Fireflies sparking on a summer’s evening. Jellyfish glowing in night waters. These are examples of bioluminescence – light emitted by living organisms. Unlike incandescent light, which is caused by heat, the light produced by luminous animals and plants results from a biochemical reaction to oxygen.
Among the thousands of species of bioluminescent marine creatures, the majority are planktonic organisms. Frequently found in immense groups, these creatures glow in the wake of swimming fish or passing ships. Certain bodies of water, such as Phosphorescent Bay* in Puerto Rico have extremely heavy concentrations, contributing to legends of spirits inhabiting the water. Some luminous species of saltwater bacteria cause decaying fish to glow in the dark. Certain animals such as the “flashlight” fish culture colonies of these bacteria on their bodies and use them as their own light source.
At levels where the sun does not penetrate, luminous animals give the effect of a starlit night. Scientists estimate that 96 percent of all creatures found at these depths possess some form of self light generation. Some deep-sea creatures develop photophores, light-producing organs which may be arranged in symmetrical rows along the fish’s body or in a single unit overhanging the mouth. Studies reveal that photophores are connected to the nervous system and are biochemically activated.
Why does bioluminescence occur? Scientists still aren’t certain, but there are several possible reasons. “Blinking” patterns observed in many species indicate that the light serves a communication or courtship purpose. The light may reveal food or lure prey. Creatures with poorly developed or nonexistent eyes may use the light to blind or startle predators.
In the past, bioluminescence has caused superstition, awe, and even fear. Today, it remains one of nature’s most fascinating phenomena.
*Bioluminescence was once confused with phosphorescence in the belief t

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