Marine habitats

Marine habitats are habitats that support marine life. Marine life depends in some way on the saltwater that is in the sea (the term marine comes from the Latin mare, meaning sea or ocean). A habitat is an ecological or environmental area inhabited by one or more living species.[1][2] The marine environment supports many kinds of these habitats.

Marine habitats can be divided into coastal and open ocean habitats. Coastal habitats are found in the area that extends from as far as the tide comes in on the shoreline out to the edge of the continental shelf. Most marine life is found in coastal habitats, even though the shelf area occupies only seven percent of the total ocean area. Open ocean habitats are found in the deep ocean beyond the edge of the continental shelf.

Alternatively, marine habitats can be divided into pelagic and demersal zones. Pelagic habitats are found near the surface or in the open water column, away from the bottom of the ocean. Demersal habitats are near or on the bottom of the ocean. An organism living in a pelagic habitat is said to be a pelagic organism, as in pelagic fish. Similarly, an organism living in a demersal habitat is said to be a demersal organism, as in demersal fish. Pelagic habitats are intrinsically shifting and ephemeral, depending on what ocean currents are doing.

Marine habitats can be modified by their inhabitants. Some marine organisms, like corals, kelp, mangroves and seagrasses, are ecosystem engineers which reshape the marine environment to the point where they create further habitat for other organisms. By volume, oceans provide about 99 percent of the living space on the planet.


Only 29 percent of the world surface is land. The rest is ocean, home to the marine habitats. The oceans are nearly four kilometres deep on average and are fringed with coastlines that run for nearly 380,000 kilometres.

In contrast to terrestrial habitats, marine habitats are shifting and ephemeral. Swimming organisms find areas by the edge of a continental shelf a good habitat, but only while upwellings bring nutrient rich water to the surface. Shellfish find habitat on sandy beaches, but storms, tides and currents mean their habitat continually reinvents itself.

The presence of seawater is common to all marine habitats. Beyond that many other things determine whether a marine area makes a good habitat and the type of habitat it makes. For example:

  • temperature – is affected by geographical latitude, ocean currents, weather, the discharge of rivers, and by the presence of hydrothermal vents or cold seeps
  • sunlight – photosynthetic processes depend on how deep and turbid the water is
  • nutrients – are transported by ocean currents to different marine habitats from land runoff, or by upwellings from the deep sea, or they sink through the sea as marine snow
  • salinity – varies, particularly in estuaries or near river deltas, or by hydrothermal vents
  • dissolved gases – oxygen levels in particular, can be increased by wave actions and decreased during algal blooms
  • acidity – this is partly to do with dissolved gases above, since the acidity of the ocean is largely controlled by how much carbon dioxide is in the water.
  • turbulence – ocean waves, fast currents and the agitation of water affect the nature of habitats
  • cover – the availability of cover such as the adjacency of the sea bottom, or the presence of floating objects
  • the occupying organisms themselves – since organisms modify their habitats by the act of occupying them, and some, like corals, kelp, mangroves and seagrasses, create further habitats for other organisms.

There are five major oceans, of which the Pacific Ocean is nearly as large as the rest put together. Coastlines fringe the land for nearly 380,000 kilometres.

Ocean Area
million km2
% Volume[3]
million cu km
% Mean depth
Max depth
% Ref
Pacific Ocean 155.6 46.4 679.6 49.6 4.37 10.924 135,663 [4]
Atlantic Ocean 76.8 22.9 313.4 22.5 4.08 8.605 111,866 [5]
Indian Ocean 68.6 20.4 269.3 19.6 3.93 7.258 66,526 [6]
Southern Ocean 20.3 6.1 91.5 6.7 4.51 7.235 17,968 [7]
Arctic Ocean 14.1 4.2 17.0 1.2 1.21 4.665 45,389 [8]
Overall 335.3 1370.8[9] 4.09 10.924 377,412
Starr 080207-2310 Acacia farnesiana
Land runoff, pouring into the sea, can contain nutrients

Altogether, the ocean occupies 71 percent of the world surface, averaging nearly four kilometres in depth. By volume, the ocean provides about 99 percent of the living space on the planet.[10] The science fiction writer Arthur C. Clarke has pointed out it would be more appropriate to refer to the planet Earth as the planet Sea or the planet Ocean.[11][12]

Marine habitats can be broadly divided into pelagic and demersal habitats. Pelagic habitats are the habitats of the open water column, away from the bottom of the ocean. Demersal habitats are the habitats that are near or on the bottom of the ocean. An organism living in a pelagic habitat is said to be a pelagic organism, as in pelagic fish. Similarly, an organism living in a demersal habitat is said to be a demersal organism, as in demersal fish. Pelagic habitats are intrinsically ephemeral, depending on what ocean currents are doing.

The land-based ecosystem depends on topsoil and fresh water, while the marine ecosystem depends on dissolved nutrients washed down from the land.[13]

Ocean deoxygenation poses a threat to marine habitats, due to the growth of low oxygen zones.[14]

Ocean currents

Oceanic gyres
Ocean gyres rotate clockwise in the north and counterclockwise in the south

In marine systems, ocean currents have a key role determining which areas are effective as habitats, since ocean currents transport the basic nutrients needed to support marine life.[15] Plankton are the life forms that inhabit the ocean that are so small (less than 2 mm) that they cannot effectively propel themselves through the water, but must drift instead with the currents. If the current carries the right nutrients, and if it also flows at a suitably shallow depth where there is plenty of sunlight, then such a current itself can become a suitable habitat for photosynthesizing tiny algae called phytoplankton. These tiny plants are the primary producers in the ocean, at the start of the food chain. In turn, as the population of drifting phytoplankton grows, the water becomes a suitable habitat for zooplankton, which feed on the phytoplankton. While phytoplankton are tiny drifting plants, zooplankton are tiny drifting animals, such as the larvae of fish and marine invertebrates. If sufficient zooplankton establish themselves, the current becomes a candidate habitat for the forage fish that feed on them. And then if sufficient forage fish move to the area, it becomes a candidate habitat for larger predatory fish and other marine animals that feed on the forage fish. In this dynamic way, the current itself can, over time, become a moving habitat for multiple types of marine life.

