Bioluminescence is the emission of photons of visible light by living organisms via the chemical reaction of molecular oxygen (O₂) with a substrate (luciferin) catalyzed by an enzyme (luciferase). Organisms that exhibit bioluminescence are diverse and widely distributed in nature, for example, bacteria, dinoflagellates, fungi, insects (such as click beetles and fireflies), and fish. (Some fish, such as deep-sea angler fish, exhibit bioluminescence through bioluminescent bacterial symbionts, whereas others, such as hatchet fish, exhibit bioluminescence on their own.) The colors of the light produced by bioluminescent organisms differ; for example, bacteria emit a blue-green light (490 nm) and fireflies emit a yellow light (560 nm). The enzymes that catalyze the bioluminescence reaction show no homology to each other, and the substrates are also chemically unrelated. Molecular oxygen is the only common feature of bioluminescence reactions, indicating that the luminescent systems in most organisms may have evolved independently. The terms “luciferase” and “luciferin” are thus generic terms for the enzymes and substrates involved in the different bioluminescent reactions.