1. Depth and Layers

The pelagic zone is subdivided into five vertical regions. From the top down, these are:

1.1. Epipelagic (Sunlight)

The illuminated zone at the surface of the sea with sufficient light for photosynthesis. Nearly all primary production in the ocean occurs here, and marine life is concentrated in this zone, including plankton, floating seaweed, jellyfish, tuna, many sharks and dolphins.

1.2. Mesopelagic (Twilight)

The most abundant organisms thriving into the mesopelagic zone are heterotrophic bacteria.^[1] Animals living in this zone include swordfish, squid, wolffish and some species of cuttlefish. Many organisms living here are bioluminescent.^[2] Some mesopelagic creatures rise to the epipelagic zone at night to feed.^[2]

1.3. Bathypelagic (Midnight)

1.4. Abyssopelagic (Abyssal Zone)

The name is derived from grc ἄβυσσος 'bottomless' - a holdover from times when the deep ocean was believed to indeed be bottomless. Among the very few creatures living in the cold temperatures, high pressures and complete darkness here are several species of squid; echinoderms including the basket star, swimming cucumber, and the sea pig; and marine arthropods including the sea spider. Many species at these depths are transparent and eyeless.^[2]

1.5. Hadopelagic (Hadal Zone)

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.

2. Pelagic Ecosystem

 

Some representative ocean animals (not drawn to scale) within their approximate depth-defined ecological habitats. Marine microorganisms also exist on the surfaces and within the tissues and organs of the diverse life inhabiting the ocean, across all ocean habitats. The animals rooted to or living on the ocean floor are not pelagic but are benthic animals.^[3] https://handwiki.org/wiki/index.php?curid=1886218

The pelagic ecosystem is based on phytoplankton. Phytoplankton manufacture their own food using a process of photosynthesis. Because they need sunlight, they inhabit the upper, sunlit epipelagic zone, which includes the coastal or neritic zone. Biodiversity diminishes markedly in the deeper zones below the epipelagic zone as dissolved oxygen diminishes, water pressure increases, temperatures become colder, food sources become scarce, and light diminishes and finally disappears.^[4]

2.1. Pelagic Invertebrates

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.^[5][6]

2.2. Pelagic Fish

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 do live on or near the bottom, and coral reef fish.^[7]

Pelagic fish are often migratory forage fish, which feed on plankton, and the larger predatory 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.

2.3. Pelagic Reptiles

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 wandering albatross (Diomedea exulans) ranges over huge areas of ocean and can circle the globe. https://handwiki.org/wiki/index.php?curid=1685921

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

2.4. Pelagic Birds

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.