Cwall99 lg
This algae bloom occupies sunlit epipelagic waters off the southern coast of England. The algae are maybe feeding on nutrients from land runoff or upwellings at the edge of the continental shelf

Ocean currents can be generated by differences in the density of the water. How dense water is depends on how saline or warm it is. If water contains differences in salt content or temperature, then the different densities will initiate a current. Water that is saltier or cooler will be denser, and will sink in relation to the surrounding water. Conversely, warmer and less salty water will float to the surface. Atmospheric winds and pressure differences also produces surface currents, waves and seiches. Ocean currents are also generated by the gravitational pull of the sun and moon (tides), and seismic activity (tsunami).[15]

The rotation of the Earth affects the direction ocean currents take, and explains which way the large circular ocean gyres rotate in the image above left. Suppose a current at the equator is heading north. The Earth rotates eastward, so the water possesses that rotational momentum. But the further the water moves north, the slower the earth moves eastward. If the current could get to the North Pole, the earth wouldn't be moving eastward at all. To conserve its rotational momentum, the further the current travels north the faster it must move eastward. So the effect is that the current curves to the right. This is the Coriolis effect. It is weakest at the equator and strongest at the poles. The effect is opposite south of the equator, where currents curve left.[15]

Marine topography

Ocean gravity map
Map of underwater topography (1995 NOAA)

Marine (or seabed or ocean) topography refers to the shape the land has when it interfaces with the ocean. These shapes are obvious along coastlines, but they occur also in significant ways underwater. The effectiveness of marine habitats is partially defined by these shapes, including the way they interact with and shape ocean currents, and the way sunlight diminishes when these landforms occupy increasing depths. Tidal networks depend on the balance between sedimentary processes and hydrodynamics however, anthropogenic influences can impact the natural system more than any physical driver.[16]

Marine topographies include coastal and oceanic landforms ranging from coastal estuaries and shorelines to continental shelves and coral reefs. Further out in the open ocean, they include underwater and deep sea features such as ocean rises and seamounts. The submerged surface has mountainous features, including a globe-spanning mid-ocean ridge system, as well as undersea volcanoes,[17] oceanic trenches, submarine canyons, oceanic plateaus and abyssal plains.

The mass of the oceans is approximately 1.35×1018 metric tons, or about 1/4400 of the total mass of the Earth. The oceans cover an area of 3.618×108 km2 with a mean depth of 3,682 m, resulting in an estimated volume of 1.332×109 km3.[18]


One measure of the relative importance of different marine habitats is the rate at which they produce biomass.

Producer Biomass productivity
Ref Total area
(million km²)
Ref Total production
(billion tonnes C/yr)
swamps and marshes 2,500 [19] Includes freshwater
coral reefs 2,000 [19] 0.28 [20] 0.56
algal beds 2,000 [19]
river estuaries 1,800 [19]
open ocean 125 [19][21] 311 39


Porto Covo pano April 2009-4
Coastlines can be volatile habitats

Marine coasts are dynamic environments which constantly change, like the ocean which partially shape them. The Earth's natural processes, including weather and sea level change, result in the erosion, accretion and resculpturing of coasts as well as the flooding and creation of continental shelves and drowned river valleys.

The main agents responsible for deposition and erosion along coastlines are waves, tides and currents. The formation of coasts also depends on the nature of the rocks they are made of – the harder the rocks the less likely they are to erode, so variations in rock hardness result in coastlines with different shapes.

Tides often determine the range over which sediment is deposited or eroded. Areas with high tidal ranges allow waves to reach farther up the shore, and areas with lower tidal ranges produce deposition at a smaller elevation interval. The tidal range is influenced by the size and shape of the coastline. Tides do not typically cause erosion by themselves; however, tidal bores can erode as the waves surge up river estuaries from the ocean.[22]

Waves erode coastline as they break on shore releasing their energy; the larger the wave the more energy it releases and the more sediment it moves. Sediment deposited by waves comes from eroded cliff faces and is moved along the coastline by the waves. Sediment deposited by rivers is the dominant influence on the amount of sediment located on a coastline.[23]

The sedimentologist Francis Shepard classified coasts as primary or secondary.[25]

  • Primary coasts are shaped by non-marine processes, by changes in the land form. If a coast is in much the same condition as it was when sea level was stabilised after the last ice age, it is called a primary coast.[25] "Primary coasts are created by erosion (the wearing away of soil or rock), deposition (the buildup of sediment or sand) or tectonic activity (changes in the structure of the rock and soil because of earthquakes). Many of these coastlines were formed as the sea level rose during the last 18,000 years, submerging river and glacial valleys to form bays and fjords."[26] An example of a primary coast is a river delta, which forms when a river deposits soil and other material as it enters the sea.[26]
  • Secondary coasts are produced by marine processes, such as the action of the sea or by creatures that live in it. Secondary coastlines include sea cliffs, barrier islands, mud flats, coral reefs, mangrove swamps and salt marshes.[26]
  The global continental shelf, highlighted in cyan, defines the extent of coastal habitats, and occupies 5% of the total world area.

