Pelagic zone

The pelagic zone consists of the water column of the open ocean, and can be further divided into regions by depth. The word "pelagic" is derived from Ancient Greek πέλαγος (pélagos), meaning 'open sea'. The pelagic zone 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. Conditions in the water column change with distance from the surface (depth), e.g. the pressure increases; the temperature, and the amount of light decreases; the salinity, the amount of disolved oxygen, and micronutrients (e.g. Fe++, Mg++, Ca++) all change. Depending on the depth, the water column, rather like the Earth's atmosphere, may be divided into different layers.

The pelagic zone occupies 1,330 million km3 (320 million mi3) with a mean depth of 3.68 km (2.29 mi) and maximum depth of 11 km (6.8 mi).[1][2][3] Fish that live in the pelagic zone are called pelagic fish. Pelagic life decreases with increasing depth. In addition to the above changes, life is affected by the submarine topography, which is called bathymetry. 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. In other contexts, coastal water 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 subsurface 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, and can be divided into benthic fish, which are denser than water so they can rest on the bottom, and benthopelagic fish, which swim in the water column just above the bottom. Demersal fish are also known as bottom feeders and groundfish.

Depth and layers

A scale diagram of the layers of the pelagic zone

Depending on how deep the sea is, the pelagic zone can extend to five vertical regions in the ocean. From the top down, these are:

Epipelagic (sunlight)

From the surface (MSL) down to around 200 m (660 ft)

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.

Mesopelagic (twilight)

From 200 m (660 ft) down to around 1,000 m (3,300 ft)

The most abundant organisms thriving into the mesopelagic zone are heterotrophic bacteria.[4] 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.[5] Some creatures living in the mesopelagic zone rise to the epipelagic zone at night to feed.[5]

Bathypelagic (midnight)

From 1,000 m (3,300 ft) down to around 4,000 m (13,000 ft)

The name stems from Ancient Greek βαθύς, meaning '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.

Abyssopelagic (lower midnight)

From around 4,000 m (13,000 ft) down to above the ocean floor

The name is derived from Ancient Greek ἄβυσσος, meaning '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.[5] 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.[5] Many of the species living at these depths are transparent and eyeless because of the total lack of light in this zone.[5]


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.

Pelagic ecosystem

Sterna fuscata flight
The pelagic sooty tern spends months at a time flying at sea, returning to land only for breeding.[6]

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.[7]

Pelagic birds

Pelagic birds, also called oceanic birds, 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 Procellariiformes such as the albatross, Procellariidae and petrels.

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

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 live on or near the bottom, and coral reef fish.[8]

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.

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.[9][10]

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.

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


  1. ^ Costello, Mark John; Cheung, Alan; De Hauwere, Nathalie (2010). "Surface Area and the Seabed Area, Volume, Depth, Slope, and Topographic Variation for the World's Seas, Oceans, and Countries". Environmental Science & Technology. 44 (23): 8821–8. Bibcode:2010EnST...44.8821C. doi:10.1021/es1012752. PMID 21033734.
  2. ^ Charette, Matthew; Smith, Walter (2010). "The Volume of Earth's Ocean". Oceanography. 23 (2): 112–4. doi:10.5670/oceanog.2010.51. hdl:1912/3862.
  3. ^ Ocean's Depth and Volume Revealed OurAmazingPlanet, 19 May 2010.
  4. ^ Mazuecos, E.; Arístegui, J.; Vázquez-Domínguez, E.; Ortega-Retuerta, E.; Gasol, J.M.; Reche, I. (2012). "Temperature control of microbial respiration and growth efficiency in the mesopelagic zone of the South Atlantic and Indian Oceans". Deep Sea Research Part I: Oceanographic Research Papers. 95: 131–138. doi:10.3354/ame01583.
  5. ^ a b c d e The Open Ocean -
  6. ^ BirdLife International (2008). "Sterna fuscata". IUCN Red List of Threatened Species. Version 2008. International Union for Conservation of Nature. Retrieved 7 August 2009.
  7. ^ Walker P and Wood E (2005) The Open Ocean (volume in a series called Life in the sea), Infobase Publishing, ISBN 978-0-8160-5705-4.
  8. ^ Lal, Brij V.; Fortune, Kate (January 2000). The Pacific Islands: An Encyclopedia. University of Hawaii Press. p. 8. ISBN 978-0-8248-2265-1.
  9. ^ Thorson, G (1957). "Bottom communities (sublittoral or shallow shelf)". In Hedgpeth, J.W. (ed.). Treatise on Marine Ecology and Palaeoecology. Geological Society of America. pp. 461–534.
  10. ^ Mileikovsky, S. A. (1971). "Types of larval development in marine bottom invertebrates, their distribution and ecological significance: a re-evaluation". Marine Biology. 10 (3): 193–213. doi:10.1007/BF00352809.

