This article describes the habitat of the deep sea. It is one of the sub-categories within the section dealing with biodiversity of marine habitats and ecosystems. It gives an overview about the characteristics, the bottom topography, the adaptations to the environment of the biota and the threats.
The deep sea floor represents the largest habitat on earth. It ranges from the edge of the continental shelf at 200m to the bottom of the ocean. At the edge of the continental shelf is the shelf break, where the gradient of the floor increases down the continental slope. Below the continental slope lies the continental rise, which has a more gently slope. At around 4,000 meters depth, the ocean floor is reached and extends over the ocean basins at depths of 5,000 metres on average. This is called the abyssal plain. The zone between the continental shelf and the abyssal plain is the bathyal zone. In some places, the sea floor drops again into elongated trenches with depths of 10 to 11 kilometres. This region is the hadal region. The ocean floor is interrupted by mountain chain known as the mid-oceanic ridge system. Other features on the ocean floor are seamounts and hydrothermal vents. 
The temperature of the waters of the deep sea varies from 4°C to -1°C. Exceptions are the Mediterranean Sea with an average temperature of 13°C in the upper 4,000 metres and the Red Sea where the bottom water can reach temperatures up to 21.5°C at depths of 2,000 metres. The lowest temperatures are found in the Antarctic Ocean and are about -1.9°C. The salinity of the seawater is relatively constant. Below 2,000 metres, the salinity is close to 35 ‰ (34.8‰ ± 0.3 ‰) and at the very deepest layers it is 34.65‰. The oxygen concentrations are near saturation except in the oxygen minimum layer between 0.5 and 0.6 kilometres depth. In enclosed basins such as the Black Sea, the water is anoxic below 250 metres. When the deep water is further from its origin the oxygen concentration declines by metabolic processes. This is e.g. the case in the deep North Pacific. The most predictable physical variable is hydrostatic pressure. Per 10 metre the depth increases, the pressure increases by 1 atmosphere ([math]10^5[/math] Pa).  In the deep ocean, light is almost absent. It is a dark environment and the organisms are adapted to it. The circulation in the deep sea is explained in the article ocean circulation. The deep sea bottom has some specific characteristics that can be found in the article about deep sea bottom.
For the animals of the deeper layers of the water column where there is no plant growth at all, finding food and avoiding predation are the main problems. Many mid-water animals have bizarre body shapes, because they never come into contact with a physical barrier. The most abundant for is more or less spherical, often with long and delicate extensions. Most animals do not need a rigid skeleton because they spend all their time suspended in water. An important adaptation is not to be seen too easily. In the upper parts of the mesopelagic zone, where there is still quite some sunlight, many animals have more or less transparent bodies. An alternative is to have silver sides that act as mirrors and light organs along their bellies. At the deeper layers, the organisms have a dark red, black or brown colour. These colours do not reflect the blue colour of most bioluminescence. In the mid-water, animals have mouths full of big teeth. In the deeper water, the anglerfish has a fishing rod with bioluminescent lures to attract preys. Many fishes also have large mouths to take up as much as possible. Another adaptation is dial vertical migration of many animals. The general pattern is that animals spend the daylight hours at depth and move upwards during the night. This is to avoid predation and to feed. Bioluminescence is another adaptation. Depending on the depth, organisms use it as camouflage to protect them from predators or to confuse or frighten predators rather than to hide from them. Other animals use it to look for a potential mate. A few deep-sea animals use it together with another light organ that produces deep red light. Other animals cannot see this red light, but the red light will be reflected very efficiently by the red bodies of the specific prey. The blue light is a recognition mark for other members of the same species. An example of such an organism is Malacosteus. 
The mid-water communities include representatives of almost every major animal group on Earth, from tiny single-celled organisms only a fraction of a millimetre, through all the invertebrate phyla, to fish several meters long. Most of them have an average size in the range of centimetres. The dominant species are crustaceans, fish and a variety of animals with soft and jelly-like bodies such as jellyfish.
