126-641: A sea is a large body of salty water. Sea or SEA may also refer to: Sea A sea is a large body of salt water . There are particular seas and the sea . The sea commonly refers to the Ocean , the interconnected body of seawaters that spans most of Earth. Particular seas are either marginal seas , second-order sections of the oceanic sea (e.g. the Mediterranean Sea ), or certain large, nearly landlocked bodies of water. The salinity of water bodies varies widely, being lower near
252-517: A biodiverse range of larger and smaller animal life. Light is only able to penetrate the top 200 metres (660 ft) so this is the only part of the sea where plants can grow. The surface layers are often deficient in biologically active nitrogen compounds. The marine nitrogen cycle consists of complex microbial transformations which include the fixation of nitrogen , its assimilation, nitrification , anammox and denitrification. Some of these processes take place in deep water so that where there
378-1054: A dagger (†); groups of uncertain placement are labelled with a question mark (?) and dashed lines (- - - - -). Jawless fishes (118 species: hagfish , lampreys ) [REDACTED] † Thelodonti , † Conodonta , † Anaspida [REDACTED] [REDACTED] [REDACTED] † Galeaspida [REDACTED] † Osteostraci [REDACTED] † Placodermi [REDACTED] † Acanthodii [REDACTED] (>1,100 species: sharks , rays , chimaeras ) [REDACTED] (2 species: coelacanths ) [REDACTED] Dipnoi (6 species: lungfish ) [REDACTED] Tetrapoda (>38,000 species, not considered fish: amphibians, reptiles, birds, mammals) [REDACTED] (14 species: bichirs , reedfish ) [REDACTED] (27 species: sturgeons , paddlefish ) [REDACTED] Ginglymodi (7 species: gars , alligator gars ) [REDACTED] Halecomorphi (2 species: bowfin , eyetail bowfin ) [REDACTED] (>32,000 species) [REDACTED] Fishes (without tetrapods) are
504-683: A mediterranean sea ) or wholly (as inland seas ) enclosed by land . However, an exception to this is the Sargasso Sea which has no coastline and lies within a circular current, the North Atlantic Gyre . Seas are generally larger than lakes and contain salt water, but the Sea of Galilee is a freshwater lake . The United Nations Convention on the Law of the Sea states that all of the ocean
630-579: A paraphyletic group and for this reason, the class Pisces seen in older reference works is no longer used in formal classifications. Traditional classification divides fish into three extant classes (Agnatha, Chondrichthyes, and Osteichthyes), and with extinct forms sometimes classified within those groups, sometimes as their own classes. Fish account for more than half of vertebrate species. As of 2016, there are over 32,000 described species of bony fish, over 1,100 species of cartilaginous fish, and over 100 hagfish and lampreys. A third of these fall within
756-422: A channel in the seabed. It may occur at a gap in a sandbar or near a man-made structure such as a groyne . These strong currents can have a velocity of 3 ft (0.9 m) per second, can form at different places at different stages of the tide and can carry away unwary bathers. Temporary upwelling currents occur when the wind pushes water away from the land and deeper water rises to replace it. This cold water
882-400: A height of less than three feet, so they often pass unnoticed at this stage. In contrast, ocean surface waves caused by winds have wavelengths of a few hundred feet, travel at up to 65 miles per hour (105 km/h) and are up to 45 feet (14 metres) high. As a tsunami moves into shallower water its speed decreases, its wavelength shortens and its amplitude increases enormously, behaving in
1008-507: A large and multidisciplinary field: it examines the properties of seawater ; studies waves , tides , and currents ; charts coastlines and maps the seabeds ; and studies marine life . The subfield dealing with the sea's motion, its forces, and the forces acting upon it is known as physical oceanography . Marine biology (biological oceanography) studies the plants , animals , and other organisms inhabiting marine ecosystems . Both are informed by chemical oceanography , which studies
1134-453: A low-pressure system, can raise the surface of the sea at high tide dramatically. The Earth is composed of a magnetic central core , a mostly liquid mantle and a hard rigid outer shell (or lithosphere ), which is composed of the Earth's rocky crust and the deeper mostly solid outer layer of the mantle. On land the crust is known as the continental crust while under the sea it is known as
1260-436: A more spherical lens . Their retinas generally have both rods and cones (for scotopic and photopic vision ); many species have colour vision , often with three types of cone. Teleosts can see polarized light ; some such as cyprinids have a fourth type of cone that detects ultraviolet . Amongst jawless fish , the lamprey has well-developed eyes, while the hagfish has only primitive eyespots. Hearing too
1386-534: A new world of creatures living on the seabed that scientists had not previously known to exist. Some like the detrivores rely on organic material falling to the ocean floor. Others cluster round deep sea hydrothermal vents where mineral-rich flows of water emerge from the seabed, supporting communities whose primary producers are sulphide-oxidising chemoautotrophic bacteria, and whose consumers include specialised bivalves, sea anemones, barnacles, crabs, worms and fish, often found nowhere else. A dead whale sinking to
SECTION 10
#17327652396951512-419: A protective bony cover or operculum . They are able to oxygenate their gills using muscles in the head. Some 400 species of fish in 50 families can breathe air, enabling them to live in oxygen-poor water or to emerge on to land. The ability of fish to do this is potentially limited by their single-loop circulation, as oxygenated blood from their air-breathing organ will mix with deoxygenated blood returning to
1638-536: A saline body of water and therefore solely a sea in the sense of the common use of the word, like all other saltwater lakes called sea. Earth is the only known planet with seas of liquid water on its surface, although Mars possesses ice caps and similar planets in other solar systems may have oceans. Earth's 1,335,000,000 cubic kilometers (320,000,000 cu mi) of sea contain about 97.2 percent of its known water and covers approximately 71 percent of its surface. Another 2.15% of Earth's water
1764-441: A sense of touch and of hearing . Blind cave fish navigate almost entirely through the sensations from their lateral line system. Some fish, such as catfish and sharks, have the ampullae of Lorenzini , electroreceptors that detect weak electric currents on the order of millivolt. Vision is an important sensory system in fish. Fish eyes are similar to those of terrestrial vertebrates like birds and mammals, but have
1890-485: A tail fin, jaws, skin covered with scales , and lays eggs. Each criterion has exceptions, creating a wide diversity in body shape and way of life. For example, some fast-swimming fish are warm-blooded, while some slow-swimming fish have abandoned streamlining in favour of other body shapes. Fish species are roughly divided equally between freshwater and marine (oceanic) ecosystems; there are some 15,200 freshwater species and around 14,800 marine species. Coral reefs in