Further reading

Abyssal zone

The abyssal zone or abyssopelagic zone is a layer of the pelagic zone of the ocean. "Abyss" derives from the Greek word ἄβυσσος, meaning bottomless. At depths of 3,000 to 6,000 metres (9,800 to 19,700 ft), this zone remains in perpetual darkness. It alone makes up over 83% of the ocean and covers 60% of the Earth. The abyssal zone has temperatures around 2 to 3 °C (36 to 37 °F) through the large majority of its mass. Due to there being no light, there are no plants producing oxygen, which primarily comes from ice that had melted long ago from the polar regions. The water along the seafloor of this zone is actually devoid of oxygen, resulting in a death trap for organisms unable to quickly return to the oxygen-enriched water above. This region also contains a much higher concentration of nutrient salts, like nitrogen, phosphorus, and silica, due to the large amount of dead organic material that drifts down from the above ocean zones and decomposes.It is the deeper part of the midnight zone which starts in the bathypelagic waters above.The area below the abyssal zone is the sparsely inhabited hadal zone. The zone above is the bathyal zone.


Arachnida () is a class of joint-legged invertebrate animals (arthropods), in the subphylum Chelicerata.

Spiders are the largest order in the class, which also includes scorpions, ticks, mites, harvestmen, and solifuges. In 2019, a molecular phylogenetic study also placed horseshoe crabs in Arachnida.Almost all adult arachnids have eight legs, although the front pair of legs in some species has converted to a sensory function, while in other species, different appendages can grow large enough to take on the appearance of extra pairs of legs. The term is derived from the Greek word ἀράχνη (aráchnē), from the myth of the hubristic human weaver Arachne who was turned into a spider.Almost all extant arachnids are terrestrial, living mainly on land. However, some inhabit freshwater environments and, with the exception of the pelagic zone, marine environments as well. They comprise over 100,000 named species.

Bathyal zone

The bathyal zone or bathypelagic – from Greek βαθύς (bathýs), deep – (also known as midnight zone) is the part of the pelagic zone that extends from a depth of 1,000 to 4,000 m (3,300 to 13,100 ft) below the ocean surface. It lies between the mesopelagic above, and the abyssopelagic below. The average temperature hovers at about 4 °C (39 °F). Although larger by volume than the euphotic zone, the bathyal zone is less densely populated. Sunlight does not reach this zone, meaning primary production, if any, is almost nonexistent. There are no known plants because of the lack of sunlight necessary for photosynthesis. It is known as the midnight (also twilight or dark) zone because of this feature.

Benthenchelys cartieri

Benthenchelys cartieri is an eel in the family Ophichthidae. It was described by Henry Weed Fowler in 1934. It is a tropical, marine eel known from the Philippines, in the western central Pacific Ocean. It is known to dwell at a maximum depth of 1168 m, and inhabits the pelagic zone.


Cardiapoda is a genus of very small floating sea snails or heteropods, pelagic gastropod molluscs or micromolluscs in the family Carinariidae.

Both tentacles are of equal size and well-developed. The external layer of the body wall is thin.

They are found worldwide in the pelagic zone of warm seas.

Demersal zone

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 of the larger profundal zone.

Being just above the ocean floor, the demersal zone is variable in depth and can be part of the photic zone where light can penetrate and photosynthetic organisms grow, or the aphotic zone, which begins between depths of roughly 200 and 1,000 m (700 and 3,300 ft) and extends to the ocean depths, where no light penetrates.

Ferromanganese nodules

Ferromanganese nodules form in the oxidizing environment of the abyssal pelagic zone. They are the result of ion exchange reactions that precipitate ore components from the water (sedimentary) or out of the interstitial water of the sediments layers (diagenetic). The composition of Manganese-bearing minerals is dependent on how the nodules are formed; sedimentary nodules, which have a lower Mn2+ content than diagenetic, are dominated by Fe-vernadite, Mn-feroxyhyte, and asbolane-buserite while diagenetic nodules are dominated by buserite I, birnessite, todorokite, and asbolane-buserite.


