The word pelagic comes from the Greek word pelagos meaning sea. The pelagic zone encompasses the entirety of the ocean except for the coastal areas.
The pelagic zone is divided into four main layers or zones. They are the epipelagic, mesopelagic, bathypelagic, abyssopelagic, and a special zone that only exists in certain places around the world is named the hadopelagic zone. Organisms that live on the ocean floor (regardless of depth) are part of the benthos or the benthic zone. Benthic ecosystems include coral reefs, seagrass beds, and other systems in shallow coastal areas and deep hydrothermal vents, the abyssal plain, and other systems in the deep sea.
The epipelagic, also known as the sunlight zone, is the upper open ocean which receives the most sunlight. There is enough sunlight for plants and protists to utilize photosynthesis (the process by which organisms use sunlight to convert carbon dioxide into nutrients and oxygen). This zone generally reaches from the sea surface down to approximately 200 m (650 ft). The primary producers that exist in this zone are responsible for much of the original food production of the entire ocean and create at least 50% of the oxygen in the earth’s atmosphere.
The mesopelagic, also known as the twilight zone, is the middle open ocean which stretches from the bottom of the epipelagic down to the point where sunlight cannot reach. This zone is approximately 1000 m (3300 ft) and is much larger than the epipelagic zone. Many of the species of fishes and invertebrates that live here migrate up into shallower, epipelagic depths to feed, but only under the cover of night.
The bathypelagic, also known as the bathyal zone, is the lower open ocean which stretches from the bottom of the mesopelagic and the upper bound of this zone is defined by a complete lack of sunlight. This zone is approximately 4000 m (13000 ft) and is 15 times the size of the epipelagic zone. This zone is the largest ecosystem on earth and organisms in the bathypelagic live in complete darkness from the sun. However, some organisms use bioluminescent light to attract prey or find a mate.
Abyssopelagic and Hadopelagic
The abyssopelagic, also known as the abyssal zone, stretches from the bottom of the bathypelagic to the ocean floor. This zone stretches approximately 6000 m (19,700 ft) and makes up over 83% of the ocean and covers 60% of the earth.
A special zone that only exists in certain places around the world is named the hadopelagic zone or the hadal zone. This is the deepest region of the ocean and lies within the oceanic trenches that occur in the otherwise flat seafloor. By this definition, all of the deepest parts of the ocean conclude in the hadopelagic. The southern end of the Mariana Trench is named Challenger Deep and it is the deepest known ocean depth which stretches nearly 11,000 m (36,000 ft or almost 7 miles).