Definition

Big fin reef squid in deep water {Sepioteuthis lessonians}

Bioluminescence is light that is produced when a chemical reaction occurs within a living organism. Unlike other sources of light, bioluminescence is considered to be a ‘cold’ reaction as it does not give off any heat. Bioluminescence has mystified humankind throughout history and initial reports of sightings were discarded as myths. Modern day scientists continue to be eluded by the function and mechanisms of the process. A high percentage of bioluminescent species are found in the oceans, although a small amount are found in terrestrial ecosystems. It is thought that ocean-living organisms evolved the ability to produce light around 150 million years ago.

A diverse array of organisms have evolved the ability to bioluminesce, and it is thought that this adaptation has evolved between 40 and 50 times. Despite a small amount of terrestrial species possessing this quality, it is thought that four out of five bioluminescent species are found in the ocean. Nine out of ten species that live in the ocean at depths below 1,000 metres can produce light.

Biofluorescence is often confused with bioluminescence, although this process is very different and involves an organism absorbing light, before transforming it and re-emitting the light at a lower-energy wavelength, rather than producing its own.

Chemical processes

Bioluminescence is the product of a chemical reaction that occurs between a substrate (luciferin) and an enzyme (luciferase) which are solely found in organisms capable of this process. When luciferin reacts with oxygen and is in the presence of luciferase, light and oxyluciferin are produced.

Fireflies are known to convert the byproduct from this reaction, oxyluciferin, back into luciferin to be used again. This recycling process has only been documented in fireflies but is likely to exist in other bioluminescent species.

Other bioluminescent species have different photoprotein substrates (coelenterazine in the reaction below) and rely on a charged ­ion to trigger the reaction, rather than an enzyme.

 

Organisms who use this second process can control the brightness of their lights by releasing more or less photoproteins. The exact formula for this reaction, and the chemicals used, vary depending on the species.

Bioluminescent organisms carefully control their ability to produce light by regulating the chemical processes in their body. Luminescence is a response triggered by the central nervous system in response to an external factor. Some animals can produce their own luciferin, such as dinoflagellates, whereas others capture bioluminescent bacteria, retaining them within their internal organs and forming a symbiotic relationship, such as the bacteria in the lure of an anglerfish.

Physical processes

Bioluminescent deep-sea Ophiuroid Brittle Star (Ophiochiton ternispinus)

The wavelength, and therefore the colour, of the light produced by an organism is dependent on the arrangement of the luciferin molecules, and these vary greatly between species and habitats. Long wavelengths (red light) cannot travel very far into deep water, so most bioluminescence in the ocean is either blue or green as these colours have shorter wavelengths. Light from the sun is able to penetrate through the euphotic and disphotic zones of the ocean, also known as the sunlight and twilight zones respectively, but does not reach the aphotic zone, also known as the midnight zone, which occurs below depths of 1,000 metres and is where most light-producing organisms live.

Red light has the longest wavelength and is therefore unable to reach the deep sea from the surface. Many organisms in the deep sea therefore have red colouration, making them completely invisible to predators. A large amount of deep-sea animals have lost their ability to see red light, although some species use this evolutionary adaptation to their advantage, such as dragonfishes. Dragonfishes are able to produce red light, which not only serves as a method of communication between individuals but also allows them to easily see their prey. As most other deep sea animals are unable to see the red light that dragonfish produce, they will not see the predator before it attacks. This deep sea fish occasionally uses its red light to expose prey items to other dragonfish.

On land, bioluminescent organisms are not restricted to producing only green or blue light although many do, and fireflies and Quantula striata, the only land snail known to bioluminesce, can produce yellow light.

Occurrence

Deep sea Anglerfish female with lure {Himantolophus sp} Atlantic ocean

Bioluminescent organisms are mostly found in the deep sea below depths of 1,000 metres, although a smaller amount are found in shallower water and at the water’s surface, as well as on land. On land, bioluminescence is rare and is only found in a few insects, including fireflies and certain species of click beetle, annelids (worms) and centipedes, as well as over 80 fungi species belonging to the order Agaricales. Bioluminescence is even rarer in freshwater environments, with very few species known to possess this quality, such as Latia neritoides, a freshwater snail species.