Continental coastlines usually have a continental shelf, a shelf of relatively shallow water, less than 200 metres deep, which extends 68 km on average beyond the coast. Worldwide, continental shelves occupy a total area of about 24 million km2 (9 million sq mi), 8% of the ocean's total area and nearly 5% of the world's total area.[27][28] Since the continental shelf is usually less than 200 metres deep, it follows that coastal habitats are generally photic, situated in the sunlit epipelagic zone. This means the conditions for photosynthetic processes so important for primary production, are available to coastal marine habitats. Because land is nearby, there are large discharges of nutrient rich land runoff into coastal waters. Further, periodic upwellings from the deep ocean can provide cool and nutrient rich currents along the edge of the continental shelf.

As a result, coastal marine life is the most abundant in the world. It is found in tidal pools, fjords and estuaries, near sandy shores and rocky coastlines, around coral reefs and on or above the continental shelf. Coastal fish include small forage fish as well as the larger predator fish that feed on them. Forage fish thrive in inshore waters where high productivity results from upwelling and shoreline run off of nutrients. Some are partial residents that spawn in streams, estuaries and bays, but most complete their life cycle in the zone.[29] There can also be a mutualism between species that occupy adjacent marine habitats. For example, fringing reefs just below low tide level have a mutually beneficial relationship with mangrove forests at high tide level and sea grass meadows in between: the reefs protect the mangroves and seagrass from strong currents and waves that would damage them or erode the sediments in which they are rooted, while the mangroves and seagrass protect the coral from large influxes of silt, fresh water and pollutants. This additional level of variety in the environment is beneficial to many types of coral reef animals, which for example may feed in the sea grass and use the reefs for protection or breeding.[30]

Coastal habitats are the most visible marine habitats, but they are not the only important marine habitats. Coastlines run for 380,000 kilometres, and the total volume of the ocean is 1,370 million cu km. This means that for each metre of coast, there is 3.6 cu km of ocean space available somewhere for marine habitats.

La Désirade
Waves and currents shape the intertidal shoreline, eroding the softer rocks and transporting and grading loose particles into shingles, sand or mud


Intertidal zones, those areas close to shore, are constantly being exposed and covered by the ocean's tides. A huge array of life lives within this zone.

Shore habitats range from the upper intertidal zones to the area where land vegetation takes prominence. It can be underwater anywhere from daily to very infrequently. Many species here are scavengers, living off of sea life that is washed up on the shore. Many land animals also make much use of the shore and intertidal habitats. A subgroup of organisms in this habitat bores and grinds exposed rock through the process of bioerosion.

Sandy shores

The White Rocks near Portrush (1) - - 785899
Sandy shores provide shifting homes to many species

Sandy shores, also called beaches, are coastal shorelines where sand accumulates. Waves and currents shift the sand, continually building and eroding the shoreline. Longshore currents flow parallel to the beaches, making waves break obliquely on the sand. These currents transport large amounts of sand along coasts, forming spits, barrier islands and tombolos. Longshore currents also commonly create offshore bars, which give beaches some stability by reducing erosion.[31]

Sandy shores are full of life, The grains of sand host diatoms, bacteria and other microscopic creatures. Some fish and turtles return to certain beaches and spawn eggs in the sand. Birds habitat beaches, like gulls, loons, sandpipers, terns and pelicans. Aquatic mammals, such sea lions, recuperate on them. Clams, periwinkles, crabs, shrimp, starfish and sea urchins are found on most beaches.[32]

Sand is a sediment made from small grains or particles with diameters between about 60 µm and 2 mm.[33] Mud (see mudflats below) is a sediment made from particles finer than sand. This small particle size means that mud particles tend to stick together, whereas sand particles do not. Mud is not easily shifted by waves and currents, and when it dries out, cakes into a solid. By contrast, sand is easily shifted by waves and currents, and when sand dries out it can be blown in the wind, accumulating into shifting sand dunes. Beyond the high tide mark, if the beach is low-lying, the wind can form rolling hills of sand dunes. Small dunes shift and reshape under the influence of the wind while larger dunes stabilise the sand with vegetation.[31]

Ocean processes grade loose sediments to particle sizes other than sand, such as gravel or cobbles. Waves breaking on a beach can leave a berm, which is a raised ridge of coarser pebbles or sand, at the high tide mark. Shingle beaches are made of particles larger than sand, such as cobbles, or small stones. These beaches make poor habitats. Little life survives because the stones are churned and pounded together by waves and currents.[31]

Rocky shores

Porto Covo February 2009-2
Tidepools on rocky shores make turbulent habitats for many forms of marine life

The relative solidity of rocky shores seems to give them a permanence compared to the shifting nature of sandy shores. This apparent stability is not real over even quite short geological time scales, but it is real enough over the short life of an organism. In contrast to sandy shores, plants and animals can anchor themselves to the rocks.[34]

Competition can develop for the rocky spaces. For example, barnacles can compete successfully on open intertidal rock faces to the point where the rock surface is covered with them. Barnacles resist desiccation and grip well to exposed rock faces. However, in the crevices of the same rocks, the inhabitants are different. Here mussels can be the successful species, secured to the rock with their byssal threads.[34]

Rocky and sandy coasts are vulnerable because humans find them attractive and want to live near them. An increasing proportion of the humans live by the coast, putting pressure on coastal habitats.[34]


Mudflats become temporary habitats for migrating birds

Mudflats are coastal wetlands that form when mud is deposited by tides or rivers. They are found in sheltered areas such as bays, bayous, lagoons, and estuaries. Mudflats may be viewed geologically as exposed layers of bay mud, resulting from deposition of estuarine silts, clays and marine animal detritus. Most of the sediment within a mudflat is within the intertidal zone, and thus the flat is submerged and exposed approximately twice daily.