2016-589: A true "land fish" as this worm-like catfish strictly lives among waterlogged leaf litter . Cavefish of multiple families live in underground lakes , underground rivers or aquifers . Like other animals, fish suffer from parasitism . Some species use cleaner fish to remove external parasites. The best known of these are the bluestreak cleaner wrasses of coral reefs in the Indian and Pacific oceans. These small fish maintain cleaning stations where other fish congregate and perform specific movements to attract
2142-422: A typical fish is adapted for efficient swimming by alternately contracting paired sets of muscles on either side of the backbone. These contractions form S-shaped curves that move down the body. As each curve reaches the tail fin, force is applied to the water, moving the fish forward. The other fins act as control surfaces like an aircraft's flaps, enabling the fish to steer in any direction. Since body tissue
2268-471: A very high range in bays or estuaries . Submarine earthquakes arising from tectonic plate movements under the oceans can lead to destructive tsunamis , as can volcanoes, huge landslides , or the impact of large meteorites . The seas have been an integral element for humans throughout history and culture. Humans harnessing and studying the seas have been recorded since ancient times and evidenced well into prehistory , while its modern scientific study
2394-414: A wide array of species including corals (only six of which contribute to reef formation). Marine primary producers – plants and microscopic organisms in the plankton – are widespread and very essential for the ecosystem. It has been estimated that half of the world's oxygen is produced by phytoplankton. About 45 percent of the sea's primary production of living material
2520-529: Is "sea". The law of the sea has at its center the definition of the boundaries of the ocean, clarifying its application in marginal seas . But what bodies of water other than the sea the law applies to is being crucially negotiated in the case of the Caspian Sea and its status as "sea", basically revolving around the issue of the Caspian Sea about either being factually an oceanic sea or only
2646-449: Is a continuous circulation of water in the oceans. Warm surface currents cool as they move away from the tropics, and the water becomes denser and sinks. The cold water moves back towards the equator as a deep sea current, driven by changes in the temperature and density of the water, before eventually welling up again towards the surface. Deep seawater has a temperature between −2 °C (28 °F) and 5 °C (41 °F) in all parts of
SECTION 20
#17327652396952772-399: Is a point of land jutting out into the sea and a larger promontory is known as a cape . The indentation of a coastline, especially between two headlands, is a bay , a small bay with a narrow inlet is a cove and a large bay may be referred to as a gulf . Coastlines are influenced by several factors including the strength of the waves arriving on the shore, the gradient of the land margin,
2898-465: Is an important sensory system in fish. Fish sense sound using their lateral lines and otoliths in their ears, inside their heads. Some can detect sound through the swim bladder. Some fish, including salmon, are capable of magnetoreception ; when the axis of a magnetic field is changed around a circular tank of young fish, they reorient themselves in line with the field. The mechanism of fish magnetoreception remains unknown; experiments in birds imply
3024-487: Is an upwelling of cold waters, and also near estuaries where land-sourced nutrients are present, plant growth is higher. This means that the most productive areas, rich in plankton and therefore also in fish, are mainly coastal. There is a broader spectrum of higher animal taxa in the sea than on land, many marine species have yet to be discovered and the number known to science is expanding annually. Some vertebrates such as seabirds , seals and sea turtles return to
3150-556: Is called oceanography and maritime space is governed by the law of the sea , with admiralty law regulating human interactions at sea. The seas provide substantial supplies of food for humans, mainly fish , but also shellfish , mammals and seaweed , whether caught by fishermen or farmed underwater. Other human uses of the seas include trade , travel, mineral extraction , power generation , warfare , and leisure activities such as swimming , sailing , and scuba diving . Many of these activities create marine pollution . The sea
3276-478: Is contributed by diatoms . Much larger algae, commonly known as seaweeds , are important locally; Sargassum forms floating drifts, while kelp form seabed forests. Flowering plants in the form of seagrasses grow in " meadows " in sandy shallows, mangroves line the coast in tropical and subtropical regions and salt-tolerant plants thrive in regularly inundated salt marshes . All of these habitats are able to sequester large quantities of carbon and support
3402-431: Is denser than water, fish must compensate for the difference or they will sink. Many bony fish have an internal organ called a swim bladder that allows them to adjust their buoyancy by increasing or decreasing the amount of gas it contains. The scales of fish provide protection from predators at the cost of adding stiffness and weight. Fish scales are often highly reflective; this silvering provides camouflage in
3528-481: Is frozen, found in the sea ice covering the Arctic Ocean , the ice cap covering Antarctica and its adjacent seas , and various glaciers and surface deposits around the world. The remainder (about 0.65% of the whole) form underground reservoirs or various stages of the water cycle , containing the freshwater encountered and used by most terrestrial life : vapor in the air , the clouds it slowly forms,
3654-401: Is likely to reduce levels of oxygen in surface waters since the solubility of oxygen in water falls at higher temperatures. Ocean deoxygenation is projected to increase hypoxia by 10%, and triple suboxic waters (oxygen concentrations 98% less than the mean surface concentrations), for each 1 °C of upper-ocean warming. The amount of light that penetrates the sea depends on the angle of
3780-530: Is near the Mariana Islands , a volcanic archipelago in the West Pacific. Its deepest point is 10.994 kilometres (nearly 7 miles) below the surface of the sea. The zone where land meets sea is known as the coast and the part between the lowest spring tides and the upper limit reached by splashing waves is the shore . A beach is the accumulation of sand or shingle on the shore. A headland
3906-541: Is often rich in nutrients and creates blooms of phytoplankton and a great increase in the productivity of the sea. Tides are the regular rise and fall in water level experienced by seas and oceans in response to the gravitational influences of the Moon and the Sun, and the effects of the Earth's rotation. During each tidal cycle, at any given place the water rises to a maximum height known as "high tide" before ebbing away again to
