Luminescence is a feature common to many marine organisms. Certain squid and fish utilise luminous bacteria as symbiotic sources of light, and the symbionts have physiological features which are of potential advantage in the association. The luciferins of some marine fishes, squids, crustaceans and coelenterates are of very similar chemical structure, though differing markedly from the luciferins of non-marine forms. Luminescence in many marine animals is probably used for ventral camouflage purposes.