Holoplankton are organisms that are planktic (they live in the water column and cannot swim against a current) for their entire life cycle. Holoplankton can be contrasted with meroplankton, which are planktic organisms that spend part of their life cycle in the benthic zone. Examples of holoplankton include some diatoms, radiolarians, some dinoflagellates, foraminifera, amphipods, krill, copepods, and salps, as well as some gastropod mollusk species. Holoplankton dwell in the pelagic zone as opposed to the benthic zone. Holoplankton include both phytoplankton and zooplankton and vary in size. The most common plankton are protists.

ISimangaliso Marine Protected Area

The iSimangaliso Marine Protected Area is a coastal and offshore marine protected area in KwaZulu-Natal from the South Africa-Mozambique border in the north to Cape St Lucia lighthouse in the south.


Larvaceans (Class Appendicularia) are solitary, free-swimming tunicates found throughout the world's oceans. Like most tunicates, appendicularians are filter feeders. Unlike most other tunicates, they live in the pelagic zone, specifically in the upper sunlit portion of the ocean (photic zone) or sometimes deeper. They are transparent planktonic animals, generally less than 1 cm (0.39 in) in body length (excluding the tail).

Lesser thrush eel

The lesser thrush eel, also known as the common worm eel and the spaghetti eel, (Moringua microchir) is an eel in the family Moringuidae (spaghetti/worm eels). It was described by Pieter Bleeker in 1853. It is a tropical, marine eel which is known from East Africa, Samoa, the Ryukyu Islands, and the southern Great Barrier Reef. It typically dwells at a depth range of 3–20 m, with juveniles inhabiting estuaries and rivers, adult females leading a benthic lifestyle in shallow oceanic waters, and adult males living in the pelagic zone. Adults breed offshore. Males can reach a maximum total length of 47 cm.The lesser thrush eel's diet consists primarily of crustaceans and bony fish.

Marine biology

Marine biology is the scientific study of marine life, organisms in the sea. Given that in biology many phyla, families and genera have some species that live in the sea and others that live on land, marine biology classifies species based on the environment rather than on taxonomy.

A large proportion of all life on Earth lives in the ocean. The exact size of this large proportion is unknown, since many ocean species are still to be discovered. The ocean is a complex three-dimensional world covering approximately 71% of the Earth's surface. The habitats studied in marine biology include everything from the tiny layers of surface water in which organisms and abiotic items may be trapped in surface tension between the ocean and atmosphere, to the depths of the oceanic trenches, sometimes 10,000 meters or more beneath the surface of the ocean. Specific habitats include coral reefs, kelp forests, seagrass meadows, the surrounds of seamounts and thermal vents, tidepools, muddy, sandy and rocky bottoms, and the open ocean (pelagic) zone, where solid objects are rare and the surface of the water is the only visible boundary. The organisms studied range from microscopic phytoplankton and zooplankton to huge cetaceans (whales) 25–32 meters (82–105 feet) in length. Marine ecology is the study of how marine organisms interact with each other and the environment.

Marine life is a vast resource, providing food, medicine, and raw materials, in addition to helping to support recreation and tourism all over the world. At a fundamental level, marine life helps determine the very nature of our planet. Marine organisms contribute significantly to the oxygen cycle, and are involved in the regulation of the Earth's climate. Shorelines are in part shaped and protected by marine life, and some marine organisms even help create new land.Many species are economically important to humans, including both finfish and shellfish. It is also becoming understood that the well-being of marine organisms and other organisms are linked in fundamental ways. The human body of knowledge regarding the relationship between life in the sea and important cycles is rapidly growing, with new discoveries being made nearly every day. These cycles include those of matter (such as the carbon cycle) and of air (such as Earth's respiration, and movement of energy through ecosystems including the ocean). Large areas beneath the ocean surface still remain effectively unexplored.


Meroplankton is a wide variety of planktonic organisms, which spend a portion of their lives in the benthic region of the ocean. These organisms do not remain as plankton permanently, rather, they are planktonic components in transition, which eventually become larger organisms. Meroplankton can be contrasted with holoplankton, which are planktonic organisms that stay in the pelagic zone as plankton throughout their entire life cycle.

