ichthyologist:

Bioluminescence in marine organisms

Bioluminescence is created by a chemical reaction.  It usually takes place within special light-producing cells called photocytes, which are located inside light organs called photophores. 

The basic chemical reaction that produces bioluminescence requires three things:  a type of molecule called a luciferin, an enzyme called luciferase, and oxygen.  The luciferase enzyme catalyzes, or speeds up, the reaction in which luciferin combines with oxygen, producing an oxidized form of luciferin called oxyluciferin.  The oxyluciferin emits light, usually blue or blue-green. 

Marine animals use bioluminescence for a variety of reasons.  Some species use the light to attract prey.  In the black depths of the ocean, the prey is not visible, so instead of hunting, predators such as the anglerfish can simply remain in one place and wait for the prey to be attracted to the light they emit. 

On the other hand, some marine animals use bioluminescence to avoid being caught by predators.  Flashlight fish are able to turn the lights on their cheeks on and off to confuse predators.  Firefly squid can release luminous ink as a smokescreen while they make a quick getaway.  Other animals use light to communicate with each other, warn of approaching enemies, or attract mates.  
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