Mudflats are typically important regions for wildlife, supporting a large population, although levels of biodiversity are not particularly high. They are of particular importance to migratory birds. In the United Kingdom mudflats have been classified as a Biodiversity Action Plan priority habitat.

Mangrove and salt marshes

Sundarban mangrove
Mangroves provide nurseries for fish

Mangrove swamps and salt marshes form important coastal habitats in tropical and temperate areas respectively.

Mangroves are species of shrubs and medium size trees that grow in saline coastal sediment habitats in the tropics and subtropics – mainly between latitudes 25° N and 25° S. The saline conditions tolerated by various species range from brackish water, through pure seawater (30 to 40 ppt), to water concentrated by evaporation to over twice the salinity of ocean seawater (up to 90 ppt).[35][36] There are many mangrove species, not all closely related. The term "mangrove" is used generally to cover all of these species, and it can be used narrowly to cover just mangrove trees of the genus Rhizophora.

Mangroves form a distinct characteristic saline woodland or shrubland habitat, called a mangrove swamp or mangrove forest'.[37] Mangrove swamps are found in depositional coastal environments, where fine sediments (often with high organic content) collect in areas protected from high-energy wave action. Mangroves dominate three quarters of tropical coastlines.[36]


Conwy Estuary - - 575854
Estuaries occur when rivers flow into a coastal bay or inlet. They are nutrient rich and have a transition zone which moves from freshwater to saltwater.

An estuary is a partly enclosed coastal body of water with one or more rivers or streams flowing into it, and with a free connection to the open sea.[38] Estuaries form a transition zone between river environments and ocean environments and are subject to both marine influences, such as tides, waves, and the influx of saline water; and riverine influences, such as flows of fresh water and sediment. The inflow of both seawater and freshwater provide high levels of nutrients in both the water column and sediment, making estuaries among the most productive natural habitats in the world.[39]

Most estuaries were formed by the flooding of river-eroded or glacially scoured valleys when sea level began to rise about 10,000-12,000 years ago.[40] They are amongst the most heavily populated areas throughout the world, with about 60% of the world's population living along estuaries and the coast. As a result, estuaries are suffering degradation by many factors, including sedimentation from soil erosion from deforestation; overgrazing and other poor farming practices; overfishing; drainage and filling of wetlands; eutrophication due to excessive nutrients from sewage and animal wastes; pollutants including heavy metals, PCBs, radionuclides and hydrocarbons from sewage inputs; and diking or damming for flood control or water diversion.[40]

Estuaries provide habitats for a large number of organisms and support very high productivity. Estuaries provide habitats for salmon and sea trout nurseries,[41] as well as migratory bird populations.[42] Two of the main characteristics of estuarine life are the variability in salinity and sedimentation. Many species of fish and invertebrates have various methods to control or conform to the shifts in salt concentrations and are termed osmoconformers and osmoregulators. Many animals also burrow to avoid predation and to live in the more stable sedimental environment. However, large numbers of bacteria are found within the sediment which have a very high oxygen demand. This reduces the levels of oxygen within the sediment often resulting in partially anoxic conditions, which can be further exacerbated by limited water flux. Phytoplankton are key primary producers in estuaries. They move with the water bodies and can be flushed in and out with the tides. Their productivity is largely dependent on the turbidity of the water. The main phytoplankton present are diatoms and dinoflagellates which are abundant in the sediment.

Kelp forests

Kelp forest
Kelp forests provide habitat for many marine organisms

Kelp forests are underwater areas with a high density of kelp. They form some of the most productive and dynamic ecosystems on Earth.[43] Smaller areas of anchored kelp are called kelp beds. Kelp forests occur worldwide throughout temperate and polar coastal oceans.[43]

Kelp forests provide a unique three-dimensional habitat for marine organisms and are a source for understanding many ecological processes. Over the last century, they have been the focus of extensive research, particularly in trophic ecology, and continue to provoke important ideas that are relevant beyond this unique ecosystem. For example, kelp forests can influence coastal oceanographic patterns[44] and provide many ecosystem services.[45]

However, humans have contributed to kelp forest degradation. Of particular concern are the effects of overfishing nearshore ecosystems, which can release herbivores from their normal population regulation and result in the over-grazing of kelp and other algae.[46] This can rapidly result in transitions to barren landscapes where relatively few species persist.[47]

Frequently considered an ecosystem engineer, kelp provides a physical substrate and habitat for kelp forest communities.[48] In algae (Kingdom: Protista), the body of an individual organism is known as a thallus rather than as a plant (Kingdom: Plantae). The morphological structure of a kelp thallus is defined by three basic structural units:[47]

  • The holdfast is a root-like mass that anchors the thallus to the sea floor, though unlike true roots it is not responsible for absorbing and delivering nutrients to the rest of the thallus;
  • The stipe is analogous to a plant stalk, extending vertically from the holdfast and providing a support framework for other morphological features;
  • The fronds are leaf- or blade-like attachments extending from the stipe, sometimes along its full length, and are the sites of nutrient uptake and photosynthetic activity.

In addition, many kelp species have pneumatocysts, or gas-filled bladders, usually located at the base of fronds near the stipe. These structures provide the necessary buoyancy for kelp to maintain an upright position in the water column.

