From MSNBC:
The definition of bioluminescence “is easier than the pronunciation and spelling of the word: It is just visible light made by living animals,” says Edith Widder, president of the Ocean Research and Conservation Association in Fort Pierce, Fla.
The word may be easy to define, but the chemical process is still poorly understood. Bioluminescence has apparently evolved independently at least 40 times in species belonging to more than 700 genera, or classifications of organisms. Widder notes in the journal Science that about 80 percent of those genera are found in the open ocean.
Examples of bioluminescent organisms include yellow-glowing Tompteris worms (upper left). Also pictured (clockwise) are the squid Abralia veranyi; northern krill, known by the scientific name Meganyctiphanes norvegica; the scaleless black dragon fish (Melanstomias bartonbeani); and deep-sea jellyfish (Atolla wyvillei).
More here.