Bioluminescence
Making light by a chemical reaction, which most deep-sea fish do to hunt, hide or signal in the dark.
Also: light production
Bioluminescent fish make their own light, usually through the reaction of a substance called luciferin with oxygen, or by housing glowing bacteria. It is common in the deep sea, where there is no sunlight. Anglerfish dangle a glowing lure in front of their mouths. Lanternfish and hatchetfish light their bellies to hide their silhouettes. Dragonfish make red light, which most deep-sea animals cannot see, and use it like a private searchlight. Flashlight fish of reefs switch light organs under their eyes on and off as they feed at night.