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The deep sea, or deep layer, is the lowest layer in the ocean
An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...

, existing below the thermocline
A thermocline is a thin but distinct layer in a large body of fluid , in which temperature changes more rapidly with depth than it does in the layers above or below...

 and above the seabed
The seabed is the bottom of the ocean.- Ocean structure :Most of the oceans have a common structure, created by common physical phenomena, mainly from tectonic movement, and sediment from various sources...

, at a depth of 1000 fathom
A fathom is a unit of length in the imperial and the U.S. customary systems, used especially for measuring the depth of water.There are 2 yards in an imperial or U.S. fathom...

s (1800 m) or more. Little or no light penetrates this part of the ocean and most of the organisms that live there rely for subsistence on falling organic matter produced in the photic zone
Photic zone
The photic zone or euphotic zone is the depth of the water in a lake or ocean that is exposed to sufficient sunlight for photosynthesis to occur...

. For this reason scientists once assumed that life would be sparse in the deep ocean but virtually every probe has revealed that, on the contrary, life is abundant in the deep ocean.
From the time of Pliny
Pliny the Elder
Gaius Plinius Secundus , better known as Pliny the Elder, was a Roman author, naturalist, and natural philosopher, as well as naval and army commander of the early Roman Empire, and personal friend of the emperor Vespasian...

 until the expedition in the ship Challenger
Challenger expedition
The Challenger expedition of 1872–76 was a scientific exercise that made many discoveries to lay the foundation of oceanography. The expedition was named after the mother vessel, HMS Challenger....

 between 1872 and 1876 to prove Pliny wrong; its deep-sea dredges and trawls brought up living things from all depths that could be reached. Yet even in the twentieth century scientists continued to imagine that life at great depth was insubstantial, or somehow inconsequential. The eternal dark, the almost inconceivable pressure, and the extreme cold that exist below one thousand meters were, they thought, so forbidding as to have all but extinguished life. The reverse is in fact true....(Below 200 meters) lies the largest habitat on earth.

In 1960 the Bathyscaphe Trieste
Bathyscaphe Trieste
The Trieste is a Swiss-designed, Italian-built deep-diving research bathyscaphe with a crew of two, which reached a record maximum depth of about , in the deepest known part of the Earth's oceans, the Challenger Deep, in the Mariana Trench near Guam, on January 23, 1960, crewed by Jacques Piccard ...

 descended to the bottom of the Mariana Trench
Mariana Trench
The Mariana Trench or Marianas Trench is the deepest part of the world's oceans. It is located in the western Pacific Ocean, to the east of the Mariana Islands. The trench is about long but has a mean width of only...

 near Guam
Guam is an organized, unincorporated territory of the United States located in the western Pacific Ocean. It is one of five U.S. territories with an established civilian government. Guam is listed as one of 16 Non-Self-Governing Territories by the Special Committee on Decolonization of the United...

, at 35,798 feet or 6.77 miles (10,911 meters), the deepest spot in any ocean. If Mount Everest
Mount Everest
Mount Everest is the world's highest mountain, with a peak at above sea level. It is located in the Mahalangur section of the Himalayas. The international boundary runs across the precise summit point...

 were submerged there, its peak would be more than a mile beneath the surface. At this great depth a small flounder-like fish was seen moving away from the bathyscaphe's spotlight. The Trieste was retired and for a while the Japanese remote-operated vehicle (ROV) Kaiko
was a remotely operated underwater vehicle built by the Japan Agency for Marine-Earth Science and Technology for exploration of the deep sea. Kaikō was the second of only three vessels ever to reach the bottom of the Challenger Deep, as of 2010...

 was the only vessel capable of reaching this depth. It was lost at sea in 2003. In May and June of 2009, the hybrid-ROV (HROV) Nereus
Nereus (underwater vehicle)
Nereus is a hybrid autonomous underwater vehicle built by the Woods Hole Oceanographic Institution . Constructed as a research vehicle to operate at depths of up to , it was designed to explore Challenger Deep, the deepest surveyed point in the global ocean...

returned to the Challenger Deep
Challenger Deep
The Challenger Deep is the deepest known point in the oceans, with a depth of to by direct measurement from submersibles, and slightly more by sonar bathymetry . It is located at the southern end of the Mariana Trench near the Mariana Islands group...

 for a series of three dives to depths exceeding 10900 meters.