The environmental factors necessary for kelp to survive include hard substrate (usually rock), high nutrients (e.g., nitrogen, phosphorus), and light (minimum annual irradiance dose > 50 E m−2[49]). Especially productive kelp forests tend to be associated with areas of significant oceanographic upwelling, a process that delivers cool nutrient-rich water from depth to the ocean's mixed surface layer.[49] Water flow and turbulence facilitate nutrient assimilation across kelp fronds throughout the water column.[50] Water clarity affects the depth to which sufficient light can be transmitted. In ideal conditions, giant kelp (Macrocystis spp.) can grow as much as 30-60 centimetres vertically per day. Some species such as Nereocystis are annual while others like Eisenia are perennial, living for more than 20 years.[51] In perennial kelp forests, maximum growth rates occur during upwelling months (typically spring and summer) and die-backs correspond to reduced nutrient availability, shorter photoperiods and increased storm frequency.[47]

Seagrass meadows

White-spotted puffer
White-spotted puffers like living in seagrass areas

Seagrasses are flowering plants from one of four plant families which grow in marine environments. They are called seagrasses because the leaves are long and narrow and are very often green, and because the plants often grow in large meadows which look like grassland. Since seagrasses photosynthesize and are submerged, they must grow submerged in the photic zone, where there is enough sunlight. For this reason, most occur in shallow and sheltered coastal waters anchored in sand or mud bottoms.

Seagrasses form extensive beds or meadows, which can be either monospecific (made up of one species) or multispecific (where more than one species co-exist). Seagrass beds make highly diverse and productive ecosystems. They are home to phyla such as juvenile and adult fish, epiphytic and free-living macroalgae and microalgae, mollusks, bristle worms, and nematodes. Few species were originally considered to feed directly on seagrass leaves (partly because of their low nutritional content), but scientific reviews and improved working methods have shown that seagrass herbivory is a highly important link in the food chain, with hundreds of species feeding on seagrasses worldwide, including green turtles, dugongs, manatees, fish, geese, swans, sea urchins and crabs.

Seagrasses are ecosystem engineers in the sense that they partly create their own habitat. The leaves slow down water-currents increasing sedimentation, and the seagrass roots and rhizomes stabilize the seabed. Their importance to associated species is mainly due to provision of shelter (through their three-dimensional structure in the water column), and due to their extraordinarily high rate of primary production. As a result, seagrasses provide coastal zones with ecosystem services, such as fishing grounds, wave protection, oxygen production and protection against coastal erosion. Seagrass meadows account for 15% of the ocean's total carbon storage.[52]

Coral reefs


Reefs comprise some of the densest and most diverse habitats in the world. The best-known types of reefs are tropical coral reefs which exist in most tropical waters; however, reefs can also exist in cold water. Reefs are built up by corals and other calcium-depositing animals, usually on top of a rocky outcrop on the ocean floor. Reefs can also grow on other surfaces, which has made it possible to create artificial reefs. Coral reefs also support a huge community of life, including the corals themselves, their symbiotic zooxanthellae, tropical fish and many other organisms.

Much attention in marine biology is focused on coral reefs and the El Niño weather phenomenon. In 1998, coral reefs experienced the most severe mass bleaching events on record, when vast expanses of reefs across the world died because sea surface temperatures rose well above normal.[53][54] Some reefs are recovering, but scientists say that between 50% and 70% of the world's coral reefs are now endangered and predict that global warming could exacerbate this trend.[55][56][57][58]

Open ocean

Earth elevation histogram 2
Elevation-area graph showing the proportion of land area at given heights and the proportion of ocean area at given depths

The open ocean is relatively unproductive because of a lack of nutrients, yet because it is so vast, it has more overall primary production than any other marine habitat. Only about 10 percent of marine species live in the open ocean. But among them are the largest and fastest of all marine animals, as well as the animals that dive the deepest and migrate the longest. In the depths lurk animal that, to our eyes, appear hugely alien.[59]

Surface waters

Starr 061214-2478 Chenopodium oahuense
In the open ocean, sunlit surface epipelagic waters get enough light for photosynthesis, but there are often not enough nutrients. As a result, large areas contain little life apart from migrating animals.[60]

The surface waters are sunlit. The waters down to about 200 metres are said to be in the epipelagic zone. Enough sunlight enters the epipelagic zone to allow photosynthesis by phytoplankton. The epipelagic zone is usually low in nutrients. This partially because the organic debris produced in the zone, such as excrement and dead animals, sink to the depths and are lost to the upper zone. Photosynthesis can happen only if both sunlight and nutrients are present.[60]

In some places, like at the edge of continental shelves, nutrients can upwell from the ocean depth, or land runoff can be distributed by storms and ocean currents. In these areas, given that both sunlight and nutrients are now present, phytoplankton can rapidly establish itself, multiplying so fast that the water turns green from the chlorophyll, resulting in an algal bloom. These nutrient rich surface waters are among the most biologically productive in the world, supporting billions of tonnes of biomass.[60]

"Phytoplankton are eaten by zooplankton - small animals which, like phytoplankton, drift in the ocean currents. The most abundant zooplankton species are copepods and krill: tiny crustaceans that are the most numerous animals on Earth. Other types of zooplankton include jelly fish and the larvae of fish, marine worms, starfish, and other marine organisms".[60] In turn, the zooplankton are eaten by filter-feeding animals, including some seabirds, small forage fish like herrings and sardines, whale sharks, manta rays, and the largest animal in the world, the blue whale. Yet again, moving up the foodchain, the small forage fish are in turn eaten by larger predators, such as tuna, marlin, sharks, large squid, seabirds, dolphins, and toothed whales.[60]

Deep sea

Scale diagram of the layers of the pelagic zone

The deep sea starts at the aphotic zone, the point where sunlight loses most of its energy in the water. Many life forms that live at these depths have the ability to create their own light a unique evolution known as bio-luminescence.

