Here are answers to 10 common questions about bioluminescence:
What are some of the different animals that make light?
Although bioluminescence may be considered rare as measured by the total number of species, it is extremely diverse in its occurrence. There are many different types of organisms that produce bioluminescence, from microscopic cells to fish and even a few sharks. But there are no luminescent animals in higher vertebrates above the fish. Overall, luminescent organisms represent most of the major phyla.
Let’s go through a short list of groups that have luminescent members (rare means that only a few species are luminescent). The most common are highlighted with an asterisk (*):
- Single celled organisms:
- *Coelenterates and Ctenophores (jellyfish): siphonophores, medusae, soft corals, (comb jellies)
- Gastropods: nudibranchs (rare), clams (rare), *squids, octopus (rare)
- Annelids (worms): *polychaetes (bristle worms), earthworms
- *Marine crustaceans: mysids (rare), copepods, ostracods (firefleas), amphipods, krill, shrimp
- Insects: *beetles (fireflies, glowworms), flies (rare), centipedes (rare), millipedes (rare)
- Echinoderms:, sealilies, seastars, *brittlestars, sea cucumbers
- *Tunicates: pyrosomes, larvaceans
- Sharks (rare)
- *Fishes – many different types
Why are so many animals in the ocean bioluminescent?
Probably bioluminescence originated in the oceans; based on the chemical structures of luciferins and luciferases, bioluminescence may have independently evolved many dozens of times.
Light emission is functionally important only if it is detected by other organisms. There are several reasons why bioluminescence is an effective means of communication in the ocean.
- First, in a large part of the ocean the transmitted sunlight is dim or absent, so bioluminescence becomes an alternative way to communicate using light.
- Second, the volume of habitat where bioluminescence is effective is vast, allowing natural selection to take place in a huge ecological context.
- Third, in most of the ocean there is no concealment, so animals “hide in the wide open.”
Some of the most common functions of bioluminescence in the ocean are for defense against predators or to find or attract prey. In the deep ocean, where sunlight is dim or absent, more than 90% of the animals are luminescent.
Did you know that a small luminescent deep-sea fish called the bristlemouth lightfish is considered the most abundant vertebrate on the planet?
Are bioluminescent animals found only in the ocean?
No. There are luminescent land animals, but they are relatively rare compared to those in the ocean. If you live east of the U.S. continental divide you may be familiar with the dusk displays of fireflies during the summer.
There are so-called railroad worms in South and Central America, which are actually beetle larvae. Their name comes from the rows of green and red lights coming from each body segment. Some mushrooms glow, as does a land snail from Malaysia, and some earthworms, millipedes, centipedes, and nematodes.
With the exception of one animal related to a clam, there are no luminescent freshwater animals.
So in general bioluminescence on land and in freshwater is rare compared to its occurrence in the ocean. We can only guess at why luminescence does not occur in freshwater environments. There are freshwater habitats with low light levels like in the deep sea but with no bioluminescence. Perhaps there is a chemical requirement that is missing? It is easier to study something that exists than something that doesn’t, so we know much more about why there is bioluminescence in the ocean than why there isn’t bioluminescence in lakes and rivers.
What is the function of bioluminescence?
Bioluminescence is light emission with an ecological context – to be detected by other organisms and change their behavior. While there are different functions of light emission, and animals can use their light for more than one function, the uses of bioluminescence can be grouped into several main types:
- Finding or attracting prey
In the dark ocean, dim glowing can be used to attract prey. Fish such as the anglerfish use a light organ filled with symbiotic bacteria that dangles from their forehead. Prey are attracted to the light in the same way that a fisherman might use a glowing lure for night fishing. When the unlucky prey gets near the anglerfish it is engulfed whole. Some fish use bioluminescence as a flashlight, which is how flashlight fish got their name. They use light, produced by symbiotic bacteria living in an organ below their eyes, to light up potential prey. On land, the glow of glowworms living in caves serves to attract insect prey, which get snared in the glowworms’ sticky mucous threads. Another example is the glow of fungi, which attracts insects not as prey but as a means of dispersing the fungal spores.
- Defense against predators
Bioluminescence can serve as a decoy. Some squid and shrimp produce a luminescent glowing cloud similar in function to the ink cloud of squid in daylight. When attacked by a predator, scaleworms and brittlestars sacrifice a part of the body, and some medusae sacrifice tentacles, that continue to flash as the animal makes its escape. Other animals living in ocean depths where the sunlight is very dim use bioluminescence to camouflage themselves. Their bioluminescence matches the color and brightness of the dim sunlight, and is called luminescent countershading, because it disrupts their shadow and makes it harder for them to be detected by predators. Small plankton including dinoflagellates use flashes of light to startle their predators in an attempt to interrupt their feeding. Dinoflagellate bioluminescence can also serve as a ‘burglar alarm’, attracting the predators of their predators (aka, the ‘police’), in this case to eat the dinoflagellate predator (aka, the ‘burglar’).
The best known example is the bioluminescence of fireflies, where there is an exchange of flashes between males and females. Females respond to the flashes of flying males, with the eventual result that the male 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. Some marine animals such as polychates (bristle worms) use bioluminescence during mating swarms, where the females attract males with a glowing mucus containing gametes. In others such as ostracods (firefleas), males flash in a sequence as they swim to attract females.
Do all jellyfish make light? What is the function of jellyfish bioluminescence?
