Bioluminescence is the light produced by a chemical reaction that occurs in an organism. It occurs at all depths in the ocean, but is most commonly observed at the surface. Bioluminescence is the only source of light in the deep ocean where sunlight does not penetrate. Amazingly, about ninety percent of the organisms that live in the ocean have the capability to produce light.

It can be used to evade predators, attract prey, communicate within their species, or advertise (Nealson, 1985). For example, the angler fish uses the Lure Effect (attracting prey). This fish has a dangling lure in which bioluminescent bacteria live. The lure hangs in front of its mouth; fish swim toward the light and may become food for the angler fish. Some fish use bioluminescence for mating signals or as territorial signals (intraspecies communication), and some use it to communicate interspecies (advertisement). Some organisms employ it for more than a single reason.

Most bioluminescence is blue for two reasons. First, blue-green light travels the farthest in water. Its wavelength is between 440-479 nm, which is mid-range in the spectrum of colors.

Second, most organisms are sensitive to only blue light. They do not have the visual pigments to absorb the longer or shorter wavelengths. Red light, which has a long wavelength, is quickly absorbed as you descend in water-this is why underwater pictures appear blue. As with every rule, exceptions exist. Each luminescent organism has a unique flash. Factors that can vary are color, rise time, decay time, and total flash time (Nealson, 1985).

Some organisms can emit light continuously, but most emit flashes with varying durations and brightness. The luminescence of one dinoflagellete lasts for 0.1 seconds and is visible to humans. Larger organisms, such as a jellyfish, can luminesce for tens of seconds.