Pelagic zone

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.

The most abundant organisms thriving into the mesopelagic zone are heterotrophic bacteria.[5] 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.[6] Some creatures living in the mesopelagic zone rise to the epipelagic zone at night to feed.[6]

The name stems from Ancient Greek βαθύς 'deep'. At this depth, the ocean is pitch black, apart from occasional bioluminescent organisms, such as anglerfish. No living plant exists here. 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 grimpoteuthis or "dumbo octopus". The giant squid is hunted here by deep-diving sperm whales.

The name is derived from Ancient Greek ἄβυσσος 'bottomless' (a holdover from the times 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.[6] 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.[6] Many of the species living at these depths are transparent and eyeless because of the total lack of light in this zone.[6]

The name is derived from the realm of Hades, the Greek underworld. This is the deepest part of the ocean at more than 6,000 m (20,000 ft) or 6,500 m (21,300 ft), depending on authority. Such depths are generally located in trenches.

Some examples of pelagic invertebrates include krill, copepods, jellyfish, decapod larvae, hyperiid amphipods, rotifers and cladocerans.

Thorson's rule states that benthic marine invertebrates at low latitudes tend to produce large numbers of eggs developing to widely dispersing pelagic larvae, whereas at high latitudes such organisms tend to produce fewer and larger lecithotrophic (yolk-feeding) eggs and larger offspring.[9][10]

Pelagic fish live in the water column of coastal, ocean, and lake waters, but not on or near the bottom of the sea or the lake. They can be contrasted with demersal fish, which live on or near the bottom, and coral reef fish.[11]

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 prey on the forage fish are billfish, tuna, and oceanic sharks.

Pelamis platura, the pelagic sea snake, is the only one of the 65 species of marine snakes to spend its entire life in the pelagic zone. It bears live young at sea and is helpless on land. The species sometimes forms aggregations of thousands along slicks in surface waters. The pelagic sea snake is the world's most widely distributed snake species.

The pelagic albatross ranges over huge areas of ocean and can circle the globe.

Many species of sea turtles spend the first years of their lives in the pelagic zone, moving closer to shore as they reach maturity.

Pelagic birds, also called oceanic birds or seabirds, live on open seas and oceans rather than inland or around more restricted waters such as rivers and lakes. Pelagic birds feed on planktonic crustaceans, squid and forage fish. Examples are the Atlantic puffin, macaroni penguins, sooty terns, shearwaters, and Procellariiformes such as the albatross, Procellariidae and petrels.