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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% of ...
characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachimorpha (or Selachii) and are the sister group to the
rays Ray may refer to: Fish * Ray (fish), any cartilaginous fish of the superorder Batoidea * Ray (fish fin anatomy), a bony or horny spine on a fin Science and mathematics * Ray (geometry), half of a line proceeding from an initial point * Ray (gra ...
. 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 tissue. ...
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 sh ...
and xenacanths. The oldest modern sharks are known from the
Early Jurassic The Early Jurassic Epoch (geology), Epoch (in chronostratigraphy corresponding to the Lower Jurassic series (stratigraphy), Series) is the earliest of three epochs of the Jurassic Period. The Early Jurassic starts immediately after the Triassic-J ...
. 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 of ...
(''Etmopterus perryi''), a deep sea species that is only in length, to the whale shark (''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 denticles that protects their skin from damage and parasites in addition to improving their fluid dynamics. 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 lev ...
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), de ...
. 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 tempera ...
, blue shark, great white shark, mako shark, thresher shark, 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 or shark fin soup. Many shark populations are threatened by human activities. Since 1970, shark populations have been reduced by 71%, mostly from overfishing.


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 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'', 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 commu ...
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 co ...
, 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. 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 of ...
records an isolated occurrence of the word ''shark'' (referring to a sea fish) in a letter written by Thomas Beckington 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. T ...
period, 450–420 million years ago, before land vertebrates 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 agnathans. The oldest generally accepted "shark" scales are from about 420 million years ago, in the Silurian period. Those animals looked very different from modern sharks. At this time the most common shark tooth is the cladodont, a style of thin tooth with three tines like a trident, apparently to help catch fish. The majority of modern sharks can be traced back to around 100 million years ago. Most fossils are of teeth, often in large numbers. Partial skeletons and even complete fossilized remains have been discovered. Estimates suggest that sharks grow tens of thousands of teeth over a lifetime, which explains the abundant fossils. The teeth consist of easily fossilized
calcium phosphate The term calcium phosphate refers to a family of materials and minerals containing calcium ions (Ca2+) together with inorganic phosphate anions. Some so-called calcium phosphates contain oxide and hydroxide as well. Calcium phosphates are whi ...
, an apatite. When a shark dies, the decomposing skeleton breaks up, scattering the apatite prisms. Preservation requires rapid burial in bottom sediments. Among the most ancient and primitive shark-like fish is '' Cladoselache'', from about 370 million years ago, which has been found within Paleozoic strata in Ohio, Kentucky, and Tennessee. At that point in Earth's history these rocks made up the soft bottom sediments of a large, shallow ocean, which stretched across much of North America. ''Cladoselache'' was only about long with stiff triangular fins and slender jaws. Its teeth had several pointed cusps, which wore down from use. From the small number of teeth found together, it is most likely that ''Cladoselache'' did not replace its teeth as regularly as modern sharks. Its caudal fins had a similar shape to the great white sharks and the pelagic shortfin and longfin makos. The presence of whole fish arranged tail-first in their stomachs suggest that they were fast swimmers with great agility. Most fossil shark-like fish from about 300 to 150 million years ago can be assigned to one of two groups. The
Xenacanthida Xenacanthida (or Xenacanthiforms) is a super-order of extinct shark-like elasmobranchs that appeared during the Lower Carboniferous period. The order includes the families Xenacanthidae, Sphenacanthidae, Diplodoselachidae, and Orthacanthidae. T ...
was almost exclusive to freshwater environments. By the time this group became extinct about 220 million years ago, they had spread worldwide. The other group, the
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 sh ...
, appeared about 320 million years ago and lived mostly in the oceans, but also in freshwater. The results of a 2014 study of the gill structure of an unusually well preserved 325-million-year-old fossil suggested that sharks are not "living fossils", but rather have evolved more extensively than previously thought over the hundreds of millions of years they have been around. It appears Selachiimorpha and Batoidea split in the
Triassic The Triassic ( ) is a geologic period and system (stratigraphy), system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago (Year#Abbreviations yr and ya, Mya), to the beginning of the Jurassic Period 201.36 ...
. Modern sharks began to appear about 100 million years ago. Fossil mackerel shark teeth date to the Early Cretaceous. One of the most recently evolved families is the hammerhead shark (family Sphyrnidae), which emerged in the
Eocene The Eocene ( ) Epoch is a geological epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period in the modern Cenozoic Era. The name ''Eocene'' comes from the Ancient Greek (''ēṓs'', " ...
. The oldest white shark teeth date from 60 to 66 million years ago, around the time of the extinction of the dinosaurs. In early white shark evolution there are at least two lineages: one lineage is of white sharks with coarsely
serrated Serration is a saw-like appearance or a row of sharp or tooth-like projections. A serrated cutting edge has many small points of contact with the material being cut. By having less contact area than a smooth blade or other edge, the applied p ...
teeth and it probably gave rise to the modern great white shark, and another lineage is of white sharks with finely serrated teeth. These sharks attained gigantic proportions and include the extinct megatoothed shark, '' Megalodon''. Like most extinct sharks, ''Megalodon'' is also primarily known from its fossil teeth and vertebrae. This giant shark reached a total length (TL) of more than . ''Megalodon'' may have approached a maxima of in total length and in mass. Paleontological evidence suggests that this shark was an active predator of large cetaceans.


