Bioluminescence: Light Emission and Function in Living Things
What Is Bioluminescence?
Bioluminescence is a process in which living organisms produce light. The light often looks beautiful to humans and has a very important function for its producer. Organisms use light for communication, for defence, or for attracting a mate or prey.
A wide variety of creatures produce light. These creatures include some types of animals, fungi, protists, and bacteria. Most bioluminescent organisms live in the deep ocean, but some live in shallow water or on land.
Bioluminescence creates a cold light that is accompanied by very little heat. The process of producing the light is a complex process that is not completely understood. Researchers have suggested that some bioluminescent organisms may one day be useful to humans.
Bioluminescence is produced by the reaction between luciferin and oxygen. The word luciferin comes from the Latin word lucifer, which means “light bearer.”
How Is Bioluminescence Produced?
Bioluminescent organisms–or at least the ones that have been discovered–contain a pigment molecule with the general name of luciferin. A chemical reaction between luciferin and oxygen releases energy in the form of light. In most bioluminescent organisms, an enzyme called luciferase triggers the reaction.
There are several kinds of luciferin, each having a different chemical structure. Examples include firefly luciferin, dinoflagellate luciferin, and bacterial luciferin. Vargulin is a type of luciferin found in the midshipman fish, which lives in deep water. Coelenterazine is the luciferin found in some fish and invertebrates.
Some bioluminescent organisms make their own luciferin. Others obtain it from the creatures that they eat. Some light emitters house bioluminescent bacteria within their body and use the light produced by the bacteria.
The light released by bioluminescence generally appears white, yellow, green or—especially in the deep ocean—a beautiful shade of blue.
The Flashlight Fish
Many bioluminescent organisms are found in deep, dark ocean water far from sunlight. In fact, scientists think that about ninety percent of animals in deep water are bioluminescent. Marine organisms usually produce light with a blue-green colour, which is the colour of light that travels best under water. The eyes of underwater animals detect blue light better than light of other colours.
The flashlight fish is one deep water inhabitant that produces light. The light-producing organ of the fish is called a photophore and is located beneath each of its eyes. The light is actually produced by bioluminescent bacteria that live in the photophores. The fish can flash its light on and off by covering the organ with a flap of skin that acts somewhat like an eyelid.
Though the term "flashlight fish" is often used in the singular, fish with this name are found in several families. The video below shows Photoblepharon palpebratus when illuminated by a video light and then when the video light is turned off.
The Deep Sea Anglerfish
The deep sea anglerfish or black sea devil (Melanocetus johnsonii) has a bioluminescent lure attached to a movable spine extending from its head. It also has a set of fearsome teeth. As in the flashlight fish, the bioluminescence is produced by bacteria.
When the anglerfish needs to eat, it lowers its “fishing rod” towards its mouth and flicks the glowing end to attract prey. The rest of the fish is hard to see in the darkness. Once the prey approaches, the anglerfish grabs hold of it with strong jaws. Since the stomach of the anglerfish can expand dramatically, the fish is able to eat large prey animals when they are available.
Bioluminescence in the Firefly Squid
The firefly squid (Watasenia scintillans) is a tiny creature with a big light display. It reaches a length of only three inches. Photophores are present in many parts of its body and produce a rich blue light. The photophores may all emit light at the same time or the light may be released non-synchronously in a pattern. The complex light patterns are believed to have several functions. They may be a signal to rivals and mates and may confuse predators.
One of the best places to see the firefly squid is at Toyama Bay in Japan. During the mating season, thousands of squid are present in the bay. They spend the night in deep water and the day at shallow depths. While they are in shallow water, they emit their light as they search for a mate. The overall effect of thousands of squid producing a blue light is said to be spectacular.
Light Production by the Vampire Squid
The vampire squid (Vampyroteuthis infernalis) doesn't suck blood as its name might suggest. The name comes from the webbing between the squid's arms and perhaps from the animal's reddish colour. The webbing produces a structure that is reminiscent of Dracula's hood. The squid is six to twelve inches long and has large, blue eyes. It also has side fins. Despite its name, it's classified in a different order from true squids.
Like the firefly squid, the vampire squid has many photophores on its body and is able to create complex patterns of flashing lights. The animal also releases a sticky, bioluminescent mucus to confuse predators. The mucus is released as a glowing cloud that gives the squid a chance to escape.
Researchers have discovered that the diet of vampire squids consists of marine snow. The "snow" is the collection of dead plankton, fecal pellets, and gelatinous material that sinks from shallower into deeper water. A squid catches the snow with two filaments that it extends from its body.
