The chemical molecule that produces light in many bioluminescent marine organisms is luciferin.
Luciferin is a light-emitting substrate that reacts with oxygen, usually with help from an enzyme called luciferase. The reaction releases energy as visible light. Different organisms use chemically different luciferins, so luciferin is a broad name for a class of light-producing compounds rather than one universal molecule shared by every species.
Marine bioluminescence occurs in bacteria, dinoflagellates, jellyfish, crustaceans, fish, and many other animals. It can attract prey, startle predators, provide camouflage through counterillumination, or help members of the same species communicate. Bioluminescence is not the same as fluorescence: bioluminescence creates light through a chemical reaction, whereas fluorescence absorbs light and re-emits it. Some animals make luciferin themselves, while others obtain it through diet or symbiotic bacteria.