Sharks are a group of

elasmobranch Elasmobranchii () is a subclass of Chondrichthyes or cartilaginous fish, including sharks (superorder Selachii), rays, skates, and sawfish (superorder Batoidea). Members of this subclass are characterised by having five to seven pairs of g ...

fish Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% ...

characterized by a cartilaginous skeleton, five to seven

gill slit Gill slits are individual openings to gills, i.e., multiple gill arches, which lack a single outer cover. Such gills are characteristic of cartilaginous fish such as sharks and rays, as well as deep-branching vertebrates such as lampreys. In co ...

s on the sides of the

head A head is the part of an organism which usually includes the ears, brain, forehead, cheeks, chin, eyes, nose, and mouth, each of which aid in various sensory functions such as sight, hearing, smell, and taste. Some very simple animals may no ...

, and

pectoral fin Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as ...

s that are not fused to the head. Modern sharks are classified within the clade Selachimorpha (or Selachii) and are the

sister group In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and ...

to the rays. However, the term "shark" has also been used to refer to all extinct members of

Chondrichthyes Chondrichthyes (; ) is a class that contains the cartilaginous fishes that have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fishes'', which have skeletons primarily composed of bone tissu ...

with a shark-like morphology, such as

hybodonts Hybodontiformes, commonly called hybodonts, are an extinct group of shark-like chondrichthyans, which existed from the late Devonian to the Late Cretaceous. They form the group of Elasmobranchii closest to neoselachians, the clade of modern shar ...

and xenacanths. The oldest modern sharks are known from the

Early Jurassic The Early Jurassic Epoch (in chronostratigraphy corresponding to the Lower Jurassic Series) is the earliest of three epochs of the Jurassic Period. The Early Jurassic starts immediately after the Triassic-Jurassic extinction event, 201.3 Ma ...

. They range in size from the small

dwarf lanternshark The dwarf lanternshark (''Etmopterus perryi'') is a species of dogfish shark in the family Etmopteridae and is the smallest shark in the world, reaching a maximum known length of . It is known to be present only on the upper continental slopes o ...

(''Etmopterus perryi''), a deep sea species that is only in length, to the

whale shark The whale shark (''Rhincodon typus'') is a slow-moving, filter-feeding carpet shark and the largest known extant fish species. The largest confirmed individual had a length of .McClain CR, Balk MA, Benfield MC, Branch TA, Chen C, Cosgrove J, ...

(''Rhincodon typus''), the largest fish in the world, which reaches approximately in length. Sharks are found in all seas and are common to depths up to . They generally do not live in freshwater, although there are a few known exceptions, such as the

bull shark The bull shark (''Carcharhinus leucas''), also known as the Zambezi shark (informally zambi) in Africa and Lake Nicaragua shark in Nicaragua, is a species of requiem shark commonly found worldwide in warm, shallow waters along coasts and in ri ...

and the

river shark ''Glyphis'' is a genus in the family Carcharhinidae, commonly known as the river sharks. They live in rivers or coastal regions in and around south-east Asia and parts of Australia. Taxonomy This genus contains only three extant species. Furth ...

, which can be found in both seawater and freshwater. Sharks have a covering of

dermal denticle A fish scale is a small rigid plate that grows out of the skin of a fish. The skin of most jawed fishes is covered with these protective scales, which can also provide effective camouflage through the use of reflection and colouration, as w ...

s that protects their skin from damage and

parasite Parasitism is a Symbiosis, close relationship between species, where one organism, the parasite, lives on or inside another organism, the Host (biology), host, causing it some harm, and is Adaptation, adapted structurally to this way of lif ...

s in addition to improving their

fluid dynamics In physics and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids—liquids and gases. It has several subdisciplines, including '' aerodynamics'' (the study of air and other gases in motion) ...

. They have numerous sets of replaceable teeth. Several species are

apex predator An apex predator, also known as a top predator, is a predator at the top of a food chain, without natural predators of its own. Apex predators are usually defined in terms of trophic dynamics, meaning that they occupy the highest trophic le ...

s, which are organisms that are at the top of their

food chain A food chain is a linear network of links in a food web starting from producer organisms (such as grass or algae which produce their own food via photosynthesis) and ending at an apex predator species (like grizzly bears or killer whales), d ...

. Select examples include the

tiger shark The tiger shark (''Galeocerdo cuvier'') is a species of requiem shark and the last extant member of the family Galeocerdonidae. It is a large macropredator, capable of attaining a length over . Populations are found in many tropical and temper ...

blue shark The blue shark (''Prionace glauca''), also known as the great blue shark, is a species of requiem shark, in the family Carcharhinidae, which inhabits deep waters in the world's temperate and tropical oceans. Averaging around and preferring c ...

great white shark The great white shark (''Carcharodon carcharias''), also known as the white shark, white pointer, or simply great white, is a species of large Lamniformes, mackerel shark which can be found in the coastal surface waters of all the major ocean ...

mako shark ''Isurus'' is a genus of mackerel sharks in the family Lamnidae, commonly known as the mako sharks. Description The two living species are the common shortfin mako shark (''I. oxyrinchus'') and the rare longfin mako shark (''I. paucus''). They ...

thresher shark Thresher sharks are large lamniform sharks of the family Alopiidae found in all temperate and tropical oceans of the world; the family contains three extant species, all within the genus ''Alopias''. All three thresher shark species have been ...