Bioluminescence in Dinoflagellates
Dinoflagellates belong to a group of relatively simple organisms known as protists. Noctiluca scintillans, also known as sea sparkle, is a bioluminescent dinoflagellate. It lives near the ocean surface and feeds on plankton (tiny organisms that drift in the ocean). Its body consists of just one cell, but the cell is large compared to the bodies of other unicellular organisms.
Organelles inside the cell of Noctiluca scintillans produce the bioluminescence. The light is produced when the dinoflagellates are mechanically stimulated, such as by the actions of nearby predators. One theory is that the ability to produce the light developed because it distracted the predators.
There are only almost no bioluminescent organisms in fresh water. So far only some insect larvae and one kind of limpet have been found to release light in fresh water.
Fireflies or Lightning Bugs
Probably the best known example of land organisms that exhibit bioluminescence is the firefly, also called a lightning bug. Despite their name, fireflies are actually nocturnal beetles, not flies. About 2,000 different species exist. Their light-producing organs are located at the end of their bodies on the bottom of their abdomens.
Fireflies generally release their light in a series of flashes. The flashes help fireflies find mates. They may also protect the insect by warning potential predators that the fireflies taste bad. In addition, scientists have discovered that the flashes may be used to attract prey. Some female fireflies can imitate the light flash pattern of the female of another firefly species. The male of the other species is attracted by the mating signal and is eaten by the attracting female.
First firefly photo: Aspisoma sp., by gailhampshire, CC BY 2.0, via Wikimedia.org
Second and third photo: The undersurface of a New Brunswick firefly photographed with and without flash, by Emmanuelm, CC BY 3.0, via Wikimedia.org
How Do Fireflies Produce Light?
It’s not known for certain how the firefly is able to turn its light on and off, but it’s thought that the insect is able to control the quantity of oxygen inside the light organ. Oxygen availability may be determined by the presence of nitric oxide and mitochondria. The mitochondria produce energy for a cell by a process called cellular respiration. Oxygen is required for this process.
According to the theory, when nitric oxide is present, it inhibits the mitochondria in the light organs from using oxygen. The oxygen is therefore available for light production. When nitric oxide is absent, the mitochondria are no longer inhibited and use oxygen for cellular respiration. The oxygen is therefore unavailable for light production.
Glow worms also produce light by bioluminescence. They aren't worms, despite their name. The term “glow worm” refers to the bioluminescent, worm-like larvae of certain flies or beetles or to adult bioluminescent beetles that have a worm-like appearance.
In some places, the term "glow worm" is restricted to the female Lampyris noctiluca, which is a type of beetle. The female looks more like an elongated larva than an adult beetle and is unable to fly. She attracts the male, who can fly, with her light. The light is produced on the underside of the last three segments of the abdomen, as shown in the photo below. The light is emitted continuously, unlike the case in fire flies. Other stages of the insect's life cycle can also emit light, but the adult female's glow is the brightest.
The body of a fungus consists of thread-like structures called hyphae. The hyphae are collectively known as a mycelium. Mushrooms are spore-bearing structures produced by the mycelium. The spores enable the fungus to reproduce.
Bioluminescent fungi produce a beautiful and often eerie glow that emanates from wood or a forested area. The glow is sometimes known as foxfire. The light is continually emitted—even during the day—but shows up best in the dark.
Fungi obtain food by secreting digestive enzymes into their food source and then absorbing the products of the digestion. Bioluminescent fungi produce light as they digest wood. In some species, only the cap or the gills of the mushroom release light, while in other species only the stem glows. Sometimes only the mycelium that produces the mushroom is bioluminescent.
As in most bioluminescent animals, the fungi produce their light by the luciferin/luciferase system. The function of the light is still a mystery. One theory is that the light attracts insects to aid in spore dispersal. Different species of fungi may glow for different reasons, however.
Other Bioluminescent Organisms
Bioluminescence plays an important role in the lives of many creatures, but no bioluminescent plant, amphibian, reptile, bird, or mammal has yet been discovered. (According to biological classification, fungi aren't plants.) There may be surprises in store for us, however.
Researchers have recently discovered that the hawksbill sea turtle is biofluorescent. It absorbs blue light and releases red and green light. Fluorescence is the process is which light of one colour (or wavelength) is absorbed and then immediately emitted as light of a different colour. While light production by the hawksbill sea turtle isn't an example of bioluminescence, it does suggest that there is more to learn about light emission in nature. It's a fascinating phenomenon.
- Information about bioluminescense in the ocean from Smithsonian's Ocean Portal
- Facts about Noctiluca scintillans from the University of Tasmania, Australia
- Bioluminescence in fireflies from Scientific American
- Facts about glow worms and other bioluminescent creatures from the Wet Tropics Management Authority of Australia
- Fungal production of light from the Smithsonian Magazine
© 2010 Linda Crampton