The pelagic zone consists of the

water column A water column is a conceptual column of water from the surface of a sea, river or lake to the bottom sediment.Munson, B.H., Axler, R., Hagley C., Host G., Merrick G., Richards C. (2004).Glossary. ''Water on the Web''. University of Minnesota-D ...

of the open ocean, and can be further divided into regions by depth (as illustrated on the right). The word ''pelagic'' is derived . The pelagic zone can be thought of as an imaginary cylinder or water column between the surface of the sea and the bottom. Conditions in the water column change with depth: pressure increases; temperature and light decrease; salinity, oxygen, micronutrients (such as iron, magnesium and calcium) all change.

Marine life Marine life, sea life, or ocean life is the plants, animals and other organisms that live in the salt water of seas or oceans, or the brackish water of coastal estuaries. At a fundamental level, marine life affects the nature of the planet. ...

is affected by

bathymetry Bathymetry (; ) is the study of underwater depth of ocean floors ('' seabed topography''), lake floors, or river floors. In other words, bathymetry is the underwater equivalent to hypsometry or topography. The first recorded evidence of water ...

(underwater topography) such as the seafloor, shoreline, or a submarine

seamount A seamount is a large geologic landform that rises from the ocean floor that does not reach to the water's surface (sea level), and thus is not an island, islet or cliff-rock. Seamounts are typically formed from extinct volcanoes that rise a ...

, as well as by proximity to the boundary between the ocean and the atmosphere at the ocean surface, which brings light for photosynthesis, predation from above, and wind stirring up waves and setting currents in motion. The pelagic zone refers to the open, free waters away from the shore, where marine life can swim freely in any direction unhindered by topographical constraints. The

oceanic zone The oceanic zone is typically defined as the area of the ocean lying beyond the continental shelf (such as the Neritic zone), but operationally is often referred to as beginning where the water depths drop to below 200 meters (660 feet), seaward ...

is the deep open ocean beyond the

continental shelf A continental shelf is a portion of a continent that is submerged under an area of relatively shallow water, known as a shelf sea. Much of these shelves were exposed by drops in sea level during glacial periods. The shelf surrounding an island ...

, which contrasts with the inshore waters near the

coast The coast, also known as the coastline or seashore, is defined as the area where land meets the ocean, or as a line that forms the boundary between the land and the coastline. The Earth has around of coastline. Coasts are important zones in n ...

, such as in

estuaries An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environme ...

or on the continental shelf. Waters in the oceanic zone plunge to the depths of the abyssopelagic and further to the hadopelagic. Coastal waters are generally the relatively shallow epipelagic. Altogether, the pelagic zone occupies 1,330 million km³ (320 million mi³) with a mean depth of and maximum depth of . Pelagic life decreases as depth increases. The pelagic zone contrasts with the

benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning " ...

and

demersal The demersal zone is the part of the sea or ocean (or deep lake) consisting of the part of the water column near to (and significantly affected by) the seabed and the benthos. The demersal zone is just above the benthic zone and forms a layer ...

zones at the bottom of the sea. The benthic zone is the ecological region at the very bottom, including the sediment surface and some subsurface layers. Marine organisms such as

clam Clam is a common name for several kinds of bivalve molluscs. The word is often applied only to those that are edible and live as infauna, spending most of their lives halfway buried in the sand of the seafloor or riverbeds. Clams have two shel ...

s and

crab Crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" (abdomen) ( el, βραχύς , translit=brachys = short, / = tail), usually hidden entirely under the thorax. They live in all th ...

s living in this zone are called

benthos Benthos (), also known as benthon, is the community of organisms that live on, in, or near the bottom of a sea, river, lake, or stream, also known as the benthic zone.Demersal fish Demersal fish, also known as groundfish, live and feed on or near the bottom of seas or lakes (the demersal zone).Walrond Carl . "Coastal fish - Fish of the open sea floor"Te Ara - the Encyclopedia of New Zealand. Updated 2 March 2009 They oc ...

can be divided into

benthic fish Demersal fish, also known as groundfish, live and feed on or near the bottom of seas or lakes (the demersal zone).Walrond Carl . "Coastal fish - Fish of the open sea floor"Te Ara - the Encyclopedia of New Zealand. Updated 2 March 2009 They o ...

, which are denser than water and rest on the bottom, and

benthopelagic fish Demersal fish, also known as groundfish, live and feed on or near the bottom of seas or lakes (the demersal zone).Walrond Carl . "Coastal fish - Fish of the open sea floor"Te Ara - the Encyclopedia of New Zealand. Updated 2 March 2009 They o ...