It has been suggested that more is known about the Moon than the deepest parts of the ocean. Until the late 1970s little was known about the extent of life on the deep ocean floor but the discovery of thriving colonies of shrimps and other organisms around hydrothermal vents changed that. Before the discovery of the undersea vents, it had been accepted that almost all life on earth obtained its energy (one way or another) from the sun. The new discoveries revealed groups of creatures that obtained nutrients and energy directly from thermal sources and chemical reactions associated with changes to mineral deposits. These organisms thrive in completely lightless and anaerobic environments, in highly saline water that may reach 300 °F (150 °C), drawing their sustenance from hydrogen sulfide
Hydrogen sulfide
Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...

, which is highly toxic to almost all terrestrial life. The revolutionary discovery that life can exist under these extreme conditions changed opinions about the chances of there being life elsewhere in the universe. Scientists now speculate that Europa
Europa (moon)
Europa Slightly smaller than Earth's Moon, Europa is primarily made of silicate rock and probably has an iron core. It has a tenuous atmosphere composed primarily of oxygen. Its surface is composed of ice and is one of the smoothest in the Solar System. This surface is striated by cracks and...

, one of Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

's moons, may be able to support life beneath its icy surface, where there is evidence of a global ocean of liquid water.


Natural light does not penetrate the deep ocean, with the exception of the upper parts of the mesopelagic
The mesopelagic is that part of the pelagic zone that extends from a depth of 200 to 1000 metres below the ocean surface. It lies between the photic epipelagic above and the aphotic bathypelagic below, where there is no light at all...

. Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

 is not possible, precluding any photosynthetically based primary productivity. Bioluminescence
Bioluminescence is the production and emission of light by a living organism. Its name is a hybrid word, originating from the Greek bios for "living" and the Latin lumen "light". Bioluminescence is a naturally occurring form of chemiluminescence where energy is released by a chemical reaction in...

 is the only light available at these depths. This lack of light means the organisms have to rely on senses other than vision. It may also have a selective effect on the locomotory habits of the animals and on their propulsive systems.


Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...

 is the greatest environmental factor acting on deep-sea organisms. Pressure increases 1 atmosphere (atm) for each 10 m in depth. In the deep sea, although most of the deep sea is under pressures between 200 and 600 atm, the range of pressure is from 20 to 1,000 atm. Pressure exhibits a great role in the distribution of deep sea organisms. Until recently, people lacked detailed information on the direct effects of pressure on most deep-sea organisms, because virtually all organisms trawled from the deep sea arrived at the surface dead or dying. With the advent of traps that incorporate a special pressure-maintaining chamber, undamaged larger metazoan animals have been retrieved from the deep sea in good condition. Some of these have been maintained for experimental purposes, and researchers are obtaining more knowledge of the biological effects of pressure.


Salinity is the saltiness or dissolved salt content of a body of water. It is a general term used to describe the levels of different salts such as sodium chloride, magnesium and calcium sulfates, and bicarbonates...

 is remarkably constant throughout the deep sea. There are some minor differences in salinity, but none that are ecologically significant, except in the Mediterranean & Red seas.


The two areas of greatest and most rapid temperature change in the oceans are the transition zone between the surface waters and the deep waters, the thermocline, and the transition between the deep-sea floor and the hot water flows at the hydrothermal vents. Thermoclines vary in thickness from a few hundred meters to nearly a thousand meters. Below the thermocline, the water mass of the deep ocean is cold and far more homogeneous. Thermoclines are strongest in the tropics, where the temperature of the epipelagic zone is usually above 20°C. From the base of the epipelagic, the temperature drops over several hundred meters to 5 or 6°C at 1,000 meters. It continues to decrease to the bottom, but the rate is much slower. Below 3,000 to 4,000 m, the water is isothermal.

