Many saltwater fish—particularly deep-sea species—glow in the dark. However, almost no fish produce their own light. Instead they rely on a symbiotic bacterium (Aliivibrio fischeri) to produce their light. Marine fish use this light several ways for several purposes.
All marine fish that produce light rely on a bacterium called Aliivibrio fischeri to produce their light. This bacterium was formerly known as Vibrio fischeri, but was recently reclassified based on genetic testing. The bacteria is rod-shaped and gram-negative. It glows greenish or bluish and can live freely in ocean water. In fact, A. fischeri is found in almost all ocean water in varying levels. It also lives as a symbiote in fish, squid and other marine organisms, allowing them to glow in the dark in exchange for a place to live.
Many fish rely on A. fischeri to produce light. Only one species of fish glows on its own—without genetic modification—and it's a freshwater eel that fluoresces only under UV light. Many deep-sea and nocturnal fish form a symbiotic relationship with A. fischeri, "culturing" the bacteria within various organs in their bodies. These include the angler fish, flashlight fish and lantern fish. Since the bacterium lives in most ocean water, fish don't have trouble finding it.
Controlling the Glow
If fish were stuck glowing constantly, they could run into problems. For example, predators could easily spot them. For this reason, most fish that have A. fischeri within their bodies have some way to to "turn off" their lights. Since the bacteria glow constantly, this usually consists of some method of obscuring the light. These fish have organs that can either blink shut like eyes or rotate away from the fish's skin, obscuring the light and giving the fish the ability to control its glow.
Using the Glow
Different species of fish use their bacteria-granted lighting in a number of ways. Some species, like deep-sea angler fish, use their light in a lure-like organ, used to attract prey in the gloomy darkness of the deep ocean. Others, like the flashlight fish, blink their lights at each other to help them find their school and mates in dark waters. Still other fish, like lantern fish, use their light for counter-illumination. Counter-illumination is a form of active camouflage. Since many predators hunt by looking up and spotting the silhouettes of fish, fish use counter-illumination to match the lighting of the water above them, making them almost invisible to predators below.