, which swim just above the bottom. Demersal fish are also known as

bottom feeder A bottom feeder is an aquatic animal that feeds on or near the bottom of a body of water. Biologists often use the terms ''benthos''—particularly for invertebrates such as shellfish, crabs, crayfish, sea anemones, starfish, snails, bristlewo ...

s and

groundfish Demersal fish, also known as groundfish, live and feed on or near the bottom of seas or lakes (the demersal zone).Walrond Carl . "Coastal fish - Fish of the open sea floor"Te Ara - the Encyclopedia of New Zealand. Updated 2 March 2009 They oc ...

Depth and layers

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

Epipelagic (sunlight)

The illuminated zone at the surface of the sea with sufficient light for photosynthesis. Nearly all

primary production In ecology, primary production is the synthesis of organic compounds from atmospheric or aqueous carbon dioxide. It principally occurs through the process of photosynthesis, which uses light as its source of energy, but it also occurs through ...

in the ocean occurs here, and marine life is concentrated in this zone, including

plankton Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a cruc ...

, floating seaweed,

jellyfish Jellyfish and sea jellies are the informal common names given to the medusa-phase of certain gelatinous members of the subphylum Medusozoa, a major part of the phylum Cnidaria. Jellyfish are mainly free-swimming marine animals with umbre ...

tuna A tuna is a saltwater fish that belongs to the tribe Thunnini, a subgrouping of the Scombridae (mackerel) family. The Thunnini comprise 15 species across five genera, the sizes of which vary greatly, ranging from the bullet tuna (max length: ...

, many

shark Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachi ...

s and

dolphin A dolphin is an aquatic mammal within the infraorder Cetacea. Dolphin species belong to the families Delphinidae (the oceanic dolphins), Platanistidae (the Indian river dolphins), Iniidae (the New World river dolphins), Pontoporiidae (the b ...

Mesopelagic (twilight)

The most abundant organisms thriving into the mesopelagic zone are heterotrophic bacteria. Animals living in this zone include

swordfish Swordfish (''Xiphias gladius''), also known as broadbills in some countries, are large, highly migratory predatory fish characterized by a long, flat, pointed bill. They are a popular sport fish of the billfish category, though elusive. Swordf ...

squid True squid are molluscs with an elongated soft body, large eyes, eight arms, and two tentacles in the superorder Decapodiformes, though many other molluscs within the broader Neocoleoidea are also called squid despite not strictly fittin ...

wolffish Anarhichadidae, the wolffishes, sea wolves or wolf eels, is a family of marine ray finned fishes belonging to the order Scorpaeniformes. These are predatory, eel shaped fishes which are native to the cold waters of the Arctic, North Pacific and ...

and some species of

cuttlefish Cuttlefish or cuttles are marine molluscs of the order Sepiida. They belong to the class Cephalopoda which also includes squid, octopuses, and nautiluses. Cuttlefish have a unique internal shell, the cuttlebone, which is used for control of ...

. Many organisms living here are

bioluminescent Bioluminescence is the production and emission of light by living organisms. It is a form of chemiluminescence. Bioluminescence occurs widely in marine vertebrates and invertebrates, as well as in some fungi, microorganisms including some b ...

.The Open Ocean - MarineBio.org
/ref> Some mesopelagic creatures rise to the epipelagic zone at night to feed.

Bathypelagic (midnight)

The name stems . The ocean is pitch black at this depth apart from occasional

organisms, such as

anglerfish The anglerfish are fish of the teleost order Lophiiformes (). They are bony fish named for their characteristic mode of predation, in which a modified luminescent fin ray (the esca or illicium) acts as a lure for other fish. The luminescence ...

. No plants live here. Most animals survive on

detritus In biology, detritus () is dead particulate organic material, as distinguished from dissolved organic material. Detritus typically includes the bodies or fragments of bodies of dead organisms, and fecal material. Detritus typically hosts comm ...

known as "

marine snow In the deep ocean, marine snow (also known as "ocean dandruff") is a continuous shower of mostly organic detritus falling from the upper layers of the water column. It is a significant means of exporting energy from the light-rich photic zone to ...

" falling from the zones above or, like the marine hatchetfish, by preying on other inhabitants of this zone. Other examples of this zone's inhabitants are

giant squid The giant squid (''Architeuthis dux'') is a species of deep-ocean dwelling squid in the family Architeuthidae. It can grow to a tremendous size, offering an example of abyssal gigantism: recent estimates put the maximum size at around Tra ...

, smaller

and the grimpoteuthis or "dumbo octopus". The giant squid is hunted here by deep-diving

sperm whale The sperm whale or cachalot (''Physeter macrocephalus'') is the largest of the toothed whales and the largest toothed predator. It is the only living member of the genus ''Physeter'' and one of three extant species in the sperm whale famil ...