At any given depth, the temperature is practically unvarying over long periods of time. There are no seasonal temperature changes, nor are there any annual changes. No other habitat on earth has such a constant temperature.

Hydrothermal vents are the direct contrast with constant temperature. In these systems, the temperature of the water as it emerges from the "black smoker" chimneys may be as high as 400°C (it is kept from boiling by the high hydrostatic pressure) while within a few meters it may be back down to 2 - 4°C.


Regions below the epipelagic
Pelagic zone
Any water in a sea or lake that is not close to the bottom or near to the shore can be said to be in the pelagic zone. The word pelagic comes from the Greek πέλαγος or pélagos, which means "open sea". The pelagic zone can be thought of in terms of an imaginary cylinder or water column that goes...

 are divided into further zones, beginning with the mesopelagic
The mesopelagic is that part of the pelagic zone that extends from a depth of 200 to 1000 metres below the ocean surface. It lies between the photic epipelagic above and the aphotic bathypelagic below, where there is no light at all...

which spans from 200 to 1000 below sea level, where a little light penetrates while still being insufficient for primary production
Primary production
400px|thumb|Global oceanic and terrestrial photoautotroph abundance, from September [[1997]] to August 2000. As an estimate of autotroph biomass, it is only a rough indicator of primary production potential, and not an actual estimate of it...

. Below this zone the deep sea proper begins, consisting of the aphotic
Aphotic zone
The aphotic zone is the portion of a lake or ocean where there is little or no sunlight. It is formally defined as the depths beyond which less than 1% of sunlight penetrates. Consequently, bioluminescence is essentially the only light found in this zone...

Bathyal zone
The bathyal zone or bathypelagic – from Greek βαθύς , deep – is that part of the pelagic zone that extends from a depth of 1000 to 4000 metres below the ocean surface. It lies between the mesopelagic above, and the abyssopelagic below. The average temperature hovers at about 39°F...

, abyssopelagic
Abyssal zone
The abyssal zone is the abyssopelagic layer or pelagic zone that contains the very deep benthic communities near the bottom of oceans. "Abyss" derives from the Greek word ἄβυσσος, meaning bottomless. At depths of 4,000 to 6,000 metres , this zone remains in perpetual darkness and never receives...

and hadopelagic
Hadal zone
The hadal zone , also known as the hadopelagic zone and trench zone, is the delineation for the deepest trenches in the ocean...

. Food consists of falling organic matter known as 'marine snow
Marine snow
In the deep ocean, marine snow is a continuous shower of mostly organic detritus falling from the upper layers of the water column. It is a significant means of exporting energy from the light-rich photic zone to the aphotic zone below. The term was first coined by the explorer William Beebe as he...

' and carcasses derived from the productive zone above, and is scarce both in terms of spatial and temporal distribution.

Instead of relying on gas for their buoyancy, many species have jelly-like flesh consisting mostly of glycosaminoglycan
Glycosaminoglycans or mucopolysaccharides are long unbranched polysaccharides consisting of a repeating disaccharide unit. The repeating unit consists of a hexose or a hexuronic acid, linked to a hexosamine .-Production:Protein cores made in the rough endoplasmic reticulum are posttranslationally...

s, which has very low density. It is also common among deep water squid
Squid are cephalopods of the order Teuthida, which comprises around 300 species. Like all other cephalopods, squid have a distinct head, bilateral symmetry, a mantle, and arms. Squid, like cuttlefish, have eight arms arranged in pairs and two, usually longer, tentacles...

 to combine the gelatinous tissue with a flotation chamber filled with a coelomic fluid made up of the metabolic waste product ammonium chloride
Ammonium chloride
Ammonium chloride NH4Cl is an inorganic compound with the formula NH4Cl. It is a white crystalline salt that is highly soluble in water. Solutions of ammonium chloride are mildly acidic. Sal ammoniac is a name of natural, mineralogical form of ammonium chloride...

, which is lighter than the surrounding water.

