Oceanography

The pelagic zone (also known as the open-ocean zone) is further divided into parts, creating 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 m) - the illuminated surface zone where there is enough light for photosynthesis. Due to this, plants and animals are largely concentrated in this zone. Here one will typically encounter fish such as tuna and many sharks.
* Mesopelagic (from 200 m down to around 1000 m) - the twilight zone. Although some light penetrates this deep, it is insufficient for photosynthesis. The name stems from Greek ?????, middle.
* Bathypelagic (from 1000 m down to around 4000 m) - 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 ????? (bathys), deep.
* Abyssopelagic (from 4000 m 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 ??????? (ábyssos), 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. This zone is 90% unknown and very few species are known to live here (in the open areas). However, many organisms live in hydrothermal vents in this and other zones.