Sea (disambiguation) - Misplaced Pages Continue
4032-465: Is the biggest part of the brain; it is small in hagfish and lampreys , but very large in mormyrids , processing their electrical sense . The brain stem or myelencephalon controls some muscles and body organs, and governs respiration and osmoregulation . The lateral line system is a network of sensors in the skin which detects gentle currents and vibrations, and senses the motion of nearby fish, whether predators or prey. This can be considered both
4158-595: Is the interconnected system of all the Earth's oceanic waters, including the Atlantic , Pacific , Indian , Southern and Arctic Oceans . However, the word "sea" can also be used for many specific, much smaller bodies of seawater, such as the North Sea or the Red Sea . There is no sharp distinction between seas and oceans , though generally seas are smaller, and are often partly (as marginal seas or particularly as
4284-419: Is turned into kinetic energy, creating a shallow wave, a tsunami, radiating outwards at a velocity proportional to the square root of the depth of the water and which therefore travels much faster in the open ocean than on a continental shelf. In the deep open sea, tsunamis have wavelengths of around 80 to 300 miles (130 to 480 km), travel at speeds of over 600 miles per hour (970 km/h) and usually have
4410-651: The Coriolis effect . The surface currents flow in a clockwise direction in the Northern Hemisphere and anticlockwise in the Southern Hemisphere. The water moving away from the equator is warm, and that flowing in the reverse direction has lost most of its heat. These currents tend to moderate the Earth's climate, cooling the equatorial region and warming regions at higher latitudes. Global climate and weather forecasts are powerfully affected by
4536-617: The Gnathostomata or (for bony fish) Osteichthyes , also contains the clade of tetrapods (four-limbed vertebrates, mostly terrestrial), which are usually not considered fish. Some tetrapods, such as cetaceans and ichthyosaurs , have secondarily acquired a fish-like body shape through convergent evolution . Fishes of the World comments that "it is increasingly widely accepted that tetrapods, including ourselves, are simply modified bony fishes, and so we are comfortable with using
4662-671: The Indo-Pacific constitute the center of diversity for marine fishes, whereas continental freshwater fishes are most diverse in large river basins of tropical rainforests , especially the Amazon , Congo , and Mekong basins. More than 5,600 fish species inhabit Neotropical freshwaters alone, such that Neotropical fishes represent about 10% of all vertebrate species on the Earth. Fish are abundant in most bodies of water. They can be found in nearly all aquatic environments, from high mountain streams (e.g., char and gudgeon ) to
4788-857: The Mediterranean and Red Sea with the Egyptian Hannu reaching the Arabian Peninsula and the African Coast around 2750 BC. In the first millennium BC, Phoenicians and Greeks established colonies throughout the Mediterranean and the Black Sea . Around 500 BC, the Carthaginian navigator Hanno left a detailed periplus of an Atlantic journey that reached at least Senegal and possibly Mount Cameroon . In
4914-572: The Paleozoic , diversifying into many forms. The earliest fish with dedicated respiratory gills and paired fins , the ostracoderms , had heavy bony plates that served as protective exoskeletons against invertebrate predators . The first fish with jaws , the placoderms, appeared in the Silurian and greatly diversified during the Devonian , the "Age of Fishes". Bony fish, distinguished by
5040-571: The Silurian , with giant armoured placoderms such as Dunkleosteus . Jawed fish, too, appeared during the Silurian: the cartilaginous Chondrichthyes and the bony Osteichthyes . During the Devonian , fish diversity greatly increased, including among the placoderms, lobe-finned fishes, and early sharks, earning the Devonian the epithet "the age of fishes". Fishes are a paraphyletic group, since any clade containing all fish, such as
5166-477: The abyssal and even hadal depths of the deepest oceans (e.g., cusk-eels and snailfish ), although none have been found in the deepest 25% of the ocean. The deepest living fish in the ocean so far found is a cusk-eel, Abyssobrotula galatheae , recorded at the bottom of the Puerto Rico Trench at 8,370 m (27,460 ft). In terms of temperature, Jonah's icefish live in cold waters of
Sea (disambiguation) - Misplaced Pages Continue
5292-584: The early Mediaeval period , the Vikings crossed the North Atlantic and even reached the northeastern fringes of North America. Novgorodians had also been sailing the White Sea since the 13th century or before. Meanwhile, the seas along the eastern and southern Asian coast were used by Arab and Chinese traders. The Chinese Ming Dynasty had a fleet of 317 ships with 37,000 men under Zheng He in
5418-614: The hundred-year wave ) they are designed against. Rogue waves, however, have been documented at heights above 25 meters (82 ft). The top of a wave is known as the crest, the lowest point between waves is the trough and the distance between the crests is the wavelength. The wave is pushed across the surface of the sea by the wind, but this represents a transfer of energy and not a horizontal movement of water. As waves approach land and move into shallow water , they change their behavior. If approaching at an angle, waves may bend ( refraction ) or wrap rocks and headlands ( diffraction ). When
5544-400: The intertidal zone , are facultative air breathers, able to breathe air when out of water, as may occur daily at low tide , and to use their gills when in water. Some coastal fish like rockskippers and mudskippers choose to leave the water to feed in habitats temporarily exposed to the air. Some catfish absorb air through their digestive tracts. The digestive system consists of a tube,
5670-422: The oceanic crust . The latter is composed of relatively dense basalt and is some five to ten kilometres (three to six miles) thick. The relatively thin lithosphere floats on the weaker and hotter mantle below and is fractured into a number of tectonic plates . In mid-ocean, magma is constantly being thrust through the seabed between adjoining plates to form mid-oceanic ridges and here convection currents within
5796-436: The photosynthetic activity of these plants produces oxygen, which dissolves in the seawater and is used by marine animals. At night, photosynthesis stops, and the amount of dissolved oxygen declines. In the deep sea, where insufficient light penetrates for plants to grow, there is very little dissolved oxygen. In its absence, organic material is broken down by anaerobic bacteria producing hydrogen sulphide . Climate change
5922-421: The rain falling from them, and the lakes and rivers spontaneously formed as its waters flow again and again to the sea. The scientific study of water and Earth's water cycle is hydrology ; hydrodynamics studies the physics of water in motion. The more recent study of the sea in particular is oceanography . This began as the study of the shape of the ocean's currents but has since expanded into
