Any water in the sea that is not close to the bottom is in the pelagic zone. The word pelagic comes from the Greek πέλαγος or pélagos, which means open sea.

It 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, like the diagram on the left. Conditions change as you go deeper down the water column; the pressure increases and there is less light. Depending on the depth, scientists further subdivide the water column, rather like the earth's atmosphere is divided into different layers.

Description

The pelagic zone occupies 1,370 million cubic kilometres (330 million cubic miles) and has a vertical range up to 11 kilometres (6.8 miles). Fish that live in the pelagic zone are called pelagic fish. Pelagic life decreases with increasing depth. It is affected by light levels, pressure, temperature, salinity, the supply of dissolved oxygen and nutrients, and the submarine topography.

In deep water the pelagic zone is sometimes called the open-ocean zone and can be contrasted with water that is near the coast or on the continental shelf. However in other contexts, coastal water that is 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 sub-surface layers. Marine organisms living in this zone, such as clams 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. They are also called bottom feeders or groundfish.

Depth and layers

Depending on how deep the sea is, there can be up to five vertical layers in the ocean. From the top down, they are:

Epipelagic (sunlit)

From the surface (MSL) down to around 200 m (656 ft).
The illuminated surface zone where there is enough light for photosynthesis. Due to this, plants and animals are largely concentrated in this zone. Nearly all primary production in the ocean occurs here. This layer is the domain of fish such as tuna, many sharks, dolphin fish, and jellyfish. This zone is also known as the surface zone.

Mesopelagic (twilight)

From 200 m down to around 1,000 m (3,280 ft).
Although some light penetrates this deep, it is insufficient for photosynthesis. The name stems from Greek μέσον, middle. At about 500 m the water becomes depleted of oxygen. Still, an abundance of life copes with more efficient gills or minimal movement. Animals such as swordfish, squids, wolffish, a few species of cuttlefish, and other semi-deep-sea creatures live here. Many bioluminescent organisms live in this zone. Due to the relative lack of nutritious food found in this zone, 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 (13,123 ft).
By this depth the ocean is pitch black, apart from the occasional thermoluminescent organism, such as lanternfish. There are no living plants, and most animals survive by consuming the snow of detritus falling from the zones above or (like the marine hatchetfish) by preying upon others. Giant squid (as well as smaller squids & Dumbo octopodes) live at this depth, and here they are hunted by deep-diving sperm whales. From Greek βαθύς (bathýs), deep.

Abyssopelagic (lower midnight)

From 4,000 m down to above the ocean floor.
The name is derived from the Greek (ábyssos), abyss, meaning bottomless (a holdover from the times when the deep ocean was believed to be bottomless). Very few creatures are sufficiently adapted to survive in the freezing temperatures and incredible pressures found at 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 crustaceans including the sea spider. Many of the species living at these depths have evolved 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 Greek Ἁΐδης (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. There are several ideas about the layers. Most scientists don't consider the Abyssal Layer an actual layer.

Pelagic fish

Pelagic fish are fish that live in the water column of coastal, ocean and lake waters, but not on the bottom of the sea or the lake. They can be contrasted with demersal fish, which live on or near the bottom, and reef fish which are associated with coral reefs.Lal BV and Fortune K (2000) Page 8. University of Hawaii Press. ISBN 9780824822651.
These fish are often migratory forage fish, which feed on plankton, and the larger fish that follow and feed on the forage fish. Examples of migratory forage fish are herring, anchovies, capelin and menhaden. Examples of larger pelagic fish which predate the forage fish are billfish, tuna and oceanic sharks.

Pelagic birds

Pelagic birds, also called oceanic birds, are birds that live on the open sea, rather than around waters adjacent to land or around inland waters. Pelagic birds feed on planktonic crustaceans, squid and forage fish. Examples are the Atlantic puffin, macaroni penguins, sooty terns, shearwaters, and procellariiforms such as the albatross, procellariids and petrels.

The term seabird includes birds which live around the sea adjacent to land, as well as pelagic birds.