In the deep ocean, the waters extend far below the epipelagic zone, and support very different types of pelagic life forms adapted to living in these deeper zones.[61]

Much of the aphotic zone's energy is supplied by the open ocean in the form of detritus. In deep water, marine snow is a continuous shower of mostly organic detritus falling from the upper layers of the water column. Its origin lies in activities within the productive photic zone. Marine snow includes dead or dying plankton, protists (diatoms), fecal matter, sand, soot and other inorganic dust. The "snowflakes" grow over time and may reach several centimetres in diameter, travelling for weeks before reaching the ocean floor. However, most organic components of marine snow are consumed by microbes, zooplankton and other filter-feeding animals within the first 1,000 metres of their journey, that is, within the epipelagic zone. In this way marine snow may be considered the foundation of deep-sea mesopelagic and benthic ecosystems: As sunlight cannot reach them, deep-sea organisms rely heavily on marine snow as an energy source.

Some deep-sea pelagic groups, such as the lanternfish, ridgehead, marine hatchetfish, and lightfish families are sometimes termed pseudoceanic because, rather than having an even distribution in open water, they occur in significantly higher abundances around structural oases, notably seamounts and over continental slopes. The phenomenon is explained by the likewise abundance of prey species which are also attracted to the structures.

PSM V23 D086 The deep sea fish eurypharynx pelecanoides
The umbrella mouth gulper eel can swallow a fish much larger than itself

The fish in the different pelagic and deep water benthic zones are physically structured, and behave in ways, that differ markedly from each other. Groups of coexisting species within each zone all seem to operate in similar ways, such as the small mesopelagic vertically migrating plankton-feeders, the bathypelagic anglerfishes, and the deep water benthic rattails. "[62]

Ray finned species, with spiny fins, are rare among deep sea fishes, which suggests that deep sea fish are ancient and so well adapted to their environment that invasions by more modern fishes have been unsuccessful.[63] The few ray fins that do exist are mainly in the Beryciformes and Lampriformes, which are also ancient forms. Most deep sea pelagic fishes belong to their own orders, suggesting a long evolution in deep sea environments. In contrast, deep water benthic species, are in orders that include many related shallow water fishes.[64]

The umbrella mouth gulper is a deep sea eel with an enormous loosely hinged mouth. It can open its mouth wide enough to swallow a fish much larger than itself, and then expand its stomach to accommodate its catch.[65]

Sea floor

Vents and seeps

Explorer Ridge Zooarium chimney
Zooarium chimney provides a habitat for vent biota

Hydrothermal vents along the mid-ocean ridge spreading centers act as oases, as do their opposites, cold seeps. Such places support unique marine biomes and many new marine microorganisms and other lifeforms have been discovered at these locations.


The deepest recorded oceanic trenches measure to date is the Mariana Trench, near the Philippines, in the Pacific Ocean at 10,924 m (35,838 ft). At such depths, water pressure is extreme and there is no sunlight, but some life still exists. A white flatfish, a shrimp and a jellyfish were seen by the American crew of the bathyscaphe Trieste when it dove to the bottom in 1960.[66]


Marine life also flourishes around seamounts that rise from the depths, where fish and other sea life congregate to spawn and feed.


Mushroom corals

Deepsea mushroom corals growing on the chimney of an Explorer Ridge hydrothermal vent

Gandalfus yunohana by OpenCage

Deep sea crab found on hydrothermal vents in the Philippine Sea

Deepsea anemone

Deep sea anemone on Blake Ridge

Wreckdiver 300

Submerged wrecks create artificial reef habitat

Greenspotrock 100

Versatile rockfish can be found living in almost any habitat from rocky bottoms to sand and mud, and from vertical faces to horizontal plains.

Otter in kelp field

Marine life within the kelp forest and rocky reef habitat

Monterey Bay STS090-705-5

Monterey Bay, the largest marine sanctuary in the United States, is home to the world's largest group of marine research institutions

Orcas off Santa Rosa island

The killer whale, apex predator of the ocean, cruises a huge range of different marine habitats

Starr 080603-5639 Boerhavia repens

Laysan albatross chick in a contemporary modified habitat, surrounded by human marine debris

See also


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External links


AlgaeBase is a global species database of information on all groups of algae, as well as one group of flowering plants, the sea-grasses.AlgaeBase evolved from Michael Guiry's seaweed website, and has grown into a database of algae from throughout the world, and in freshwater, terrestrial, and brackish as well as marine habitats. By 2005, the database contained about 65,000 names, and by September 2006, 122,240 species and infraspecific names were in the database, with 5,826 images, 38,290 bibliographic items, 138,706 distributional records. Currently, the data for the marine algae, particularly seaweeds, are the most complete. About 30,000 species of algae are included, of which the Rhodophyta (6000 species), marine Chlorophyta (1500 species), and Phaeophyceae (1755 species) are the most complete. The diatoms and the smaller freshwater green algae are currently (August 2010) the most incomplete groups.

Programming is carried out by VisualID ( (Pier Kuipers and Caoilte Guiry) and the compilation of the data was funded by the Irish government Department of Education and Science's PRTLI 3 and 4 programmes (


The Alismatales (alismatids) are an order of flowering plants including about 4500 species. Plants assigned to this order are mostly tropical or aquatic. Some grow in fresh water, some in marine habitats.

Atlantis Paradise Island

Atlantis Paradise Island is an ocean-themed resort on Paradise Island in the Bahamas. It features a variety of accommodations built around Aquaventure, a 154-acre waterscape, which includes fresh and saltwater lagoons, pools, marine habitats, and water slides and river rides.

