Why Do Some Animals Glow? The Chemistry and Uses of Living Light

The reaction is simple in outline and varied in detail. A molecule called a luciferin is oxidised with the help of an enzyme called a luciferase, and the energy released comes out as a photon rather than as heat, which is why it is described as cold light and why it is far more efficient than a filament bulb, converting most of the energy into visible output. The names are generic rather than specific: at least eleven chemically distinct luciferins are known, and the enzymes differ completely between groups, which is the clearest evidence that the ability has arisen many separate times rather than being inherited from a common ancestor. Some animals do not make the chemistry themselves and instead house luminous bacteria in dedicated organs, feeding them and controlling the output with shutters, lenses and reflectors, which is how the anglerfish lure and the light organs of many squid work.

The uses divide into a handful of strategies, and the same species often runs several:

• •Counter-illumination, the commonest use of all: an animal in mid-water lights its underside to match the dim light filtering from above, erasing its silhouette from predators looking up
• •Luring prey, as the anglerfish does with a bacterial lamp on a modified fin spine, and as the larvae of a New Zealand fungus gnat do with sticky glowing threads on cave ceilings
• •Startling or distracting an attacker, by a sudden flash or by ejecting a cloud of glowing fluid, the deep-sea equivalent of squid ink
• •The burglar alarm, in which a captured animal lights up brightly to attract a larger predator that may attack the one holding it, which is thought to explain much dinoflagellate flashing
• •Mating signals, most familiar in fireflies, where each species has its own flash pattern and timing, and where females of some genera mimic the pattern of another species to lure and eat the males that answer
• •Illumination, used by a few deep-sea fish that produce red light, invisible to almost everything else down there, as a private searchlight

The distribution is lopsided. In the open ocean below the surface layer, surveys with submersibles have found that around three quarters of the animals observed can produce light, and the figure rises in some depth bands; the sea is not dark so much as sparsely lit. On land the ability is comparatively rare and concentrated in a few groups, chiefly beetles, including fireflies, glow-worms and click beetles, some fungi, springtails, millipedes and a small number of earthworms and snails. In fresh water it is almost entirely absent, and why that should be is an open question, with the leading suggestion being that fresh water systems are geologically young and often turbid, so the signal has less value. The blue-green wavelengths that dominate marine bioluminescence are those that travel furthest through seawater, while terrestrial light is typically yellower, which fits the different medium.

Milky seas are the strangest. Mariners have reported the ocean glowing uniformly to the horizon for nights on end, and satellite imagery has now confirmed patches of luminous water thousands of square kilometres in extent, caused by vast populations of bacteria, the mechanism of which is not fully understood. Bays in Puerto Rico, Jamaica and Vietnam glow when disturbed because of dense dinoflagellate populations, the same organisms responsible for the blue surf that appears occasionally on ordinary beaches. The firefly squid of Japan spawns in such numbers that the shoreline is lit. Certain fungi produce a steady glow in rotting wood known as foxfire, recorded by Aristotle and used, according to one account, to mark the inside of an early submarine's instruments. And the flashing of certain firefly populations in Thailand, Malaysia and Tennessee synchronises across whole trees, an emergent effect of many individuals adjusting to their neighbours rather than any coordination.

The most consequential result came from a jellyfish. Osamu Shimomura, working on Aequorea victoria from the 1960s, isolated a protein that glows green, and it turned out to fluoresce on its own without any additional chemistry. Once its gene was identified, it could be attached to the gene for any other protein, so that the protein of interest lights up wherever it is made inside a living cell. Green fluorescent protein transformed cell biology, letting researchers watch nerve cells grow, tumours spread and infections progress in living animals rather than in fixed slides, and Shimomura shared the Nobel Prize in Chemistry in 2008 with Martin Chalfie and Roger Tsien, who extended the palette to a range of colours. The luciferase system is separately used to measure gene activity and to detect bacterial contamination in food and on surfaces, since the test reacts to a molecule present in all living cells. A study of why the sea glows has ended up as standard laboratory equipment.

Living light comes from oxidising a luciferin with a luciferase, a reaction that has evolved independently more than ninety times and releases almost all its energy as light rather than heat, with some animals culturing luminous bacteria instead. In the open ocean roughly three quarters of animals can do it, using the light mainly to erase their silhouette from below, to lure prey, to startle attackers, to summon a predator onto whatever has grabbed them and to signal to mates. A glowing jellyfish protein became the standard tool for watching processes inside living cells.