Early Miocene extinction event

A study published in 2021 provided evidence for a major shark extinction event that occurred 19 million years ago. This is 5 million years before the established Middle Miocene disruption. The cause of this extinction event is not yet known, however, the study suggests that pelagic shark diversity decreased by over 70% and abundance by over 90%, and that modern sharks never recovered from this event. The authors also state that prior to the extinction event, "sharks played a much larger role in the open-ocean ecosystem than they do today." In present times, only 53 open-ocean shark species remains.


Taxonomy

Sharks belong to the superorder Selachimorpha in the subclass
Elasmobranchii 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 ...
in the class
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 tissue. ...
. The Elasmobranchii also include
rays Ray may refer to: Fish * Ray (fish), any cartilaginous fish of the superorder Batoidea * Ray (fish fin anatomy), a bony or horny spine on a fin Science and mathematics * Ray (geometry), half of a line proceeding from an initial point * Ray (gra ...
and skates; the Chondrichthyes also include Chimaeras. It was thought that the sharks form a
polyphyletic A polyphyletic group is an assemblage of organisms or other evolving elements that is of mixed evolutionary origin. The term is often applied to groups that share similar features known as homoplasies, which are explained as a result of conver ...
group: some sharks are more closely related to rays than they are to some other sharks, but current molecular studies support monophyly of both groups of sharks and batoids. The superorder Selachimorpha is divided into Galea (or Galeomorphii), and Squalea (or Squalomorphii). The Galeans are the Heterodontiformes, Orectolobiformes, Lamniformes, and Carcharhiniformes. Lamnoids and Carcharhinoids are usually placed in one clade, but recent studies show the Lamnoids and Orectoloboids are a clade. Some scientists now think that Heterodontoids may be Squalean. The Squaleans are divided into Hexanchiformes and Squalomorpha. The former includes cow shark and frilled shark, though some authors propose both families to be moved to separate orders. The Squalomorpha contains the Squaliformes and the Hypnosqualea. The Hypnosqualea may be invalid. It includes the Squatiniformes, and the Pristorajea, which may also be invalid, but includes the Pristiophoriformes and the
Batoidea Batoidea is a superorder of cartilaginous fishes, commonly known as rays. They and their close relatives, the sharks, comprise the subclass Elasmobranchii. Rays are the largest group of cartilaginous fishes, with well over 600 species in 26 fa ...
. There are more than 500 species of sharks split across thirteen orders, including four orders of sharks that have gone extinct: * Carcharhiniformes: Commonly known as ground sharks, the order includes the
blue Blue is one of the three primary colours in the RYB colour model (traditional colour theory), as well as in the RGB (additive) colour model. It lies between violet and cyan on the spectrum of visible light. The eye perceives blue when ...
, tiger,
bull A bull is an intact (i.e., not castrated) adult male of the species ''Bos taurus'' (cattle). More muscular and aggressive than the females of the same species (i.e., cows), bulls have long been an important symbol in many religions, includin ...
, grey reef, blacktip reef, Caribbean reef, blacktail reef, whitetip reef, and oceanic whitetip sharks (collectively called the requiem sharks) along with the
houndshark Houndsharks, the Triakidae, are a family of ground sharks, consisting of about 40 species in nine genera. In some classifications, the family is split into two subfamilies, with '' Mustelus'', '' Scylliogaleus'', and ''Triakis'' in the subfamily ...
s,
catshark Catsharks are ground sharks of the family Scyliorhinidae. They are the largest family of sharks with around 160 species placed in 17 genera. Although they are generally known as catsharks, some species can also be called dogfish due to previous ...
s, 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 ...
s. They are distinguished by an elongated snout and a nictitating membrane which protects the eyes during an attack. * Heterodontiformes: They are generally referred to as the bullhead or
horn shark The horn shark (''Heterodontus francisci'') is a species of bullhead shark, in the family Heterodontidae. It is endemic to the coastal waters off the western coast of North America, from California to the Gulf of California. Young sharks are seg ...
s. * Hexanchiformes: Examples from this group include the cow sharks and frilled sharks, which somewhat resembles a marine snake. * Lamniformes: They are commonly known as the mackerel sharks. They include the goblin shark,
basking shark The basking shark (''Cetorhinus maximus'') is the second-largest living shark and fish, after the whale shark, and one of three plankton-eating shark species, along with the whale shark and megamouth shark. Adults typically reach in leng ...
,
megamouth shark The megamouth shark (''Megachasma pelagios'') is a species of deepwater shark. It is rarely seen by humans and is the smallest of the three extant filter-feeding sharks alongside the relatively larger whale shark and basking shark. Since its d ...
, the thresher sharks, shortfin and longfin mako sharks, and great white shark. They are distinguished by their large jaws and ovoviviparous reproduction. The Lamniformes also include the extinct megalodon, ''Otodus megalodon''. * Orectolobiformes: They are commonly referred to as the carpet sharks, including zebra sharks,
nurse shark The nurse shark (''Ginglymostoma cirratum'') is an elasmobranch fish in the family Ginglymostomatidae. The conservation status of the nurse shark is globally assessed as Vulnerable in the IUCN List of Threatened Species. They are considered t ...
s,
wobbegong The wobbegong is the common name given to the 12 species of carpet sharks in the family Orectolobidae. They are found in shallow temperate and tropical waters of the western Pacific Ocean and eastern Indian Ocean, chiefly around Australia and I ...
s, and the whale shark. * Pristiophoriformes: These are the
sawshark A sawshark or saw shark is a member of a shark order (Pristiophoriformes ) bearing a unique long, saw-like rostrum (snout or bill) edged with sharp teeth, which they use to slash and disable their prey. There are eight species within the Pristi ...
s, with an elongated, toothed snout that they use for slashing their prey. * Squaliformes: This group includes the dogfish sharks and roughsharks. * Squatiniformes: Also known as
angel shark The angelsharks are a group of sharks in the genus ''Squatina'' of the family Squatinidae. They commonly inhabit sandy seabeds close to in depth. Many species are now classified as critically endangered by the International Union for Conservat ...
s, they are flattened sharks with a strong resemblance to
stingrays Stingrays are a group of sea rays, which are cartilaginous fish related to sharks. They are classified in the suborder Myliobatoidei of the order Myliobatiformes and consist of eight families: Hexatrygonidae (sixgill stingray), Plesiobatidae ...
and skates. * Echinorhiniformes: This group includes the prickly shark and bramble shark. Phylogenetic placement of this group has been ambiguous in scientific studies. They are sometimes given their own order, Echinorhiniformes. *†
Cladoselachiformes Cladoselachidae is an extinct family of holocephalian cartilaginous fish Chondrichthyes (; ) is a class that contains the cartilaginous fishes that have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes o ...
*†
Hybodontiformes 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 ...
*† Symmoriida *†
Xenacanthida Xenacanthida (or Xenacanthiforms) is a super-order of extinct shark-like elasmobranchs that appeared during the Lower Carboniferous period. The order includes the families Xenacanthidae, Sphenacanthidae, Diplodoselachidae, and Orthacanthidae. T ...
(Xenacantiformes)


