 | The bloodybelly comb jelly swims by wiggling hundreds of tiny paddles that resemble the teeth on a comb. Its red belly conceals the glowing animals it swallows, which could otherwise make the jelly a target for predators. © AMNH |
In order to survive in this sunless world, 90 percent of all deep-sea animals living below 700 meters (2,300 feet) have the ability to generate their own light. Without this light, known as bioluminescence, these denizens of the dark would probably be unable to find food or mates. Using light for a variety of purposes, the animals of the deep create a dazzling light show that resembles a starry night sky.
Bioluminescence is light created by living things. This light can be used to attract a mate, find prey or elude predators. Many fish hide by matching the glow of the dim light around them, and various animals use bright flashes to distract and evade predators. Some squirt out clouds of bioluminescent particles—or even cast off entire, glowing body parts to distract predators while they sneak away.
Bioluminescent light is produced by chemical reactions that occur inside living things.
The precise chemicals involved vary, but in general, bioluminescence requires a light-producing molecule, or luciferin, and an enzyme, luciferase.
The luciferase attaches an oxygen molecule to luciferin, creating a new molecule in an excited state. When this molecule returns to a lower energy level, it releases energy in the form of light.
Some animals create their own luciferin and luciferase, while others rely on bacteria that live inside their light-producing organs, or photophores.
 | The glowing photophores of this dragonfish (Melanostomias bartonbeani) emit blue light—the color that travels farthest underwater. Many deep-sea fishes have distinctive patterns of photophores that help them escape predators or advertise their species and gender to potential mates.
© Edith Widder / Harbor Branch Oceanographic Institution |
Though bioluminescent lights occur in a variety of colors, most are blue, because blue light travels the farthest underwater.
In the ocean, 99 percent of red light is blocked at a depth of just 4 meters (13 feet), yellow light in 50 m (165 ft), and green in 110 m (360 ft). But blue light can travel 250 m (820 ft) before almost all of it has been scattered or absorbed. Many deep-dwelling organisms have eyes most sensitive to blue light, making use of the only light available in deep water.
Many deep-sea fish have body parts that look like small animals or morsels of food. They use them as lures to attract prey, just as fishermen use a rod and lure. When other animals approach to eat these living lures, they get snapped up themselves. Lures can be attached to the hunter's head, back, tail or—as in the case of the wolftrap angler—even inside its mouth. Many can be jiggled to imitate the swimming motion of a small fish or shrimp, and some glow in the dark. If bitten off, the lures can often grow back.
Many fish in the twilight zone have huge eyes to collect as much of the dim light as possible; some have eyes thirty times more light sensitive than a human's. The amphipod Cystisoma's eyes cover the entire top of its head. Many midwater fishes, including the tube-eye, have long, tube-shaped eyes, which provide greater light sensitivity and depth perception. Some fishes hide by glowing to match the dim light around them. The eyes of certain predators defeat this camouflage by filtering out all but bioluminescent light.
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