A Glow-in-the-Dark World Beneath the Sea

Who's the brightest of them all? Ocean dwellers in the know will tell you it's the many species of "glow-in-the-dark" fish that inhabit the deepest depths of the ocean. Most fish that live in the deepest depths are able to give off their own glow, a process called bioluminescence. In fact, in the sea, bioluminescence is everywhere – in fish, sea slugs, squid, jellyfish and many other deep-sea dwellers.

What is Bioluminescence?

Bioluminescence is a glow that is the result of a chemical reaction within the tissues. This reaction takes place when a special enzyme and a special protein inside the cells are exposed to oxygen and water at a temperature of about 75 degrees Fahrenheit (25 degrees Celsius). The result is energy that gives off light and creates patches of bioluminescent tissue, photophores, which are kidney-shaped organs arranged in distinct groups on the organism. It's the same process that causes a firefly to light up in the summer night.

Almost all of marine bioluminescence are blue in color. There are two reasons for this: First, blue-green light travels furthest in the water. Second, most organisms are sensitive only to blue light – they lack visual ability to absorb longer red or shorter ultraviolet light.

Survival Tricks

Photophores help fish survive in the deep sea in three ways:

• Camouflage. Some fish, like the lantern fish or lampfish, from the family Myctophidae, use photophores as camouflage by producing a counter-illumination. Their photophores are located on their undersides, heads and tails. They spend their days deep in the ocean, but often migrate to shallower depths at night. They often form schools of hundreds of thousands of fish, and deep in the ocean, they are preyed upon by tuna, bonito, albacore, dolphin fish and others, but they are invisible from below because their glowing undersides match the light of the sunlit or moonlit sea surface. At night in shallower water, their dark unlighted backs blend into the darkness of the deep ocean water and they are invisible from above, keeping them safe from sea birds, like penguins and seals.
• Lure. Some fish use their glow as lure to attract their dinner. For example, bioluminescent bacteria live in pockets under some fish's eyes, making the eyes look like tiny headlights, which attracts smaller fish. Luckily, when a predator swims by, however, the fish closes his eyes.

The female angler fish eats fish and shrimp that are attracted by what looks like a fishing rod growing out of the top of her head with a light at the end. She also attracts prey by vibrating the lure. This is much like the use of colorful lure by fishermen.

• Identification and attraction. Fish may use their glow to signal other fish, and the distinct groupings of photophores may help them to identify others in their species. Some experts believe fish use bioluminescence in the same fashion as a peacock displaying his colors. And what female wouldn't be attracted to a glowing mate?