A photophore is a glandular organ that appears as luminous spots on various marine animals, including fish and cephalopods. The organ can be simple, or as complex as the human eye; equipped with lenses, shutters, color filters and reflectors, however unlike an eye it is optimized to produce light, not absorb it. The bioluminescence can variously be produced from compounds during the digestion of prey, from specialized mitochondrial cells in the organism called photocytes ("light producing" cells), or, similarly, associated with symbiotic bacteria in the organism that are cultured.
The character of photophores is important in the identification of deep sea fishes. Photophores on fish are used for attracting food or for camouflage from predators by counter-illumination.
Photophores are found on some cephalopods including the firefly squid, which can create impressive light displays, as well as numerous other deep sea organisms such as the pocket shark Mollisquama mississippiensis and the strawberry squid.