Bioluminescence is light produced by a chemical reaction within a living organism. Bioluminescence is a type of chemiluminescence, which is simply the term for a chemical reaction where light is produced. (Bioluminescence is chemiluminescence that takes place inside a living organism.) 
 
Bioluminescence is a "cold light." Cold light means less than 20% of the light generates thermal radiation, or heat.
 
Most bioluminescent organisms are found in the ocean. These bioluminescent marine species include fish, bacteria, and jellies. Some bioluminescent organisms, including fireflies and fungi, are found on land. There are almost no bioluminescent organisms native to freshwater habitats.
 
Chemistry
 
The chemical reaction that results in bioluminescence requires two unique chemicals: luciferin and either luciferase or photoprotein. Luciferin is the compound that actually produces light. In a chemical reaction, luciferin is called the substrate. The bioluminescent color (yellow in fireflies, greenish in lanternfish) is a result of the arrangement of luciferin molecules.
 
Some bioluminescent organisms produce (synthesize) luciferin on their own. Dinoflagellates, for instance, bioluminesce in a bluish-green color. Bioluminescent dinoflagellates are a type of plankton—tiny marine organisms that can sometimes cause the surface of the ocean to sparkle at night. 
 
Some bioluminescent organisms do not synthesize luciferin. Instead, they absorb it through other organisms, either as food or in a symbiotic relationship. Some species of midshipman fish, for instance, obtain luciferin through the "seed shrimp" they consume. Many marine animals, such as squid, house bioluminescent bacteria in their light organs. The bacteria and squid have a symbiotic relationship. 
 
Luciferase is an enzyme. An enzyme is a chemical (called a catalyst) that interacts with a substrate to affect the rate of a chemical reaction. The interaction of the luciferase with oxidized (oxygen-added) luciferin creates a byproduct, called oxyluciferin. More importantly, the chemical reaction creates light.
 
Bioluminescent dinoflagellates produce light using a luciferin-luciferase reaction. The luciferase found in dinoflagellates is related to the green chemical chlorophyll found in plants. 
 
Bioluminescent dinoflagellate ecosystems are rare, mostly forming in warm-water lagoons with narrow openings to the open sea. Bioluminescent dinoflagellates gather in these lagoons or bays, and the narrow opening prevents them from escaping. The whole lagoon can be illuminated at night. Biologists identified a new bioluminescent dinoflagellate ecosystem in the Humacao Natural Reserve, Puerto Rico, in 2010.
 
Most bioluminescent reactions involve luciferin and luciferase. Some reactions, however, do not involve an enzyme (luciferase). These reactions involve a chemical called a photoprotein. Photoproteins combine with luciferins and oxygen, but need another agent, often an ion of the element calcium, to produce light.
 
Photoproteins were only recently identified, and biologists and chemists are still studying their unusual chemical properties. Photoproteins were first studied in bioluminescent crystal jellies found off the west coast of North America. The photoprotein in crystal jellies is called "green fluorescent protein" or GFP.
 
Bioluminescence is not the same thing as fluorescence, however. Florescence does not involve a chemical reaction. In fluorescence, a stimulating light is absorbed and re-emitted. The fluorescing light is only visible in the presence of the stimulating light. The ink used in highlighter pens is fluorescent. Phosphorescence is similar to florescence, except the phosphorescent light is able to re-emit light for much longer periods of time. Glow-in-the-dark stickers are phosphorescent. 
 
Bioluminescent Light
 
The appearance of bioluminescent light varies greatly, depending on the habitat and organism in which it is found. 
 
Most marine bioluminescence, for instance, is expressed in the blue-green part of the visible light spectrum. These colors are more easily visible in the deep ocean. Also, most marine organisms are sensitive only to blue-green colors. They are physically unable to process yellow, red, or violet colors. 
 
Most land organisms also exhibit blue-green bioluminescence. However, many glow in the yellow spectrum, including fireflies and the only known land snail to bioluminesce, Quantula striata, native to the tropics of Southeast Asia.
 
Few organisms can glow in more than one color. The so-called railroad worm (actually the larva of a beetle) may be the most familiar. The head of the railroad worm glows red, while its body glows green. Different luciferases cause the bioluminescence to be expressed differently.
 
Some organisms emit light continuously. Some species of fungi present in decaying wood, for instance, emit a fairly consistent glow, called foxfire.
 
Most organisms, however, use their light organs to flash for periods of less than a second to about 10 seconds. These flashes can occur in specific spots, such as the dots on a squid. Other flashes can illuminate the organism's entire body.
 
Adaptations
 
Bioluminescence is used by living things to hunt prey, defend against predators, find mates, and execute other vital activities.
 
Defensive Adaptations
Some species luminesce to confuse attackers. Many species of squid, for instance, flash to startle predators, such as fish. With the startled fish caught off guard, the squid tries to quickly escape.
 
The vampire squid exhibits a variation of this defensive behavior. Like many deep-sea squid, the vampire squid lacks ink sacs. (Squid that live near the ocean surface eject dark ink to leave their predators in the dark.) Instead, the vampire squid ejects sticky bioluminescent mucus, which can startle, confuse, and delay predators, allowing the squid to escape.
 
