Epipelagic (sunlit)

From the surface down to around 200 m (650 ft).
This is the illuminated zone at the surface of the sea where there is enough light 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 200 meters down to around 1,000 meters (3,300 ft).
The name for this zone stems from the Ancient Greek: μέσον (méson), "middle".
Although some light penetrates this second layer, it is insufficient for photosynthesis. At about 500 m the water also becomes depleted of oxygen. Still, life copes, with gills that are more efficient or by minimizing movement.
Examples of animals that live here are: swordfish, squid, wolffish and some species of cuttlefish. Many organisms that live in this zone are bioluminescent. Some creatures living in the mesopelagic zone will rise to the epipelagic zone at night in order to feed.



Bathypelagic (midnight)

From 1,000 m down to around 4,000 m (13,000 ft).
The name stems from the Ancient Greek: βαθύς (bathýs), "deep".
At this depth the ocean is pitch black, apart from occasional bioluminescent organisms, such as lanternfish. There is no living plant life.
Most animals living here survive by consuming the detritus falling from the zones above, which is known as "marine snow", or, like the marine hatchetfish, by preying on other inhabitants of this zone.
Other examples of this zone's inhabitants are giant squid, smaller squids and the dumbo octopus. The giant squid is hunted here by deep-diving sperm whales.



Abyssopelagic (lower midnight)

From 4,000 m down to above the ocean floor.
The name is derived from the Ancient Greek: ἄβυσσος (ábyssos), "bottomless" (a holdover from the times when the deep ocean, or abyss, was believed to be bottomless). Very few creatures are sufficiently adapted to survive 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 have adapted to be transparent and eyeless as a result of the total lack of light in this zone.



Hadopelagic

The deep water in ocean trenches.
The name is derived from the Ancient Greek: Ἁδης (Haidēs), "Hades", the classical Greek underworld. This zone is mostly unknown, and very few species are known to live here (in the open areas). However, many organisms live in hydrothermal vents in this and other zones. Some define the hadopelagic as waters below 6,000 m (19,685 ft), whether in a trench or not.
The bathypelagic, abyssopelagic, and hadopelagic zones are very similar in character, and some marine biologists combine them into a single zone or consider the latter two to be the same. The abyssal plain is covered with soft sludge composed of dead organisms from above.