The colour and patterns of bioluminescent light vary greatly between species and are often a reflection of the habitat in which an individual is found. Albeit relatively rare, some organisms emit light continuously, such as foxfire fungi and glow worms. Bioluminescence occurs in many patterns and sequences and can vary between a light that glows continuously, such as that seen in the ghost fungus, or flashing, as with fireflies, or can form mesmerising light displays across the entire body of the individual, as seen in squids and dinoflagellates. Most frequently, light is usually produced in flashes that last between 0.1 and 10 seconds and occurs in specific areas on the body.

Function

The function of bioluminescence in many species continues to elude scientists who can only take a knowledgeable guess as to why most organisms produce light. The function of light production in certain species; however, is more obvious and it has been observed being used for hunting prey, defence, finding a mate and communication, and many species use light for more than one of these purposes.

Reproductive

Light production is an important factor in finding a mate both for marine and terrestrial bioluminescent species. Males can use light to advertise their interest in a female, while the female can use it to let the male know that she is receptive to mating. Some species even judge the virility of a potential partner by the brightness of their light, and will ignore the advances of any less impressive individuals. Certain species also use their bioluminescence to publicise their gender to the opposite sex.

Defensive

One of the most common uses of bioluminescence is as a defence mechanism, with light often being produced to startle or confuse a predator.

Many marine species use counterillumination to protect themselves from predators. Similar in function to countershading, counterillumination is a method of camouflage but relies on light rather than colouration. Light is produced by photophores on the underside of the body, which from underneath, make the outline of the individual disappear against the lighter, shallower water. This prevents many predators who hunt from below, such as sharks, from seeing their prey and therefore being unable to attack.

Flashes of light can be an extremely effective way of deterring predators from attacking, as not only can it startle an individual and make it confused about the location of its prey, but it can also alert bigger predators to the location of the original predator and initiate an attack on them. This type of defence bioluminescence is often referred to as a ‘burglar alarm’. When predators are present, they create a disturbance in the water which makes the plankton at the surface of the water glow. The light alerts larger predators, such as whales and sharks, to their location, making them an easy target. The plankton then stops glowing when the water is calm.

Some deep water species are able to detach bioluminescent parts of their body as a distraction technique, so that the predator chases after the limb rather than attacking the main body. Some species even detach limbs onto other animals so that the predator attacks them rather than its original target. If a bioluminescent animal is successfully attacked by a predator, or its detached limb is consumed, often the light will continue being produced within its stomach, which can alert larger animals to its location and trigger an attack.

Offensive

As well as being used as a defence mechanism, bioluminescence is also used as a way of luring prey, as many deep sea creatures are attracted to light and will pursue it. Many species have lures containing bioluminescent bacteria, which are usually located around their mouthparts. Once the attracted prey comes close enough, the predator can easily consume it, preventing it from having to expel energy searching for food. Bioluminescence is also used by predators to improve their sight, and therefore locate their next meal more easily. 

Bioluminescent species

Viperfish

  • Species in the Chauliodus genus are deep sea-living fish that inhabit tropical and temperate waters at depths of up to 2,800 metres.
  • Viperfish species possess a long, fishing rod-like appendage with a bioluminescent lure on the tip. This lure is used to entice prey towards its mouth which is filled with numerous sharp, fang-like teeth.
  • The underside of viperfish is covered with bioluminescent photophores which help to disguise its outline against the lighter, shallower water above, protecting it from predators.

Flashlight fish

  • Fishes in the Anomalopidae family are commonly known as ‘flashlight’ or ‘lantern’ fish and are mainly found in the Indo-Pacific in clear, tropical water.
  • The common name of flashlight fish is related to the light-producing organs beneath their eyes which contain bioluminescent bacteria. The fish hide in caves during the day and emerge at night to feed when these lights are the only part of the body that can be seen by predators. The fish can cover their light-emitting organs with a flap of skin, and can either flash their lights to confuse predators or turn their lights off and change direction to escape.
  • Flashlight fish also use their bioluminescent organs to improve their sight whilst searching for food, and some scientists believe that they are also used for communication.