Bouddi National Park

The Bouddi National Park is a protected national park that is located in the Central Coast region of New South Wales, in eastern Australia. The 1,532-hectare (3,790-acre) national park is situated 46 kilometres (29 mi) northeast of Sydney. A section of the national park extends into the Tasman Sea, creating fully protected land, shore and marine habitats. The park contains one of the last mature temperate rainforests on the Central Coast, Fletchers Glen.


The Chromatiaceae are the main family of purple sulfur bacteria. Many members conduct an anoxygenic photosynthesis. They are distinguished from the Ectothiorhodospiraceae by producing sulfur globules and storing them inside their cells. Most species of the Ectothiorhodospiraceae are also purple sulfur bacteria, but they store the globules outside their cells. The sulfur is an intermediate in the oxidization of sulfide, which is ultimately converted into sulfate, and may serve as a reserve.Nitrosococcus belongs to the nitrifying bacteria.Members are found in both anoxygenic parts of fresh and salt water, and are especially common in stagnant pools, marine habitats, sulfur springs, and soda and salt lakes.Chromatiaceae bacteria have been the cause for some low-oxygen lakes turning "blood red", such as O.C. Fisher Reservoir in San Angelo, Texas, USA.


Corallimorpharia is an order of marine cnidarians closely related to stony or reef building corals (Scleractinia). They are mostly tropical, with a narrow column topped with a wide oral disc. The tentacles are usually short or very short, arranged in rows radiating from the mouth. Many species occur together in large groups. In many respects, they resemble the stony corals, except for the absence of a stony skeleton.

Corallimorpharians occur in a wide range of marine habitats, and are associated with phase shifts in coral reef ecosystems that result in a change from a hard-coral dominated reef to a soft-coral dominated one. Many species are also common invertebrates kept in marine aquaria.

Corallimorphus niwa

Corallimorphus niwa is a species of corals in the genus Corallimorphus. It was classified by Fautin in 2011. Corallimorphus niwa lives in marine habitats.

Corallimorphus profundus

Corallimorphus profundus is a species of corals in the genus Corallimorphus. It lives in marine habitats. This species can be found in the Southern Ocean and in New Zealand.

Gulf of Tadjoura

The Gulf of Tadjoura (Arabic: خليج تدجورا‎), (Somali: Badda Tajuura) is a gulf or basin of the Indian Ocean in the Horn of Africa. It lies south of the straits of Bab-el-Mandeb, or the entrance to the Red Sea, at 11.7°N 43.0°E / 11.7; 43.0. The gulf has many fishing grounds, extensive coral reefs, and abundant pearl oysters. Most of its coastline is the territory of Djibouti, except for a short stretch on the southern shore, which is part of the territory of Somalia.

The Gulf other marine habitats include sea grass beds, salt pans and mangroves.


In ecology, a habitat is the type of natural environment in which a particular species of organism lives. It is characterized by both physical and biological features. A species' habitat is those places where it can find food, shelter, protection and mates for reproduction.

The physical factors are for example soil, moisture, range of temperature, and light intensity as well as biotic factors such as the availability of food and the presence or absence of predators. Every organism has certain habitat needs for the conditions in which it will thrive, but some are tolerant of wide variations while others are very specific in their requirements. A habitat is not necessarily a geographical area, it can be the interior of a stem, a rotten log, a rock or a clump of moss, and for a parasitic organism it is the body of its host, part of the host's body such as the digestive tract, or a single cell within the host's body.

Habitat types include polar, temperate, subtropical and tropical. The terrestrial vegetation type may be forest, steppe, grassland, semi-arid or desert. Fresh water habitats include marshes, streams, rivers, lakes, and ponds, and marine habitats include salt marshes, the coast, the intertidal zone, estuaries, reefs, bays, the open sea, the sea bed, deep water and submarine vents.

Habitats change over time. This may be due to a violent event such as the eruption of a volcano, an earthquake, a tsunami, a wildfire or a change in oceanic currents; or the change may be more gradual over millennia with alterations in the climate, as ice sheets and glaciers advance and retreat, and as different weather patterns bring changes of precipitation and solar radiation. Other changes come as a direct result of human activities; deforestation, the plowing of ancient grasslands, the diversion and damming of rivers, the draining of marshland and the dredging of the seabed. The introduction of alien species can have a devastating effect on native wildlife, through increased predation, through competition for resources or through the introduction of pests and diseases to which the native species have no immunity.


The Indo-Pacific, sometimes known as the Indo-West Pacific or Indo-Pacific Asia, is a biogeographic region of Earth's seas, comprising the tropical waters of the Indian Ocean, the western and central Pacific Ocean, and the seas connecting the two in the general area of Indonesia. It does not include the temperate and polar regions of the Indian and Pacific oceans, nor the Tropical Eastern Pacific, along the Pacific coast of the Americas, which is also a distinct marine realm.

The term is especially useful in marine biology, ichthyology, and similar fields, since many marine habitats are continuously connected from Madagascar to Japan and Oceania, and a number of species occur over that range, but are not found in the Atlantic Ocean.

The region has an exceptionally high species richness, including 3000 species of fish, compared with around 1200 in the next richest marine region, the Western Atlantic, and around 500 species of reef building corals, compared with about 50 species in the Western Atlantic.

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.


Myxozoa (etymology: Greek: μύξα myxa "slime" or "mucus" + thematic vowel o + ζῷον zoon "animals") is a class of aquatic, obligately parasitic cnidarian animals. Over 1300 species have been described and many have a two-host lifecycle, involving a fish and an annelid worm or a bryozoan. The average size of a myxosporean spore usually ranges from 10 μm to 20 μm, whereas that of a malacosporean (a subclade of the Myxozoa) spore can be up to 2 mm. Myxozoans can live in both freshwater and marine habitats.