Many marine species use a technique called counterillumination to protect themselves. Many predators, such as sharks, hunt from below. They look above, where sunlight creates shadows beneath prey. Counterillumination is a type of camouflage against this predatory behavior. 
 
Hatchetfish use counterillumination. Hatchetfish have light-producing organs that point downward. They adjust the amount of light coming from their undersides to match the light coming from above. By adjusting their bioluminescence, they disguise their shadows and become virtually invisible to predators looking up.
 
Some bioluminescent animals, such as brittle stars, can detach body parts to distract predators. The predator follows the glowing arm of the brittle star, while the rest of the animal crawls away in the dark. (Brittle stars, like all sea stars, can re-grow their arms.)
 
When some animals detach body parts, they detach them on other animals. When threatened, some species of sea cucumber can break off the luminescent parts of their bodies onto nearby fish. The predator will follow the glow on the fish, while the sea cucumber crawls away.
 
Biologists think that some species of sharks and whales may take advantage of defensive bioluminescence, even though they are not bioluminescent themselves. A sperm whale, for instance, may seek out a habitat with large communities of bioluminescent plankton, which are not part of the whale's diet. As the plankton's predators (fish) approach the plankton, however, their glowing alerts the whale. The whale eats the fish. The plankton then turn out their lights.
 
Some insect larvae (nicknamed "glow worms") light up to warn predators that they are toxic. Toads, birds, and other predators know that consuming these larvae will result in illness and possible death.
 
Offensive Adaptations
Bioluminescence may be used to lure prey or search for prey.
 
The most famous predator to use bioluminescence may be the anglerfish, which uses bioluminescence to lure prey. The anglerfish has a huge head, sharp teeth, and a long, thin, fleshy growth (called a filament) on the top of its head. On the end of the filament is a ball (called the esca) that the anglerfish can light up. Smaller fish, curious about the spot of light, swim in for a closer look. By the time the prey sees the enormous, dark jaws of the anglerfish behind the bright esca, it may be too late.
 
Other fish, such as a type of dragonfish called loosejaws, use bioluminescence to search for prey. Loosejaws have adapted to emit red light; most fish can only see blue light, so loosejaws have an enormous advantage when they light up a surrounding area. They can see their prey, but their prey can't see them.
 
Attraction
Adult fireflies, also called lightning bugs, are bioluminescent. They light up to attract mates. Although both male and female fireflies can luminesce, in North America most flashing fireflies are male. The pattern of their flashes tells nearby females what species of firefly they are and that they're interested in mating.
 
Other Bioluminescence
Organisms can luminesce when they are disturbed. Changes in the environment, such as a drop in salinity, can force bioluminescent algae to glow, for instance. These living lanterns can be seen as spots of pink or green in the dark ocean.
 
"Milky seas" are another example of bioluminescence. Unlike bioluminescent algae, which flash when their environment is disturbed, milky seas are continuous glows, sometimes bright and large enough to be visible from satellites in orbit above the Earth. 
 
Scientists think milky seas are produced by bioluminescent bacteria on the surface of the ocean. Millions of bacteria must be present for milky seas to form, and conditions must be right for the bacteria to have enough chemicals to light up. Satellite imagery of milky seas have been captured in tropical waters such as the Indian Ocean.
 
Bioluminescence and People
 
Biologists and engineers are studying the chemicals and circumstances involved in bioluminescence to understand how people can use the process to make life easier and safer.
 
Green fluorescent protein (GFP), for instance, is a valuable "reporter gene." Reporter genes are chemicals (genes) that biologists attach to other genes they are studying. GFP reporter genes are easily identified and measured, usually by their fluorescence. This allows scientists to trace and monitor the activity of the studied gene—its expression in a cell, or its interaction with other chemicals.
 
Other uses are more experimental. Bioluminescent trees, for instance, could help light city streets and highways. This would reduce the need for electricity. Bioluminescent crops and other plants could luminesce when they needed water or other nutrients, or when they were ready to be harvested. This would reduce costs for farmers and agribusiness.
bioluminescence
Glow, little glow worms.
Dark Freshwater
Most scientists point to two reasons why so few freshwater organisms exhibit bioluminescence. 
  • First, freshwater habitats have not been around as long as marine habitats—evolution is a slow process and freshwater habitats do not yet have the biodiversity of oceans. 
  • Second, freshwater species wouldn't really benefit from bioluminescence. Freshwater habitats are often murkier, and deepwater species use other adaptations (such as a catfish's sensitive "whiskers") to hunt and defend in the environment. 
Bioluminescent Defense
Bioluminescence is of great interest to the Navy. When objects move through  the ocean, they can cause bioluminescent organisms to flash. This can put projects and people at risk by revealing the presence of submarines, or even Navy SEALs swimming around covertly. 
adapt
Verb

to adjust to new surroundings or a new situation.

agribusiness
Noun

the strategy of applying profit-making practices to the operation of farms and ranches.

algae
Plural Noun

(singular: alga) diverse group of aquatic organisms, the largest of which are seaweeds.

