Sea

Coastal sea waves at Paracas National Reserve, Ica, Peru
Animated map exhibiting the world's oceanic waters. A continuous body of water encircling Earth, the World Ocean is divided into a number of principal areas with relatively uninhibited interchange among them. Five oceanic divisions are usually defined: Pacific, Atlantic, Indian, Arctic, and Southern; the last two listed are sometimes consolidated into the first three.
Marginal seas as defined by the International Maritime Organization
Composite images of the Earth created by NASA in 2001
Salinity map taken from the Aquarius Spacecraft. The rainbow colours represent salinity levels: red = 40 ‰, purple = 30 ‰
When the wave enters shallow water, it slows down and its amplitude (height) increases.
The 2004 tsunami in Thailand
Surface currents: red–warm, blue–cold
The global conveyor belt shown in blue with warmer surface currents in red
High tides (blue) at the nearest and furthest points of the Earth from the Moon
Three types of plate boundary
Praia da Marinha in Algarve, Portugal
The Baltic Sea in the archipelago of Turku, Finland
Coral reefs are among the most biodiverse habitats in the world.
A thornback cowfish
Map showing the seaborne migration and expansion of the Austronesians beginning at around 3000 BC
Gerardus Mercator's 1569 world map. The coastline of the old world is quite accurately depicted, unlike that of the Americas. Regions in high latitudes (Arctic, Antarctic) are greatly enlarged on this projection.
Naval warfare: The explosion of the Spanish flagship during the Battle of Gibraltar, 25 April 1607 by Cornelis Claesz van Wieringen, formerly attributed to Hendrik Cornelisz Vroom
Shipping routes, showing relative density of commercial shipping around the world
German factory ship, 92 m long
Fishing boat in Sri Lanka
Scuba diver with face mask, fins and underwater breathing apparatus
Tidal power: the 1 km Rance Tidal Power Station in Brittany generates 0.5 GW.
Minerals precipitated near a hydrothermal vent
Reverse osmosis desalination plant
Great wave off the coast of Kanagawa by Katsushika Hokusai, c. 1830
Dutch Golden Age painting: The Y at Amsterdam, seen from the Mosselsteiger (mussel pier) by Ludolf Bakhuizen, 1673
The Oceanids (The Naiads of the Sea), a painting by Gustave Doré (c. 1860)

This article focuses on human experience, history and culture of the collective seas of Earth.

- Sea

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List of seas

List of seas of the World Ocean, including marginal seas, areas of water, various gulfs, bights, bays, and straits.

Marginal seas as defined by the International Maritime Organization
The Norwegian Sea
Aegean, Adriatic, Ionian, and Tyrrhenian seas
The Irish Sea
The Arabian Sea as a marginal sea of the Indian Ocean.
Coral Sea

Sea has several definitions:

Body of water

Any significant accumulation of water on the surface of Earth or another planet.

The Aubach (Wiehl) in Germany (Watercourse)
A fjord (Lysefjord) in Norway
River Gambia, Niokolokoba National Park
Port Jackson, Sydney, New South Wales
The Canal Grande in Venice, one of the major water-traffic corridors in the city. View from the Accademia bridge.
A tide pool in Santa Cruz, California with sea anemones and sea stars
A weir in Toledo, Spain. Weirs are frequently used to change the height of a riverlevel, prevent floodings, and measure water discharge.

The term most often refers to oceans, seas, and lakes, but it includes smaller pools of water such as ponds, wetlands, or more rarely, puddles.

Oceanography

Scientific study of the oceans.

Thermohaline circulation
Map of the Gulf Stream by Benjamin Franklin, 1769–1770. Courtesy of the NOAA Photo Library.
1799 map of the currents in the Atlantic and Indian Oceans, by James Rennell
undertook the first global marine research expedition in 1872.
Ocean currents (1911)
Writer and geographer John Francon Williams FRGS commemorative plaque, Clackmannan Cemetery 2019
Oceanographic frontal systems on the Southern Hemisphere
The Applied Marine Physics Building at the University of Miami's Rosenstiel School of Marine and Atmospheric Science on Virginia Key, September 2007
Oceanographic Museum

Humans first acquired knowledge of the waves and currents of the seas and oceans in pre-historic times.

