Biological luminescence is seen in some kinds of plants and in most of the great subdivisions of deep-sea animals
Many bacteria, which are plants, can emit light and they often join up with animals, which collect them in certain areas of their bodies and make them shine on command. Some fungi produce light too, including toadstools and simpler kinds that emit a ghostly glow on rotting logs.
In oceanic waters, among the animals that exhibit luminescence are one-celled radiolarians and flagellates, sponges, corals; especially, the soft corals like sea fans, hydroids and jelllyfish, the comb jellies, many crustaceans, worms, some clams and snails, many squids, and fish.
Bioluminescence is chemiluminescence which is produced when a chemical called luciferin combines with oxygen producing oxyluciferin and water.
On land, centipedes, millipedes, and many insects can luminesce. For some reason, no species emitting light is found in any of the higher vertebrate classes. The occurrence of luminescence is sporadic, often with one species producing light and a closely related form lacking this resource.
It is strange, but true, the only major habitat that is without self-lighting organisms, or almost so, is fresh water, where only one snail and a glow-worm are luminescent and it is difficult to determine why this is so.
The most familiar and best studied of luminescent creature are the "fireflies", which are not flies at all, but a group of specially endowed beetles. They have enchanted people with their nocturnal pyrotechnics since for centuries, and fascinated scientists have studied the mystery of their light-production mechanism for many years
Luminescence is a natural chemical reaction
It is in the sea that luminescence blossoms in its full glory. Few people who live near the ocean have failed to be interested by what ordinarily goes by the name of phosphorescence.
It took awhile, but it was discovered that what is commonly called phosphorescence has nothing to do with phosphorus, but is a result of the ability of living creatures to produce light.
Careful studies of fish and other animals, but especially of fireflies, have revealed that light production by living creatures is a complex chemical phenomenon; bioluminescence is chemiluminescence. Inside the cells of the light-producing organs, or in some cases, in the areas between the cells, a chemical called luciferin combines with oxygen, yielding oxyluciferin and water.
In the process, energy is released in the form of light. The reaction can not take place unless a third chemical, luciferase is present to stimulate the change. This catalytic, or "helping-hand", chemical is called an enzyme, and it does its job without itself being altered.
The conversion of luciferin to oxyluciferin is not a simple oxidation reaction. Instead, a whole series of rapid steps take place, and a sequence of chemicals is formed and quickly altered to the next phase. The aspect of the chemical reaction of greatest significance is the fact that light energy is released.
The reaction is a reversible one; the oxyluciferin can combine with water and thereby be "reduced" to the original form, luciferin, ready to produce light the next time is is triggered.
As early as 1887, a French scientist, Raphael Dubois, expressed the opinion that luciferase was an enzyme. He worked with the boring clam, Pholas dactylus, and found that extracts of the clam would fluoresce only if they were not heated, and that it required two substances to create the light.
It was he who named them luciferin, from Lucifer the "light bearer", and luciferase, the -ase ending being the chemist's shorthand denoting an enzyme. It is characteristic of this class of chemicals that they are destroyed by heat.
In the 20th century, Dr. E. Newton Harvey of Princeton University, spent most of his active life studying luminescence.
Dr. E. Newton Harvey showed that there were several kinds of luciferin-luciferase reactions in different animals, and that they were clearly enzyme reactions and he came to the conclusion that the light-producing chemicals of different animals were not exactly the same.
He could produce light by mixing luciferin and luciferase if both were extracted from the same animal, but not if they came from different species unless they were very closely related. It may be that every luminescing creature has light-producing chemicals with slight differences in structure.
In 1940, Dr. Harvey wrote the following about "Light-Producing Organisms".
Luminous organs have often been mistaken for eyes. The reason is obvious; they are eyes in reverse. Chemical production of light is the converse of the chemical detection of light. The lantern of fishes is an organ of chemiphotic change; the eye an organ of photochemical change.
Moreover, photochemical reactions and chemiluminescent reactions have this in common, that they are largely but not exclusively oxidations. In light production by living things some material is oxidized.
In general, we may divide luminous organisms into two great classes according as to whether the oxidizable material is burned within the cell where it is formed or is secreted to the exterior and is burned outside; such as, intracellular and extracellular luminescence. Animals with intracellular luminescence
have the most complicated luminous organs.
Even the evolution of complex light organs and eyes is similar. In the simplest unicellular forms, certain pigment granules within the cell serve as the photochemical detectors of light, while in luminous protozoa, similarly, photogenic granules scattered throughout the cell are oxidized with light production.
In the higher forms, the eye contains groups of photo sensitive rods and cones connected with afferent nerves, lenses, and accessory structures for properly adjusting the light, while luminous organs contain groups of photogenic cells in connection with efferent nerves, lenses, and accessory structures for properly directing the light.
One of the most significant facts about "living light" is its efficiency
Man-made light comes with a lot of heat, as anyone knows who has tried to unscrew a light bulb which has been burning for any length of time. Since the object is to produce illumination, the heat is a waste product, dissipating some of the electrical energy and reducing the efficiency of the light source.
In most kinds of man-made light more than half the energy is lost in the form of heat, and sometimes the loss is considerably over this fraction. It had been suspected that in some forms of living light; such as that of fireflies, only as little as one per cent of the energy was wasted as heat.