Bioluminescence in the Deep Ocean

Far below the sunlit surface of the sea lies a vast region of permanent darkness. Sunlight is absorbed and scattered so quickly by seawater that almost none of it reaches beyond a few hundred metres, and past roughly one thousand metres the ocean is effectively black. Yet this dark world is not lifeless, nor is it entirely without light. Many of the animals that live here produce their own glow through a chemical process called bioluminescence. In the deep ocean, living light is not a rare curiosity but one of the most common ways in which organisms communicate, hunt and survive.

Bioluminescence is the production of light by a living organism through a chemical reaction. In most cases the reaction involves a light-emitting molecule, generally known as a luciferin, and an enzyme called luciferase that speeds the reaction along. When the luciferin is combined with oxygen in the presence of the enzyme, energy is released in the form of light rather than heat. Because so little of the energy is lost as warmth, the glow is often described as cold light. This efficiency matters greatly to animals that cannot afford to waste the limited energy available to them in the food-poor depths. The reaction is also cool to the touch, so unlike a flame or an electric bulb it poses no danger of scorching the delicate tissues of the creature that produces it.

The colours that deep-sea creatures produce are not random. The overwhelming majority of marine bioluminescence is blue or blue-green. There is a clear physical reason for this: blue light travels farther through seawater than any other colour, so a blue signal can be seen across a greater distance in the open ocean. Most deep-sea animals have also lost the ability to detect other colours and are sensitive mainly to blue. A striking exception is a group of fishes sometimes called dragonfishes, which can produce and perceive red light. Because almost no other animal can see red at these depths, such a fish effectively carries a private searchlight, able to illuminate and stalk its prey without being noticed. Its own glow reveals nothing to the animals around it, which are simply blind to that part of the spectrum, and so the fish hunts in what is, for its victims, complete darkness.

The uses of living light are remarkably varied. One of the most widespread strategies is counter-illumination, a form of camouflage. Many small animals that swim in the upper layers of the deep sea carry rows of light organs on their undersides. By glowing gently downward, they erase the dark silhouette that a predator looking up from below would otherwise see against the faint light filtering from above. The animal matches the background light so closely that it seems to vanish. Other creatures use light in the opposite way, as a sudden flash to startle an attacker or to briefly blind it while the prey escapes into the darkness.

Light is also a powerful tool for finding food and attracting mates. The anglerfish is perhaps the most famous example: a female carries a glowing lure on a modified fin spine that dangles in front of her mouth, drawing curious prey close enough to be seized. In many anglerfishes the light itself is not made by the fish at all. Instead, the glowing tip houses colonies of bioluminescent bacteria, which the fish shelters and feeds in exchange for their light. This kind of partnership, in which two very different organisms depend on one another, is common in the ocean and is one of the reasons bioluminescence is so widespread.

Some of the most spectacular displays are defensive. Certain shrimps and other animals, when threatened, spew a cloud of glowing material into the water. The bright cloud confuses the predator and may even attract a larger animal that will attack the original hunter, giving the victim a chance to flee. A comparable trick is seen in a small deep-sea jellyfish that produces spinning circles of blue light when disturbed. Scientists sometimes call this a burglar alarm, because rather than hiding the animal, the light advertises the attack in the hope of summoning something bigger and more dangerous to the scene.

Studying these phenomena is difficult. The animals are fragile, the pressures are crushing, and bringing specimens to the surface alive is nearly impossible. For a long time our knowledge came mainly from creatures hauled up dead in nets, their light long extinguished. The development of remotely operated vehicles and sensitive low-light cameras has transformed the field, allowing researchers to record living animals glowing in their natural surroundings. What these observations reveal is a world in which light is a language, spoken in flashes and glows that carry messages of warning, deception and desire through the deepest darkness on Earth.