, and

hammerhead shark The hammerhead sharks are a group of sharks that form the family Sphyrnidae, so named for the unusual and distinctive structure of their heads, which are flattened and laterally extended into a "hammer" shape called a cephalofoil. Most hammerhe ...

. Sharks are caught by humans for

shark meat Shark meat is a seafood consisting of the flesh of sharks. Several sharks are fished for human consumption, such as porbeagles, shortfin mako shark, requiem shark, and thresher shark, among others. Shark meat is popular in Asia, where it is ...

shark fin soup Shark fin soup is a traditional soup or stewed dish served in parts of China, Taiwan, and Southeast Asia. The shark fins provide texture, while the taste comes from the other soup ingredients. It is commonly served at special occasions such as ...

. Many shark populations are threatened by human activities. Since 1970, shark populations have been reduced by 71%, mostly from

overfishing Overfishing is the removal of a species of fish (i.e. fishing) from a body of water at a rate greater than that the species can replenish its population naturally (i.e. the overexploitation of the fishery's existing fish stock), resulting in the ...

Etymology

Until the 16th century, sharks were known to mariners as "sea dogs". This is still evidential in several species termed " dogfish," or the

porbeagle The porbeagle (''Lamna nasus'') is a species of mackerel shark in the family Lamnidae, distributed widely in the cold and temperate marine waters of the North Atlantic and Southern Hemisphere. In the North Pacific, its ecological equivalent is ...

. The etymology of the word ''shark'' is uncertain, the most likely etymology states that the original sense of the word was that of "predator, one who preys on others" from the

Dutch Dutch commonly refers to: * Something of, from, or related to the Netherlands * Dutch people () * Dutch language () Dutch may also refer to: Places * Dutch, West Virginia, a community in the United States * Pennsylvania Dutch Country People E ...

, meaning 'villain, scoundrel' (

cf. The abbreviation ''cf.'' (short for the la, confer/conferatur, both meaning "compare") is used in writing to refer the reader to other material to make a comparison with the topic being discussed. Style guides recommend that ''cf.'' be used onl ...

'' card shark'', ''

loan shark A loan shark is a person who offers loans at extremely high interest rates, has strict terms of collection upon failure, and generally operates outside the law. Description Because loan sharks operate mostly illegally, they cannot reasonably ...

'', etc.), which was later applied to the fish due to its predatory behaviour. A now disproven theory is that it derives from the

Yucatec Maya Yucatec Maya (; referred to by its speakers simply as Maya or as , is one of the 32 Mayan languages of the Mayan language family. Yucatec Maya is spoken in the Yucatán Peninsula and northern Belize. There is also a significant diasporic co ...

word (), meaning 'shark'. Evidence for this etymology came from the

Oxford English Dictionary The ''Oxford English Dictionary'' (''OED'') is the first and foundational historical dictionary of the English language, published by Oxford University Press (OUP). It traces the historical development of the English language, providing a com ...

, which notes ''shark'' first came into use after Sir John Hawkins' sailors exhibited one in London in 1569 and posted "''sharke''" to refer to the large sharks of the

Caribbean Sea The Caribbean Sea ( es, Mar Caribe; french: Mer des Caraïbes; ht, Lanmè Karayib; jam, Kiaribiyan Sii; nl, Caraïbische Zee; pap, Laman Karibe) is a sea of the Atlantic Ocean in the tropics of the Western Hemisphere. It is bounded by Mexico ...

. However, the

Middle English Dictionary ''The Middle English Dictionary'' is a dictionary of Middle English published by the University of Michigan. "Its 15,000 pages offer a comprehensive analysis of lexicon and usage for the period 1175–1500, based on the analysis of a collection o ...

records an isolated occurrence of the word ''shark'' (referring to a sea fish) in a letter written by

Thomas Beckington Thomas Beckington (also spelt Beckynton; c. 139014 January 1465) was the Bishop of Bath and Wells and King's Secretary in medieval England under Henry VI. Life Beckington was born at Beckington in Somerset, and was educated at Winchester a ...

in 1442, which rules out a New World etymology.

Evolutionary history

Fossil record

Evidence for the existence of shark-like chondrichthyans dates from the

Ordovician The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya. ...

period, 450–420 million years ago, before land

vertebrates Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, wi ...

existed and before a variety of plants had colonized the continents. Only scales have been recovered from the first supposed elasmobranchians and not all paleontologists agree that these are from true sharks, suspecting that these scales are actually those of

thelodont Thelodonti (from Greek: "feeble teeth")Maisey, John G., Craig Chesek, and David Miller. Discovering fossil fishes. New York: Holt, 1996. is a class of extinct jawless fishes with distinctive scales instead of large plates of armor. There is muc ...

agnathans. The oldest generally accepted "shark" scales are from about 420 million years ago, in the

Silurian</