6048-425: The stout infantfish . Swimming performance varies from fish such as tuna, salmon , and jacks that can cover 10–20 body-lengths per second to species such as eels and rays that swim no more than 0.5 body-lengths per second. A typical fish is cold-blooded , has a streamlined body for rapid swimming, extracts oxygen from water using gills, has two sets of paired fins, one or two dorsal fins, an anal fin and
6174-412: The swash moves beach material seawards. Under their influence, the sand and shingle on the beach is ground together and abraded. Around high tide, the power of a storm wave impacting on the foot of a cliff has a shattering effect as air in cracks and crevices is compressed and then expands rapidly with release of pressure. At the same time, sand and pebbles have an erosive effect as they are thrown against
6300-420: The water , carbon , and nitrogen cycles . The surface of water interacts with the atmosphere, exchanging properties such as particles and temperature, as well as currents . Surface currents are the water currents that are produced by the atmosphere's currents and its winds blowing over the surface of the water, producing wind waves , setting up through drag slow but stable circulations of water, as in
6426-569: The Austronesian " Lapita " peoples displayed great feats of navigation, reaching out from the Bismarck Archipelago to as far away as Fiji , Tonga , and Samoa . Their descendants continued to travel thousands of miles between tiny islands on outrigger canoes , and in the process they found many new islands, including Hawaii , Easter Island (Rapa Nui), and New Zealand. The Ancient Egyptians and Phoenicians explored
SECTION 50
#17327652396956552-405: The Earth , clarified the process of sedimentation , and assisted the study of volcanism and earthquakes . A characteristic of seawater is that it is salty. Salinity is usually measured in parts per thousand ( ‰ or per mil), and the open ocean has about 35 grams (1.2 oz) solids per litre, a salinity of 35 ‰. The Mediterranean Sea is slightly higher at 38 ‰, while the salinity of
6678-420: The Earth is closest to the Moon because it is also where the effect of the Moon's gravity is stronger. On the opposite side of the Earth, the lunar force is at its weakest and this causes another bulge to form. As the Moon rotates around the Earth, so do these ocean bulges move around the Earth. The gravitational attraction of the Sun is also working on the seas, but its effect on tides is less powerful than that of
6804-411: The Earth to make a complete revolution and return the Moon to its previous position relative to an observer. The Moon's mass is some 27 million times smaller than the Sun, but it is 400 times closer to the Earth. Tidal force or tide-raising force decreases rapidly with distance, so the moon has more than twice as great an effect on tides as the Sun. A bulge is formed in the ocean at the place where
6930-413: The Moon, and when the Sun, Moon and Earth are all aligned (full moon and new moon), the combined effect results in the high "spring tides". In contrast, when the Sun is at 90° from the Moon as viewed from Earth, the combined gravitational effect on tides is less causing the lower "neap tides". A storm surge can occur when high winds pile water up against the coast in a shallow area and this, coupled with
7056-556: The Southern Hemisphere in the Roaring Forties , long, organised masses of water called swell roll across the ocean. If the wind dies down, the wave formation is reduced, but already-formed waves continue to travel in their original direction until they meet land. The size of the waves depends on the fetch , the distance that the wind has blown over the water and the strength and duration of that wind. When waves meet others coming from different directions, interference between
7182-601: The Southern Ocean, including under the Filchner–Ronne Ice Shelf at a latitude of 79°S, while desert pupfish live in desert springs, streams, and marshes, sometimes highly saline, with water temperatures as high as 36 C. A few fish live mostly on land or lay their eggs on land near water. Mudskippers feed and interact with one another on mudflats and go underwater to hide in their burrows. A single undescribed species of Phreatobius has been called
7308-540: The Spanish Magellan-Elcano expedition which would be the first to sail around the world. Fish A fish ( pl. : fish or fishes ) is an aquatic , anamniotic , gill -bearing vertebrate animal with swimming fins and a hard skull , but lacking limbs with digits . Fish can be grouped into the more basal jawless fish and the more common jawed fish , the latter including all living cartilaginous and bony fish , as well as
7434-517: The added CO 2 is absorbed by the oceans, forming carbonic acid and lowering the pH (now below 8.1 ) through a process called ocean acidification . The extent of further ocean chemistry changes, including ocean pH, will depend on climate change mitigation efforts taken by nations and their governments. The amount of oxygen found in seawater depends primarily on the plants growing in it. These are mainly algae, including phytoplankton , with some vascular plants such as seagrasses . In daylight,
7560-556: The attention of the cleaners. Cleaning behaviors have been observed in a number of fish groups, including an interesting case between two cichlids of the same genus, Etroplus maculatus , the cleaner, and the much larger E. suratensis . Fish occupy many trophic levels in freshwater and marine food webs . Fish at the higher levels are predatory , and a substantial part of their prey consists of other fish. In addition, mammals such as dolphins and seals feed on fish, alongside birds such as gannets and cormorants . The body of
7686-411: The behavior of elements and molecules within the oceans: particularly, at the moment, the ocean's role in the carbon cycle and carbon dioxide 's role in the increasing acidification of seawater. Marine and maritime geography charts the shape and shaping of the sea, while marine geology (geological oceanography) has provided evidence of continental drift and the composition and structure of
SECTION 60
#17327652396957812-550: The body, and produce a concentrated urine. The reverse happens in freshwater fish : they tend to gain water osmotically, and produce a dilute urine. Some fish have kidneys able to operate in both freshwater and saltwater. Fish have small brains relative to body size compared with other vertebrates, typically one-fifteenth the brain mass of a similarly sized bird or mammal. However, some fish have relatively large brains, notably mormyrids and sharks , which have brains about as large for their body weight as birds and marsupials . At
7938-529: The bottom of the ocean provides food for an assembly of organisms which similarly rely largely on the actions of sulphur-reducing bacteria. Such places support unique biomes where many new microbes and other lifeforms have been discovered. Humans have travelled the seas since they first built sea-going craft. Mesopotamians were using bitumen to caulk their reed boats and, a little later, masted sails . By c. 3000 BC, Austronesians on Taiwan had begun spreading into maritime Southeast Asia . Subsequently,