Marine life

General characteristics of a large marine ecosystem (Gulf of Alaska)
Jupiter's moon Europa may have an underground ocean which supports life.
Phylogenetic and symbiogenetic tree of living organisms, showing a view of the origins of eukaryotes and prokaryotes
The range of sizes shown by prokaryotes (bacteria and archaea) and viruses relative to those of other organisms and biomolecules
Sea spray containing marine microorganisms can be swept high into the atmosphere where they become aeroplankton, and can travel the globe before falling back to earth.
Under a magnifier, a splash of seawater teems with life.
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Vibrio vulnificus, a virulent bacterium found in estuaries and along coastal areas
Pelagibacter ubique, the most abundant bacteria in the ocean, plays a major role in the global carbon cycle.
Archaea were initially viewed as extremophiles living in harsh environments, such as the yellow archaea pictured here in a hot spring, but they have since been found in a much broader range of habitats.
Lichen on a rock in a marine splash zone. Lichens are mutualistic associations between a fungus and an alga or cyanobacterium.
A sea snail, Littoraria irrorata, covered in lichen. This snail farms intertidal ascomycetous fungi.
Dickinsonia may be the earliest animal. They appear in the fossil record 571 million to 541 million years ago.
Kimberella, an early mollusc important for understanding the Cambrian explosion. Invertebrates are grouped into different phyla (body plans).
Taxonomic biodiversity of accepted marine species, according to WoRMS, 18 October 2019.
Opabinia, an extinct stem group arthropod appeared in the Middle Cambrian.
Sponges are perhaps the most basal animals. They have no nervous, digestive or circulatory system.
Together with sponges, brilliantly bioluminescent ctenophores (comb jellies) are the most basal animals.
The beroid ctenophore, mouth gaping, preys on other ctenophores.
Cnidarians, like this starlet sea anemone, are the simplest animals to organise cells into tissue. Yet they have the same genes that form the vertebrate (including human) head.
Idealised wormlike bilaterian body plan. With a cylindrical body and a direction of movement the animal has head and tail ends. Sense organs and mouth form the basis of the head. Opposed circular and longitudinal muscles enable peristaltic motion.
Many marine worms are related only distantly, so they form a number of different phyla. The worm shown is an arrow worm, found worldwide as a predatory component of plankton.
First known air-breathing animal to colonise land, the millipede Pneumodesmus newmani, lived in the Early Devonian.
Adult echinoderms have fivefold symmetry but as larvae have bilateral symmetry. This is why they are in the Bilateria.
The lancelet, like all cephalochordates, has a head. Adult lancelets retain the four key features of chordates: a 
notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. Water from the mouth enters the pharyngeal slits, which filter out food particles. The filtered water then collects in the atrium and exits through the atriopore.
In chordates, the four above labelled common features appear at some point during development.
The Tully monster, a strange looking extinct animal with eyes like a hammerhead protruding from its back, may be an early jawless fish.
Guiyu oneiros, the earliest-known bony fish lived during the Late Silurian 419 million years ago.
Lobe fins are bedded into the body by bony stalks. They evolved into the legs of the first tetrapod land vertebrates.
Ray fins have spines (rays) which can be erected to stiffen the fin for better control of swimming performance.
Tiktaalik, an extinct lobe-finned fish, developed limb-like fins that could take it onto land.
Sea otter, a classic keystone species which controls sea urchin numbers
Composite image showing the global distribution of photosynthesis, including both oceanic phytoplankton and terrestrial vegetation. Dark red and blue-green indicate regions of high photosynthetic activity in the ocean and on land, respectively.
Plankton are drifting or floating organisms that cannot swim against a current, and include organisms from most areas of life: bacteria, archaea, algae, protozoa and animals.
The drainage basins of the principal oceans and seas of the world are marked by continental divides. The grey areas are endorheic basins that do not drain to the ocean.
Apparent marine fossil diversity during the Phanerozoic
The marine Thiomargarita namibiensis, the largest known bacterium
Cyanobacteria blooms can contain lethal cyanotoxins.
The chloroplasts of glaucophytes have a peptidoglycan layer, evidence suggesting their endosymbiotic origin from cyanobacteria.<ref name="keeling">{{cite journal|journal =American Journal of Botany|year = 2004|volume = 91|pages = 1481–1493|title = Diversity and evolutionary history of plastids and their hosts| vauthors = Keeling PJ |url = http://www.amjbot.org/cgi/content/full/91/10/1481|doi = 10.3732/ajb.91.10.1481|issue=10|pmid=21652304}}</ref>
