The Abyssal Zone
The abyssal zone is the ocean habitat 4,000 to 6,000 meters (13,000 to 20,000 ft) below sea level. This region is defined by complete darkness, extremely high pressure, and average temperatures of 2-3 degrees C (35 degrees F). Reaching down yet farther, oceanic trenches extend to depths exceeding 30,000 feet. Sometimes called the last frontier, the extreme depths of the ocean have only begun to be explored.
Oceanic trenches are located at the boundaries where tectonic plates collide. When one plate sinks down beneath an adjoining plate in the process of subduction, a long, narrow and very deep trench may form. Several such trenches are located in the Pacific Ocean. The deepest recorded point on Earth at 11,034 meters (36,200 ft) is Challenger Deep, located within the Mariana Trench, near Guam.
While the temperature of the water in the abyssal zone is close to freezing, there are openings through which superheated water flows up from beneath the ocean floor. These hydrothermal vents occur at geologically active sites such as underwater volcanoes and mid-ocean ridges, boundaries where tectonic plates are pulled apart and magma wells up from the mantle. The water coming out of the hydrothermal vents may be in excess of 400 degrees C (750 degrees F), far above the boiling point, yet it does not boil due to the enormous pressure.
The water released by hydrothermal vents often has a very high mineral content, and builds up into tall chimney-like structures around the vent. Black smokers are vents where the water is rich in iron and sulfide, forming a dark-colored plume. The water emitted by white smokers contains high amounts of lighter colored minerals such as silicon, barium and calcium, and is seen as a white plume.
Creatures of the Deep
The organisms that live at the extreme depths of the ocean are highly specialized to survive in conditions of high pressure and total darkness. They have slow metabolisms and are able to live in water with very little dissolved oxygen. Many of the fish living in the abyssal zone are ambush predators such as anglerfish, black swallowers and gulper eels, who wait patiently for their prey to come to them, rather than expending energy by swimming. They have long, needle-sharp teeth, jaws that come unhinged and mouths and stomachs that can stretch like balloons in order to swallow prey that may be larger than the fish itself. It is common for deep sea predators such as flashlight fish to lure their prey by bioluminescence, light that is generated by a chemical reaction.
The greatest amount of biological diversity in the abyssal zone is found near hydrothermal vents. Since there is no sunlight, chemosynthetic (chemoautotrophic) bacteria convert compounds such as hydrogen sulfide into organic matter. These microorganisms are the primary producers and basis of the food chain. They are fed upon by small crustaceans, which in turn are eaten by a variety of predators including crabs, shrimp, snails and bivalves.
Tube worms are a species uniquely adapted to the abyssal zone. They have no mouth or digestive system, but absorb inorganic compounds through their skin and rely on the chemosynthetic bacteria within their bodies to provide them with food.