Pelagic zone

The pelagic zone consists of the water column of the open ocean, and can be further divided into regions by depth. The word "pelagic" is derived from Greek πέλαγος (pélagos), meaning 

'open sea'. The pelagic zone can be thought of in terms of an imaginary cylinder or water column that goes from the surface of the sea almost to the bottom. Conditions differ deeper in the water column such that as pressure increases with depth, the temperature drops and less light penetrates. Depending on the depth, the water column, rather like the Earth's atmosphere, may be divided into different layers. 

The pelagic zone occupies 1,330 million km³ (320 million mi³) with a mean depth of^[각주:1] 3.68 km (2.29 mi) and maximum depth of 11 km (6.8 mi). Fish that live in the pelagic zone are called pelagic fish. Pelagic life decreases with increasing depth. It is affected by light intensity, pressure, temperature, salinity, the supply of dissolved oxygen and nutrients, and the submarine topography, which is called bathymetry^[각주:2]. In deep water, the pelagic zone is sometimes called the open-ocean zone and can be contrasted with^[각주:3] water that is near the coast or on the continental shelf. In other contexts, coastal water not near the bottom is still said to be in the pelagic zone. 

The pelagic zone can be contrasted with the benthic and demersal zones at the bottom of the sea. The benthic zone is the ecological region at the very bottom of the sea. It includes the sediment surface and some subsurface layers. Marine organisms living in this zone, such as clam and crabs, are called benthos. The demersal zone is just above the benthic zone. It can be significantly affected by the seabed and the life that lives there. Fish that live in the demersal zone are called demersal fish, and can be divided into benthic fish, which are denser than water so they can rest on the bottom, and benthopelagic fish, which swim in the water column just above the bottom. Demeral fish are also known as bottom feeders and groudfish. 

Depth and layers

Depending on how deep the sea is, the pelagic zone can extend over up to five horizontal regions in the ocean. From the top down, these are:

Epipelagic (sunlight)

From the surface (MSL) down to around 200 m (600ft)

This is the illuminated zone at the surface of the sea where enough light is available for photosynthesis. Nearly all primary production in the ocean occurs here. Consequently, plants and animals are largely concentrated in this zone. Examples of organisms living in this zone are plankton, floating seaweed, jellyfish, tuna, many sharks and dolphins. 

Mesopelagic (twilight) 

From 200m (660 ft) down to around 1,000 m (3,300 ft)

The most abundant organisms thriving into the mesopelagic zone are heterotrophic bacteria. Examples of animals that live here are swordfish, squid, Anarhichadidae or "wolffish" and some species of cuttlefish. Many organisms that live in this zone are bioluminescent^[각주:4]. Some creatures living in the mesopelagic zone rise to the epipelagic zone at night to feed. 

Bathypelagic (midnight) 

From 1,000 m (3,300 ft) down to around 4,000 m (13,000 ft)

The name stems from Greek βαθύς, meaning 'deep'. at this depth, the ocean is pitch black^[각주:5], apart from occasional bioluminescent organisms, such as anglerfish. No living plant exists here. Most animals living here survive by consuming the detritus^[각주:6] falling from the zones above, which is known as "marine snow", or, like the marine hatchetfish, by preying on other inhabits of this zone. Other examples of this zone's inhabitants are giant squid, smaller squids and the grimpoteuthis or "dumbo octopus". The giant squid is hunted here by deep-diving sperm whales. 

Abyssopelagic (lower midnight) 

From around 4,000 m (13,000 ft) down to above the ocean floor

The name is derived from Greek ἄβυσσος, meaning 'bottomless' (a holdover from the time when the deep ocean, or abyss, was believed to be bottomless). Very few creatures live in the cold temperatures, high pressures and complete darkness of this depth. Among the species found in this zone are several species of squid; echinoderms including the basket star, swimming cucumber, and the sea pig; and marine arthropods including the sea spider. Many of the species living at these depths are transparent and eyeless because of the total lack of light in this zone. 

Hadopelagic 

The deep water in ocean trenches 

The name is derived from the realm of Hades, the Greek underworld. However, many organisms live in hydrothermal vents in this and other zones. Some define the hadopelagic as waters below 6,000 m (20,000 ft), whether in a trench or not. 

1. mean depth ; [명사] (수문학) 평균 수심(平均水深) [본문으로]
2. bathymetry ; [명사] 수심 측량술, 측심학(測深學) ;; 미국·영국 [bəθímətri] [본문으로]
3. contrast ; 1. [타동사][VN] ~ (A and/with B) 대조하다, 대비시키다 [본문으로]
4. bioluminescent ; [형용사 ;; [형용사] 생물 발광(發光)의, 생물 발광(發光)하는. ;; bioluminescence ; (개똥벌레·버섯·심해어<深海魚> 따위의) 생물 발광(發光). ;; 미국식 [|baɪoʊlu:mɪ|nesns] 영국식 [|baɪəʊlu:mɪ|nesns] [본문으로]
5. pitch black ; [형용사] 칠흑같이 새까만[어두운] [본문으로]
6. detritus ; [U] 1. (전문 용어) (생물체 등에 의한 자연발생적인) 쓰레기 ;; 2. (격식) (모든 종류의) 쓰레기, 폐기물 ;; 미국∙영국 [dɪ|traɪtəs] [본문으로]