It is estimated that about 50% of jellyfish are bioluminescent. There are many different types represented, including siphonophores (related to the Portuguese man-o-war), medusae, sea pens and other soft corals, and ctenophores (comb jellies). The greatest diversity of luminescent jellyfish occurs in the deep sea, where just about every kind of jellyfish is luminescent. Most jellyfish bioluminescence is used for defense against predators. Jellyfish such as comb jellies produce bright flashes to startle a predator, others such as siphonophores can produce a chain of light or release thousands of glowing particles into the water as a mimic of small plankton to confuse the predator. Others produce a glowing slime that can stick to a potential predator and make it vulnerable to its predators. Some jellyfish can release their tentacles as glowing decoys. So you see that there are many strategies for using bioluminescence by jellyfish.
Some deep-sea jellyfish such as the comb jellies, which can get as large as a basketball, are in some cases so fragile that they are almost impossible to collect intact.
Also spectacular are siphonophores, some of which can reach several meters in length. Siphonophores deploy many tentacles like a gill net casting for small fish.
How do animals use chemistry to make light?
All bioluminescence comes from energy released from a chemical reaction. This is very different from other sources of light, such as from the sun or a light bulb, where energy is also generated as heat. Hence bioluminescence is sometimes called ‘cold light’, indicating that are the energy is released as visible light and none as heat. In a luminescent reaction, two types of chemicals, called luciferin and luciferase, combine together. The protein luciferase acts as a catalyst, binding the luciferin and facilitating its oxidation, which results in a release of energy. The color of the light depends on the chemical structures of the chemicals. There are more than a dozen known chemical luminescent systems, indicating that bioluminescence evolved independently in different groups of organisms. One type of luciferin is called coelenterazine, found in jellyfish, shrimp, and fish. Dinoflagellates and krill share another class of unique luciferins, while ostracods (firefleas) and some fish have a completely different luciferin. The occurrence of identical luciferins for different types of organisms suggests a dietary source for some groups. Laboratory experiments have demonstrated that if some animals don’t get luciferin in their diet then they aren’t luminescent. Organisms such as bacteria and fireflies have unique luminescent chemistries. In many other groups the chemistry is still unknown. For more information on luminescent chemistry visit the Bioluminescence web site.
Does bioluminescence occur in just one color, or are there different colors? If so, how are the different colors produced?
Bioluminescence does come in different colors, from blue through red. The color is based on the chemistry, which involves the substrate molecule luciferin, which releases energy as it is oxidized in a chemical reaction that is facilitated by the protein luciferase. In land animals such as fireflies and other beetles, the color is most commonly green or yellow, and sometimes red. In the ocean, though, bioluminescence is mostly blue-green or green. All colors of light do not transmit equally through ocean water, so if the purpose of bioluminescence is to provide a signal that is detected by other organisms, then it is important that the light be transmitted through seawater and not absorbed or scattered. Blue-green light transmits best through seawater, so it makes sense that this is the most common color of bioluminescence in the ocean.
There are some exceptions to the blue-green/green color rule for ocean bioluminescence. Some worms make yellow light, and a deep-sea fish called the black loosejaw produce red light in addition to blue. We believe the red light functions as an invisible searchlight of sorts, because the eyes of most animals in the ocean cannot detect red light, while the eyes of the black loosejaw are red sensitive. Thus it can shine its red light to find prey while the prey wouldn’t even know they are being lit up!
What is a photon?
Light is a form of electromagnetic radiation, like radio or microwaves. Some aspects of light, such as its frequency (color), are based on its wave properties. Light can also be considered a stream of particles called photons, each of which contains energy. This concept is called the quantum theory. So there are two ways to express how much light there is. One is based on energy (in units of watts, joules, or calories), and the other is based on the number of photons. For example, the wavelength of green light is less than 1 millionth of an inch, and the energy of one photon of green light is equivalent to 1 million billionth of a calorie! Even though photons are particles, they are particles of energy and are different from particles in a cell such as molecules.
A typical dinoflagellate bioluminescent flash contains about 100 million photons and lasts about a tenth of a second.
Is a glowworm the same as a firefly?
Glowworms are not worms, but they do glow. Glowworms are actually fly larvae, and they live in caves such as Waitomo Cave in New Zealand. Their glowing attracts insects that get stuck in mucous threads hanging from the ceiling and are then eaten. So in this case, the glowing acts as a lure to attract prey.
Through gene splicing, would any species of plants or animals stand to benefit from an artificially induced bioluminescence capability?
All cells have the ability to produce ultra-low levels of light due to oxidation of organic molecules such as proteins, nucleic acids, etc. Through a very long process of natural selection, the organisms we call bioluminescent have developed the ability to enhance light production through physiological, molecular, anatomical, and behavioral adaptations. All this because the bioluminescence imparts an important ecological advantage to the organism. It is the ecological context that provides the driving force for natural selection.
In order for an organism to use bioluminescence that has been artificially induced, several criteria need to be met:
- First, there should be an ecological role for the light emission.
- Second, there needs to be control of light emission. We know from the study of luminescent organisms that with the exception of bacteria, all organisms have precise control of light emission. To produce light for the wrong reason or at the wrong time is a deadly mistake. There are futuristic visions of glowing Christmas trees, plants that light up along highways, or even crops that glow when they are thirsty, but this type of light emission doesn’t have an ecological context.
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