![]() David Wharton, author of Life at the Limits: Organisms in Extreme Environments, notes "Biochemical reactions are accompanied by changes in volume. In addition to differences in internal pressure, these organisms have developed a different balance between their metabolic reactions from those organisms that live in the epipelagic zone. ![]() Fluidity in cell membranes increases efficiency of biological functions, most importantly the production of proteins, so organisms have adapted to this circumstance by increasing the proportion of unsaturated fatty acids in the lipids of the cell membranes. Their high internal pressure, however, results in the reduced fluidity of their membranes because molecules are squeezed together. Deep-sea organisms have the same pressure within their bodies as is exerted on them from the outside, so they are not crushed by the extreme pressure. Hydrostatic pressure increases by 1 atm for every 10 m in depth. ![]() The phenomenon is explained by the likewise abundance of prey species which are also attracted to the structures. Some deep-sea pelagic groups, such as the lanternfish, ridgehead, marine hatchetfish, and lightfish families are sometimes termed pseudoceanic because, rather than having an even distribution in open water, they occur in significantly higher abundances around structural oases, notably seamounts and over continental slopes. Therefore, most organisms in the bathypelagic rely on the marine snow from regions higher in the vertical column. Since there is no light in the deep sea (aphotic), there is a lack of primary producers. In this way marine snow may be considered the foundation of deep-sea mesopelagic and benthic ecosystems: As sunlight cannot reach them, deep-sea organisms rely heavily on marine snow as an energy source. However, most organic components of marine snow are consumed by microbes, zooplankton and other filter-feeding animals within the first 1000 m of their journey, that is, within the epipelagic zone. ![]() The "snowflakes" grow over time and may reach several centimetres in diameter, travelling for weeks before reaching the ocean floor. Marine snow includes dead or dying plankton, protists ( diatoms), fecal matter, sand, soot and other inorganic dust. Its origin lies in activities within the productive photic zone. In deep water, marine snow is a continuous shower of mostly organic detritus falling from the upper layers of the water column. In the deep ocean, the waters extend far below the epipelagic zone, and support very different types of pelagic fishes adapted to living in these deeper zones. Scale diagram of the layers of the pelagic zone The deep sea is also an extremely hostile environment, with temperatures that rarely exceed 3 ☌ (37 ☏) and fall as low as −1.8 ☌ (29 ☏) (with the exception of hydrothermal vent ecosystems that can exceed 350 ☌, or 662 ☏), low oxygen levels, and pressures between atm (between 2 and 100 MPa). Because this typically extends only a few hundred meters below the water, the deep sea, about 90% of the ocean volume, is in darkness. The epipelagic zone (0–200 m) is the area where light penetrates the water and photosynthesis occurs. These zones make up about 75% of the inhabitable ocean space. The bathypelagic and abyssopelagic zones are aphotic, meaning that no light penetrates this area of the ocean. This area is also where nutrients are most abundant. The oxygen minimum layer exists somewhere between a depth of 700 m and 1000 m deep depending on the place in the ocean. The mesopelagic zone is the disphotic zone, meaning light there is minimal but still measurable. However, characteristics of deep-sea organisms, such as bioluminescence can be seen in the mesopelagic (200–1000 m deep) zone as well. Deep-sea organisms generally inhabit bathypelagic (1000–4000 m deep) and abyssopelagic (4000–6000 m deep) zones. This means that they live in the water column as opposed to the benthic organisms that live in or on the sea floor. Only about 2% of known marine species inhabit the pelagic environment. Other deep sea fishes include the flashlight fish, cookiecutter shark, bristlemouths, anglerfish, viperfish, and some species of eelpout. The lanternfish is, by far, the most common deep-sea fish. Deep-sea fish are fish that live in the darkness below the sunlit surface waters, that is below the epipelagic or photic zone of the sea.
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