Bioluminescence

Bioluminescence is light made by living things. It comes from a chemical reaction in which a light-emitting molecule called a luciferin is oxidized, usually with the help of an enzyme called a luciferase. The reaction releases its energy mostly as visible light rather than heat, so it is often called cold light. Organisms either make their own luciferin, obtain it from their food, or host glowing bacteria in special light organs.

Bioluminescence is uncommon on land, where fireflies, glow-worms and some fungi are the main examples, and almost absent in fresh water, but it is very common in the ocean. Most animals living in the dim middle depths of the sea can produce light, including fish, squid, shrimp and jellies, and the light is usually blue-green because that color travels farthest through seawater. Animals use it to lure prey, startle or distract predators, signal to mates, and hide their silhouettes from below by matching the faint light from above, a trick called counterillumination. Because the chemistry differs between groups, it appears to have evolved independently many times.

It differs from fluorescence, in which a substance absorbs light and gives it out again. In the lab, luciferase genes are widely used as reporters that show when and where a gene is switched on.