Anatomy


Teeth

Shark teeth are embedded in the gums rather than directly affixed to the jaw, and are constantly replaced throughout life. Multiple rows of replacement teeth grow in a groove on the inside of the jaw and steadily move forward in comparison to a conveyor belt; some sharks lose 30,000 or more teeth in their lifetime. The rate of tooth replacement varies from once every 8 to 10 days to several months. In most species, teeth are replaced one at a time as opposed to the simultaneous replacement of an entire row, which is observed in the
cookiecutter shark The cookiecutter shark (''Isistius brasiliensis''), also called the cigar shark, is a species of small squaliform shark in the family Dalatiidae. This shark occurs in warm, oceanic waters worldwide, particularly near islands, and has been rec ...
. Tooth shape depends on the shark's diet: those that feed on mollusks and
crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean group can ...
s have dense and flattened teeth used for crushing, those that feed on fish have needle-like teeth for gripping, and those that feed on larger prey such as mammals have pointed lower teeth for gripping and triangular upper teeth with
serrated Serration is a saw-like appearance or a row of sharp or tooth-like projections. A serrated cutting edge has many small points of contact with the material being cut. By having less contact area than a smooth blade or other edge, the applied p ...
edges for cutting. The teeth of plankton-feeders such as the basking shark are small and non-functional.


Skeleton

Shark skeletons are very different from those of bony fish and terrestrial vertebrates. Sharks and other cartilaginous fish ( skates and
rays Ray may refer to: Fish * Ray (fish), any cartilaginous fish of the superorder Batoidea * Ray (fish fin anatomy), a bony or horny spine on a fin Science and mathematics * Ray (geometry), half of a line proceeding from an initial point * Ray (gra ...
) have skeletons made of cartilage and connective tissue. Cartilage is flexible and durable, yet is about half the normal density of bone. This reduces the skeleton's weight, saving energy. Because sharks do not have rib cages, they can easily be crushed under their own weight on land.