Anglerfish

  • Anglerfish have been found at depths of 1,600 metres below sea level in the Atlantic and Antarctic Oceans.
  • Female anglerfish have a long, rod-like appendage, known as a filament, with a bioluminescent lure at the tip, which is flashed and moved forwards and backwards to entice its prey, and show potential mates that it is a female.
  • The male anglerfish does not possess a lure and is extremely diminutive in comparison to the female. A male must find a female and attach himself to her, fusing their bodies until he becomes little more than a sperm sac which fertilises the female’s eggs.

Firefly squid

  • Firefly squid are usually found in the twilight zone of the ocean, at depths of around 365 metres.
  • This cephalopod produces one of the most impressive light shows of any bioluminescent species, with light-producing photophores covering a large majority of its body and flashing in mesmerising patterns.
  • The light produced by firefly squid serve many purposes, including preventing predation, communicating with potential mates and attracting prey.

Odd bobtail squid

  • The odd bobtail squid is found in the Atlantic and Pacific Oceans as well as the Mediterranean and Black Sea and adults have been recorded at depths of up to 1,588 metres.
  • When under attack, the odd bobtail squid releases a bioluminescent ‘ink’, scaring and confusing the predator, similarly to their shallow water relatives.
  • The ‘ink’ of the odd bobtail squid is mucus that contains bioluminescent bacteria.

Starfish and brittle stars

  • There are over 2,000 species of brittle star, of which over 70 are known to be bioluminescent.
  • Brittle stars use their bioluminescence for defence and can detach their limbs to distract predators from their main body.
  • Brittle stars often use their light to alert larger animals to the location of a predator that is trying to attack them, turning the tables and allowing them to escape.

Dragonfish

  • Dragonfish live in the deep sea and have been recorded at depths of over 2,000 metres.
  • The long, fleshy appendage that extends from underneath the mouth of dragonfish has a bioluminescent lure on the tip which is used to attract prey.
  • The light-producing organs underneath the eyes of dragonfish are used to search for prey.
  • An extremely clever adaptation allows dragonfish to emit and see red light and as most deep sea fish can only see blue light, they are able to search an area for prey without being detected.

Hatchetfish

  • Hatchetfish are found in the deep sea at depths of up to 1,500 metres.
  • The light-producing cells along the side of hatchetfish produce impressive patterns which dazzle and confuse predators.
  • The underside of hatchetfish is interspersed with bioluminescent cells which help to disguise the outline of an individual against the light coming from the water’s surface, making them invisible to predators below.

Ostracods

  • Ostracods are microscopic crustaceans in the order Ostracoda and are found in almost every aquatic ecosystem.
  • There are currently over 33,000 ostracod species known to science.
  • Some male ostracods produce trails of light to attract females and indicate their location. Once a single male begins this mating ritual, many other males join in, creating spectacular light displays. Each ostracod species has a unique light pattern.
  • Ostracods are prey for many sea creatures. When a predator attacks an ostracod and swallows it, it releases a flash of light which is known to be one of the brightest incidences of bioluminescence and can be seen through the body of the fish. If this happens, the attacker will immediately regurgitate the ostracod, although the predator’s whereabouts may have been advertised to a larger predator swimming nearby which will often then attack.

Dinoflagellates

  • Dinoflagellates are single-celled eukaryotic protists that occur in all aquatic environments around the world.
  • Dinoflagellates bioluminesce whenever the water around them moves, producing a blue-green light. In certain areas, such as the Humacao Nature Reserve in Puerto Rico, there is such a high concentration of dinoflagellates, the whole lagoon glows at night when the water is constantly moving.
  • The function of the light produced by dinoflagellates is for defence. By glowing when they are physically disturbed due to water movement, the location of the predator is advertised to larger predators, quickly deterring it from attacking.

