Animal bioluminescence

Bioluminescence is the visible light produced by luminous animals, plants, fungi, protists, and bacteria that results from a biochemical reaction with oxygen. Unlike incandescent light from electric light bulbs, bioluminescence is produced without accompanying heat. Bioluminescence was first described in 500 BCE, but the chemical mechanism of bioluminescence was not elucidated until the beginning of the twentieth century. The ability to luminesce appears to have arisen as many as thirty times during evolution. The chemical systems used by luminescent organisms are similar but not exactly the same. Most organisms use a luciferin/luciferase system. The luciferin molecules are oxidized through catalysis by an oxidase enzyme (luciferase). The oxidized form of luciferin is in an excited electronic state that relaxes to the ground state through light emission.

Animals may produce light in one of three ways. The bioluminescence may be intracellular: chemical reactions within specialized cells result in the emission of visible light. These specialized cells are often found within photophores. These light-producing organs may be arranged in symmetrical rows along the animal’s body, in a single unit overhanging the mouth, or in patches under the eyes and are connected to the nervous system. Alternatively, the bioluminescence may be extracellular: the animals secrete chemicals that react in their surroundings to produce light. The third option involves a symbiotic relationship between an animal and bioluminescent bacteria. Several species of fish and squid harbor bioluminescent bacteria in specialized light organs. The symbiotic relationship is specific—each type of fish or squid associates with a certain type of bacteria. The bacteria-filled organ is continuously luminous. The animal regulates the emission of light either with melanophores scattered over the organ's surface or with a black membrane that may be mechanically drawn over the organ.

Although bioluminescence is widespread in animals, its occurrence is sporadic. Most of the bioluminescent animal species are invertebrates. Among the vertebrates, only fish exhibit bioluminescence; there are no known luminous amphibians, reptiles, birds, or mammals. Although bioluminescence is found in terrestrial and freshwater environments, most luminous organisms are marine. Scientists estimate that 75 percent of all creatures in the deep sea possess some form of light generation.

Functions of Bioluminescence

There appear to be three main uses of bioluminescence: finding or attracting prey, defense against predators, and communication. Although visible light penetrates into the ocean to one thousand meters at most, most fish living below one thousand meters possess eyes or other photoreceptors. Many deep-sea fishes have dangling luminous light organs to attract prey. Terrestrial flies have also exploited bioluminescence for predation. The glow of glowworms (fly larvae) living in caves attracts insect prey, which get snared in the glowworms’ sticky mucous threads. Fungus gnats (carnivorous flies) attract small arthropods through light emission and capture the prey in webs of mucous and silk.

Bioluminescence can serve as a decoy or camouflage. For example, jellyfish such as comb jellies produce bright flashes to startle a predator, while siphonophores can release thousands of glowing particles into the water to mimic small plankton to confuse the predator. Other jellyfish produce a glowing slime that can stick to a potential predator and make it vulnerable to its predators. Many squid and some fishes possess photophores that project light downward, regardless of the orientation of the squid’s body. The emitted light matches that of ambient light when viewed from below, rendering the squid invisible to both predators and prey.

The best-known example of bioluminescence used as communication is in fireflies, the common name for any of a large family of luminescent beetles. Luminescent glands are located on the undersides of the rear abdominal segments. There is an exchange of flashes between males and females. Females respond to the flashes of flying males, with the result that the male eventually approaches the female for the purpose of mating. To avoid confusion between members of different types of fireflies, the signals of each species are coded in a unique temporal sequence of flashing, the timing of which is controlled by the abundant nerves in the insect’s light-making organ. Females of one genus of fireflies (Photuris) take advantage of this by mimicking the response of females of another genus (Photinus) to lure Photinus males that the Photuris females then kill and eat.

Some marine animals also utilize bioluminescence for communication. For example, lanternfishes and hatchet fishes (the most abundant vertebrates on earth) possess distinct arrangements of light organs on their bodies that can serve as species- and sex-recognition patterns; female fire worms release luminescent chemicals into the water during mating, beginning one hour after sundown on the three nights following the full moon; and deep-sea dragonfish emit red light that is undetectable except by other dragonfish.

Scientists continued to make new discoveries regarding bioluminescence in animals in the twenty-first century. In 2021, scientists discovered three new species of deep-sea sharks which produced bioluminescence through special cells in their skin. The study of bioluminescence does not just provide insight into the animal world—researchers study animal bioluminescence to understand how the science can be used to aid in human medical technology and advances as well.

Principal Terms

Luciferase: one of a group of enzymes that catalyzes the oxidation of a luciferin

Luciferin: one of a group of organic compounds that emits visible light when oxidized

Melanophore: a melanin-containing cell

Photophore: a light-emitting organ consisting of a lens, reflector, and light-emitting photogenic cells

Symbiosis: the intimate living together of two dissimilar organisms in a mutually beneficial relationship

Bibliography

Haddock, Steve. “Illuminating the Facts of Deep-Sea Bioluminescence.” Monterey Bay Aquarium, 15 Nov. 2021, www.montereybayaquarium.org/stories/bioluminescence. Accessed 29 June 2023.

Langlois, Jill. “How Studying Bioluminescent Creatures Is Transforming Medical Science.” Smithsonian Magazine, 5 Dec. 2019, www.smithsonianmag.com/science-nature/how-bioluminescent-creatures-are-transforming-medical-science-180973690. Accessed 29 June 2023.

Presnall, Judith Janda. Animals That Glow. Salem, Mass.: Franklin Watts, 1993. Aimed at the juvenile audience, this book’s strengths are the illustrations and the explanations of specific cases of bioluminescence in fish, squid, and insects. Includes practical applications of bioluminescence.

Robison, Bruce H. “Light in the Ocean’s Midwaters: Bioluminescent Marine Animals.” Scientific American 273 (July, 1995): 60-64. Discusses the functions of bioluminescence, photoreception in eyeless animals, and human exploration of the midwater zone.

Roth, Annie. “This Deep-Sea Shark Is One of the World's Largest Glowing Animals.” National Geographic, 5 Mar. 2021, www.nationalgeographic.com/animals/article/deep-sea-shark-largest-glowing-animal-bioluminescence. Accessed 29 June 2023.

Silverstein, Alvin, and Virginia Silverstein. Nature’s Living Lights: Fireflies and Other Bioluminescent Creatures. Boston: Little, Brown, 1988. Describes the functions of bioluminescence in insects, plants, and sea animals for a young audience.

Toner, Mike. “When Squid Shine and Mushrooms Glow, Fish Twinkle, and Worms Turn into Stars.” International Wildlife 24 (May/June, 1994): 30-37. A nice overview of bioluminescent species. Includes a description of the chemical reaction, the purposes of bioluminescence, and descriptions of several bioluminescent species.

Tweit, Susan J. “Dance of the Fireflies.” Audubon 101 (July/August, 1999): 26-30. Easily understood examination of the role of bioluminescence in firefly mating rituals. Includes a discussion of firefly habitat destruction.