is from luminous bacteria that infect the shrimp and kill it in about 24 hours.

In most bioluminescent organisms, the essential light-emitting components are the oxidizable organic molecule luciferin and the enzyme luciferase, which are specific for different organisms. The present custom is to use generic names according to origin—e.g., firefly luciferin and luciferase, Cypridina luciferin and luciferase. The luciferin-luciferase reaction is actually an enzyme-substrate reaction in which luciferin, the substrate, is oxidized by molecular oxygen, the reaction being catalyzed by the enzyme luciferase, with the consequent emission of light. The light emission continues until all the luciferin is oxidized. This type of reaction is found in fireflies, Cypridina, Latia, and many types of fish, such as lantern fish, hatchetfish, Apogon, and Parapriaeanthus.

In firefly luminescence, the substance adenosine triphosphate (ATP) initially reacts with firefly luciferase, magnesium ion, and firefly luciferin to form a complex (luciferase-luciferyl-adenylate) and pyrophosphate. This complex then reacts with molecular oxygen to emit light. Enough energy is liberated in the last step to convert the electronic configuration of the luciferase-luciferyl-adenylate complex from a low-energy ground state to a high-energy excited state. The high-energy complex then loses energy by radiating a photon of visible light and returns to the ground state.

Luminescent bacteria employ the enzymatic oxidation of reduced flavin mononucleotide (FMNH₂). In the complete reaction, bacterial luciferase reacts with FMNH₂ and oxygen to form a long-lived intermediate complex, which then reacts with a long-chain aliphatic aldehyde molecule (e.g., decanal) to emit light.

The luminescent reaction of the firefly has been used as an assay (assaying) method for the determination of adenosine triphosphate (ATP), an important metabolic substance used by all living cells in numerous reactions in which energy is either stored or expended. The glow of a specially blended extract of firefly lanterns eventually dims and disappears as ATP is broken down. The addition of fresh ATP, either as a pure chemical or as a constituent of a tissue extract, immediately restores the luminescence. The intensity of the glow is a direct measure of the amount of ATP present in the extract. This assay method has been widely used in medical and biological research to determine the amount of ATP present in extracts of cells and tissues. The study of reactions involving ATP has led to a detailed understanding of the mechanisms of energy conversion in cells. The firefly reaction is one of the few reactions in which ATP is directly involved with light emission. All other bioluminescent reactions involve compounds that are chemically distinct from ATP.