Bioluminescence, which is the ability of an organism to emit visible light, is a common attribute of marine creatures. The phenomenon is relatively rare on land, where fireflies are the best-known example. In the oceans it is ubiquitous, and is found at all depths. The most common sources in the marine environment are bacteria, dinoflagellates, jellyfish, crustaceans, cephalopods, and fish. Among cephalopods, which include squids, cuttlefish, and octopods, the expression of bioluminescence is extremely diverse. Out of 100 genera of squids and cuttlefish, 63 have been found to include bioluminescent species, but in octopods only 3 out of 43 genera do.