The ability of certain organisms to produce their own light has fascinated observers since antiquity. Aristotle remarked on the glow of dead fish, and Chinese farmers of the Tang dynasty kept jars of fireflies as portable lamps. Yet bioluminescence, as the phenomenon is now known, remained mysterious until the twentieth century, when researchers began to isolate the chemical reactions responsible. What they found has proved far more useful than anyone expected: today, the same reactions are routinely employed in medical imaging, food-safety testing, and the search for new antibiotics.

A. Most luminous organisms rely on a family of compounds called luciferins, which are oxidised in the presence of an enzyme called luciferase. The reaction produces water, carbon dioxide, and a photon of visible light. Because almost none of the chemical energy is lost as heat, bioluminescence is the most efficient light-producing process known. A firefly, for example, converts more than ninety percent of the input energy into light, compared with less than ten percent for an incandescent bulb. This difference explains why the firefly's abdomen can glow for hours without burning the insect.

B. The chemistry varies considerably between species. The luciferin used by fireflies is quite different from the coelenterazine of deep-sea jellyfish, and different again from the reactions of luminous bacteria. This diversity suggests that bioluminescence has evolved independently at least forty times. Marine biologists point out that the phenomenon is overwhelmingly a feature of the ocean: as many as ninety percent of the animals living below a depth of two hundred metres are thought to produce light of some kind. In this environment of permanent darkness, a flash can serve as warning, lure, camouflage, or mating signal, depending on its colour and duration.

C. Camouflage seems a paradoxical function for light, but it is in fact among the most common. Many small fish and shrimp living in the twilight zone, between two hundred and one thousand metres, produce a faint downward glow that matches the residual sunlight filtering from above. A predator looking up therefore sees no silhouette, only a uniform bluish background. This strategy, known as counter-illumination, was first described in the 1930s by the American zoologist Harvey Clarke, whose pressure-resistant chambers allowed live specimens to be observed at surface.

D. Bioluminescence also plays a role in defence. When disturbed, the dinoflagellates responsible for the famous glow of tropical bays produce sudden flashes that startle potential predators. Some squid go further, ejecting a cloud of luminous mucus that confuses an attacker while the animal escapes into the dark. The Australian marine biologist Emily Trent has argued that the colour of the flash is significant: red light travels poorly in seawater, so many deep-sea predators have lost the ability to detect it. A few species of dragonfish exploit this gap, emitting red beams that illuminate their prey without betraying their own position.

E. On land, the best-known examples are the various species of firefly, but glow-worms and certain fungi also use light to attract attention. The New Zealand glow-worm is actually the larva of a fungus gnat; it suspends sticky threads from the roof of a cave and lures flying insects towards the faint greenish glow of its body. A single cave can contain tens of thousands of these larvae, producing a spectacle that has become a significant tourist attraction. Managing the caves presents difficulties, however, because artificial light or even excessive exhaled carbon dioxide can cause the larvae to dim their displays for weeks.

F. The practical applications of bioluminescence rest on two properties: its specificity and its sensitivity. Because each luciferase reacts only with its matching luciferin, molecular biologists can fuse the gene for luciferase to another gene of interest and then track the activity of that gene inside a living cell simply by watching for light. The technique has been used, for example, to follow the spread of tuberculosis bacteria through tissue, to test whether experimental drugs reach their targets, and to screen hospital equipment for traces of contamination at concentrations of a single bacterial cell. Trent has estimated that the global market for luminescence-based diagnostic kits doubled between 2014 and 2022.

G. Not every claim made for bioluminescence survives scrutiny. Early reports that luminous seas could guide ships safely through coral reefs have been abandoned, since the glow is too diffuse and unreliable. Nonetheless, the study of living light continues to yield surprises. In 2021, a team working on the Caribbean reef revealed that a common tube-building worm uses its light to signal the sex of nearby individuals during mass-spawning nights. The discovery suggests that the communicative functions of bioluminescence may be considerably richer than biologists have yet recognised, and that many more reactions remain to be uncovered.