While the evolutionary history of myxozoans is still an active area of research, it is now understood that myxozoans are highly modified cnidarians that have undergone dramatic evolution from a free swimming, self-sufficient jellyfish-like creature into their current form of obligate parasites composed of a mere handful of cells. As myxozoans evolved into microscopic parasites, they lost many genes responsible for multicellular development, coordination, and cell-cell communication. The genomes of some myxozoans are now among the smallest genomes of any known animal species.


Ophidiiformes is an order of ray-finned fish that includes the cusk-eels (family Ophidiidae), pearlfishes (family Carapidae), viviparous brotulas (family Bythitidae), and others. Members of this order have small heads and long slender bodies. They have either smooth scales or no scales, a long dorsal fin and an anal fin that typically runs into the caudal fin. They mostly come from the tropics and subtropics, and live in both freshwater and marine habitats, including abyssal depths. They have adopted a range of feeding methods and lifestyles, including parasitism. The majority are egg-laying, but some are viviparous.

Oyster Bay National Wildlife Refuge

The Oyster Bay National Wildlife Refuge on the north shore of Long Island consists of high quality marine habitats that support a variety of aquatic-dependent wildlife. The refuge's waters and marshes surround Sagamore Hill National Historic Site, home of Theodore Roosevelt - father of the National Wildlife Refuge System. Subtidal (underwater up to mean high tide line) habitats are abundant with marine invertebrates, shellfish and fish.

Marine invertebrate and fish communities support a complex food web from waterfowl to fish-eating birds, to marine mammals. Waterfowl use of the Refuge peaks from October through April. Over 20,000 ducks have been documented on the refuge during one survey. Over 25 species of waterfowl, along with numerous other waterbirds, depend on Oyster Bay for survival. The most common marine mammals at the refuge are harbor seals - which have become more noticeable during recent years. Sea turtles and diamondback terrapins can also be sighted at the Refuge.

New York State's only remaining commercial oyster farm operates on the refuge, producing 90% of the State's oyster harvest.


The periostracum is a thin organic coating or "skin" which is the outermost layer of the shell of many shelled animals, including molluscs and brachiopods. Among molluscs it is primarily seen in snails and clams, i.e. in gastropods and bivalves, but it is also found in cephalopods such as Allonautilus scrobiculatus. Periostracum is an integral part of the shell, and it forms as the shell forms, along with the other shell layers.

Periostracum is visible as the outer layer of the shell of many molluscan species from terrestrial, freshwater and marine habitats, and may be seen in land snails, river mussels and other kinds of freshwater bivalves, as well as in many kinds of marine shelled molluscs.

The word "periostracum" means "around the shell", meaning that the periostracum is wrapped around what is usually the more calcareous part of the shell. Technically the calcareous part of the shell can (at least in theory) be referred to as the "ostracum", but that term is only very rarely used.

Sponge ground

Sponge grounds, also known as sponge aggregations, are intertidal to deep-sea habitats formed by large accumulations of sponges (glass sponges and/or demosponges), often dominated by a few massive species. Sponge grounds were already reported more than 150 years ago, but the habitat was first fully recognized, studied and described in detail around the Faroe Islands during the inter-Nordic BIOFAR 1 programme 1987–90. These were called Ostur (meaning "cheese" and referring to the appearance of the sponges) by the local fishermen and this name has to some extent entered the scientific literature. Sponge grounds were later found elsewhere in the Northeast Atlantic and in the Northwest Atlantic, as well as near Antarctica. They are now known from many other places worldwide and recognized as key marine habitats.Sponge grounds are important habitats supporting diverse ecosystems. During a study of outer shelf and upper slope sponge grounds at the Faroe Islands, 242 invertebrate species were found in the vicinity and 115 were associated with the sponges. In general, fish fauna associated with sponge grounds are poorly known, but include rockfish and gadiforms. Sponge grounds are threatened, especially by bottom trawling and other fishing gear, dredging, oil and gas exploration and undersea cables, but potentially also by deep sea mining, carbon dioxide sequestration, pollution and climate change.

Swimmer's itch

Swimmer’s itch or cercarial dermatitis, is a short-term allergic immune reaction occurring in the skin of humans that have been infected by water-borne schistosomes. Symptoms, which include itchy, raised papules, commonly occur within 1–2 days of infection and do not generally last more than 2–3 weeks. However, people repeatedly exposed to cercariae develop heavier symptoms with faster onset. Cercarial dermatitis is common in freshwater, brackish and marine habitats worldwide. Incidence may be on the rise, although this may also be attributed to better monitoring. Nevertheless, the condition has been regarded as emerging infectious disease.There are no permanent effects to people from this condition. Orally administered hydroxyzine, an antihistamine, is sometimes prescribed to treat swimmer's itch and similar dermal allergic reactions. In addition, bathing in oatmeal, baking soda, or Epsom salts can also provide relief of symptoms.

The Naze Nature Reserve

The Naze Nature Reserve is a 45 hectare nature reserve on The Naze peninsula north of Walton-on-the-Naze in Essex. It is managed by the Essex Wildlife Trust,. Part of it, The Naze SSSI, is a geological Site of Special Scientific Interest and a Geological Conservation Review site.This coastal site has a variety of terrestrial and marine habitats, and many migrating and nesting birds, such as dark bellied brent geese, sedge warblers, barn owls and whitethroats. The cliff is eroding at one to two metres a year.There is access from Old Hall Lane, where the visitor centre is located.

Aquatic ecosystems

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