8064-467: The boundaries between the land and sea, the slightly denser oceanic plates slide beneath the continental plates and more subduction trenches are formed. As they grate together, the continental plates are deformed and buckle causing mountain building and seismic activity. The Earth's deepest trench is the Mariana Trench which extends for about 2,500 kilometres (1,600 miles) across the seabed. It
8190-516: The case of the ocean sustaining deep-sea ocean currents . Deep-sea currents, known together as the global conveyor belt , carry cold water from near the poles to every ocean and significantly influence Earth's climate. Tides , the generally twice-daily rise and fall of sea levels , are caused by Earth's rotation and the gravitational effects of the Moon and, to a lesser extent, of the Sun . Tides may have
8316-437: The coast scour out channels and transport sand and pebbles away from their place of origin. Sediment carried to the sea by rivers settles on the seabed causing deltas to form in estuaries. All these materials move back and forth under the influence of waves, tides and currents. Dredging removes material and deepens channels but may have unexpected effects elsewhere on the coastline. Governments make efforts to prevent flooding of
8442-446: The composition and hardness of the coastal rock, the inclination of the off-shore slope and the changes of the level of the land due to local uplift or submergence. Normally, waves roll towards the shore at the rate of six to eight per minute and these are known as constructive waves as they tend to move material up the beach and have little erosive effect. Storm waves arrive on shore in rapid succession and are known as destructive waves as
8568-598: The continental shelf. Alternatively, marine habitats can be divided vertically into pelagic (open water), demersal (just above the seabed) and benthic (sea bottom) habitats. A third division is by latitude : from polar seas with ice shelves, sea ice and icebergs, to temperate and tropical waters. Coral reefs, the so-called "rainforests of the sea", occupy less than 0.1 percent of the world's ocean surface, yet their ecosystems include 25 percent of all marine species. The best-known are tropical coral reefs such as Australia's Great Barrier Reef , but cold water reefs harbour
8694-522: The currents. Most of the smallest organisms are the larvae of fish and marine invertebrates which liberate eggs in vast numbers because the chance of any one embryo surviving to maturity is so minute. The zooplankton feed on phytoplankton and on each other and form a basic part of the complex food chain that extends through variously sized fish and other nektonic organisms to large squid , sharks , porpoises , dolphins and whales . Some marine creatures make large migrations, either to other regions of
8820-410: The destruction may be caused by the flood water draining back into the sea after the tsunami has struck, dragging debris and people with it. Often several tsunami are caused by a single geological event and arrive at intervals of between eight minutes and two hours. The first wave to arrive on shore may not be the biggest or most destructive. Wind blowing over the surface of the sea causes friction at
8946-413: The direction of the wind. The friction between air and water caused by a gentle breeze on a pond causes ripples to form. A strong blow over the ocean causes larger waves as the moving air pushes against the raised ridges of water. The waves reach their maximum height when the rate at which they are travelling nearly matches the speed of the wind. In open water, when the wind blows continuously as happens in
9072-571: The early fifteenth century, sailing the Indian and Pacific Oceans. In the late fifteenth century, Western European mariners started making longer voyages of exploration in search of trade. Bartolomeu Dias rounded the Cape of Good Hope in 1487 and Vasco da Gama reached India via the Cape in 1498. Christopher Columbus sailed from Cadiz in 1492, attempting to reach the eastern lands of India and Japan by
9198-570: The exact root is unknown; some authorities reconstruct a Proto-Indo-European root * peysk- , attested only in Italic , Celtic , and Germanic . About 530 million years ago during the Cambrian explosion , fishlike animals with a notochord and eyes at the front of the body, such as Haikouichthys , appear in the fossil record . During the late Cambrian , other jawless forms such as conodonts appear. Jawed vertebrates appear in
9324-445: The extinct placoderms and acanthodians . Most fish are cold-blooded , their body temperature varying with the surrounding water, though some large active swimmers like white shark and tuna can hold a higher core temperature . Many fish can communicate acoustically with each other, such as during courtship displays . The earliest fish appeared during the Cambrian as small filter feeders ; they continued to evolve through
9450-422: The food chain or precipitated into the deeper, more carbon-rich layers as dead soft tissue or in shells and bones as calcium carbonate . It circulates in this layer for long periods of time before either being deposited as sediment or being returned to surface waters through thermohaline circulation. The oceans are home to a diverse collection of life forms that use it as a habitat. Since sunlight illuminates only
9576-405: The front of the brain are the olfactory lobes , a pair of structures that receive and process signals from the nostrils via the two olfactory nerves . Fish that hunt primarily by smell, such as hagfish and sharks, have very large olfactory lobes. Behind these is the telencephalon , which in fish deals mostly with olfaction. Together these structures form the forebrain. Connecting the forebrain to
9702-580: The gap and a circular movement of surface currents known as a gyre is formed. There are five main gyres in the world's oceans: two in the Pacific, two in the Atlantic and one in the Indian Ocean. Other smaller gyres are found in lesser seas and a single gyre flows around Antarctica . These gyres have followed the same routes for millennia, guided by the topography of the land, the wind direction and
9828-412: The gills flows in the opposite direction to the water, resulting in efficient countercurrent exchange . The gills push the oxygen-poor water out through openings in the sides of the pharynx. Cartilaginous fish have multiple gill openings: sharks usually have five, sometimes six or seven pairs; they often have to swim to oxygenate their gills. Bony fish have a single gill opening on each side, hidden beneath
9954-532: The gills. Oxygen-rich blood then flows without further pumping, unlike in mammals, to the body tissues. Finally, oxygen-depleted blood returns to the heart. Fish exchange gases using gills on either side of the pharynx . Gills consist of comblike structures called filaments. Each filament contains a capillary network that provides a large surface area for exchanging oxygen and carbon dioxide . Fish exchange gases by pulling oxygen-rich water through their mouths and pumping it over their gills. Capillary blood in