Bacteria can be beneficial. This Pompeii worm, an extremophile found only at hydrothermal vents, has a protective cover of bacteria.
Halobacteria, found in water near saturated with salt, are now recognised as archaea.
Flat, square-shaped cells of the archaea Haloquadratum walsbyi
Methanosarcina barkeri, a marine archaea that produces methane
Thermophiles, such as Pyrolobus fumarii, survive well over 100 °C.
Drawing of another marine thermophile, Pyrococcus furiosus
Diatoms are a major algae group generating about 20% of world oxygen production.<ref name="livescience.com">{{cite web | vauthors = Alverson A | date = 11 June 2014 | work = Live Science | url = https://www.livescience.com/46250-teasing-apart-the-diatom-genome.html | title = The Air You're Breathing? A Diatom Made That }}</ref>
Diatoms have glass like cell walls made of silica and called frustules.<ref>{{cite web | title=More on Diatoms | website=University of California Museum of Paleontology | url=http://www.ucmp.berkeley.edu/chromista/diatoms/diatommm.html | access-date=27 June 2019 | archive-url=https://web.archive.org/web/20121004130024/http://www.ucmp.berkeley.edu/chromista/diatoms/diatommm.html | archive-date=4 October 2012 | url-status=dead }}</ref>
Fossil diatom frustule from 32 to 40 mya
Radiolarian
Single-celled alga, Gephyrocapsa oceanica
Two dinoflagellates
Zooxanthellae is a photosynthetic algae that lives inside hosts like coral.
A single-celled ciliate with green zoochlorellae living inside endosymbiotically.
Euglenoid
This ciliate is digesting cyanobacteria. The cytostome or mouth is at the bottom right.
The single-celled giant amoeba has up to 1000 nuclei and reaches lengths of 5 mm.
Gromia sphaerica is a large spherical testate amoeba which makes mud trails. Its diameter is up to 3.8 cm.<ref name="Matz2008">{{cite journal | vauthors = Matz MV, Frank TM, Marshall NJ, Widder EA, Johnsen S | title = Giant deep-sea protist produces bilaterian-like traces | journal = Current Biology | volume = 18 | issue = 23 | pages = 1849–54 | date = December 2008 | pmid = 19026540 | doi = 10.1016/j.cub.2008.10.028 | publisher = Elsevier Ltd | s2cid = 8819675 }}</ref>
Spiculosiphon oceana, a unicellular foraminiferan with an appearance and lifestyle that mimics a sponge, grows to 5 cm long.
The xenophyophore, another single-celled foraminiferan, lives in abyssal zones. It has a giant shell up to 20 cm across.<ref>{{Cite journal| vauthors = Gooday AJ, Da Silva AA, Pawlowski J |date=2011-12-01|title=Xenophyophores (Rhizaria, Foraminifera) from the Nazaré Canyon (Portuguese margin, NE Atlantic)|journal=Deep-Sea Research Part II: Topical Studies in Oceanography|series=The Geology, Geochemistry, and Biology of Submarine Canyons West of Portugal|volume=58|issue=23–24|pages=2401–2419|doi=10.1016/j.dsr2.2011.04.005|bibcode=2011DSRII..58.2401G}}</ref>
Giant kelp, a brown algae, is not a true plant, yet it is multicellular and can grow to 50m.
Over 10,000 marine species are copepods, small, often microscopic crustaceans
Darkfield photo of a gastrotrich, a worm-like animal living between sediment particles
Armoured Pliciloricus enigmaticus, about 0.2 mm long, live in spaces between marine gravel.
Drawing of a tardigrade (water bear) on a grain of sand
Rotifers, usually 0.1–0.5 mm long, may look like protists but have many cells and belongs to the Animalia.
Sponge biodiversity. There are four sponge species in this photo.
Branching vase sponge
Venus' flower basket at a depth of 2572 meters
Barrel sponge
The long-living Monorhaphis chuni
Light diffracting along the comb rows of a cydippid, left tentacle deployed, right retracted
Deep-sea ctenophore trailing tentacles studded with tentilla (sub-tentacles)
Egg-shaped cydippid ctenophore
Group of small benthic creeping comb jellies streaming tentacles and living symbiotically on a starfish.
Lobata sp. with paired thick lobes
The sea walnut has a transient anus which forms only when it needs to defecate.<ref>{{cite magazine| title=Animal with an anus that comes and goes could reveal how ours evolved| vauthors = Le Page M | magazine=New Scientist| url=https://www.newscientist.com/article/2195656-animal-with-an-anus-that-comes-and-goes-could-reveal-how-ours-evolved/|  date=March 2019}}</ref>
Sea anemones are common in tidepools.
Their tentacles sting and paralyse small fish.
Close up of polyps on the surface of a coral, waving their tentacles.
If an island sinks below the sea, coral growth can keep up with rising water and form an atoll.
The mantle of the red paper lantern jellyfish crumples and expands like a paper lantern.<ref>{{cite web|title=Red Paper Lantern Jellyfish|url=http://www.realmonstrosities.com/2012/04/red-paper-lantern-jellyfish.html|website=Real Monstrosities|access-date=25 October 2015}}</ref>
The Portuguese man o' war is a colonial siphonophore
Marrus orthocanna another colonial siphonophore, assembled from two types of zooids.
Porpita porpita consists of a colony of hydroids<ref name="Blue Button">{{cite web | url = http://www.beachhunter.net/thingstoknow/jellyfish/blue-button-jellyfish.htm | title = Blue Buttons in Florida | work = BeachHunter.net }}</ref>
