Why do some deep-sea fish produce their own light?

Below about 200 meters, sunlight fades quickly and the deep sea becomes nearly black, so many animals have evolved bioluminescence. Fish can produce light with chemical reactions in light organs, or by hosting glowing bacteria, and use it as camouflage, lures, signals, or counterillumination. Anglerfish are famous for using a glowing bait to draw prey close, while lanternfish use light in schools and for species recognition. Bioluminescence is especially common in the mesopelagic zone, where faint residual light still exists but is not enough for normal vision alone. This adaptation has evolved independently in many marine groups.