10080-570: The globe. Seawater with a typical salinity of 35 ‰ has a freezing point of about −1.8 °C (28.8 °F). When its temperature becomes low enough, ice crystals form on the surface. These break into small pieces and coalesce into flat discs that form a thick suspension known as frazil . In calm conditions, this freezes into a thin flat sheet known as nilas , which thickens as new ice forms on its underside. In more turbulent seas, frazil crystals join into flat discs known as pancakes. These slide under each other and coalesce to form floes . In
10206-447: The gut, leading from the mouth to the anus. The mouth of most fishes contains teeth to grip prey, bite off or scrape plant material, or crush the food. An esophagus carries food to the stomach where it may be stored and partially digested. A sphincter, the pylorus, releases food to the intestine at intervals. Many fish have finger-shaped pouches, pyloric caeca , around the pylorus, of doubtful function. The pancreas secretes enzymes into
10332-457: The heart from the rest of the body. Lungfish, bichirs, ropefish, bowfins, snakefish, and the African knifefish have evolved to reduce such mixing, and to reduce oxygen loss from the gills to oxygen-poor water. Bichirs and lungfish have tetrapod-like paired lungs, requiring them to surface to gulp air, and making them obligate air breathers. Many other fish, including inhabitants of rock pools and
10458-442: The interface between air and sea. Not only does this cause waves to form, but it also makes the surface seawater move in the same direction as the wind. Although winds are variable, in any one place they predominantly blow from a single direction and thus a surface current can be formed. Westerly winds are most frequent in the mid-latitudes while easterlies dominate the tropics. When water moves in this way, other water flows in to fill
10584-455: The intestine to digest the food; other enzymes are secreted directly by the intestine itself. The liver produces bile which helps to break up fat into an emulsion which can be absorbed in the intestine. Most fish release their nitrogenous wastes as ammonia . This may be excreted through the gills or filtered by the kidneys . Salt is excreted by the rectal gland. Saltwater fish tend to lose water by osmosis ; their kidneys return water to
10710-547: The land by the building of breakwaters , seawalls , dykes and levees and other sea defences. For instance, the Thames Barrier is designed to protect London from a storm surge, while the failure of the dykes and levees around New Orleans during Hurricane Katrina created a humanitarian crisis in the United States. The sea plays a part in the water or hydrological cycle , in which water evaporates from
10836-523: The land to breed but fish, cetaceans and sea snakes have a completely aquatic lifestyle and many invertebrate phyla are entirely marine. In fact, the oceans teem with life and provide many varying microhabitats. One of these is the surface film which, even though tossed about by the movement of waves, provides a rich environment and is home to bacteria, fungi , microalgae , protozoa , fish eggs and various larvae. The pelagic zone contains macro- and microfauna and myriad zooplankton which drift with
10962-648: The levels of salinity in different seas, the relative composition of the dissolved salts is stable throughout the world's oceans. Seawater is too saline for humans to drink safely, as the kidneys cannot excrete urine as salty as seawater. Although the amount of salt in the ocean remains relatively constant within the scale of millions of years, various factors affect the salinity of a body of water. Evaporation and by-product of ice formation (known as "brine rejection") increase salinity, whereas precipitation , sea ice melt, and runoff from land reduce it. The Baltic Sea , for example, has many rivers flowing into it, and thus
11088-508: The mantle tend to drive the two plates apart. Parallel to these ridges and nearer the coasts, one oceanic plate may slide beneath another oceanic plate in a process known as subduction . Deep trenches are formed here and the process is accompanied by friction as the plates grind together. The movement proceeds in jerks which cause earthquakes, heat is produced and magma is forced up creating underwater mountains, some of which may form chains of volcanic islands near to deep trenches. Near some of
11214-419: The midbrain is the diencephalon ; it works with hormones and homeostasis . The pineal body is just above the diencephalon; it detects light, maintains circadian rhythms, and controls color changes. The midbrain contains the two optic lobes . These are very large in species that hunt by sight, such as rainbow trout and cichlids . The hindbrain controls swimming and balance.The single-lobed cerebellum
11340-416: The minimum "low tide" level. As the water recedes, it uncovers more and more of the foreshore , also known as the intertidal zone. The difference in height between the high tide and low tide is known as the tidal range or tidal amplitude. Most places experience two high tides each day, occurring at intervals of about 12 hours and 25 minutes. This is half the 24 hours and 50 minute period that it takes for
11466-443: The nine largest families; from largest to smallest, these are Cyprinidae , Gobiidae , Cichlidae , Characidae , Loricariidae , Balitoridae , Serranidae , Labridae , and Scorpaenidae . About 64 families are monotypic , containing only one species. Fish range in size from the huge 16-metre (52 ft) whale shark to some tiny teleosts only 8-millimetre (0.3 in) long, such as the cyprinid Paedocypris progenetica and
11592-539: The northern Red Sea can reach 41‰. In contrast, some landlocked hypersaline lakes have a much higher salinity, for example, the Dead Sea has 300 grams (11 oz) dissolved solids per litre (300 ‰). While the constituents of table salt ( sodium and chloride ) make up about 85 percent of the solids in solution, there are also other metal ions such as magnesium and calcium , and negative ions including sulphate , carbonate , and bromide . Despite variations in
11718-730: The novel means of travelling westwards. He made landfall instead on an island in the Caribbean Sea and a few years later, the Venetian navigator John Cabot reached Newfoundland . The Italian Amerigo Vespucci , after whom America was named, explored the South American coastline in voyages made between 1497 and 1502, discovering the mouth of the Amazon River . In 1519 the Portuguese navigator Ferdinand Magellan led
11844-484: The ocean depths caused by the movement of deep water masses. A main deep ocean current flows through all the world's oceans and is known as the thermohaline circulation or global conveyor belt. This movement is slow and is driven by differences in density of the water caused by variations in salinity and temperature. At high latitudes the water is chilled by the low atmospheric temperature and becomes saltier as sea ice crystallizes out. Both these factors make it denser, and
