In the pitch-black environment of the deep sea, light is a precious thing. In fact, making light is essential for the survival of many deep-sea animals. At least nine major groups of deep-sea animals, including jellies, corals, brittle stars, squids, and fish, use the same light-emitting chemical, coelenterazine, to power their displays. Surprisingly, many of these animals can't actually produce coelenterazine, but must get it by eating other glowing animals.
source https://phys.org/news/2020-12-jellies-compounds-food.html
