What is bioluminescence?
Bioluminescence is light produced by a living organism through a chemical reaction. In most cases a light-emitting molecule called a luciferin reacts with oxygen, helped by an enzyme called a luciferase. The reaction releases energy as visible light rather than heat, which is why it is sometimes called “cold light”. Some animals make these chemicals themselves; others obtain them through their diet or host glowing bacteria.
How common is it?
On land, bioluminescence is rare — fireflies and some fungi are the best-known examples. In the ocean it is everywhere. A 2017 study of deep-sea video from Monterey Bay found that about three quarters of the animals observed in the water column were capable of producing light. Fish, squid, shrimp, jellyfish, worms and single-celled plankton all do it.
Why is the light usually blue?
Seawater absorbs red and yellow light quickly but lets blue-green light, at wavelengths around 470 nanometres, travel farthest. Most deep-sea animals have therefore evolved both to produce blue light and to see it. Many cannot see red light at all — which is why researchers often use red lights on deep-sea cameras to film animals without disturbing them.
There is a famous exception: some dragonfish, such as the loosejaw, produce red light from organs below their eyes. Because their prey can't see red, it works like a private searchlight.
Four reasons to glow
1. Camouflage
Seen from below, any animal is a dark silhouette against the faint light coming from the surface. Hatchetfish, lanternfish and many squid produce light on their bellies that matches the brightness above, making themselves effectively invisible. This is called counter-illumination.
2. Hunting
The anglerfish is the classic example: a glowing lure dangles in front of its mouth to attract curious prey. Other predators use light like a torch to find food.
3. Defence
Some animals flash to startle attackers. Others release clouds of glowing fluid instead of ink — the vampire squid can eject a cloud of glowing mucus. The alarm jellyfish's spinning blue display may even call in bigger predators to attack whatever is attacking it.
4. Communication
Species-specific patterns of photophores, like those of lanternfish, may help animals find mates and recognise their own kind in total darkness.
Bioluminescence and science
Glowing sea life has changed laboratory biology. Green fluorescent protein (GFP), first isolated from the jellyfish Aequorea victoria, is now used worldwide to tag and track cells and proteins. Its discovery and development earned the 2008 Nobel Prize in Chemistry for Osamu Shimomura, Martin Chalfie and Roger Tsien.
How the game uses it
In Abyssal Drift the water darkens as you descend, and below a few hundred metres only your spotlight shows the full shape of anything. Species that glow in real life — lanternfish, hatchetfish, anglerfish, viperfish, jellyfish — keep their glowing marks visible even outside the beam. Learning to recognise those points of light is a real skill in the game, just as it is for animals of the deep.
Related: the real animals behind the game · ocean depth zones