Lion's mane jellyfish, largest known jellyfish<ref>{{cite book | vauthors = Karleskint G, Turner R, Small J | date = 2012 | url = https://books.google.com/books?id=uBXTCQAAQBAJ&q=%22lion%27s+mane+jellyfish%22+%22Introduction+to+Marine+Biology%22&pg=PA445 | title = Introduction to Marine Biology | publisher = Cengage Learning | edition = 4th | page = 445 | isbn = 978-1-133-36446-7 }}</ref>
Turritopsis dohrnii achieves biological immortality by transferring its cells back to childhood.<ref>{{cite journal| vauthors = Bavestrello G, Sommer C, Sarà M |year=1992|title=Bi-directional conversion in Turritopsis nutricula (Hydrozoa)|journal=Scientia Marina|volume=56|issue=2–3|pages=137–140}}</ref><ref name="piraino-96">{{cite journal| vauthors = Piraino S, Boero F, Aeschbach B, Schmid V |year=1996|title=Reversing the life cycle: medusae transforming into polyps and cell transdifferentiation in Turritopsis nutricula (Cnidaria, Hydrozoa)|journal=Biological Bulletin|volume=190|issue=3|pages=302–312|doi=10.2307/1543022|pmid=29227703|jstor=1543022|s2cid=3956265|url=https://semanticscholar.org/paper/a5d7d34b1d9c09dc5747a8b45b89451b94b18ff1}}</ref>
The sea wasp is the most lethal jellyfish in the world.<ref name="Fenner1996">{{Cite journal|vauthors=Fenner PJ, Williamson JA |title=Worldwide deaths and severe envenomation from jellyfish stings |journal=The Medical Journal of Australia |volume=165 |issue=11–12 |pages=658–61 |year=1996 |pmid=8985452 |doi=10.5694/j.1326-5377.1996.tb138679.x |s2cid=45032896 }}</ref>
Giant tube worms cluster around hydrothermal vents.
Nematodes are ubiquitous pseudocoelomates which can parasite marine plants and animals.
Bloodworms are typically found on the bottom of shallow marine waters.
Marine gastropods are sea snails or sea slugs. This nudibranch is a sea slug.
The sea snail Syrinx aruanus has a shell up to 91 cm long, the largest of any living gastropod.
Molluscs usually have eyes. Bordering the edge of the mantle of a scallop, a bivalve mollusc, can be over 100 simple eyes.
Common mussel, another bivalve
The nautilus is a living fossil little changed since it evolved 500 million years ago as one of the first cephalopods.<ref name=Callaway2008>{{cite magazine|url=https://www.newscientist.com/article/dn14033-simpleminded-nautilus-reveals-flash-of-memory.html|title=Simple-Minded Nautilus Shows Flash of Memory| vauthors = Callaway E |date=2 June 2008|magazine=New Scientist|access-date=7 March 2012}}</ref><ref name=Phillips2008>{{cite journal|title=Living Fossil Memories| vauthors = Phillips K |date=15 June 2008|page=iii|volume=211|doi=10.1242/jeb.020370|journal=Journal of Experimental Biology|issue=12|s2cid=84279320}}</ref><ref name=Crook2008>{{cite journal| vauthors = Crook R, Basil J |year=2008|title=A biphasic memory curve in the chambered nautilus, Nautilus pompilius L. (Cephalopoda: Nautiloidea)|journal=Journal of Experimental Biology|volume=211|pages=1992–1998|doi=10.1242/jeb.018531|issue=12|pmid=18515730|s2cid=6305526}}</ref>
Reconstruction of an ammonite, a highly successful early cephalopod that appeared 400 mya.
Cephalopods, like this cuttlefish, use their mantle cavity for jet propulsion.
Colossal squid, the largest of all invertebrates<ref name=BBC2008>{{cite news| vauthors = Black R |title=Colossal squid out of the freezer|work=BBC News|date=26 April 2008|url=http://news.bbc.co.uk/1/hi/sci/tech/7367774.stm }}</ref>
Fossil trilobite. Trilobites first appeared about 521 Ma. They were highly successful and were found everywhere in the ocean for 270 Ma.<ref name=firstlife>{{cite web |url=http://firstlifeseries.com/learn-more/ |title= David Attenborough's First Life |access-date=2011-03-10 |url-status=dead |archive-url=https://web.archive.org/web/20110126162514/http://firstlifeseries.com/learn-more/ |archive-date=26 January 2011 |df=dmy-all }}</ref>
The Anomalocaris ("abnormal shrimp") was one of the first apex predators and first appeared about 515 Ma.
The largest known arthropod, the sea scorpion Jaekelopterus rhenaniae, has been found in estuarine strata from about 390 Ma. It was up to {{convert|2.5|m|ft|abbr=on}} long.<ref name=Braddy2007>{{cite journal | vauthors = Braddy SJ, Poschmann M, Tetlie OE | title = Giant claw reveals the largest ever arthropod | journal = Biology Letters | volume = 4 | issue = 1 | pages = 106–9 | date = February 2008 | pmid = 18029297 | pmc = 2412931 | doi = 10.1098/rsbl.2007.0491 }}</ref><ref name=Cressey2007>{{cite journal| vauthors = Daniel C |title=Giant sea scorpion discovered|journal=Nature|date=21 November 2007|access-date=10 June 2013|url=http://www.nature.com/news/2007/071120/full/news.2007.272.html|doi=10.1038/news.2007.272}}</ref>
Horseshoe crabs are living fossils, essentially unchanged for 450 Ma.
Many crustaceans are very small, like this tiny amphipod, and make up a significant part of the ocean's zooplankton.
The Japanese spider crab has the longest leg span of any arthropod, reaching {{convert|5.5|m|ft}} from claw to claw.<ref>{{cite journal|year = 1920|title = An ugly giant crab of Japan|url = https://books.google.com/books?id=xCkDAAAAMBAJ&pg=PA42|journal = Popular Science|volume = 96|issue = 6|page = 42 }}</ref>