Glowing bacteria

  • Bioluminescent bacteria are known to create a natural spectacle called ‘milky sea’, where billions of individuals gather, causing the water to glow continuously.
  • Some incidences of ‘milky sea’ are so bright and large that they can be seen from space.
  • Bioluminescent bacteria are harvested and kept captive by other organisms who cannot produce their own light, such as anglerfish and flashlight fish.

Fireflies

  • Fireflies have a large distribution and are found in North and South America, Europe and Asia, where they usually inhabit forested areas with open water, although they are occasionally found in drier habitats.
  • Fireflies use bioluminescence as a method of communication between males and females during the mating season, with females using the quality and brightness of a male’s light to determine their virility. If the female is receptive to mating, it will flash its lantern towards the male. Each firefly species has its own series of flashes which help them to distinguish between their own and other species.
  • Female fireflies will often mimic the flashes of other species to entice males. Once the male is close by, the female eats the male and harvests his toxins as it cannot produce its own.
  • Female fireflies have also been known to use their bioluminescence to acquire prey. On seeing a male caught in a spider’s web, female fireflies have been known to carefully approach the web and flash their lantern at the spider, before stealing and devouring their prey.

Glow worms

  • Lampyris noctiluca individuals are more commonly known as ‘glow worms’, despite actually being beetles.
  • The female glow worm is wingless and is able to produce a strong yellow-green light to attract males who fly close by.
  • Rather than producing light in flashes like fireflies, the female glow worm produces a constant light that it turns off once it has mated.

Foxfire fungi

  • Bioluminescent fungi are found throughout the world, although they are at their highest concentrations in the tropics.
  • There are thought to be at least 80 light-producing fungi species.
  • The light produced by fungi is neon green and constantly glows rather than flashing.
  • Foxfire fungi use their light to attract insects to their gills, where their spores become attached to the insect's body. The insects then spread the spores throughout the forest. However, the fruit body of certain species glows rather than the gills where spores are found, and the reason for these species’ bioluminescence is still a mystery.

Glossary

Aphotic zone
The deepest section of a waterbody where there is very little or no sunlight.
Countershading
A type of camouflage most often exhibited by aquatic animals where an individual’s underside is light and upperparts are dark, causing it to blend in against light coming from the surface and the dark ocean below and protect it from predators or prevent it being seen by its prey.
Dinoflagellates
Single-celled organisms with two flagella, occurring in large numbers in marine plankton and also found in fresh water. Belong to the order Dinoflagellata.
Disphotic
(zone) the central layer of water within a waterbody which receives a small amount of light, also known as the ‘twilight zone’.
Enzyme
Protein that triggers, or accelerates, activity in the cells of the body, for example, breaking down foods during digestion and building new proteins.
Eukaryotic
Organisms with complex cells that have an organised nucleus and organelles which may include mitochondria, chloroplasts and an endoplasmic reticulum.
Euphotic
(zone) the surface layer in a waterbody which receives the most amount of light, allowing photosynthesis to occur.
Fruit body
In fungi, the fruit body is the visible part of the fungus which bears spores (microscopic particles involved in reproduction).
Ion
Electrically charged particles formed when atoms lose or gain electrons.
Photophores
Light-producing organs which either secrete a compound that glows or contain colonies of bioluminescent bacteria.
Protists
Species belonging to the Protista kingdom which is made up of eukaryotic single-celled organisms, aside from a very small amount of multi-celled species, including protozoans, dinoflagellates, amoebas, most algae and some fungi.
Spores
Microscopic particles produced by many non-flowering plants and fungi that are capable of developing into a new individual. Spores are adapted for dispersal and surviving for long periods of time in unfavourable conditions.
Substrate
The surface a plant or animals lives upon.
Symbiotic relationship
Relationship in which two organisms form a close association. The term is now usually used only for associations that benefit both organisms (a mutualism).