The midwater fish have special adaptations to cope with these conditions—they are small, usually being under 25 centimetres (10 in); they have slow metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...

s and unspecialized diets, preferring to sit and wait for food rather than waste energy searching for it. They have elongated bodies with weak, watery muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...

s and skeletal
The skeleton is the body part that forms the supporting structure of an organism. There are two different skeletal types: the exoskeleton, which is the stable outer shell of an organism, and the endoskeleton, which forms the support structure inside the body.In a figurative sense, skeleton can...

 structures. They often have extendable, hinged jaws with recurved teeth. Because of the sparse distribution and lack of light, finding a partner with which to breed is difficult, and many organisms are hermaphroditic
In biology, a hermaphrodite is an organism that has reproductive organs normally associated with both male and female sexes.Many taxonomic groups of animals do not have separate sexes. In these groups, hermaphroditism is a normal condition, enabling a form of sexual reproduction in which both...


Because light is so scarce, fish often have larger than normal, tubular eyes with only rod cell
Rod cell
Rod cells, or rods, are photoreceptor cells in the retina of the eye that can function in less intense light than can the other type of visual photoreceptor, cone cells. Named for their cylindrical shape, rods are concentrated at the outer edges of the retina and are used in peripheral vision. On...

s. Their upward field of vision allows them to seek out the silhouette of possible prey. Prey fish however also have adaptations to cope with predation
In ecology, predation describes a biological interaction where a predator feeds on its prey . Predators may or may not kill their prey prior to feeding on them, but the act of predation always results in the death of its prey and the eventual absorption of the prey's tissue through consumption...

. These adaptations are mainly concerned with reduction of silhouette, a form of camouflage
Camouflage is a method of concealment that allows an otherwise visible animal, military vehicle, or other object to remain unnoticed, by blending with its environment. Examples include a leopard's spotted coat, the battledress of a modern soldier and a leaf-mimic butterfly...

. The two main methods by which this is achieved are reduction in the area of their shadow by lateral compression of the body, and counter illumination via bioluminescence
Bioluminescence is the production and emission of light by a living organism. Its name is a hybrid word, originating from the Greek bios for "living" and the Latin lumen "light". Bioluminescence is a naturally occurring form of chemiluminescence where energy is released by a chemical reaction in...

. This is achieved by production of light from ventral photophore
A photophore is a light-emitting organ which appears as luminous spots on various marine animals, including fish and cephalopods. The organ can be simple, or as complex as the human eye; equipped with lenses, shutters, color filters and reflectors...

s, which tend to produce such light intensity to render the underside of the fish of similar appearance to the background light. For more sensitive vision in low light
Night vision
Night vision is the ability to see in low light conditions. Whether by biological or technological means, night vision is made possible by a combination of two approaches: sufficient spectral range, and sufficient intensity range...

, some fish have a retroreflector
A retroreflector is a device or surface that reflects light back to its source with a minimum scattering of light. An electromagnetic wave front is reflected back along a vector that is parallel to but opposite in direction from the wave's source. The device or surface's angle of incidence is...

 behind the retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...

. Flashlight fish
Flashlight fish
The flashlight fish are a family, the Anomalopidae, of beryciform fish. There are some unrelated fish with similar features, some of which are also called flashlight fish. Notable among these are the deep sea lanternfish, of the family Myctophidae, of which there are over 200 species.Flashlight...

 have this plus photophore
A photophore is a light-emitting organ which appears as luminous spots on various marine animals, including fish and cephalopods. The organ can be simple, or as complex as the human eye; equipped with lenses, shutters, color filters and reflectors...

s, which combination they use to detect eyeshine
Tapetum lucidum
The tapetum lucidum is a layer of tissue in the eye of many vertebrate animals....

 in other fish (see Tapetum lucidum
Tapetum lucidum
The tapetum lucidum is a layer of tissue in the eye of many vertebrate animals....