The Tasmanian giant crab is long-lived and slow-growing, making it vulnerable to overfishing.<ref name=Currie2009>{{cite book| vauthors = Currie DR, Ward TM |year=2009 |url=http://www.sardi.sa.gov.au/__data/assets/pdf_file/0019/109522/No_345_SA_Giant_Crab_Fishery_Assessment_Report_2007_08Final_Report_published_alias.pdf|title=South Australian Giant Crab (Pseudocarcinus gigas) Fishery|id=Fishery Assessment Report for PIRSA|publisher=South Australian Research and Development Institute|access-date=9 December 2013|archive-url=https://web.archive.org/web/20120328121906/http://www.sardi.sa.gov.au/__data/assets/pdf_file/0019/109522/No_345_SA_Giant_Crab_Fishery_Assessment_Report_2007_08Final_Report_published_alias.pdf|archive-date=28 March 2012|url-status=dead}}</ref>
Mantis shrimp have the most advanced eyes in the animal kingdom,<ref name=Kilday2005>{{cite news|url=http://archive.dailycal.org/article/19671/mantis_shrimp_boasts_most_advanced_eyes|title=Mantis shrimp boasts most advanced eyes | vauthors = Kilday P |newspaper=The Daily Californian|date=28 September 2005}}</ref> and smash prey by swinging their club-like raptorial claws.<ref name="Patek and Caldwell">{{cite journal| vauthors = Patek SN, Caldwell RL |year=2005|journal=Journal of Experimental Biology|volume=208|pages=3655–3664|title=Extreme impact and cavitation forces of a biological hammer: strike forces of the peacock mantis shrimp|doi=10.1242/jeb.01831|pmid=16169943|issue=19|s2cid=312009}}</ref>
Echinoderm literally means "spiny skin", as this water melon sea urchin illustrates.
The ochre sea star was the first keystone predator to be studied. They limit mussels which can overwhelm intertidal communities.<ref name=Holsinger>Holsinger, K. (2005). Keystone species. Retrieved 10 May 2010, from {{cite web |url=http://darwin.eeb.uconn.edu/eeb310/lecture-notes/interactions/node2.html | vauthors = Holsinger K | date = 11 October 2005 | work = University of Connecticut |title=Keystone species |access-date=2010-05-12 |url-status=dead |archive-url=https://web.archive.org/web/20100630134633/http://darwin.eeb.uconn.edu/eeb310/lecture-notes/interactions/node2.html |archive-date=30 June 2010 |df=dmy-all }}</ref>
Colorful sea lilies in shallow waters
Sea cucumbers filter feed on plankton and suspended solids.
The sea pig, a deep water sea cucumber, is the only echinoderm that uses legged locomotion.
A benthopelagic and bioluminescent swimming sea cucumber, 3200 metres deep
The lancelet, a small translucent fish-like cephalochordate, is one of the closest living invertebrate relative of the vertebrates.<ref>{{cite journal| vauthors = Gewin V |year = 2005|title = Functional genomics thickens the biological plot|journal = PLOS Biology|volume = 3|issue = 6|page = e219|doi = 10.1371/journal.pbio.0030219|pmid=15941356|pmc=1149496}}</ref><ref>{{cite web | vauthors = Timmer J | url = https://arstechnica.com/science/2008/06/lancelet-amphioxus-genome-and-the-origin-of-vertebrates/ | title = Lancelet (amphioxus) genome and the origin of vertebrates | work = Ars Technica | date = 19 June 2008 }}</ref>
Tunicates, like these fluorescent-colored sea squirts, may provide clues to vertebrate and therefore human ancestry.<ref>{{cite journal| vauthors = Lemaire P |year = 2011|title = Evolutionary crossroads in developmental biology: the tunicates|journal = Development|volume = 138|issue = 11|pages = 2143–2152|doi = 10.1242/dev.048975|pmid=21558365|s2cid = 40452112}}</ref>
Pyrosomes are free-floating bioluminescent tunicates made up of hundreds of individuals.
Salp chain
Hagfish are the only known living animals with a skull but no vertebral column.
Lampreys are often parasitic and have a toothed, funnel-like sucking mouth.
The extinct Pteraspidomorphi, ancestral to jawed vertebrates
Cartilaginous fishes may have evolved from spiny sharks.
Stingray
Manta ray, the largest ray
Sawfish, rays with long rostrums resembling a saw. All species are now endangered.<ref name=BBC2007>{{cite news | vauthors = Black R |date=11 June 2007|title=Sawfish protection acquires teeth|work=BBC News|url=http://news.bbc.co.uk/2/hi/science/nature/6740609.stm}}</ref>
The extinct megalodon resembled a giant great white shark.
The Greenland shark lives longer than any other vertebrate.
The largest extant fish, the whale shark, is now a vulnerable species.
Sailfish
Mahi-mahi
Eel
[[Seahorse|Sea-
Ocean sunfish
Anglerfish
Pufferfish
Clown triggerfish
Mandarin dragonet
Marine iguana
Leatherback sea turtle
Saltwater crocodile
Marine snakes have flattened tails.
The ancient Ichthyosaurus communis independently evolved flippers similar to dolphins.
European herring gull attack herring schools from above.
Gentoo penguin swimming underwater
Gannets "divebomb" at high speed.
Albatrosses range over huge areas of ocean and regularly circle the globe.
Endangered blue whale, the largest living animal<ref>{{cite web|url=http://wwf.panda.org/what_we_do/endangered_species/cetaceans/about/blue_whale/|title=Blue whale|publisher=World Wide Fund For Nature|access-date=15 August 2016}}</ref>
