Have you ever seen mystical flashes of blue-green light in the night? If you are one of the lucky ones who say “yes”, then you have observed bioluminescence, or biologically produced light. It exists in a variety of living forms: fungi (foxfire), fireflies, marine fish, squids, worms, and jellyfish. It is known on land and in the sea, but not in freshwater. Below the zone of sunlight penetration in the sea, it is the rule, with virtually all organisms bioluminescing.
The purpose? Most authorities believe it is used by animals for communication, including warning and courtship. But what is the mechanism? Do they have light bulbs? Little nuclear plants? In all known cases, the light results when compounds called luciferins combine with oxygen in the presence of an special enzyme and produce light. The most unique quality of this reaction is that the energy released is totally in the form of light - there is no heat produced, so it is much more efficient than any light we make.
On a recent trip to Santa Rosa Sound in Florida, bioluminescent comb jellies (a non-stinging type of jellyfish) were so abundant that simply wading in knee-deep water at night produced flashes of light. While sailing at night in the Gulf, bands of luminescent organisms were seen in different depths and currents. This made it clear how the captains of ancient Polynesian sailing vessels used these animals to navigate in the open sea!
King, F. B. 1983. Bioluminesence. Living Museum 45(3):36-38.
Russell, F. S. 1928. The Seas. pp. 182-183.