On the deep-sea floor, many mounds and depressions are formed by benthic animals such as worms, mollusks, crustaceans, starfish, brittlestars, shrimps, fishes sea cucumbers and sea urchins. Because the deep-sea floor is poor in food resources, it cannot support large populations of big animals. This causes bigger animals to be distributed over a greater range than elsewhere. For this reason, many animals adopt a ‘sit and wait’ strategy. This strategy means that the animals stay at one place and wait until food falls down on them or are carries to them in the currents. These animals are sponges, sea anemones, tube worms and barnacles. Smaller animals can also be found.
There are five groups that are dominated in terms of numbers. These are: foraminiferans, Radiolaria, nematodes, crustaceans (amphipod shrimps, isopods, copepods, bivalves) and polychaete worms.
Hydrothermal communities may be hundreds of times more abundant than on the adjacent sea floor. This is because the animals have their own rich source of food. This food source is totally independent of the input from the photosynthesizing plants in the overlying surface waters. Organisms that can be found in association with these vents are bacteria, mussels, cockles, oysters, sea anemones, shrimps, crabs, fish and tube worms. An example of a tube worm is Riftia pachyptila. It has a red plume of gills sticking out of the top of the worm’s white tube. They are also found around cold hydrocarbon seeps. It has no mouth or gut and it has special chemosynthetic bacteria in the tissues of its body called the trophosome. The chemicals that the bacteria need are delivered by the blood of the worm. 
Cold seeps have a lower metazoan biodiversity but a higher productivity. This is because of the high biomass of megabenthic organisms. The communities are often dominated by the bivalves that have symbiontic relationships with chemosynthetic bacteria. Organisms that can be found here are tube worms, bivalves, crabs, anemones and soft corals. An example of an organism is the vesicomyid clam that has a symbiontic relationship with sulfur oxidizing bacteria.
Seamounts have high concentrations of plankton. Also fishes are abundant, because of the high concentration of plankton and the constant influx of prey organisms. Marine mammals such as sharks, tuna and cephalopods all congregate over the seamounts to fee. Even seabirds have been shown to be more abundant in the zones with shallow seamounts.
Carcasses are hot-spots for food. When the carcass reaches the bottom, it attracts within minutes amphipods. They have powerful jaws, which can slice flesh. They can detect the odor of a carcass carried on the water currents. After them several fishes arrive and feed on the carcass. Also shrimps can occur on it. When these animals have eaten enough or when there is nothing suitable left for them, the remainders will attract slow-moving animals such as mollusks, starfish, brittlestars and sea cucumbers. Finally, bacteria will finish the job. They can even break down the bones. Within weeks the carcass is cleared away.
Giant animals and strange creatures are common in the deep sea. These are sometimes called monsters of the sea. There are a lot of examples. Many myths of monsters must be based on sightings of squid. Sharks are also seen as monster and aggressive animals. Other animals can be seen as monsters because they are very large in comparison to the species that we know. An example is the giant isopod. This phenomenon is called gigantism.
Human activities can cause serious damage to the deep sea. Examples of human activities that have a negative impact are :
- Fishing activities: overfishing, destructive practices and illegal, unreported and unregulated fishing activities (IUU)
- Bycath or incidental mortality of non-target species
- Noise pollution such as powerful sonar systems and airguns
- Shipping: noise, accidental spills of oil, discharge of garbage, oily wastes, sewage, chemical residues and ballast water, anti-fouling products
- Submarine cables and pipelines
- Climate change and ozone depletion
- Kaiser M. et al. 2005. Marine ecology: Processes, systems and impacts. Oxford University Press. p.584
- Gage J.D. and Tyler P.A. 1991. Deep-sea biology – A natural history of organisms at the deep-sea floor. Cambridge University Press. p. 504
- Rice T. 2000. Deep Ocean. The natural history museum, London. p. 96
- Photo credit NOAA
- Photo credit Wim Van Egmont
- Photo credit Chris Perridas
- UNEP Regional Seas Report and Studies No.178. 2006. Ecosystems and Biodiversity in Deep Waters and High Seas. p. 60
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