Bioluminescent Fish: How Fish Use Light To Their Advantage

Bioluminescence means light produced by living organisms.

The possession of light producing organs is quite common in fish. Particularly, but not only, in those species that live in deeper waters.

45% of fish that live at depths below 300 metres and 75% of all Bathypelagic fish have some light emitting organs.

Although many groups of organisms (fish, insects, molluscs, etc.) possess bioluminescent organs, they are all normally dependent on bacteria that is maintained in special cells, for the production of the light.

However, some fish do produce a chemical that reacts with water to produce light.

The bacterial light is usually produced as a result of an enzyme (often luciferase) mediated oxidation reaction, in which a molecule (often luciferan) changes its shape and emits a single photon of light in the process.

Luciferan is a large, complex molecule that can later be returned to its original shape through a reduction reaction. During which, it gains an amount of energy equivalent to the single photon of light it emitted earlier.

Because the essential reaction is an oxidation reaction, the host organism (in this case the fish) can control the amount of light emitted, by controlling the blood flow – and hence the oxygen supply to the cells containing the bacteria.

Light Glands

fish light organ diagram

The light glands of different fish species show quite a wide variety in complexity.

The simplest are just a few cells embedded in the skin. More complex systems involve a reflecting layer behind the light source (black or silver), the light producing gland, a color filter and a lens.

In some species, the lens is derived from a modified fish scale.

The positioning of the light organs is also highly variable between species. So much so, that scientists can use the pattern of the light organs to recognize an individual species.

Most of the species that possess light organs are pelagic. Some exceptions to this can be found in the benthic Macrouridae (Rat-tail fish).

How Do Fish Use Bioluminescent Light?

Many species of bioluminescent fish have lights on the lower parts of their bodies. Scientists believe they use these lights to prevent their bodies becoming silhouetted against the surface.

A fish looking up from below, will see the body of the fish above it against the haze of light that is the surface. By having lights on their underside, fish can break up the pattern of their body and effectively hide in the haze.

Lights along the sides of the body are more likely to be for communicating with other members of their own species – either whilst shoaling, or to attract potential mates.

In other bioluminescent fish, such as the well known Angler Fish, the light organs seem designed to attract prey. Nearly all angler Fish possess luminous baits. The same applies to other types of fish with lights near their jaws or at the ends of fins or barbels.

angler fish light
Angler fish, Edridolychnus schmidti, famous for its bioluminesent lamp

Malacocephalus laevis has light producing bacteria in a gland near its anus. When it is disturbed, it can eject some of these bacteria into the water through its anus. The sudden appearance of this luminescent mass would easily disorientate a predator.

light gland in fish diagram

It is possible that fish with lights distant from the centre of their body – such as on or near their tail – may use these lights to disorientate predators.

Of course it is highly likely that fish use their light organs for more than one purpose.

It is often difficult to unwind time and discern whether or not an adaptation had a single primary cause.

While it remains true that an adaptation with more than one positive function is more likely to be selected for than one with only a single positive function, it is also true that Nature is prudent of existing adaptations and often works to add functions to an existing adaptation.

Some bioluminescent fish can actually produce a beam of light and such a beam would obviously help them find prey.

Many bathypelagic fish with light producing organs also have additional light organs that shine into their own eyes.  Scientists believe that this may work to accustom their eyes to a certain level of light, so that when they turn on their main lights they do not blind themselves.

Most luminous fish are teleosts, but there are some luminous sharks as well. For example Euprotomicrus bispinatus, which is also the smallest shark in the world.

Some more examples of bioluminescent fish are:

  • Isistius brasiliensis, which has thousands of small luminescent organs on its belly;
  • Anamalops koptroton and Photoblepharon palpebratus which are coastal;
  • Giganticus vanhoffeni and Dolopichthys niger which are both deep-sea angler fish.

Gordon Ramel

Gordon is an ecologist with two degrees from Exeter University. He's also a teacher, a poet and the owner of 1,152 books. Oh - and he wrote this website.


    1. You mean to collect electricity from them commercially I suppose, we wouldn’t be generating the electricity, the fish do that. The answer is I shouldn’t think so, they need to live and move around. Also to generate a sustained flow of any use you would need tens of thousands of fish connected to your system. I doubt it would be cost effective once you started paying for housing and food. More importantly I think it would be highly immoral, to get the fish to regularly release electricity would require constantly stressing them, which would be the equivalent of torture. Most of the environmental problems on our planet stem from humanity’s attitude of casual abuse towards nature. Fixing this attitude is actually a problem of huge importance for the future of our species and for all life on this planet. There are many better ways of generating electricity.

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