Deep sea is the largest yet least-known habitat on Earth. It covers about two thirds of Earth’s surface and includes all waters more than 3,300 feet (1,000 meters) below sea level. For centuries, most people assumed that the cold, black depths of the ocean supported little or no life. In the 1900’s, however, new types of undersea vessels and other technological developments enabled people to explore this vast frontier. Scientists have discovered a great variety of living things in the deep sea. Some deepwater creatures are among the most bizarre on Earth.
Giant isopod
Environment
Environmental conditions in the deep ocean are like conditions nowhere else on Earth. The water is bitterly cold. Temperatures hover just above freezing, from 34 to 39 °F (1 to 4 °C). The pressure in the deep sea is 100 to 1,000 times as great as the pressure on the surface. This pressure results from the weight of the waters above. It would crush the unprotected bodies of most surface animals. No sunlight penetrates to the deep sea, so its creatures live in almost total darkness.
Food is scarce in the deep sea because plants, which depend on sunlight, cannot grow in the dark waters. On land and in shallow waters, plants serve as the basis of the food chain, the system by which energy is transferred from one living thing to another in the form of food. Plant-based food chains begin with energy from sunlight. Plants use this energy to make food in a process called photosynthesis. Other living things rely on the plants. Because there are no plants in the ocean depths, most deep-sea creatures depend on marine snow, a constant fall of plant and animal fragments from the waters above.
Parts of the deep sea contain more animals than can possibly be supported by marine snow alone. In these areas, tiny living things called microorganisms form the basis of the food chain. The microorganisms manufacture nutrients from chemicals in a process called chemosynthesis.
The deep-sea floor
The ocean floor is a landscape of low mountains, broad plains, and deep trenches. Underwater mountains called mid-ocean ridges cover about 80 percent of the Pacific Ocean floor and about 50 percent of the Atlantic Ocean floor. Most peaks of the mid-ocean ridges rise less than 5,000 feet (1,500 meters) above the sea floor. Scientists believe many of them are extinct volcanoes.
When explorers discovered the mountain chains in the Atlantic, Indian, and Pacific oceans, they gave them different names, including the Mid-Atlantic Ridge in the Atlantic Ocean, the Mid-Indian Ridge in the Indian Ocean, and the East Pacific Rise in the Pacific Ocean. Today, scientists realize that the ridges form a single mountain chain that runs through the three oceans. Estimates of the chain’s total length range from 30,000 to 50,000 miles (50,000 to 80,000 kilometers).
The sides of mid-ocean ridges slope down into flat stretches of ocean floor called abyssal plains. Accumulated sediment (deposits of sand and mud) makes the abyssal plains some of the flattest areas on Earth.
Trenches are the deepest parts of the ocean. Many trenches occur in the Pacific Ocean. Most trenches are long, narrow, and deep, 2 to 2.5 miles (3 to 4 kilometers) below the surrounding sea floor. The greatest depth in the oceans is found in the Mariana Trench southeast of Japan. It plunges more than 6.8 miles (11 kilometers) below sea level. Frequent earthquakes and volcanic eruptions occur along the trenches. Many trenches lie near chains of volcanic islands called island arcs.
Among the most fascinating parts of the deep sea are underwater hot springs called hydrothermal vents. Hydrothermal vents shoot out heated water rich in sulfides and other chemicals. Microorganisms, such as bacteria and archaea, use chemosynthesis to make food from the sulfides. The microorganisms, in turn, support other life forms. Hydrothermal vents contain thousands of times more living matter than animal communities elsewhere in the deep sea. The best-known vents occur near mid-ocean ridges.
Diverse communities of life also exist near underwater cold springs called cold seeps, where the water welling up from the sea floor is cold instead of hot. Sulfides in these waters support the same kinds of life forms found near hydrothermal vents.
Hydrothermal vents are short-lived, lasting from a few years to perhaps 100 years. When they stop flowing, nearby animals die. Cold seeps can last hundreds of years.
Deep-sea life
Deep-sea life is unfamiliar, even to most biologists. Oceanographers believe that as many kinds of animals live in the deep sea as live in tropical rain forests and coral reefs. In fact, there may be 10 million or more deepwater animal species. Many creatures have beautiful but bizarre bodies shaped by the extreme cold, lack of light, and tons of pressure in the ocean depths.
Deep-sea life can be divided into three groups: (1) microorganisms, (2) invertebrates (animals without backbones), and (3) fish.
Microorganisms
make up the largest group of living things in the deep sea. Common deepwater microorganisms include single-celled bacteria and archaea, which provide food for many other creatures. Bacteria are so numerous in some places that they form colonies of huge, ruglike mats.
Invertebrates
are the most common animals in the deep sea. Numerous worms, crabs, and other small invertebrates live beneath the sediment on the sea floor. Many other invertebrates inhabit the waters near the floor.
Sponges are a diverse group of deep-sea invertebrates. Some deepwater sponges grow quite large. The giant barrel sponge, for example, may reach 6 feet (1.8 meters) in height. The weak, slow currents in deep waters enable sponges to have delicate bodies. Glass sponges, for example, have skeletons that consist of fine threads of silica, a glasslike mineral. Their cup-shaped bodies sit on a stalk that attaches them to the ocean floor. Many sponges of the deep sea are white or gray rather than the yellow or orange of their shallow-water relatives. Shallow-water sponges have pigments to protect them from harmful sunrays. However, deep-sea sponges do not need such protection because no sunlight reaches their environment.
