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Introduction; How Light Is Produced; Light-Producing Organs; Uses for Bioluminescence; Medical and Scientific Use
Bioluminescence, production of light by living organisms as a result of chemical reactions. Organisms that produce bioluminescence or “living light” include bacteria, dinoflagellates, fungi, jellyfish, worms, insects, crustaceans, millipedes, squid, and fish. Bioluminescence is common in some sea creatures, but rare in organisms found on land apart from fireflies. Most of the animals that live in the deepest regions of the ocean have some type of bioluminescence.
Bioluminescent light is produced when a substance called a luciferin is oxidized in a chemical reaction with molecular oxygen. The luciferin releases light and becomes an inactive oxyluciferin that must be replaced by the organism. The organism may replenish luciferin by synthesizing the chemical itself or by eating other organisms that contain luciferin. An enzyme called a luciferase acts as a catalyst and helps speed up the chemical reaction that releases light but the luciferase is not chemically changed itself. The chemical reaction that releases light produces virtually no heat. The chemicals that occur as luciferins and luciferases are extremely diverse. Such variety suggests that bioluminescence has evolved independently many times among different types of living things. The chemical reactions may have first evolved as mechanisms to protect cells from oxidative damage (harmful chemical reactions with oxygen). Bioluminescence was a side effect that turned out to be useful to the organism in some way. Bioluminescence may be blue, green, yellow, orange, or red. Land-living organisms tend to produce yellow or green light, and marine organisms blue or green light. In the deep sea most bioluminescence is blue, the wavelength of light transmitted best by ocean waters. Most deep-sea animals can only see blue light but a few fish, such as the black dragonfish, can see and produce both blue and red light. Because their red bioluminescence is invisible to most deep-sea animals, such fishes may use their red light for communication or predation. In shallow waters bioluminescence tends to be blue-green. Bioluminescence should not be confused with phosphorescence or fluorescence, which occurs when certain materials absorb energy and give off light (see Luminescence). Bioluminescence is also distinct from iridescence in which colors are produced by interference patterns that break up white light.
Creatures that use bioluminescence occur in two main categories: organisms that produce their own light and organisms that borrow light from symbiotic bioluminescent bacteria. Many bioluminescent animals have special cells (called photocytes) that produce light using the species’ own enzymes and luciferins, or using luciferins obtained by eating other bioluminescent organisms. The luminescent tissue is often organized into complex light-producing structures called photophores. Photophores may include mirrors, lenses, shutters, and colored filters that modify and control the output of light. Most bioluminescent organisms produce flashes of light. Flashes may be a few milliseconds long or may be sustained for many seconds, with most lasting for a tenth of a second to a full second. In large animals, such as some jellyfish medusas, the flashes may involve spectacular patterns and whorls of light. Fish such as flashlight fish found in the Philippines and squid such as the Hawaiian bobtail squid do not have light-producing organs of their own. Instead these animals have a symbiotic relationship with bioluminescent bacteria. The host animals keep the bacteria in special baglike organs where the microbes thrive on nutrients and oxygen from the animal's blood. In return, the light produced by the bacteria can serve as a way for the host to attract prey or mates, or to confuse predators. Bacterial luminescence is usually a continuous glow so these squid and fish have evolved special mechanisms to turn off or block the borrowed light.
Bioluminescence can provide organisms with protection. It can also be a means of communicating with members of their own species, often to attract mates, and of finding and luring prey. Examples of using light for protection include producing sudden flashes to startle or distract predators, sometimes by ejecting blobs of luminous material. Small crustaceans called copepods and some types of squid use these tactics. Sea surface luminescence, commonly seen in the wakes of ships, is usually caused by blooms of microscopic dinoflagellates that flash in response to turbulence. The flashes are probably to attract fish that eat the zooplankton that attack dinoflagellates. Bioluminescence can also act as camouflage for animals such as deep-sea fish, squid, and crustaceans that have downward-shining photophores. These light-producing organs can act as counter-illumination to disguise the animals’ silhouettes against light from the surface when seen by predators below. On land, species of Motyxia millipedes glow in the dark as a warning to predators that they contain noxious chemicals. Some fish that hunt or travel in schools use bioluminescence to help coordinate the movements of the school. Other fish and certain squid display distinctive patterns of bioluminescence to find members of their own species. Flashing bioluminescent displays in species-specific patterns to attract mates occur among fireflies (a type of beetle) on land, as well as in some small copepod crustaceans and in some species of squid in the sea. Finding food and luring prey are other common uses for bioluminescence. Flashlight fish have light-producing photophores on their heads that may light their way in the darkness. Deep-sea anglerfish have luminescent lures that attract potential prey within reach of their mouths. Some cave-dwelling insect larvae have glowing tails that attract flying insects into sticky dangling strings that the larvae secrete to catch prey. A few species of fireflies mimic the mating flashes of other firefly species then prey on the unsuspecting victims who respond to the false mating signals. Other forms of bioluminescence are harder to explain. A small number of mushrooms and other fungi glow in the dark, possibly to attract insects to disperse their spores. However, it may be that such bioluminescence is simply incidental and has no special function or benefit for the fungi.
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