Bioluminescence of a jellyfish seen with the lights off.
The light emitted by a bioluminescent organism is produced by energy released from chemical reactions occurring inside (or ejected by) the organism.
If you’ve ever seen a firefly, you have encountered a bioluminescent organism. In the ocean, bioluminescence is not as rare as you might think. In fact, most types of animals, from bacteria to sharks, include some bioluminescent members. Also, bioluminescent are found throughout marine habitats, from the ocean surface to the deep sea floor.
While the functions of bioluminescence are not known for all animals, typically bioluminescence is used to warn or evade predators), to lure or detect prey, for communication between members of the same species.