Many invertebrates share the ocean bottom with sponges. Scattered among the deep-sea sponges are various other animals, including stalked sea lilies and featherstars. Many of these deep-sea animals look like flowers or giant, hairy spiders.
Some deep-sea crabs, starfish, and worms grow larger than similar animals in the shallows. For example, one deep-sea starfish grows larger than a basketball.
The black deep-sea waters hide and protect many creatures with brightly colored bodies. One such animal is the giant ostracod, a relative of crabs and shrimp. It has a bold orange-red body about the size and shape of a cherry. This color would make it difficult for the animal to hide from predators in shallow waters. But in the dark deep sea, the giant ostracod blends almost completely into the blackness.
Deep-sea shrimp, like many other deepwater animals, are bioluminescent—that is, they emit light from their bodies. A chemical reaction in the shrimp’s light organs produces greenish-yellow flashing lights, which act as a signal to attract mates.
Among the most common deep-sea invertebrates are sea cucumbers. These animals have been found in the deepest parts of the ocean. Many sea cucumbers live atop the mud, wriggling like fat worms. A few deep-sea species, however, swim gracefully through the dark water.
Many invertebrates live in waters just above the ocean bottom. Among the most unusual of these animals is a species of siphonophore. This jellylike colony of creatures is shaped like a string of glowing pearls and lined with stinging cells. Extremely large colonies may reach more than 100 feet (30 meters) in length.
One of the most graceful animals of the deep sea also lives just above the ocean floor. The cirrate octopod, a deep-sea octopus, is so jellylike that it was originally mistaken for a jellyfish. Cirrate octopods are named for the fingerlike projections called cirri that line their webbed arms. These cirri sense food in a dark world. The animal’s skin consists of about 95 percent water, which is almost impossible to compress. It cushions the animal’s body against the pressure of the deep sea.
Different kinds of squids also live in the deep sea. The fearsome giant squid is famous, but little is known about it. Though the animal may grow 60 feet (18 meters) long, few giant squids have been seen alive. Scientists know about these creatures mostly from dead individuals that have washed onto beaches after storms.
A great variety of invertebrates live around hydrothermal vents. Giant tubeworms are among the best-known of these animals. These white giants grow up to about 5 feet (1.5 meters) long and have red gill-like organs that they retract when disturbed. Giant tubeworms have no digestive tract. Instead, they get their food from bacteria that live within their organs. In return, they provide the bacteria with a place to live. Giant tubeworms also extract sulfides from the water, which the bacteria need for food. This biological relationship is an example of symbiosis, in which two creatures live together in a way that benefits one or both of them.
Near the tubeworms live dinner-plate-sized clams and mussels, as well as the huge mats of bacteria upon which these animals depend. A species of siphonophore is often found floating above the bacteria. This unusual organism resembles a dandelion flower. Many of the invertebrates that live around hydrothermal vents have not yet been classified or even discovered.
Fish
are the only vertebrates (backboned animals) that inhabit the deep ocean. Deep-sea fish are very different from those found in shallow waters. Most deepwater fish are extremely small, growing only a few inches or centimeters in length. Like many invertebrates, deep-water fish often have jellylike skin to withstand pressure in the ocean depths. Deep-sea fishes are usually weak swimmers. Most drift slowly through the weak deep-sea currents, expending as little energy as possible.
One deep-sea fish, called the rattail or grenadier, is thought to be the most widespread animal in the world. This fish has been found in deep waters from the equator to the polar seas.
Like deep-sea shrimp, numerous deepwater fishes are bioluminescent. The anglerfish has a light-producing organ above its mouth to lure prey. The organ is on a long, flexible spine that looks like a fishing pole.
Anglerfish and another deep-sea predator, the viperfish, have mouths full of long, sharp teeth to capture prey. Like many other deepwater creatures, these fish have rates of digestion many times slower than those of upper-ocean animals. Food is so scarce in the ocean depths that deep-sea animals must use nourishment slowly to survive.
One of the most unusual-looking fish is the pelican eel. It has an enormous mouth and a long, thin body. It may use its mouth like a net, capturing large numbers of animals at once.
Another strange deep-sea creature, the tripod fish, rests on the bottom of the ocean on three elongated fins. This posture gives it the appearance of a tripod or three-legged stool.
Exploration
Undersea explorers and scientists are only beginning to understand the deep sea. Until the mid-1800’s, most scientists believed there was little or no life at the bottom of the ocean. Then in the 1870’s, people began to find deep-sea creatures by using dredges. They lowered these scoops under the ocean surface and brought up samples from the sea floor.
In the 1930’s, scientists began to develop diving vehicles that enabled them to explore the ocean at greater and greater depths. In 1967, marine biologists first used the epibenthic sled, an underwater collecting device resembling a scoop. Deep-sea samples collected by this machine contained many more animals than samples gathered in shallower waters. These findings gave scientists a much more accurate idea of the vast array of life on the ocean floor.
In the late 1970’s, scientists began to get firsthand looks at sea-floor life through the windows of underwater vessels called submersibles. Scientists also explored deep waters with remotely operated vehicles and robots lowered from research ships. Despite this progress, researchers estimate that less than 1 percent of the deep sea had been explored by the beginning of the 2000’s.
