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The pelagic zone is the part of the open sea or ocean that is not near the coast or sea floor. The name comes from the Greek πέλαγος (pélagos), which might be roughly translated as "sea" but is more accurately translated as "open sea."
The pelagic (or open-ocean) zone is divided into a number of sub-zones:
- Epipelagic (surface to 200m): the illuminated surface zone where there is enough light for photosynthesis. Because of this, plankton, and the food chain which feeds on it, are concentrated in this zone. The top predators are fish such as tuna and many sharks.
- Mesopelagic (from 200m to 1000m): the twilight zone. Although some light penetrates this deep, it is insufficient for photosynthesis. The name comes from Greek μέσον, middle.
- Bathyal zone (from 1000m to 4000m): 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. From Greek βαθύς (bathys), deep.
- Abyssal zone (from 4000m to the ocean floor): no light penetrates to this depth, and some creatures are blind and colourless. Bioluminescence also occurs in this zone.
- Hadal zone (the deep water in ocean trenches) – the name comes from Hades, the classical Greek underworld. This zone is less well known. Very few species live in the open areas, but many organisms live in hydrothermal vents.
The last three zones are similar in character, and some marine biologists count them as a single zone or consider the latter two to be the one zone. Some define the hadopelagic as waters below 6000 meters, whether in a trench or not.