Bioluminescence

Bioluminescence is simply light produced by a chemical reaction which originates in an organism.

It can be expected anytime and in any region or depth in the sea. Its most common occurrence to the sailor is in the often brilliantly luminescent bow wave or wake of a surface ship. In these instances the causal organisms are almost always dinoflagellates, single-cell algae, often numbering many hundreds per liter.

They are mechanically excited to produce light by the ship's passage or even by the movement of porpoises and smaller fish.

Bioluminescence is a primarily marine phenomenon. It is the predominant source of light in the largest fraction of the habitable volume of the earth, the deep ocean . In contrast, bioluminescence is essentially absent (with a few exceptions) in fresh water, even in Lake Baikal. On land it is most commonly seen as glowing fungus on wood (called foxfire), or in the few families of luminous insects.

Bioluminescence has evolved many times in the sea as evidenced by the several distinct chemical mechanisms by which light is emitted and the large number of only distantly related taxonomic groups that have many bioluminescent members.

Bioluminescent bacteria occur nearly everywhere, and probably most spectacularly as the rare "milky sea" phenomenon, particularly in the Indian Ocean where mariners report steaming for hours through a sea glowing with a soft white light as far as the eye can see.

(c) http://www.lifesci.ucsb.edu/~biolum/