Jaw

The
jaw The jaw is any opposable articulated structure at the entrance of the mouth, typically used for grasping and manipulating food. The term ''jaws'' is also broadly applied to the whole of the structures constituting the vault of the mouth and serv ...
s of sharks, like those of rays and skates, are not attached to the cranium. The jaw's surface (in comparison to the shark's vertebrae and gill arches) needs extra support due to its heavy exposure to physical stress and its need for strength. It has a layer of tiny hexagonal plates called " tesserae", which are crystal blocks of calcium salts arranged as a mosaic. This gives these areas much of the same strength found in the bony tissue found in other animals. Generally sharks have only one layer of tesserae, but the jaws of large specimens, such as the bull shark, tiger shark, and the great white shark, have two to three layers or more, depending on body size. The jaws of a large great white shark may have up to five layers. In the
rostrum Rostrum may refer to: * Any kind of a platform for a speaker: **dais **pulpit * Rostrum (anatomy), a beak, or anatomical structure resembling a beak, as in the mouthparts of many sucking insects * Rostrum (ship), a form of bow on naval ships * Ros ...
(snout), the cartilage can be spongy and flexible to absorb the power of impacts.


Fins

Fin skeletons are elongated and supported with soft and unsegmented rays named ceratotrichia, filaments of elastic protein resembling the horny keratin in hair and feathers. Most sharks have eight fins. Sharks can only drift away from objects directly in front of them because their fins do not allow them to move in the tail-first direction.


Dermal denticles

Unlike bony fish, sharks have a complex dermal corset made of flexible collagenous fibers and arranged as a helical network surrounding their body. This works as an outer skeleton, providing attachment for their swimming muscles and thus saving energy. Their dermal teeth give them
hydrodynamic 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) a ...
advantages as they reduce turbulence when swimming. Some species of shark have pigmented denticles that form complex patterns like spots (e.g. Zebra shark) and stripes (e.g.
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 tempera ...
). These markings are important for
camouflage Camouflage is the use of any combination of materials, coloration, or illumination for concealment, either by making animals or objects hard to see, or by disguising them as something else. Examples include the leopard's spotted coat, the b ...
and help sharks blend in with their environment, as well as making them difficult for prey to detect. For some species, dermal patterning returns to healed denticles even after they have been removed by injury.


Tails

Tails provide thrust, making speed and acceleration dependent on tail shape. Caudal fin shapes vary considerably between shark species, due to their evolution in separate environments. Sharks possess a heterocercal caudal fin in which the
dorsal Dorsal (from Latin ''dorsum'' ‘back’) may refer to: * Dorsal (anatomy), an anatomical term of location referring to the back or upper side of an organism or parts of an organism * Dorsal, positioned on top of an aircraft's fuselage * Dorsal c ...
portion is usually noticeably larger than the
ventral Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...
portion. This is because the shark's
vertebral column The vertebral column, also known as the backbone or spine, is part of the axial skeleton. The vertebral column is the defining characteristic of a vertebrate in which the notochord (a flexible rod of uniform composition) found in all chordate ...
extends into that dorsal portion, providing a greater surface area for muscle attachment. This allows more efficient
locomotion Locomotion means the act or ability of something to transport or move itself from place to place. Locomotion may refer to: Motion * Motion (physics) * Robot locomotion, of man-made devices By environment * Aquatic locomotion * Flight * Locomo ...
among these negatively buoyant cartilaginous fish. By contrast, most bony fish possess a
homocercal 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 see ...
caudal fin. Tiger sharks have a large upper lobe, which allows for slow cruising and sudden bursts of speed. The tiger shark must be able to twist and turn in the water easily when hunting to support its varied diet, whereas the
porbeagle shark 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 ...
, which hunts schooling fish such as mackerel and herring, has a large lower lobe to help it keep pace with its fast-swimming prey. Other tail adaptations help sharks catch prey more directly, such as the thresher shark's usage of its powerful, elongated upper lobe to stun fish and squid.


Physiology


Buoyancy

Unlike bony fish, sharks do not have gas-filled swim bladders for buoyancy. Instead, sharks rely on a large liver filled with oil that contains
squalene Squalene is an organic compound. It is a triterpenoid with the formula C30H50. It is a colourless oil, although impure samples appear yellow. It was originally obtained from shark liver oil (hence its name, as ''Squalus'' is a genus of sharks). A ...
, and their cartilage, which is about half the normal density of bone. Their liver constitutes up to 30% of their total body mass. The liver's effectiveness is limited, so sharks employ
dynamic lift A fluid flowing around an object exerts a force on it. Lift is the Vector (geometric)#Vector components, component of this force that is perpendicular to the oncoming flow direction. It contrasts with the drag (physics), drag force, which is th ...
to maintain depth while swimming. Sand tiger sharks store air in their stomachs, using it as a form of swim bladder. Bottom-dwelling sharks, like the
nurse shark The nurse shark (''Ginglymostoma cirratum'') is an elasmobranch fish in the family Ginglymostomatidae. The conservation status of the nurse shark is globally assessed as Vulnerable in the IUCN List of Threatened Species. They are considered t ...
, have negative buoyancy, allowing them to rest on the ocean floor. Some sharks, if inverted or stroked on the nose, enter a natural state of tonic immobility. Researchers use this condition to handle sharks safely.