Bottlenose dolphin, which has the highest encephalization of any animal after humans<ref>{{cite journal| vauthors = Marino L |title=Cetacean Brain Evolution: Multiplication Generates Complexity|journal=International Society for Comparative Psychology|issue=17|pages=1–16 |year=2004 |url=http://www.cogs.indiana.edu/spackled/2005readings/CetaceanBrainEvolution.pdf |access-date=15 August 2016|archive-url= https://web.archive.org/web/20180916132752/http://www.cogs.indiana.edu/spackled/2005readings/CetaceanBrainEvolution.pdf |archive-date=16 September 2018|url-status=dead}}</ref>
Beluga whale
Dugong grazing on seagrass
Walrus
Polar bear
Chlamydomonas globosa, a unicellular green alga with two flagella just visible at bottom left
Chlorella vulgaris, a common green microalgae, in endosymbiosis with a ciliate<ref>{{cite journal | vauthors = Duval B, Margulis L | year = 1995 | title = The microbial community of Ophrydium versatile colonies: endosymbionts, residents, and tenants | journal = Symbiosis | volume = 18 | pages = 181–210 | pmid = 11539474 }}</ref>
Centric diatom
Dinoflagellates
A seaweed is a macroscopic form of
Sargassum seaweed is a planktonic brown alga with air bladders that help it float.
Sargassum fish are camouflaged to live among drifting Sargassum seaweed.
The unicellular bubble algae lives in tidal zones. It can have a 4 cm diameter.<ref>{{cite book | vauthors = Tunnell JW, Chávez EA, Withers K |year=2007 |title=Coral reefs of the southern Gulf of Mexico |publisher=Texas A&M University Press |isbn=978-1-58544-617-9 |page=91 |url=https://books.google.com/books?id=tu0sqBp8eAAC&pg=PA91}}</ref>
The unicellular mermaid's wineglass are mushroom-shaped algae that grow up to 10 cm high.
Killer algae are single-celled organisms, but look like ferns and grow stalks up to 80 cm long.<ref>{{cite web | url = https://www.cabi.org/isc/datasheet/29292 | work = Invasive Species Compendium | title = Caulerpa taxifolia (killer algae) | publisher = Centre for Agriculture and Bioscience International | date = 6 November 2018 }}</ref>
Mangroves
Seagrass meadow
Sea dragons camouflaged to look like floating seaweed live in kelp forests and seagrass meadows.<ref>{{FishBase |genus=Phycodurus |species=eques |year=2009 |month=July}}</ref>
Phytoplankton are the foundation of the ocean food chain.
Phytoplankton come in many shapes and sizes.
Diatoms are one of the most common types of phytoplankton.
Colonial phytoplankton
The cyanobacterium Prochlorococcus accounts for much of the ocean's primary production.
Green cyanobacteria scum washed up on a rock in California
Gyrodinium, one of the few naked dinoflagellates which lack armour
Zoochlorellae (green) living inside the ciliate Stichotricha secunda
{{center|Coccolithophores named after the BBC documentary series
The coccolithophore Emiliania huxleyi
Algae bloom of Emiliania huxleyi off the southern coast of England
Guinardia delicatula, a diatom responsible for algal blooms in the North Sea and the English Channel<ref>{{cite journal | vauthors = Arsenieff L, Simon N, Rigaut-Jalabert F, Le Gall F, Chaffron S, Corre E, Com E, Bigeard E, Baudoux AC | title = First Viruses Infecting the Marine Diatom Guinardia delicatula | journal = Frontiers in Microbiology | volume = 9 | issue = | pages = 3235 | date = 2018 | pmid = 30687251 | pmc = 6334475 | doi = 10.3389/fmicb.2018.03235 | doi-access = free }}</ref>
Radiolarians come in many shapes.
Group of planktic foraminiferans
Copepods eat phytoplankton. This one is carrying eggs.
The dinoflagellate, Protoperidinium extrudes a large feeding veil to capture prey.
Moon jellyfish
Venus girdle, a ctenophore
Arrow worm
Tomopteris, a planktonic segmented worm with unusual yellow bioluminescence<ref>{{cite book| vauthors = Harvey EN |title = Bioluminescence|publisher = Academic Press|year = 1952 }}</ref>
Marine amphipod
Krill
Pelagic sea cucumber
Salmon larva hatching from its egg
Ocean sunfish larva
Juvenile planktonic squid
{{center|Larva stage of a spiny lobster}}
Tintinnid ciliate Favella
Euglena mutabilis, a photosynthetic flagellate
Noctiluca scintillans, a bioluminescence dinoflagellate
Marine carbon cycle<ref name="Prentice_2001">{{Cite web| vauthors = Prentice IC |title=The carbon cycle and atmospheric carbon dioxide|url =http://ir.anet.ua.ac.be/irua/handle/10067/381670151162165141?show=full|publisher=Climate change 2001: the scientific basis: contribution of Working Group I to the Third Assessment Report of the Intergouvernmental Panel on Climate Change / Houghton, J.T. [edit.]|year=2001|access-date=31 May 2012 }}</ref>
Oxygen cycle
{{center|Marine nitrogen cycle}}
{{center|Marine phosphorus cycle}}
An elaborate mineral skeleton of a radiolarian made of silica.
Diatoms, major components of marine plankton, also have silica skeletons called frustules.
Coccolithophores have plates or scales made with calcium carbonate called coccoliths
Calcified test of a planktic foraminiferan
A diatom microfossil from 40 million years ago
Diatomaceous earth is a soft, siliceous, sedimentary rock made up of microfossils in the form of the frustules (shells) of single cell diatoms (click to magnify).
Illustration of a Globigerina ooze
Shells (tests), usually made of calcium carbonate, from a foraminiferal ooze on the deep ocean floor