11970-440: The ocean on a seasonal basis or vertical migrations daily, often ascending to feed at night and descending to safety by day. Ships can introduce or spread invasive species through the discharge of ballast water or the transport of organisms that have accumulated as part of the fouling community on the hulls of vessels. The demersal zone supports many animals that feed on benthic organisms or seek protection from predators and
12096-471: The ocean, travels through the atmosphere as vapour, condenses , falls as rain or snow , thereby sustaining life on land, and largely returns to the sea. Even in the Atacama Desert , where little rain ever falls, dense clouds of fog known as the camanchaca blow in from the sea and support plant life. In central Asia and other large land masses, there are endorheic basins which have no outlet to
12222-500: The ocean. Fish are caught for recreation , or raised by fishkeepers as ornaments for private and public exhibition in aquaria and garden ponds . Fish have had a role in human culture through the ages, serving as deities , religious symbols, and as the subjects of art, books and movies. The word fish is inherited from Proto-Germanic , and is related to German Fisch , the Latin piscis and Old Irish īasc , though
12348-423: The open pelagic zone. The organisms living in the sea range from whales 30 metres (98 feet) long to microscopic phytoplankton and zooplankton , fungi, and bacteria. Marine life plays an important part in the carbon cycle as photosynthetic organisms convert dissolved carbon dioxide into organic carbon and it is economically important to humans for providing fish for use as food. Life may have originated in
12474-402: The open ocean. Because the water all around is the same colour, reflecting an image of the water offers near-invisibility. Fish have a closed-loop circulatory system . The heart pumps the blood in a single loop throughout the body; for comparison, the mammal heart has two loops, one for the lungs to pick up oxygen, one for the body to deliver the oxygen. In fish, the heart pumps blood through
12600-463: The presence of swim bladders and later ossified endoskeletons , emerged as the dominant group of fish after the end-Devonian extinction wiped out the apex placoderms. Bony fish are further divided into the lobe-finned and ray-finned fish . About 96% of all living fish species today are teleosts , a crown group of ray-finned fish that can protrude their jaws . The tetrapods , a mostly terrestrial clade of vertebrates that have dominated
12726-430: The process of freezing, salt water and air are trapped between the ice crystals. Nilas may have a salinity of 12–15 ‰, but by the time the sea ice is one year old, this falls to 4–6 ‰. Seawater is slightly alkaline and had an average pH of about 8.2 over the past 300 million years. More recently, climate change has resulted in an increase of the carbon dioxide content of the atmosphere; about 30–40% of
12852-418: The quality of the inflowing water. Oceans contain the greatest quantity of actively cycled carbon in the world and are second only to the lithosphere in the amount of carbon they store. The oceans' surface layer holds large amounts of dissolved organic carbon that is exchanged rapidly with the atmosphere. The deep layer's concentration of dissolved inorganic carbon is about 15 percent higher than that of
12978-417: The rocks. This tends to undercut the cliff, and normal weathering processes such as the action of frost follows, causing further destruction. Gradually, a wave-cut platform develops at the foot of the cliff and this has a protective effect, reducing further wave-erosion. Material worn from the margins of the land eventually ends up in the sea. Here it is subject to attrition as currents flowing parallel to
13104-409: The same way as a wind-generated wave in shallow water but on a vastly greater scale. Either the trough or the crest of a tsunami can arrive at the coast first. In the former case, the sea draws back and leaves subtidal areas close to the shore exposed which provides a useful warning for people on land. When the crest arrives, it does not usually break but rushes inland, flooding all in its path. Much of
13230-479: The sea and all the major groups of animals are represented there. Scientists differ as to precisely where in the sea life arose: the Miller-Urey experiments suggested a dilute chemical "soup" in open water, but more recent suggestions include volcanic hot springs, fine-grained clay sediments, or deep-sea " black smoker " vents, all of which would have provided protection from damaging ultraviolet radiation which
13356-436: The sea could be considered as brackish . Meanwhile, the Red Sea is very salty due to its high evaporation rate. Sea temperature depends on the amount of solar radiation falling on its surface. In the tropics, with the sun nearly overhead, the temperature of the surface layers can rise to over 30 °C (86 °F) while near the poles the temperature in equilibrium with the sea ice is about −2 °C (28 °F). There
13482-537: The sea level has been higher than it is today. The main factor affecting sea level over time is the result of changes in the oceanic crust, with a downward trend expected to continue in the very long term. At the last glacial maximum , some 20,000 years ago, the sea level was about 125 metres (410 ft) lower than in present times (2012). For at least the last 100 years, sea level has been rising at an average rate of about 1.8 millimetres (0.071 in) per year. Most of this rise can be attributed to an increase in
13608-399: The sea under the influence of gravity. A tsunami is an unusual form of wave caused by an infrequent powerful event such as an underwater earthquake or landslide, a meteorite impact, a volcanic eruption or a collapse of land into the sea. These events can temporarily lift or lower the surface of the sea in the affected area, usually by a few feet. The potential energy of the displaced seawater
13734-671: The sea, separated from the ocean by mountains or other natural geologic features that prevent the water draining away. The Caspian Sea is the largest one of these. Its main inflow is from the River Volga , there is no outflow and the evaporation of water makes it saline as dissolved minerals accumulate. The Aral Sea in Kazakhstan and Uzbekistan, and Pyramid Lake in the western United States are further examples of large, inland saline water-bodies without drainage. Some endorheic lakes are less salty, but all are sensitive to variations in
13860-657: The seabed provides a range of habitats on or under the surface of the substrate which are used by creatures adapted to these conditions. The tidal zone with its periodic exposure to the dehydrating air is home to barnacles , molluscs and crustaceans . The neritic zone has many organisms that need light to flourish. Here, among algal-encrusted rocks live sponges , echinoderms , polychaete worms, sea anemones and other invertebrates. Corals often contain photosynthetic symbionts and live in shallow waters where light penetrates. The extensive calcareous skeletons they extrude build up into coral reefs which are an important feature of