Abyssopelagic (abyssal zone)

The name is derived - 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;

echinoderm An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the ...

s including the basket star, swimming cucumber, and the sea pig; and marine arthropods including the

sea spider Sea spiders are marine arthropods of the order Pantopoda ( ‘all feet’), belonging to the class Pycnogonida, hence they are also called pycnogonids (; named after ''Pycnogonum'', the type genus; with the suffix '). They are cosmopolitan, fou ...

. Many species at these depths are transparent and eyeless.

Hadopelagic (hadal zone)

The name is derived from the realm of

Hades Hades (; grc-gre, ᾍδης, Háidēs; ), in the ancient Greek religion and myth, is the god of the dead and the king of the underworld, with which his name became synonymous. Hades was the eldest son of Cronus and Rhea, although this also ...

, the

Greek underworld In mythology, the Greek underworld, or Hades, is a distinct realm (one of the three realms that makes up the cosmos) where an individual goes after death. The earliest idea of afterlife in Greek myth is that, at the moment of death, an individu ...

. This is the deepest part of the ocean at more than or , depending on authority. Such depths are generally located in

trenches A trench is a type of excavation or in the ground that is generally deeper than it is wide (as opposed to a wider gully, or ditch), and narrow compared with its length (as opposed to a simple hole or pit). In geology, trenches result from erosi ...

Pelagic ecosystem

The pelagic ecosystem is based on

phytoplankton Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater ecosystems. The name comes from the Greek words (), meaning 'plant', and (), meaning 'wanderer' or 'drifter'. ...

. Phytoplankton manufacture their own food using a process of

photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in ...

. Because they need sunlight, they inhabit the upper, sunlit epipelagic zone, which includes the coastal or

neritic zone The neritic zone (or sublittoral zone) is the relatively shallow part of the ocean above the drop-off of the continental shelf, approximately in depth. From the point of view of marine biology it forms a relatively stable and well-illuminated ...

. 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.Walker P and Wood E (2005
''The Open Ocean''
(volume in a series called ''Life in the sea''), Infobase Publishing, .

Pelagic invertebrates

Some examples of pelagic invertebrates include

krill Krill are small crustaceans of the order Euphausiacea, and are found in all the world's oceans. The name "krill" comes from the Norwegian word ', meaning "small fry of fish", which is also often attributed to species of fish. Krill are consi ...

copepod Copepods (; meaning "oar-feet") are a group of small crustaceans found in nearly every freshwater and saltwater habitat. Some species are planktonic (inhabiting sea waters), some are benthic (living on the ocean floor), a number of species have ...

, decapod

larvae A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle. T ...

hyperiid The Hyperiidea are a suborder of amphipods, small aquatic crustaceans. Unlike the other suborders of Amphipoda, hyperiids are exclusively marine and do not occur in fresh water. Hyperiids are distinguished by their large eyes and planktonic habi ...

amphipod Amphipoda is an order of malacostracan crustaceans with no carapace and generally with laterally compressed bodies. Amphipods range in size from and are mostly detritivores or scavengers. There are more than 9,900 amphipod species so far descri ...

rotifer The rotifers (, from the Latin , "wheel", and , "bearing"), commonly called wheel animals or wheel animalcules, make up a phylum (Rotifera ) of microscopic and near-microscopic pseudocoelomate animals. They were first described by Rev. John H ...

s and

cladocera The Diplostraca or Cladocera, commonly known as water fleas, are a superorder of small crustaceans that feed on microscopic chunks of organic matter (excluding some predatory forms). Over 1000 species have been recognised so far, with many more ...

ns.

Thorson's rule Thorson's rule (named after Gunnar Thorson by S. A. Mileikovsky in 1971) Mileikovsky, S. A. 1971. ''Types of larval development in marine bottom invertebrates, their distribution and ecological significance: a reevaluation.'' Marine Biology 19: 193 ...

states that

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 Oviparous animals are animals that lay their eggs, with little or no other embryonic development within the mother. This is the reproductive method of most fish, amphibians, most reptiles, and all pterosaurs, dinosaurs (including birds), and m ...

(yolk-feeding) eggs and larger offspring.

Pelagic fish

Pelagic fish live in the

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 Coral reef fish are fish which live amongst or in close relation to coral reefs. Coral reefs form complex ecosystems with tremendous biodiversity. Among the myriad inhabitants, the fish stand out as colourful and interesting to watch. Hundreds ...

. Pelagic fish are often migratory

forage fish Forage fish, also called prey fish or bait fish, are small pelagic fish which are preyed on by larger predators for food. Predators include other larger fish, seabirds and marine mammals. Typical ocean forage fish feed near the base of the f ...

