The oceanic zone refers to the region of the marine environment where water depths are greater than or equal to 200 meters. The region can be further divided into epipelagic, mesopelagic and bathypelagic zones, according to the amount of sunlight penetrating each. A wide array of marine organisms can be found throughout, with some living in the more extreme deep-water environments where pressures are high and there is very little light.
The Epipelagic Zone
Also called the euphotic zone, the epipelagic zone is the upper 30 meter layer of the ocean. This zone contains the majority of marine life -- about 90 percent of all organisms. This is the only zone to support plant life, which requires sunlight for photosynthesis. Sunlight is responsible for the relative elevated temperatures in this zone, another factor conducive to the biodiversity found here.
The Mesopelagic Zone
The mesopelagic zone, also called the twilight zone, is the layer of the ocean directly below the epipelagic zone. This zone extends to a depth of about 1,100 meters. There is no plant life here, due to the absence of appreciable light. Life here must cope with more extreme conditions, including decreased temperature and increased pressure with depth. Animals here include many cephalopods, such as squid and octopus. Because of the lack of plant life, many animals in this zone are predators, and must rely on hunting and scavenging for nutrition.
The Bathypelagic Zone
The bathypelagic zone, also referred to as the midnight zone, lies below the mesopelagic zone. The lower limit of this zone ranges from about 2,000 to 4,000 meters, where temperatures are near freezing, and no sunlight penetrates. Because of the absence of light, animals here often display bioluminescence. The pressure here is extreme, often exceeding 5,800 lbs. per square inch.
The Abyssopelagic Zone
The abyssopelagic zone refers to the region below the bathypelagic zone, exceeding a depth of 4,000 meters. The extreme temperature and crushing pressure greatly limit biodiversity here, and only a few different invertebrates inhabit the region. Tiny squid, tubeworms and seastars are among the organisms found here. These are mainly scavengers, feeding on organic material that sinks down from the oceanic zones above.