Open ocean, significantly distant from the continental shelves, is collectively known as the pelagic zone of the ocean. The name is derived from the Greek pélagos which means "open ocean". The pelagic zone is divided into five sub-zones of increasing depth: the epipelagic zone, mesopelagic zone, bathypelagic zone, abyssoplegic zone, and — reserved for deep sea trenches only — the hadeoplegic zone.
The epipelagic zone extends from the surface down to 200 meters (656 ft), and is the brightest of the zones (and one of the only two with much light at all). This is where most of the familiar creatures of the sea hang out, like much of the fish we eat and the coral reefs which catch our ships by surprise. The area is rich with photosynthesizing microorganisms, which are consumed by larger organisms all the way up the food chain to creatures like sharks. The name "epipelagic" roughly means "top zone of the ocean."
The mesopelagic zone (from 200 m down to around 1,000 m or 3,280 feet) is the twilight zone. The name roughly means "middle of the ocean". The biodensity here is less than that of the epipelagic zone above because the light penetration rapidly drops off in the lower regions of this zone. Collectively, the epipelagic and top of the mesopelagic zones are known as the photic zone, meaning light gets to them. Semi-deep sea creatures such as the Swordfish and Wolf Eels live here.
The bathypelagic zone extends from 1,000 m underwater to around 4,000 m (13,123 feet) underwater. Very little light reaches this depth, and as such no living plants can be found here. The deep sea animals which live here are adapted to consuming the snow of organic detritus that continually falls from above. Giant and Colossal Squid can be found here, as well as sperm whales.
The deepest zones are the abyssopelagic and hadeopelagic, meaning "bottomless ocean" and "hellish ocean" respectively. The abyssopelagic receives no light whatsoever, and is the region located deeper than 4,000 m underwater. Bottom-feeders live here, many of which have scoop-shaped jaws to lift detritus from the ocean floor. The very bottom of the oceans tends to be filled with a layer of organic muck a few inches in depth, like a forest floor. Very little is known about these regions, however, as only the hardiest deep-diving robots can make it down here.
By Michael Anissimov