Respiration

Like other fish, sharks extract oxygen from seawater as it passes over their gills. Unlike other fish, shark gill slits are not covered, but lie in a row behind the head. A modified slit called a spiracle lies just behind the eye, which assists the shark with taking in water during
respiration Respiration may refer to: Biology * Cellular respiration, the process in which nutrients are converted into useful energy in a cell ** Anaerobic respiration, cellular respiration without oxygen ** Maintenance respiration, the amount of cellul ...
and plays a major role in bottom–dwelling sharks. Spiracles are reduced or missing in active pelagic sharks. While the shark is moving, water passes through the mouth and over the gills in a process known as "ram ventilation". While at rest, most sharks pump water over their gills to ensure a constant supply of oxygenated water. A small number of species have lost the ability to pump water through their gills and must swim without rest. These species are ''obligate ram ventilators'' and would presumably asphyxiate if unable to move. Obligate ram ventilation is also true of some pelagic bony fish species. The respiration and circulation process begins when deoxygenated blood travels to the shark's two-chambered
heart The heart is a muscular organ in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as carbon dioxide to t ...
. Here the shark pumps blood to its gills via the ventral
aorta The aorta ( ) is the main and largest artery in the human body, originating from the left ventricle of the heart and extending down to the abdomen, where it splits into two smaller arteries (the common iliac arteries). The aorta distributes o ...
artery where it branches into afferent brachial arteries. Reoxygenation takes place in the gills and the reoxygenated blood flows into the efferent brachial arteries, which come together to form the
dorsal aorta The dorsal aortae are paired (left and right) embryological vessels which progress to form the descending aorta. The paired dorsal aortae arise from aortic arches that in turn arise from the aortic sac. The primary dorsal aorta is located deep ...
. The blood flows from the dorsal aorta throughout the body. The deoxygenated blood from the body then flows through the posterior cardinal veins and enters the posterior cardinal sinuses. From there blood enters the heart ventricle and the cycle repeats.


Thermoregulation

Most sharks are "cold-blooded" or, more precisely,
poikilotherm A poikilotherm () is an animal whose internal temperature varies considerably. Poikilotherms have to survive and adapt to environmental stress. One of the most important stressors is temperature change, which can lead to alterations in membrane ...
ic, meaning that their internal
body temperature Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different. A thermoconforming organism, by contrast, simply adopts the surrounding temperature ...
matches that of their ambient environment. Members of the family Lamnidae (such as the
shortfin mako shark The shortfin mako shark (; ; ''Isurus oxyrinchus''), also known as the blue pointer or bonito shark, is a large mackerel shark. It is commonly referred to as the mako shark, as is the longfin mako shark (''Isurus paucus''). The shortfin mako can ...
and the great white shark) are
homeothermic Homeothermy, homothermy or homoiothermy is thermoregulation that maintains a stable internal body temperature regardless of external influence. This internal body temperature is often, though not necessarily, higher than the immediate environmen ...
and maintain a higher body temperature than the surrounding water. In these sharks, a strip of
aerobic Aerobic means "requiring air," in which "air" usually means oxygen. Aerobic may also refer to * Aerobic exercise, prolonged exercise of moderate intensity * Aerobics, a form of aerobic exercise * Aerobic respiration, the aerobic process of cel ...
red muscle located near the center of the body generates the heat, which the body retains via a countercurrent exchange mechanism by a system of blood vessels called the rete mirabile ("miraculous net"). The
common thresher The common thresher (''Alopias vulpinus''), also known as Atlantic thresher, is the largest species of thresher shark, family Alopiidae, reaching some in length. About half of its length consists of the elongated upper lobe of its caudal fin. Wi ...
and
bigeye thresher The bigeye thresher (''Alopias superciliosus'') is a species of thresher shark, family Alopiidae, found in temperate and tropical oceans worldwide. Like the other thresher sharks, nearly half its total length consists of the elongated upper lobe ...
sharks have a similar mechanism for maintaining an elevated body temperature.


