The open ocean is called the pelagic zone and is divided into five sub-zones: epipelagic, mesopelagic, bathypelagic, abyssopelagic, and hadopelagic. Each zone has different characteristics and inhabitants adapted to the conditions.
The open ocean, significantly distant from continental shelves, is collectively known as the pelagic zone of the ocean. The name derives from the Greek pélagos which means “open ocean”. The pelagic zone is divided into five sub-zones of increasing depth: the epipelagic zone, the mesopelagic zone, the bathypelagic zone, the abyssoplegic zone and, reserved only for deep trenches, the edeoplegic zone.
The epipelagic zone extends from the surface down to 200 meters (656 ft) and is the brightest of the zones (and one of only two with a lot of light). This is where most of the sea’s familiar creatures are found, such as much of the fish we eat and the coral reefs that take our ships by surprise. The area is rich in photosynthesizing microorganisms, which are consumed by larger organisms all the way up the food chain to creatures such as sharks. The name “epipelagic” roughly means “upper area of the ocean”.
The mesopelagic zone (from 200 m up to about 1,000 m or 3,280 ft) is the twilight zone. The name roughly means “middle of the ocean”. The biodensity here is lower than that of the upper epipelagic zone because light penetration decreases rapidly in the lower regions of this zone. Collectively, the epipelagic and upper mesopelagic zones are known as the photic zone, which means that light reaches them. Here live creatures of the semi-deep sea such as swordfish and wolf eel.
The bathypelagic zone extends from 1,000 m underwater to approximately 4,000 m (13,123 ft) underwater. Very little light reaches this depth, and as such no living plants can be found here. The deep-sea animals living here are adapted to consume organic debris snow that continuously 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 “infernal ocean” respectively. The abyssopelagic receives no light and is the region located deeper than 4,000 m underwater. This is where bottom feeders live, many of which have scoop-shaped jaws for lifting debris from the ocean floor. The bottom of the oceans tends to be filled with a layer of organic manure a few inches deep, like a forest floor. However, very little is known about these regions, as only the hardiest deep diving robots can make it here.
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