Photophore
A light-making organ in the skin of lanternfish, hatchetfish and many other deep-sea fish.
Also: light organ
Photophores are small organs that produce light, either through chemical reactions in the fish's own cells or by housing glowing bacteria. Lanternfish carry rows of them along the belly in patterns that differ by species, so they can recognise each other in the dark. Many mid-water fish light their bellies to match the faint light from above, which hides their outline from predators looking up, a trick called counter-illumination. The anglerfish's lure holds glowing bacteria to attract prey.