Osmoregulation

In contrast to bony fish, with the exception of the
coelacanth The coelacanths ( ) are fish belonging to the order Actinistia that includes two extant species in the genus ''Latimeria'': the West Indian Ocean coelacanth (''Latimeria chalumnae''), primarily found near the Comoro Islands off the east coast ...
, the blood and other tissue of sharks and
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 tissue. ...
is generally isotonic to their marine environments because of the high concentration of urea (up to 2.5%) and trimethylamine N-oxide (TMAO), allowing them to be in osmotic balance with the seawater. This adaptation prevents most sharks from surviving in freshwater, and they are therefore confined to marine environments. A few exceptions exist, 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 ...
, which has developed a way to change its kidney function to excrete large amounts of urea. When a shark dies, the urea is broken down to ammonia by bacteria, causing the dead body to gradually smell strongly of ammonia. Research in 1930 by Homer W. Smith showed that sharks' urine doesn't contain sufficient sodium to avoid
hypernatremia Hypernatremia, also spelled hypernatraemia, is a high concentration of sodium in the blood. Early symptoms may include a strong feeling of thirst, weakness, nausea, and loss of appetite. Severe symptoms include confusion, muscle twitching, and b ...
, and it was postulated that there must be an additional mechanism for salt secretion. In 1960 it was discovered at the Mount Desert Island Biological Laboratory in Salsbury Cove, Maine that sharks have a type of
salt gland The salt gland is an organ for excreting excess salts. It is found in the cartilaginous fishes subclass elasmobranchii (sharks, rays, and skates), seabirds, and some reptiles. Salt glands can be found in the rectum of sharks. Birds and reptiles ...
located at the end of the intestine, known as the "rectal gland", whose function is the secretion of chlorides.


Digestion

Digestion can take a long time. The food moves from the mouth to a J-shaped stomach, where it is stored and initial digestion occurs. Unwanted items may never get past the stomach, and instead the shark either vomits or turns its stomachs inside out and ejects unwanted items from its mouth. One of the biggest differences between the digestive systems of sharks and mammals is that sharks have much shorter intestines. This short length is achieved by the
spiral valve A spiral valve or scroll valve is the corkscrew-shaped lower portion of the intestine of some sharks, Acipenseriformes (sturgeon and paddlefish), rays, skates, bichirs, Lepisosteiformes (gars), and lungfishes. A modification of the ileum, the s ...
with multiple turns within a single short section instead of a long tube-like intestine. The valve provides a long surface area, requiring food to circulate inside the short gut until fully digested, when remaining waste products pass into the cloaca.


Fluorescence

A few sharks appear fluorescent under blue light, such as the
swell shark The swell shark (''Cephaloscyllium ventriosum'') is a catshark in the family Scyliorhinidae. It is found in the tropical and subtropical eastern Pacific Ocean from between central California to southern Mexico, with an additional population off ...
and the
chain catshark The chain catshark or chain dogfish (''Scyliorhinus retifer'') is a small, reticulated catshark that is biofluorescent. The species is common in the Northwest Atlantic, Gulf of Mexico and Caribbean. It is harmless and rarely encountered by human ...
, where the fluorophore derives from a metabolite of
kynurenic acid Kynurenic acid (KYNA or KYN) is a product of the normal metabolism of amino acid -tryptophan. It has been shown that kynurenic acid possesses neuroactive activity. It acts as an antiexcitotoxic and anticonvulsant, most likely through acting as an ...
.


Senses


Smell

Sharks have keen olfactory senses, located in the short duct (which is not fused, unlike bony fish) between the anterior and posterior nasal openings, with some species able to detect as little as one
part per million In science and engineering, the parts-per notation is a set of pseudo-units to describe small values of miscellaneous dimensionless quantities, e.g. mole fraction or mass fraction. Since these fractions are quantity-per-quantity measures, they ...
of blood in seawater. The size of the olfactory bulb varies across different shark species, with size dependent on how much a given species relies on smell or vision to find their prey. In environments with low visibility, shark species generally have larger olfactory bulbs. In reefs, where visibility is high, species of sharks from the family Carcharhinidae have smaller olfactory bulbs. Sharks found in deeper waters also have larger olfactory bulbs. Sharks have the ability to determine the direction of a given scent based on the timing of scent detection in each nostril. This is similar to the method mammals use to determine direction of sound. They are more attracted to the chemicals found in the intestines of many species, and as a result often linger near or in sewage outfalls. Some species, such as
nurse shark The nurse shark (''Ginglymostoma cirratum'') is an elasmobranch fish in the family Ginglymostomatidae. The conservation status of the nurse shark is globally assessed as Vulnerable in the IUCN List of Threatened Species. They are considered t ...
s, have external
barbels In fish anatomy and turtle anatomy, a barbel is a slender, whiskerlike sensory organ near the mouth. Fish that have barbels include the catfish, the carp, the goatfish, the hagfish, the sturgeon, the zebrafish, the black dragonfish and some s ...
that greatly increase their ability to sense prey.


