The deep ocean begins where the sunlight fades, at 200 meters (656 feet), and extends to where no light penetrates. The water contains little oxygen, and the temperature is just above freezing. Pressures reach up to 1,000 times greater than the pressure at the surface. Ocean researchers estimate 100 million species exist in the deep ocean — more biological diversity than all terrestrial ecosystems combined.
Approximately half of the seafloor consists of soft mud flats dotted with aggregates of minerals forming hard surfaces. Bottom-dwelling fish and invertebrates including worms, barnacles, brittle stars, crabs, mollusks, star fish, sea anemones, sponges, sea pens and sea cucumbers thrive on the soft bottom. Larger species live on stones and nodules of manganese, thriving on nutrient-rich currents that circulate just above the seafloor, while bacteria thrive on ancient sediment hundreds of meters beneath it.
Deep sea corals capture food particles from the water column and thrive without sunlight. One of the most biologically diverse ecosystems in the deep ocean, cold-water corals provide habitat for more than 1,300 species in the northeastern Atlantic alone. Anemones, bristleworms, clams, crabs, hydroids, limpets, nudibranchs, octopi, sea mats, snails and sponges thrive among the corals. Some corals form extensive, complex reefs. One of the largest Lophelia reefs off the Norwegian coast is more than 8,000 years old.
Cold Seep Beds
Chemosynthetic communities form where gas hydrates seep from the ocean bottom and some cold seep beds cover more than a kilometer. These beds have high microbial productivity that allows tube worms, clams and mussels to form structures as high as 2 meters (6.6 feet). Similar to the communities that form around hydrothermal vents, organisms living in cold seep beds rely on hydrocarbons or hydrogen sulfide gas for their carbon and energy needs.
Individual sponges may reach 100 years of age, weighing up to 80 kilograms (176 pounds). Sponge fields often grow into massive reefs. Aggregations of massive sponges create three-dimensional structures that provide habitat, hunting and shelter for other marine organisms, including important commercial fish such as cod, ling and redfish. A 9,000-year-old glass sponge reef off the coast of British Columbia covers 700 square kilometers (270 square miles), and in some areas is 19 meters (62 feet) high.
Water near hydrothermal vents consists of toxic chemicals and reaches 400 degrees Celsius (752 degrees Fahrenheit). Communities of anemones, crabs, mollusks, shrimps, soft corals and worms thrive by converting sulfur-rich emissions into energy. The world’s fastest growing invertebrate, a giant worm that lacks a mouth and digestive system, has bacteria in a special organ that allows it to grow about 2.5 centimeters (1 inch) in 10 days. More than 75 percent of the species living around hydrothermal vents are endemic, existing nowhere else on the planet.
Mauna Kea in Hawaii, the highest mountain on the planet, is an underwater mountain more than 9,144 meters (30,000 feet) high. Until their discovery in 2000, seamount ecosystems were largely unknown, and researchers have just begun to understand their ecological importance. Seamounts may act as oases for life amid barren areas along the deep ocean floor. Scientists are also studying the relationships between ocean currents and underwater mountains and the role they may play in the creation biological hotspots, isolated areas critical to the development of new species, endemism and biodiversity.