Marine life, sea life, or ocean life is the plants, animals and other organisms that live in the salt water of the sea or ocean, or the brackish water of coastal estuaries.

Littoral zone

The littoral zone of an ocean is the area close to the shore and extending out to the edge of the continental shelf.
The intertidal zone of a beach is also part of the littoral zone.
Estuaries are also in the littoral zone.
The three primary zones of a lake are the littoral zone, the open-water (also called the photic or limnetic) zone, and the deep-water (also called the aphotic or profundal) zone.
Shoreline of a lake with nearly unvegetated littoral zone

The littoral zone or nearshore is the part of a sea, lake, or river that is close to the shore.

Mediterranean Sea

Map of the Mediterranean Sea
Greek (red) and Phoenician (yellow) colonies in antiquity c. the 6th century BC
The Roman Empire at its farthest extent in AD 117
The Battle of Lepanto, 1571, ended in victory for the European Holy League against the Ottoman Turks.
The bombardment of Algiers by the Anglo-Dutch fleet in support of an ultimatum to release European slaves, August 1816
Borders of the Mediterranean Sea
Approximate extent of the Mediterranean drainage basin (dark green). Nile basin only partially shown
Map of the Mediterranean Sea from open Natural Earth data, 2020
Alexandria, the largest city on the Mediterranean
Barcelona, the second largest metropolitan area on the Mediterranean Sea (after Alexandria) and the headquarters of the Union for the Mediterranean
The Acropolis of Athens with the Mediterranean Sea in the background
The ancient port of Jaffa (now in Tel Aviv-Yafo), from which the biblical Jonah set sail before being swallowed by a whale
Catania, Sicily, Italy, with Mount Etna in the background
İzmir, the third metropolis of Turkey (after Istanbul and Ankara)
Africa (left, on horizon) and Europe (right), as seen from Gibraltar
Positano, Italy, Tyrrhenian Sea
View of the Saint George Bay, and snow-capped Mount Sannine from a tower in the Beirut Central District
The Port of Marseille seen from L'Estaque
Sarandë, Albania, stands on an open-sea gulf of the Ionian sea in the central Mediterranean.
The two biggest islands of the Mediterranean: Sicily and Sardinia (Italy)
Predominant surface currents for June
A submarine karst spring, called vrulja, near Omiš; observed through several ripplings of an otherwise calm sea surface.
Messinian salinity crisis before the Zanclean flood
The thermonuclear bomb that fell into the sea recovered off Palomares, Almería, 1966
Stromboli volcano in Italy
The reticulate whipray is one of the species that colonised the Eastern Mediterranean through the Suez Canal as part of the ongoing Lessepsian migration.
A cargo ship cruises towards the Strait of Messina
Port of Trieste
Kemer Beach in Antalya on the Turkish Riviera (Turquoise Coast). In 2019, Turkey ranked sixth in the world in terms of the number of international tourist arrivals, with 51.2 million foreign tourists visiting the country.
Coast of Alexandria, view From Bibliotheca Alexandrina, Egypt
Beach of Hammamet, Tunisia
The beach of la Courtade in the Îles d'Hyères, France
Sardinia's south coast, Italy
Pretty Bay, Malta
Panoramic view of Piran, Slovenia
Panoramic view of Cavtat, Croatia
View of Neum, Bosnia and Herzegovina
A view of Sveti Stefan, Montenegro
Ksamil Islands, Albania
Navagio, Greece
Ölüdeniz, Turquoise Coast, Turkey
Paphos, Cyprus
Burj Islam Beach, Latakia, Syria
A view of Raouché off the coast of Beirut, Lebanon
A view of Haifa, Israel
Old city of Ibiza Town, Spain
Les Aiguades near Béjaïa, Algeria
El Jebha, a port town in Morocco
Europa Point, Gibraltar
Panoramic view of La Condamine, Monaco
Sunset at the Deir al-Balah beach, Gaza Strip

The Mediterranean Sea is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean Basin and almost completely enclosed by land: on the north by Western and Southern Europe and Anatolia, on the south by North Africa, and on the east by the Levant.

