Bioluminescence

Bioluminescence is light made by a living thing through a chemical reaction, rather than reflected or absorbed light. In most known systems a light-emitting molecule, generically called a luciferin, reacts with oxygen with the help of an enzyme called a luciferase, releasing energy as visible light. The reaction is often described as efficient, producing light with very little heat, which is why it is sometimes called cold light.

There is no single shared chemistry behind it. Luciferases differ widely from group to group, and reviews note that bioluminescence has arisen independently many times across the tree of life, by one count more than forty separate times. Some animals make the necessary ingredients themselves, as fireflies do. Others host light-producing bacteria in special organs. The deep-sea anglerfishes, for example, are thought to light their lures with symbiotic bacteria, and the Hawaiian bobtail squid (Euprymna scolopes) houses luminous bacteria such as Aliivibrio fischeri in a dedicated light organ.

It is worth separating two ideas that are easy to confuse. The precise step-by-step chemistry is well understood in some model species and only partly described in others, so broad statements about exactly how a given animal's light works should be treated with caution unless that species has been studied directly.

Bioluminescence and biofluorescence are different optical phenomena, and the terms are often mixed up. In bioluminescence the animal generates light itself through a chemical reaction; the glow can appear in complete darkness. In biofluorescence the animal absorbs light at one wavelength and re-emits it at another, so it produces no light of its own and only appears to glow when an outside light source is present.

A practical way to keep them apart: a bioluminescent animal can light up in a sealed dark room, while a biofluorescent one needs incoming light, often blue, to re-emit as green or red. This guide is about the first phenomenon, where the animal makes the light, though some species are reported to do both.

Most bioluminescent animals live in the ocean, and the deep sea is where the trait is concentrated. Surveys of deep-water communities have found that a large share of the animals there produce light; figures of roughly three-quarters of deep-sea animals are commonly cited, though such estimates depend on the depths and groups sampled. Below the reach of sunlight, self-made light is one of the few sources of illumination available.

Marine bioluminescence is usually blue or blue-green. Water absorbs longer wavelengths quickly, so red light does not travel far, while blue-green passes through more easily and matches the visual sensitivity of many deep-sea animals. There are striking exceptions, such as dragonfishes reported to emit red light that most of their prey apparently cannot see, but blue-green dominates.

On land, bioluminescence is far less widespread. It is found mainly in certain fungi, some bacteria, and scattered groups of invertebrates, with the luminous beetles known as fireflies (family Lampyridae) being the most familiar. Beyond fireflies and a few relatives, glowing land animals are the exception rather than the rule.

Among well-documented light-producers are the deep-sea anglerfishes, whose modified dorsal fin spine forms a glowing lure, or esca, held in front of the mouth. Lanternfishes (family Myctophidae) carry rows of light organs along their bodies. Many squid and jellyfish are luminous, including the firefly squid (Watasenia scintillans) and the vampire squid (Vampyroteuthis infernalis), which can release a cloud of glowing fluid. Hatchetfishes and dragonfishes are other frequently cited deep-sea examples.

Light is also produced by very small organisms. Single-celled dinoflagellates are responsible for the blue glow sometimes seen in disturbed seawater at night. These examples span different animal groups and very different chemistries, which is consistent with bioluminescence having evolved many separate times rather than from one common origin. No single species should be read as representing how its whole family or class behaves.

Biologists describe several roles for bioluminescence, and a single species may use it for more than one. These are functions inferred from observation and experiment; they are not equally proven in every animal, and motives should not be read into them. The main categories below are widely supported, but the details vary by species and some remain under study.

Communication and courtship is the role most visible on land. Many fireflies use species-specific patterns of flashes, and in some the timing differs between males and females, which is thought to help individuals locate compatible mates. In the sea, lanternfishes have been reported to show different light patterns in males and females, which may serve a similar recognition role.

Other roles centre on predators and prey. In counter-illumination, an animal produces light on its underside to match the dim light coming from above, reducing its silhouette when seen from below, a form of camouflage reported in some squid and mid-water fishes. Luring is seen in anglerfishes, whose glowing esca is thought to draw prey close. Defence takes several forms: sudden bright flashes that may startle a predator, glowing clouds released as a distraction or smokescreen, and in some animals a body part that is shed and keeps flashing while the animal escapes.

Bioluminescence is well established as a phenomenon, but it is not fully mapped. The exact chemistry has been worked out in detail only for a subset of species, and the molecules involved differ across groups. For many deep-sea animals, which are hard to observe and to keep alive for study, the proposed functions rest on limited observations and reasonable inference rather than complete behavioural evidence.

Because of this, broad claims should be made carefully. It is accurate to say that many deep-sea animals produce light and that the trait appears to serve camouflage, signalling, luring, and defence in various species. It is not accurate to assume every glowing animal uses its light the same way, or to describe these adaptations as deliberate strategy. Ongoing deep-sea research continues to refine the picture.