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The Open Ocean
The pelagic zone occupies 1,370 million cubic kilometres (330 million cubic miles) and has a vertical range up to 11 kilometres (6.8 miles). Fish that live in the pelagic zone are called pelagic fish. Pelagic life decreases with increasing depth. It is affected by light levels, pressure, temperature, salinity, the supply of dissolved oxygen and nutrients, and the submarine topography.

In deep water the pelagic zone is sometimes called the open-ocean zone and can be contrasted with water that is near the coast or on the continental shelf. However in other contexts, coastal water that is not near the bottom is still said to be in the pelagic zone.

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The pelagic zone is further divided into a number of sub-zones, based on their different ecological characteristics (which is roughly a function of depth):
  • Epipelagic (from the surface down to around 200 metres) - the illuminated surface zone where there is enough light for photosynthesis, and thus plants and animals are largely concentrated in this zone. Here one will typically encounter fish such as tuna and many sharks.
  • Mesopelagic (from 200 metres down to around 1000 metres) - the twilight zone. Although some light penetrates this deep, it is insufficient for photosynthesis.
  • Bathypelagic (from 1000 metres down to around 4000 metres) - by this depth the ocean is almost entirely dark (with only the occasional bioluminescent organism). There are no living plants, and most animals survive by consuming the snow of detritus falling from the zones above, or (like the marine hatchetfish) by preying upon others. Giant squid live at this depth, and here they are hunted by deep-diving sperm whales.
  • Abyssopelagic (from 4000 metres down to above the ocean floor) - no light whatsoever penetrates to this depth, and most creatures are blind and colourless. The name is derived from the greek abyss meaning bottomless (a holdover from the times when the deep ocean was believed to be bottomless).
  • Hadopelagic (the deep water in ocean trenches) - the name is derived from hades, the classical greek underworld.

The epipelagic and (arguably) the mesopelagic zones between them comprise the open ocean's photic zone. The remaining (lower) zones comprise the open ocean's aphotic zone.

The bathypelagic, abyssopelagic, and hadopelagic zones are very similar in character, and some marine biologistsMarine biology is the study of animal and plant life within saltwater ecosystems. Given that in biology many phyla, families and genera have some species that live in the sea and others that live on land, marine biology deals with those species in which l elide them into a single zone or consider the latter two to be the same.

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