Have you ever gone out to the ocean and seen the water glowing in the dark? This mysterious glow in the water has been seen in as early as 500 B.C. by Aniximenes. He described of light glowing when he struck the sea with an oar. People have observed this light countless times since.

This mysterious light is called bioluminescence. Bioluminescence is light produced by a chemical reaction in an organism and this is the predominant source of light in the ocean. They are not found in fresh water and on land, they are most commonly seen in the form of a glowing fungus, earthworms, centipedes and fireflies.

In the Mesopelagic zone of the ocean, which is about 200m to 1000m in depth, almost 90% of marine organisms are bioluminescent. The most abundant vertebrate on earth is a fish called bent tooth bristle-mouth, a bioluminescent fish. Some other bioluminescent organisms in the water include lanternfish, squids, shrimps, krills, copepods, ostracods, amphipods and zooplanktons.

Marine organisms create this light for a number of reasons. Bioluminescence helps the fish to find food, find mates and defend against predators. Each of these factors plays an important role in an organisms survival.

For many fish, bioluminescence is the only way to find food. For example, Black Dragonfish have light organs under their eyes that can produce bioluminescent light to use like flashlights to search for prey in the dark ocean. Anglerfish also use bioluminescence to search for food, but in a different way. They have a glowing lure that dangles from its head and attracts prey.

Finding mates is equally important for the fish. They send sex signals using special flash codes. Most people only know of fireflies using light to find mates, but many fish do the same.

Many fish are defenseless if not for their bioluminescence. Some fish use it as a camouflage to hide themselves from predators. They have bioluminescent light organs in their bellies and use it to disguise their silhouettes.