Sight

Shark eyes are similar to the eyes of other vertebrates, including similar lenses, corneas and
retina The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which then ...
s, though their eyesight is well adapted to the marine environment with the help of a tissue called
tapetum lucidum The ''tapetum lucidum'' ( ; ; ) is a layer of tissue in the eye of many vertebrates and some other animals. Lying immediately behind the retina, it is a retroreflector. It reflects visible light back through the retina, increasing the light a ...
. This tissue is behind the
retina The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which then ...
and reflects light back to it, thereby increasing visibility in the dark waters. The effectiveness of the tissue varies, with some sharks having stronger nocturnal adaptations. Many sharks can contract and dilate their
pupil The pupil is a black hole located in the center of the Iris (anatomy), iris of the Human eye, eye that allows light to strike the retina.Cassin, B. and Solomon, S. (1990) ''Dictionary of Eye Terminology''. Gainesville, Florida: Triad Publishing ...
s, like humans, something no teleost fish can do. Sharks have eyelids, but they do not blink because the surrounding water cleans their eyes. To protect their eyes some species have nictitating membranes. This membrane covers the eyes while hunting and when the shark is being attacked. However, some species, including the great white shark (''Carcharodon carcharias''), do not have this membrane, but instead roll their eyes backwards to protect them when striking prey. The importance of sight in shark hunting behavior is debated. Some believe that electro- and
chemoreception A chemoreceptor, also known as chemosensor, is a specialized sensory receptor which transduces a chemical substance (endogenous or induced) to generate a biological signal. This signal may be in the form of an action potential, if the chemorecept ...
are more significant, while others point to the nictating membrane as evidence that sight is important. Presumably, the shark would not protect its eyes were they unimportant. The use of sight probably varies with species and water conditions. The shark's field of vision can swap between
monocular A monocular is a compact refracting telescope used to magnify images of distant objects, typically using an optical prism to ensure an erect image, instead of using relay lenses like most telescopic sights. The volume and weight of a mono ...
and stereoscopic at any time. A
micro-spectrophotometry Microspectrophotometry is the measure of the spectra of microscopic samples using different wavelengths of electromagnetic radiation (e.g. ultraviolet, visible and near infrared, etc.) It is accomplished with ''microspectrophotometers'', ''cytospect ...
study of 17 species of shark found 10 had only rod photoreceptors and no cone cells in their
retina The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which then ...
s giving them good night vision while making them
colorblind Color blindness or color vision deficiency (CVD) is the decreased ability to see color or differences in color. It can impair tasks such as selecting ripe fruit, choosing clothing, and reading traffic lights. Color blindness may make some aca ...
. The remaining seven species had in addition to rods a single type of cone photoreceptor sensitive to green and, seeing only in shades of grey and green, are believed to be effectively colorblind. The study indicates that an object's contrast against the background, rather than colour, may be more important for object detection.


Hearing

Although it is hard to test the hearing of sharks, they may have a sharp sense of hearing and can possibly hear prey from many miles away. The hearing sensitivity for most shark species lies between 20 and 1000 Hz. A small opening on each side of their heads (not the spiracle) leads directly into the inner ear through a thin channel. The lateral line shows a similar arrangement, and is open to the environment via a series of openings called lateral line pores. This is a reminder of the common origin of these two vibration- and sound-detecting organs that are grouped together as the acoustico-lateralis system. In bony fish and tetrapods the external opening into the inner ear has been lost.


Electroreception

The ampullae of Lorenzini are the electroreceptor organs. They number in the hundreds to thousands. Sharks use the ampullae of Lorenzini to detect the electromagnetic fields that all living things produce. This helps sharks (particularly the
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 ...
) find prey. The shark has the greatest electrical sensitivity of any animal. Sharks find prey hidden in sand by detecting the electric fields they produce. Ocean currents moving in the
magnetic field of the Earth Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from the Sun. The magnetic fi ...
also generate electric fields that sharks can use for orientation and possibly navigation.


Lateral line

This system is found in most fish, including sharks. It is a tactile sensory system which allows the organism to detect water speed and pressure changes near by. The main component of the system is the neuromast, a cell similar to
hair cell Hair cells are the sensory receptors of both the auditory system and the vestibular system in the ears of all vertebrates, and in the lateral line organ of fishes. Through mechanotransduction, hair cells detect movement in their environment. ...
s present in the vertebrate ear that interact with the surrounding aquatic environment. This helps sharks distinguish between the currents around them, obstacles off on their periphery, and struggling prey out of visual view. The shark can sense frequencies in the range of 25 to 50  Hz.


Life history

Shark lifespans vary by species. Most live 20 to 30 years. The
spiny dogfish The spiny dogfish (''Squalus acanthias''), spurdog, mud shark, or piked dogfish is one of the best known species of the Squalidae (dogfish) family of sharks, which is part of the Squaliformes order. While these common names may apply to several ...
has one of the longest lifespans at more than 100 years. Whale sharks (''Rhincodon typus'') may also live over 100 years. Earlier estimates suggested the Greenland shark (''Somniosus microcephalus'') could reach about 200 years, but a recent study found that a specimen was 392 ± 120 years old (i.e., at least 272 years old), making it the longest-lived vertebrate known.


Reproduction

Unlike most bony fish, sharks are K-selected reproducers, meaning that they produce a small number of well-developed young as opposed to a large number of poorly developed young.
Fecundity Fecundity is defined in two ways; in human demography, it is the potential for reproduction of a recorded population as opposed to a sole organism, while in population biology, it is considered similar to fertility, the natural capability to pr ...
in sharks ranges from 2 to over 100 young per reproductive cycle. Sharks mature slowly relative to many other fish. For example, lemon sharks reach sexual maturity at around age 13–15.


