Discover the Deep Sea

The deep sea, often defined as the water column and seabed below 200 meters in depth, occupies 90% of the marine environment and is the largest biome on the planet. It functions as the regulatory body of the entire biosphere, buffering biogeochemical circulation, regenerating nutrients and hosting infinitely rich biodiversity, making it essential to life on Earth. The key deep-sea habitats are the abyssal plains, hydrothermal vents and cold seeps, cold-water corals, seamounts (underwater mountains) and the deep-water column. These all have distinct faunas with widely divergent ecological and life history characteristics. Most deep-sea species have low productivity and are extremely vulnerable to human disturbance. This vulnerability is recognized in United Nations resolutions and regulations, requiring that it be protected from harmful fishing activities.

Spanning both the deep-seabed area and continental shelves within national jurisdiction, the deep sea is the most difficult area on Earth to access and one of the hardest to manage. Its unique, extreme and three-dimensional environment requires specialized approaches.

Deep-Sea Life

Scientists have speculated that as many as 10 million species may inhabit the deep sea – biodiversity comparable to the world’s richest tropical rainforests. Because deep-sea species live in rarely disturbed environments and tend to be slow growing, late maturing and endemic, they are exceptionally vulnerable to extinction. Deep-sea coral and sponge communities are largely untapped sources of natural products with enormous potential as pharmaceuticals, enzymes, pesticides, cosmetics and other commercial products.

Hydrothermal vents

Hydrothermal vents are found along the mid-ocean ridge system, a 60,000 km-long, underwater mountain range stretching around the planet. This underwater chain of mountains is formed by geologic processes associated with seafloor spreading and plate tectonics. Cold seawater seeps into the crust through cracks in the seafloor. As it gets closer to molten rock, it heats up. As it does, it undergoes several chemical reactions. Eventually, the hot, mineral-rich fluid rises again and gushes out of openings in the seafloor – hydrothermal vents – at temperatures up to about 400 degrees Centigrade.

Photo credit: NOAA Okeanos Explorer Program, Gulf of Mexico 2012 Expedition.

Hydrothermal vents were first discovered in 1977. The ecosystems associated with them were recognized to be unique on the planet. The microbes (bacteria and Archaea) which form the base of the food chain are chemosynthetic, using energy from hydrogen sulfide in the vent fluid, and oxygen and carbon dioxide from the seawater, to create simple sugars. Some live symbiotically with organisms such as tubeworms and mussels. Others are free-living and may be found growing on many different surfaces, including rocks, tubeworm tubes and other animals, and even inside chimneys. Many larger organisms have been found living near hydrothermal vents, including tubeworms, mussels, crabs, squat lobsters, limpets, alvinella worms, scaleworms, zoarcid fish, and octopus. Given the absence of sunlight, no plants are found here.

Abyssal plains

Abyssal plains begin at the edge of the continental margin and continue into the ocean depths. These plains are the flattest places on earth, covering about half of the deep-ocean floor. Their flatness is the result of the accumulation of a blanket of sediments – sometimes up to 5 kilometers thick – which overlies the basaltic rocks of the oceanic crust. Abyssal hills can occur where the sediments is not thick enough to cover the underlying rock completely. These hills are usually extinct volcanoes or small formations of rock which were originally in molten form. Abyssal hills often run parallel to mid-ocean ridges. They are are most common in the Atlantic Ocean and least common in the Pacific where the trenches at plate boundaries trap most of the sediment.


Photo credit: Mountains in the Sea Research Team; the IFE Crew; and NOAA/OAR/OER.

A great deal of deep-seas biodiversity is concentrated around seamounts which are underwater mountains that rise 1,000 meters or higher from the seabed without breaking the ocean’s surface. It is estimated that there may be as many as 30,000 to 100,000 seamounts worldwide. They are home to cold-water coral reefs and forests, sponge beds and hydrothermal vents, as well as the many millions of species dependent on these. And because many seamounts are located in remote surroundings – underwater islands, essentially – virtually every study finds species that were previously unknown and are endemic, meaning that they are unique to that area.

Seamounts are not only physically impressive, but like an oasis in the desert, provide an important source of food. Because of their physical characteristics and strong localized currents, they accumulate enormous quantities of plankton. The plankton, in turn, attracts a vast array of marine life, providing feeding as well as spawning grounds for myriad pelagic species, including some that have migrated across wide oceanic areas.

Home to large marine mammals, such as dolphins and whales and an extraordinary diversity of fish, together with exotic sponge and coral ecosystems, seamounts are among the world’s greatest marine-biological treasures.

Cold-water corals

The deep sea is also home to remarkably rich coral systems. Once thought to inhabit only the warm and shallow waters of tropical and subtropical regions, corals have apparently been thriving in deep, dark and cold waters throughout the world for millions of years. Indeed, it is now thought that there are more coral species living in the dark ocean depths than in the tropical shallows.

Carbon dating of living cold-water coral reefs has revealed that the oldest may be 8,000 years old or more. Several of the coral species create complex reefs and ornate three-dimensional, forest-like structures that rival tropical coral systems in their size and complexity. Indeed, the oldest and tallest reef yet observed is 35 meters high.

Although scientists have only just begun to explore the ecological aspects of cold-water corals, it is clear that cold-water reefs are bustling with life, providing essential sanctuaries and nursing grounds for countless species.

Deep-sea fish

Photo credit: NOAA Okeanos Explorer Program, Gulf of Mexico 2012 Expedition.

Seamounts, and the cold-water corals they sustain, provide habitats for several commercial bottom-dwelling fish species, such as orange roughy, roundnose grenadier, blue ling, mirror dory and silver dory. Other species, for example, alfonsino, boar fish and blue-eye trevalla, are also attracted to these habitats. The concentrations of these fish around seamounts make them very attractive fishing grounds.

Sadly, studies show that the long life cycles and slow sexual maturation of deep-sea fish makes them particularly vulnerable to large-scale fishing activities. These species have dwelled in ecosystems that are rarely disturbed and that recover from disturbances at an exceedingly slow rate, if at all. Whole populations can be quickly fished out.