Ocean

World map of the five-ocean model with approximate boundaries
The Atlantic, one component of the system, makes up 23% of the "global ocean".
Surface view of the Atlantic Ocean
World distribution of mid-oceanic ridges; USGS
Map of large underwater features (1995, NOAA)
Ocean chlorophyll concentration is a proxy for phytoplankton biomass. In this map, blue colors represent lower chlorophyll and reds represent higher chlorophyll. Satellite-measured chlorophyll is estimated based on ocean color by how green the color of the water appears from space.
The major oceanic zones, based on depth and biophysical conditions
Ocean surface currents
A map of the global thermohaline circulation; blue represents deep-water currents, whereas red represents surface currents.
Map of the Gulf Stream, a major ocean current that transports heat from the equator to northern latitudes and moderates the climate of Europe.
High tide and low tide in the Bay of Fundy, Canada.
The ocean is a major driver of Earth's water cycle.
Annual mean sea surface salinity in practical salinity units (psu) from the World Ocean Atlas.
Sea surface oxygen concentration in moles per cubic meter from the World Ocean Atlas.
Diagram of the ocean carbon cycle showing the relative size of stocks (storage) and fluxes.
Residence time of elements in the ocean depends on supply by processes like rock weathering and rivers vs. removal by processes like evaporation and sedimentation.
400px

The ocean (also the sea or the world ocean) is the body of salt water that covers approximately 70.8% of the surface of Earth and contains 97% of Earth's water.

North Sea

Ocean currents mainly entering via the north entrance exiting along Norwegian coast
• Localization of the tide-gauges listed • Tide times after Bergen (negative = before) • The three amphidromic centers • Coasts: marshes = green mudflats = greenish blue  lagoons = bright blue  dunes = yellow  sea dikes= purple  moraines near the coast= light brown  rock-based coasts = greyish brown
The German North Sea coast
The Afsluitdijk (Closure-dike) is a major dam in the Netherlands
Zuid-Beveland, North Sea flood of 1953
Pacific oysters, blue mussels and cockles in the Wadden Sea in the Netherlands
European seagull on the coast of North Sea
A female bottlenose dolphin with her young in Moray Firth, Scotland
Phytoplankton bloom in the North Sea
Painting of the Four Days' Battle of 1666 by Willem van de Velde the Younger
German cruiser SMS Blücher sinks in the Battle of Dogger Bank on 25 January 1915.
The exclusive economic zones in the North Sea
Oil platform Statfjord A with the flotel Polymarine
A trawler in Nordstrand, Germany
Unpolished amber stones, in varying hues
The beach in Scheveningen, Netherlands in c. 1900
Rotterdam, Netherlands
Map showing hypothetical extent of Doggerland (c. 8,000 BC), which provided a land bridge between Great Britain and continental Europe
North Sea from De Koog, Texel island
The North Sea between {{ma|34}} and {{ma|28}}, as Central Europe became dry land
thumb|A 1482 recreation of a map from Ptolemy's Geography showing the "Oceanus Germanicus"
thumb|Edmond Halley's solar eclipse 1715 map showing The German Sea

The North Sea is a sea of the Atlantic Ocean between Great Britain, Norway, Denmark, Germany, the Netherlands, Belgium and France.

Seawater

Seawater off San Andrés
Temperature-salinity diagram of changes in density of water
Ocean salinity at different latitudes in the Atlantic and Pacific
Annual mean sea surface salinity expressed in the Practical Salinity Scale for the World Ocean. Data from the World Ocean Atlas
Diagram showing concentrations of various salt ions in seawater. The composition of the total salt component is: 55%, 30.6%,  7.7%,  3.7%,  1.2%,  1.1%,
Other 0.7%. Note that the diagram is only correct when in units of wt/wt, not wt/vol or vol/vol.

Seawater, or salt water, is water from a sea or ocean.

River

The Amazon River (dark blue) and the rivers which flow into it (medium blue).
The start of a mountain stream.
Melting toe of Athabasca Glacier, Jasper National Park, Alberta, Canada
The Colorado River at Horseshoe Bend, Arizona
The Porvoo River (Porvoonjoki) in the medieval town of Porvoo, Finland
Nile River delta, as seen from Earth orbit. The Nile is an example of a wave-dominated delta that has the classic Greek letter delta (Δ) shape after which river deltas were named.
A radar image of a 400 km river of methane and ethane near the north pole of Saturn's moon Titan
River meandering course
Flash flooding caused by a large amount of rain falling in a short amount of time
The mouth of the River Seaton in Cornwall after heavy rain caused flooding and significant erosion of the beach.
Frozen river in Alaska
Leisure activities on the River Avon at Avon Valley Country Park, Keynsham, United Kingdom. A boat giving trips to the public passes a moored private boat.
Watermill in Belgium.
River bank repair

A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river.