After a period of time in the plankton, meroplankton either graduate to the nekton or adopt a benthic (often sessile) lifestyle on the seafloor. Meroplankton consists of larval stages of organisms such as sea urchins, starfish, and crustaceans. Success of meroplankton populations depends on many factors, such as adult fecundity, fertilization success, growth and larval stage duration, behaviour, dispersal, and settlement. Mortality depends on many factors, such as predation, competition, disease, parasites, and physiological stresses. Survival and mortality of meroplankton has a direct effect on adult population numbers of many species.

Many of the common, well-known animals found on the Great Barrier Reef spend time as free-swimming meroplankton, bearing little or no resemblance to the adult they will become. The differences between the appearance of larval and adult stages led to much confusion in the past when larval forms were often believed to be completely different species from the adults. Larvae spend varying amounts of time in the plankton, from minutes to over a year. However, just how long these tiny animals can be considered truly planktonic is under some debate.

Neritic zone

The neritic zone is the relatively shallow part of the ocean above the drop-off of the continental shelf, approximately 200 meters (660 ft) in depth.

From the point of view of marine biology it forms a relatively stable and well-illuminated environment for marine life, from plankton up to large fish and corals, while physical oceanography sees it as where the oceanic system interacts with the coast.


The omul, Coregonus migratorius, also known as Baikal omul (Russian: байкальский омуль), is a whitefish species of the salmon family endemic to Lake Baikal in Siberia, Russia. It is considered a delicacy and is the object of one of the largest commercial fisheries on Lake Baikal. In 2004, it was listed in Russia as an endangered species.


Phronima is a genus of small, deep sea hyperiid amphipods of the family Phronimidae. It is found throughout the world's oceans, except in polar regions. Phronima species live in the pelagic zone of the deep ocean. Their bodies are semitransparent. Although commonly known as parasites, they are more technically correctly called parasitoids. Instead of constantly feeding on a live host, females attack salps, using their mouths and claws to eat the animal and hollow out its gelatinous shell. Phronima females then enter the barrel and lay their eggs inside, and then propels the barrel through the water as the larvae develop, providing them with fresh food and water.


Plankton are the diverse collection of organisms that live in large bodies of water and are unable to swim against a current. The individual organisms constituting plankton are called plankters. They provide a crucial source of food to many small and large aquatic organisms, such as bivalves, fish and whales.

Planktonic organisms include bacteria, archaea, algae, protozoa and drifting or floating animals that inhabit—for example—the pelagic zone of oceans, seas, or bodies of fresh water. Essentially, plankton are defined by their ecological niche rather than any phylogenetic or taxonomic classification.

Though many planktonic species are microscopic in size, plankton includes organisms over a wide range of sizes, including large organisms such as jellyfish.

Technically the term does not include organisms on the surface of the water, which are called pleuston—or those that swim actively in the water, which are called nekton. The distribution of plankton can be affected by the wind driven Langmuir circulation and their biological effects.


The Siphonophorae or Siphonophora, the siphonophores, are an order of the hydrozoans, a class of marine animals belonging to the phylum Cnidaria. According to the World Register of Marine Species, the order contains 188 species. Although a siphonophore may appear to be a single organism, each specimen is in fact a colonial organism composed of small individual animals called zooids that have their own special function for survival. Most colonies are long, thin, transparent floaters living in the pelagic zone. Some siphonophores, such as the venomous Portuguese man o' war and the Indo-Pacific man o' war, superficially resemble jellyfish.

Another species of siphonophore, Praya dubia, is one of the longest animals in the world, with a body length of 40–50 m (130–160 ft). The term originates from the Greek siphōn 'tube' + pherein 'to bear'.

South Pacific garbage patch

The South Pacific garbage patch is an area of elevated levels of marine debris and plastic particle pollution, most of which is concentrated within the ocean's pelagic zone. It is located within the South Pacific Gyre, which itself spans from waters east of Australia to the South American continent, as far north as the Equator, and south until reaching the Antarctic Circumpolar Current. The degradation of plastics in the ocean also leads to a rise in the level of toxics in the area. This garbage patch is the most recently discovered having only been confirmed in mid-2017. The South Pacific garbage patch has been compared in nature to the Great Pacific garbage patch's state in 2007, making the former ten years younger. The garbage patch is impossible to detect using satellites, or other visual means as most particles are smaller than a grain of rice.

Ocean zones
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Aquatic ecosystems


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