Krill
Except for Bentheuphausia amblyops, krill are bioluminescent. They have organs called photophores which emit light. The light is made by an enzyme-catalysed chemiluminescence reaction. A luciferin (a kind of pigment) is activated by a luciferase enzyme. Krill probably get this substance as part of their diet, which contains dinoflagellates. Krill photophores are complex organs with lenses and focusing abilities, and can be rotated by muscles. The exact function of these organs is not known. The possibilities include mating, social interaction or as a form of camouflage to compensate for their shadow against the overhead light.