The deep sea is often described as "a world of eternal darkness." That is a lie. While it is true that sunlight does not penetrate below 1,000 meters, that does not mean that it is a lightless world down there. In fact, there are lots of lights—billions and billions of them. These are animal lights and they serve many life-sustaining functions. There are lights for finding food, lights for attracting mates, and lights used for defense. All these lights are generated by a chemical process called bioluminescence. There are only a few creatures on land that can make light. Fireflies and glowworms are some of the best known examples, but there are a handful of others such as some earthworms, click beetles, snails, centipedes, and fungi. These, however, are relatively rare and they do not play a significant role in the balance of nature. By contrast, in the oceans there are so many animals that make light that there are vast regions where as many as 80 to 90 percent of the animals collected in nets are bioluminescent. In the ocean, bioluminescence is the rule rather than the exception.
The reason that there are so many animals in the oceans that make light has to do with the nature of the oceanic visual environment. Out away from shore, in the vast open ocean that forms the largest living space on our planet, there are no trees or bushes for animals to hide behind. But just as on land, prey need to hide from predators. Some animals hide by being transparent. Others hide by descending into the dark depths during the day and only ascending into food-rich surface waters under cover of darkness. And still others remain at depths below the penetration of sunlight and survive on food that sinks or swims into the depths. It is because so many animals in the ocean survive by hiding in darkness that the ability to make light is so prevalent.
For animals that spend their lives avoiding sunlight, a built-in headlight can be a very handy device. There are many fish, shrimp, and squid that use headlights to search for prey and to signal to mates. Headlights may occur below the eye, behind the eye, or in front of the eye. Many headlights have a highly reflective surface that helps direct the light outward, much like a car's headlights. And, as with some cars, some headlights can be rolled down and out of sight when they are not in use—a handy way of hiding that reflective surface and allowing the fish to better blend into the darkness. Most headlights in the ocean are blue, which is the color that travels furthest through seawater and the only color that most deep-sea animals can see. But there are some very interesting exceptions like dragonfish with red headlights that are invisible to most other animals, but that the dragonfish can see and use like a sniper scope to sneak up on unsuspecting, unseeing prey. Dragonfish also have blue headlights that they can use like high beams to see into the distance.
Other animals use glowing lures to attract prey. Much of the fecal matter and decaying foodstuffs that rain down from above are covered with glowing bacteria, which is why a glowing lure can easily be mistaken for dinner, when it instead signals an untimely death in a toothy jaw. Lures may dangle from fishing rods that poke out of the top of the head or out of the chin; they may even be found at the tip of a very long tail.
Light is also used for defense. Many animals that live in the twilight depths between 200 and 1,000 meters use a camouflage trick called counterillumination to obliterate their silhouettes with bioluminescence. At a distance, individual belly lights called photophores blur into a light field that exactly matches the color and intensity of the dim filtered sunlight overhead. And if a cloud passes over the sun, the fish, shark, squid, or shrimp either dims its belly lights or swims upward to maintain that perfect match. One of the fish that uses this camouflage trick is called the benttooth bristlemouth (Cyclothone acclinidens); it is so common that it is believed to be the most abundant vertebrate on the planet. Imagine that! The most abundant animal with a backbone, and most people have never seen or heard of it.
Another common defensive trick is for the prey to release its bioluminescent chemicals into the face of a predator, just as a squid or an octopus releases an ink cloud. The light either blinds or distracts the predator, allowing the prey to flee into the darkness. Many jellies use this trick, as do shrimp and squid. There is even a fish, called the shining tube shoulder, that can shoot the equivalent of photon torpedoes out of a fleshy, backward-pointing pipe located just above its pectoral fin.
Still another use of light for defense is as a burglar alarm. Blaring horns and flashing lights on your car are meant to discourage a burglar because of the unwanted attention they attract; brilliant displays of bioluminescence serve the same purpose. When caught in the clutches of a predator, a prey's only hope of escape may be to attract the attention of a larger predator that will attack the attacker. Some of the most spectacular light shows in the ocean are burglar alarms. One of the best examples is the pinwheel display of the common deep-sea jellyfish, Atolla. It is a display that has to be seen to be believed; in the dark depths of the ocean it can attract the attention of a predator over 100 meters away.
Bioluminescence occurs in all the world's oceans from surface to bottom and from coast to coast. Appreciating how animals use their lights is important to understanding this ecosystem that represents more than 99 percent of our biosphere. Various light-producing chemicals extracted from different animals have also proved enormously valuable in medical and genetic research. Living lights in the ocean are beautiful, mysterious, useful to humans, and absolutely essential to the animals that possess them.
Edith Widder is senior scientist at the Ocean Research and Conservation Association and was a MacArthur Fellow for 2006.
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