Plural Noun

(singular: bacterium) single-celled organisms found in every ecosystem on Earth.

biologist
Noun

scientist who studies living organisms.

Noun

light emitted by living things through chemical reactions in their bodies.

Noun

tactic that organisms use to disguise their appearance, usually to blend in with their surroundings.

catalyst
Noun

substance that causes or quickens a chemical reaction, without being affected by it.

cell
Noun

smallest working part of a living organism.

chemical reaction
Noun

process that involves a change in atoms, ions, or molecules of the substances (reagents) involved.

chemiluminescence
Noun

emission of light as a result of a chemical reaction.

Noun

plants' green pigment that is essential to photosynthesis.

circumstance
Noun

condition or situation.

cold light
Noun

light emitted by a source that produces very little thermal radiation.

compound
Noun

substance having at least two chemical elements held together with chemical bonds.

consume
Verb

to use up.

counterillumination
Noun

type of bioluminescence used by organisms to hide from predators below by blending in with the light patterns above.

decay
Verb

to rot or decompose.

decoy
Noun

object used to attract an animal or other organism.

delay
Verb

to put off until a later time.

detach
Verb

to break apart from something else.

dinoflagellate
Noun

one-celled marine organism that is a major component of plankton.

Noun

community and interactions of living and nonliving things in an area.

electricity
Noun

set of physical phenomena associated with the presence and flow of electric charge.

emit
Verb

to give off or send out.

engineer
Noun

person who plans the building of things, such as structures (construction engineer) or substances (chemical engineer).

enzyme
Noun

proteins produced in living cells that act as catalysts to accelerate the vital processes of an organism.

esca
Noun

long, thin, fleshy growth from the head of an anglerfish.

filament
Noun

very thin fiber or thread-like structure.

flash
Verb

to emit a short burst of light.

fluorescence
Noun

emission of light by a substance during exposure to another source of light.

foxfire
Noun

light emitted by some fungi as they decay wood.

freshwater
Noun

water that is not salty.

freshwater
Noun

water that is not salty.

fungus
Noun

(plural: fungi) type of organism that survives by decomposing and absorbing the material in which it grows.

GFP
Noun

(green fluorescent protein) chemical (protein) that exhibits bright green fluorescence when exposed to light in the blue to ultraviolet range.

glow
Verb

to emit a continuous stream of light over a long period of time.

Noun

environment where an organism lives throughout the year or for shorter periods of time.

Noun

environment where an organism lives throughout the year or for shorter periods of time.

harvest
Noun

the gathering and collection of crops, including both plants and animals.

ion
Noun

electrically charged atom or group of atoms, formed by the atom having gained or lost an electron.

Noun

shallow body of water that may have an opening to a larger body of water, but is also protected from it by a sandbar or coral reef.

larva
Noun

a new or immature insect or other type of invertebrate.

luciferase
Noun

enzyme (catalyst) that reacts with a substrate (luciferin) in a chemical reaction that results in bioluminescence.

luciferin
Noun

organic substances that upon oxidation produce a virtually heatless light (bioluminescence).

luminesce
Verb

to emit light.

lure
Noun

object used to attract an animal or other organism.

marine
Adjective

having to do with the ocean.

milky sea
Noun

phenomenon of bioluminescent bacteria on the surface of the ocean.

mucus
adjective, noun

slimy, fluid secretion of some animals.

Noun

substance an organism needs for energy, growth, and life.

orbit
Noun

path of one object around a more massive object.

organism
Noun

living or once-living thing.

phosphorescence
Noun

long-lasting emission of light following exposure to and removal of stimulating light (incident radiation).

photoprotein
Noun

chemical (protein) that interacts with luciferin and other chemicals to create bioluminescence.

Plural Noun

(singular: plankton) microscopic aquatic organisms.

predator
Noun

animal that hunts other animals for food.

prey
Noun

animal that is hunted and eaten by other animals.

radiation
Noun

energy, emitted as waves or particles, radiating outward from a source.

reporter gene
Noun

gene with an easily disginguishable presence, used to track the expression and behavior of other genes.

salinity
Noun

saltiness.

satellite
Noun

object that orbits around something else. Satellites can be natural, like moons, or made by people.

satellite imagery
Noun

photographs of a planet taken by or from a satellite.

sea star
Noun

marine animal (echinoderm) with many arms radiating from its body. Also called a starfish.

startle
Verb

to suddenly surprise.

substrate
Noun

substance acted upon by an enzyme in a chemical reaction.

symbiotic
Adjective

associating with another organism, not always to the mutual benefit of either species.

synthesize
Verb

to create or manufacture.

technique
Noun

method of doing something.

toxic
Adjective

poisonous.

Plural Noun

region generally located between the Tropic of Cancer (23 1/2 degrees north of the Equator) and the Tropic of Capricorn (23 1/2 degrees south of the Equator).

visible light spectrum
Noun

light and colors that can be seen by human beings.

vital
Adjective

necessary or very important.