13986-427: The seabed. These provide a biodiverse habitat for reef-dwelling organisms. There is less sea life on the floor of deeper seas but marine life also flourishes around seamounts that rise from the depths, where fish and other animals congregate to spawn and feed. Close to the seabed live demersal fish that feed largely on pelagic organisms or benthic invertebrates. Exploration of the deep sea by submersibles revealed
14112-481: The seas, which offers a wide range of marine habitats and ecosystems , ranging vertically from the sunlit surface and shoreline to the great depths and pressures of the cold, dark abyssal zone , and in latitude from the cold waters under polar ice caps to the warm waters of coral reefs in tropical regions . Many of the major groups of organisms evolved in the sea and life may have started there. The ocean moderates Earth's climate and has important roles in
14238-418: The slope under the effect of gravity. The larger the breaking waves, the longer the beach and the more oblique the wave approach, the stronger is the longshore current. These currents can shift great volumes of sand or pebbles, create spits and make beaches disappear and water channels silt up. A rip current can occur when water piles up near the shore from advancing waves and is funnelled out to sea through
14364-429: The sun, the weather conditions and the turbidity of the water. Much light gets reflected at the surface, and red light gets absorbed in the top few metres. Yellow and green light reach greater depths, and blue and violet light may penetrate as deep as 1,000 metres (3,300 ft). There is insufficient light for photosynthesis and plant growth beyond a depth of about 200 metres (660 ft). Over most of geologic time,
14490-430: The surface and loops back on itself. It takes a thousand years for this circulation pattern to be completed. Besides gyres, there are temporary surface currents that occur under specific conditions. When waves meet a shore at an angle, a longshore current is created as water is pushed along parallel to the coastline. The water swirls up onto the beach at right angles to the approaching waves but drains away straight down
14616-495: The surface and the mouths of large rivers and higher in the depths of the ocean; however, the relative proportions of dissolved salts vary little across the oceans. The most abundant solid dissolved in seawater is sodium chloride . The water also contains salts of magnesium , calcium , potassium , and mercury , amongst many other elements, some in minute concentrations. A wide variety of organisms , including bacteria , protists , algae , plants, fungi , and animals live in
14742-473: The surface layer and it remains there for much longer periods of time. Thermohaline circulation exchanges carbon between these two layers. Carbon enters the ocean as atmospheric carbon dioxide dissolves in the surface layers and is converted into carbonic acid , carbonate , and bicarbonate : It can also enter through rivers as dissolved organic carbon and is converted by photosynthetic organisms into organic carbon. This can either be exchanged throughout
14868-415: The taxon Osteichthyes as a clade, which now includes all tetrapods". The biodiversity of extant fish is unevenly distributed among the various groups; teleosts , bony fishes able to protrude their jaws , make up 96% of fish species. The cladogram shows the evolutionary relationships of all groups of living fishes (with their respective diversity ) and the tetrapods. Extinct groups are marked with
14994-453: The temperature of the sea due to climate change , and the resulting slight thermal expansion of the upper 500 metres (1,600 ft) of water. Additional contributions, as much as one quarter of the total, come from water sources on land, such as melting snow and glaciers and extraction of groundwater for irrigation and other agricultural and human needs. Wind blowing over the surface of a body of water forms waves that are perpendicular to
15120-702: The top trophic levels in both aquatic and terrestrial ecosystems since the Late Paleozoic , evolved from lobe-finned fish during the Carboniferous , developing air-breathing lungs homologous to swim bladders. Despite the cladistic lineage, tetrapods are usually not considered to be fish, making "fish" a paraphyletic group. Fish have been an important natural resource for humans since prehistoric times, especially as food . Commercial and subsistence fishers harvest fish in wild fisheries or farm them in ponds or in breeding cages in
15246-432: The two can produce broken, irregular seas. Constructive interference can cause individual (unexpected) rogue waves much higher than normal. Most waves are less than 3 m (10 ft) high and it is not unusual for strong storms to double or triple that height; offshore construction such as wind farms and oil platforms use metocean statistics from measurements in computing the wave forces (due to for instance
15372-431: The upper layers, the major part of the ocean exists in permanent darkness. As the different depth and temperature zones each provide habitat for a unique set of species, the marine environment as a whole encompasses an immense diversity of life. Marine habitats range from surface water to the deepest oceanic trenches , including coral reefs, kelp forests , seagrass meadows , tidepools , muddy, sandy and rocky seabeds, and
15498-460: The water sinks. From the deep sea near Greenland, such water flows southwards between the continental landmasses on either side of the Atlantic. When it reaches the Antarctic, it is joined by further masses of cold, sinking water and flows eastwards. It then splits into two streams that move northwards into the Indian and Pacific Oceans. Here it is gradually warmed, becomes less dense, rises towards
15624-426: The wave reaches a point where its deepest oscillations of the water contact the seabed , they begin to slow down. This pulls the crests closer together and increases the waves' height , which is called wave shoaling . When the ratio of the wave's height to the water depth increases above a certain limit, it " breaks ", toppling over in a mass of foaming water. This rushes in a sheet up the beach before retreating into
15750-445: The world ocean, so global climate modelling makes use of ocean circulation models as well as models of other major components such as the atmosphere , land surfaces, aerosols and sea ice. Ocean models make use of a branch of physics, geophysical fluid dynamics , that describes the large-scale flow of fluids such as seawater. Surface currents only affect the top few hundred metres of the sea, but there are also large-scale flows in
15876-407: Was not blocked by the early Earth's atmosphere. Marine habitats can be divided horizontally into coastal and open ocean habitats. Coastal habitats extend from the shoreline to the edge of the continental shelf . Most marine life is found in coastal habitats, even though the shelf area occupies only 7 percent of the total ocean area. Open ocean habitats are found in the deep ocean beyond the edge of
#694305