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. Four main uses for an organism to bioluminesce have been hypothesized. This fish has a dangling lure in which bioluminescent bacteria live. 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. Second, most organisms are sensitive to only blue light. They do not have the visual pigments to absorb the longer or shorter wavelengths. As with every rule, exceptions exist. The red light they produce is almost infrared and not visible to the human eye.