It is important to realize that organisms in the deep sea are almost entirely reliant upon sinking living and dead organic matter which falls at approximately 100 meters per day. In addition to this, only about 1-3% of the production from the surface reaches the sea bed mostly in the form of marine snow - as mentioned above. Larger food falls, such as whale carcasses
Whale fall
Whale fall is the term used for a whale carcass that has fallen to the ocean floor. Whale falls were first observed in the 1980s, with the advent of deep-sea robotic exploration....

, also occur and studies have shown that these may happen more often than currently believed. There are many scavengers that feed primarily or entirely upon large food falls and the distance between whale carcasses is estimated to only be 8 kilometers. In addition, there are a number of filter feeders that feed upon organic particles using tentacles, such as Freyella elegans.

Marine bacteriophage
Marine bacteriophage
Marine bacteriophages or marine phages are viruses that live as obligate parasitic agents in marine bacteria such as cyanobacteria. Their existence was discovered through electron microscopy and epifluorescence microscopy of ecological water samples, and later through metagenomic sampling of...

s play an important role in cycling nutrients in deep sea sediments. They are extremely abundant (between 5x1012 and 1x1013 phages per square meter) in sediments around the world.


There are a number of species that do not primarily rely upon dissolved organic matter for their food and these are found at hydrothermal vents. One example is the symbiotic relationship between the tube worm Riftia and chemosynthetic bacteria. It is this chemosynthesis
In biochemistry, chemosynthesis is the biological conversion of one or more carbon molecules and nutrients into organic matter using the oxidation of inorganic molecules or methane as a source of energy, rather than sunlight, as in photosynthesis...

 that supports the complex communities that can be found around hydrothermal vents. These complex communities are one of the only ecosystems on the planet that do not rely upon sunlight
Sunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is obvious as daylight when the Sun is above the horizon.When the direct solar radiation is not blocked...

 for the supply of energy.


The deep sea is an environment completely unfriendly to humankind, and it should come as no surprise that it represents one of the least explored areas on Earth. Pressures even in the mesopelagic become too great for traditional exploration methods, demanding alternative approaches for deep sea research. Baited camera stations, small manned submersibles and ROVs (remotely operated vehicle
Remotely operated vehicle
A remotely operated vehicle is a tethered underwater vehicle. They are common in deepwater industries such as offshore hydrocarbon extraction. An ROV may sometimes be called a remotely operated underwater vehicle to distinguish it from remote control vehicles operating on land or in the air. ROVs...

s) are three methods utilized to explore the ocean's depths. Because of the difficulty and cost of exploring this zone, current knowledge is limited. Pressure increases at approximately one atmosphere
Atmosphere (unit)
The standard atmosphere is an international reference pressure defined as 101325 Pa and formerly used as unit of pressure. For practical purposes it has been replaced by the bar which is 105 Pa...

 for every 10 meters meaning that some areas of the deep sea can reach pressures of above 1,000 atmospheres. This not only makes great depths very difficult to reach without mechanical aids, but also provides a significant difficulty when attempting to study any organisms that may live in these areas as their cell chemistry will be adapted to such vast pressures.

See also

  • Deep sea fish
    Deep sea fish
    Deep sea fish is a term for any fish that lives below the photic zone of the ocean. The lanternfish is, by far, the most common deep sea fish. Other deep sea fish include the flashlight fish, cookiecutter shark, bristlemouths, anglerfish, and viperfish....

  • Deep ocean water
    Deep ocean water
    Deep Ocean Water is the name for cold, salty water found deep below the surface of Earth's oceans. Ocean water differs in temperature and salinity, with warm, relatively non-salty water found at the surface, and very cold salty water found deeper below the surface layer. Deep ocean water makes...

  • Submarine landslide
    Submarine landslide
    Submarine landslides are marine landslides that transport sediment across the continental shelf and into the deep ocean. A submarine landslide is initiated when the downwards driving stress exceeds the resisting stress of the seafloor slope material causing movements along one or more concave to...

  • The Blue Planet
    The Blue Planet
    The Blue Planet is a BBC nature documentary series narrated by David Attenborough, first transmitted in the UK from 12 September 2001.Described as "the first ever comprehensive series on the natural history of the world's oceans", each of the eight 50-minute episodes examines a different aspect of...

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