, which feed on

, and the larger

predatory fish Predatory fish are hypercarnivorous fish that actively prey upon other fish or aquatic animals, with examples including shark, billfish, barracuda, pike/ muskellunge, walleye, perch and salmon. Some omnivorous fish, such as the red-bellie ...

that follow and feed on the forage fish. Examples of migratory forage fish are

herring Herring are forage fish, mostly belonging to the family of Clupeidae. Herring often move in large schools around fishing banks and near the coast, found particularly in shallow, temperate waters of the North Pacific and North Atlantic Ocean ...

anchovies An anchovy is a small, common forage fish of the family Engraulidae. Most species are found in marine waters, but several will enter brackish water, and some in South America are restricted to fresh water. More than 140 species are placed in 1 ...

capelin The capelin or caplin (''Mallotus villosus'') is a small forage fish of the smelt family found in the North Atlantic, North Pacific and Arctic oceans. In summer, it grazes on dense swarms of plankton at the edge of the ice shelf. Larger capelin ...

, and

menhaden Menhaden, also known as mossbunker and bunker and "the most important fish in the sea", are forage fish of the genera ''Brevoortia'' and ''Ethmidium'', two genera of marine fish in the family Clupeidae. ''Menhaden'' is a blend of ''poghaden' ...

. Examples of larger pelagic fish which

prey Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill ...

on the forage fish are

billfish The term billfish refers to a group of saltwater predatory fish characterised by prominent pointed bills (rostra), and by their large size; some are longer than . Extant billfish include sailfish and marlin, which make up the family Istioph ...

, and oceanic

Pelagic reptiles

Pelamis platura The yellow-bellied sea snake (''Hydrophis platurus'') is a venomous species of snake from the subfamily Hydrophiinae (the sea snakes) found in tropical oceanic waters around the world except for the Atlantic Ocean. For many years, it was plac ...

'', 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. Diomedea exulans in flight - SE Tasmania

Diomedea exulans in flight - SE Tasmania

Many species of

sea turtles Sea turtles (superfamily Chelonioidea), sometimes called marine turtles, are reptiles of the order Testudines and of the suborder Cryptodira. The seven existing species of sea turtles are the flatback, green, hawksbill, leatherback, loggerhe ...

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

Pelagic birds

Pelagic birds, also called ''oceanic birds'' or

seabird Seabirds (also known as marine birds) are birds that are adapted to life within the marine environment. While seabirds vary greatly in lifestyle, behaviour and physiology, they often exhibit striking convergent evolution, as the same envir ...

s, live on open seas and oceans rather than inland or around more restricted waters such as rivers and lakes. Pelagic birds feed on planktonic

crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapoda, decapods, ostracoda, seed shrimp, branchiopoda, branchiopods, argulidae, fish lice, krill, remipedes, isopoda, isopods, barnacles, copepods, ...

and

. Examples are the

Atlantic puffin The Atlantic puffin ('), also known as the common puffin, is a species of seabird in the auk family. It is the only puffin native to the Atlantic Ocean; two related species, the tufted puffin and the horned puffin is found in the northeastern ...

macaroni penguin The macaroni penguin (''Eudyptes chrysolophus'') is a species of penguin found from the Subantarctic to the Antarctic Peninsula. One of six species of crested penguin, it is very closely related to the royal penguin, and some authorities consid ...

sooty tern The sooty tern (''Onychoprion fuscatus'') is a seabird in the family Laridae. It is a bird of the tropical oceans, returning to land only to breed on islands throughout the equatorial zone. Taxonomy The sooty tern was described by Carl Linnae ...

shearwater Shearwaters are medium-sized long-winged seabirds in the petrel family Procellariidae. They have a global marine distribution, but are most common in temperate and cold waters, and are pelagic outside the breeding season. Description These tub ...

s, and

Procellariiformes Procellariiformes is an order of seabirds that comprises four families: the albatrosses, the petrels and shearwaters, and two families of storm petrels. Formerly called Tubinares and still called tubenoses in English, procellariiforms are oft ...

such as the

albatross Albatrosses, of the biological family Diomedeidae, are large seabirds related to the procellariids, storm petrels, and diving petrels in the order Procellariiformes (the tubenoses). They range widely in the Southern Ocean and the North Pac ...

Procellariidae The family Procellariidae is a group of seabirds that comprises the fulmarine petrels, the gadfly petrels, the diving petrels, the prions, and the shearwaters. This family is part of the bird order Procellariiformes (or tubenoses), which als ...

and

petrel Petrels are tube-nosed seabirds in the bird order Procellariiformes. Description The common name does not indicate relationship beyond that point, as "petrels" occur in three of the four families within that group (all except the albatross f ...

Depth and layers

Epipelagic (sunlight)

Mesopelagic (twilight)

Bathypelagic (midnight)

Abyssopelagic (abyssal zone)

Hadopelagic (hadal zone)

Pelagic ecosystem

Pelagic invertebrates

Pelagic fish

Pelagic reptiles

Pelagic birds

References

Further reading