Bioluminescence is a subset of chemiluminescent reactions occurring in the natural world. A simple chemical reaction, usually including only two chemicals, luciferin and luciferase, is responsible for the release of energy as light.
Luciferase is used as a catalyst whilst luciferin is known as the light emitting chemical. In fireflies, the base oxidation of luciferin yields a 1,2-diooxacyclobutanone intermediate which decomposes to givea complex heterocycle, carbon dioxide and light
Sometimes the luciferin, luciferase and cofactor (oxygen), are bound together in a single unit called a photoprotein. This molecule can be triggered to produce light when a particular type of ion (frequently calcium) is added to the system.
Although there are many hundreds of types of luminous creatures in the oceans and on land, there are only a few basic luciferins known. These few known light emitters are found across many species, some of which have aquired it through the food chain whilst others have shown signs of being able to synthesise it on their own.