Published in 1859 in Charles Darwin’s Origin of Species, a most controversial work which provocatively stunned the religious societies, Darwin brought forth his knowledge, or, at best, his annotations, regarding bioelectricity as observed in electric fish and a brief mention addressing luminous organs. Though not understood during the 19^th century, advances in science and technology have provided for the analysis of luminous creatures in order to explain light generated by living organisms and now, as the 21^st century begins to unfold, the light phenomena may be explained as, "bioluminescence’.

Found most exclusively in photogenic zoological subjects (but also in two groups of botanical groups), bioluminescence- or chemiluminescence- may be defined as the voluntarily or involuntarily emission of visible light at relatively low temperatures by living organisms which lacks the generation of much heat, the result of the exergonic oxidation process of chemical compound, luciferin, as produced by cells, where energy is converted into light. A second type of light emission is also recognized as either photoluminescence, fluorescence, or phosphorescence: the involuntary emission of light of an organism, dependant upon the prior absorption of light, including ultraviolet wavelengths (Halsey3 181).

The production of bioluminescence is the result of an oxidation reaction (or, ‘burning reaction’) involving a chemical compound, luciferin, and a specific enzyme, luciferase, which aids in the oxidation process (22). Luciferin, believed to exist in six types as organisms use different forms of luciferin, is a light emitting compound which may be acquired by an organism through internal synthesis or externally through diet (22). It is a water-soluble compound, produced in the cells of the organism, known as intracellular luminescence, or it also may be generated outside the cell in extracelluar luminescence (Halsey3 181). Luciferin may be stored under an area of transparent cuticle in the light organ, where a layer of dense tissue may be found behind the luciferin, acting as a reflector. ³

In order to produce light in the chemical reaction, luciferin must be oxidized with the aid of the natural enzyme, luciferase, an oxygenase (adds oxygen to compounds) which requires a substrate (luciferin) and oxygen (also may require salt) to excite electrons to make them transfer to a higher energy orbital. It is as such electrons fall back into their initial rest state that light is emitted.

The reaction responsible for bioluminescence may be expressed thus:

luciferin + luciferase + O₂ + salt ---------> oxyluciferin + H₂O + light + heat

The reactants of the reaction may be bound collectively as a photoprotein and may be triggered to generate light by a calcium ion (Ca²⁺)(22) . It may also be noted that oxyluciferin reverts into luciferin in order to repeat the process to produce light and that although heat is a by-product of the reaction, there is little heat emitted from the reaction in relation to artificial methods of light production. The firefly, for use of an example, is known to be one of the most efficient lighting systems: though dim, the light of the firefly is approximately 90.0% efficient, with the release of 3.0% to heat. The average lightbulb, however, is only approximately 3.0% efficient, with a loss of heat of 97.0% (Chinery). In efforts to harness the efficiency of chemiluminescence, it has been discovered that the process is too expensive for human use. ⁴

The application of bioluminescence in living organisms is believed to be based upon four general uses: defense, offense, mating and courtship, and communication (22).

The production of light may be a way in which bioluminescent organisms defend themselves, as some generate light when in danger or threatened, where the sudden light may blind or, at least, confuse predators. It is also a possibility for fish to employ counter-illumination: if the light is produced and directed downward, positioned at a particular angle, with correct wavelength and intensity, it is possible for some species to make their shadows seemingly disappear from the ocean floor, which would lead astray the predators who depend on shadows to locate prey. The light may also be employed in offense, to catch prey by use of dangling lights which act as lures in front of the mouth (22). 

Chemiluminescence may also be used to attract mates during the breeding seasons, where particular glows distinguish between the male and female of many species as well as displaying the willingness to mate (22).

Finally, light may be used as a method of communication: patterns of flashing light, controlled by the neural system, may be used as interspecies signals. In the case of bacteria, a particular population density in certain groups triggers bioluminescence, perhaps through intermolecular messages between cells which may be transmitted through organisms (22). 

In species of shrimp, squid, and fish, there are present luminous organs of varying complexity, which may include reflectors, screens, and lenses. In the case of deep-sea fish, such characteristics of organs create a small lantern which the fish may use to navigate and find prey, as no light reaches the depth of sea in which they live (Halsey3 181).

Some species may possess a photophore, a luminous organ found in the skin of fish, a modified mucous gland which allows for the production of blue-green flashes to orange light at the will of the organism. A compound is secreted from the photophore which glows when activated. ⁷

Though understood scientifically for centuries, however believed to perhaps evolved more than thirty times in its evolutionary history, bioluminescence is now recognized as a property of various organisms who seemingly employ the ability to produce light through an oxidation reaction for several reasons, whether to locate prey or a mate or to defend themselves from predators. The light generated by such organisms is more efficient than artificial products, however, it is expensive to harness the chemiluminescence, which is found to be dim in luminous organisms. The organism with the most brilliant light is the Cucujo Beetle of South America (Pyrophorus noctilucus) which produces light one-fortieth of that of a candle flame (Chinery). However dim, the light is perceived by the human eye as being much brighter due to the wavelength at which the light is emitted, which is a sensitive wavelength and thus, it is possible for fireflies to be employed as a book light with which it is comparatively easy to read (Chinery).