Sexual

Sharks practice
internal fertilization Internal fertilization is the union of an egg and sperm cell during sexual reproduction inside the female body. Internal fertilization, unlike its counterpart, external fertilization, brings more control to the female with reproduction. For inte ...
. The posterior part of a male shark's pelvic fins are modified into a pair of intromittent organs called
clasper In biology, a clasper is a male anatomical structure found in some groups of animals, used in mating. Male cartilaginous fish have claspers formed from the posterior portion of their pelvic fin which serve to channel semen into the female's c ...
s, analogous to a mammalian penis, of which one is used to deliver sperm into the female.
Mating In biology, mating is the pairing of either opposite- sex or hermaphroditic organisms for the purposes of sexual reproduction. ''Fertilization'' is the fusion of two gametes. ''Copulation'' is the union of the sex organs of two sexually reprod ...
has rarely been observed in sharks. The smaller
catshark Catsharks are ground sharks of the family Scyliorhinidae. They are the largest family of sharks with around 160 species placed in 17 genera. Although they are generally known as catsharks, some species can also be called dogfish due to previous ...
s often mate with the male curling around the female. In less flexible species the two sharks swim parallel to each other while the male inserts a clasper into the female's
oviduct The oviduct in mammals, is the passageway from an ovary. In human females this is more usually known as the Fallopian tube or uterine tube. The eggs travel along the oviduct. These eggs will either be fertilized by spermatozoa to become a zygote, o ...
. Females in many of the larger species have bite marks that appear to be a result of a male grasping them to maintain position during
mating In biology, mating is the pairing of either opposite- sex or hermaphroditic organisms for the purposes of sexual reproduction. ''Fertilization'' is the fusion of two gametes. ''Copulation'' is the union of the sex organs of two sexually reprod ...
. The bite marks may also come from courtship behavior: the male may bite the female to show his interest. In some species, females have evolved thicker skin to withstand these bites.


Asexual

There have been a number of documented cases in which a female shark who has not been in contact with a male has conceived a pup on her own through parthenogenesis. The details of this process are not well understood, but
genetic fingerprinting DNA profiling (also called DNA fingerprinting) is the process of determining an individual's DNA characteristics. DNA analysis intended to identify a species, rather than an individual, is called DNA barcoding. DNA profiling is a forensic t ...
showed that the pups had no paternal genetic contribution, ruling out sperm storage. The extent of this behavior in the wild is unknown. Mammals are now the only major
vertebrate 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, with c ...
group in which asexual reproduction has not been observed. Scientists say that asexual reproduction in the wild is rare, and probably a last-ditch effort to reproduce when a mate is not present. Asexual reproduction diminishes genetic diversity, which helps build defenses against threats to the species. Species that rely solely on it risk extinction. Asexual reproduction may have contributed to the blue shark's decline off the
Irish Irish may refer to: Common meanings * Someone or something of, from, or related to: ** Ireland, an island situated off the north-western coast of continental Europe ***Éire, Irish language name for the isle ** Northern Ireland, a constituent unit ...
coast.


Brooding

Sharks display three ways to bear their young, varying by species, oviparity, viviparity and ovoviviparity.


Ovoviviparity

Most sharks are ovoviviparous, meaning that the eggs hatch in the
oviduct The oviduct in mammals, is the passageway from an ovary. In human females this is more usually known as the Fallopian tube or uterine tube. The eggs travel along the oviduct. These eggs will either be fertilized by spermatozoa to become a zygote, o ...
within the mother's body and that the egg's yolk and fluids secreted by glands in the walls of the oviduct nourishes the embryos. The young continue to be nourished by the remnants of the yolk and the oviduct's fluids. As in viviparity, the young are born alive and fully functional. Lamniforme sharks practice '' oophagy'', where the first embryos to hatch eat the remaining eggs. Taking this a step further, sand tiger shark pups cannibalistically consume neighboring embryos. The survival strategy for ovoviviparous species is to
brood Brood may refer to: Nature * Brood, a collective term for offspring * Brooding, the incubation of bird eggs by their parents * Bee brood, the young of a beehive * Individual broods of North American Periodical Cicadas: ** Brood X, the largest b ...
the young to a comparatively large size before birth. The whale shark is now classified as ovoviviparous rather than oviparous, because extrauterine eggs are now thought to have been aborted. Most ovoviviparous sharks give birth in sheltered areas, including bays, river mouths and shallow reefs. They choose such areas for protection from predators (mainly other sharks) and the abundance of food. Dogfish have the longest known gestation period of any shark, at 18 to 24 months.
Basking shark The basking shark (''Cetorhinus maximus'') is the second-largest living shark and fish, after the whale shark, and one of three plankton-eating shark species, along with the whale shark and megamouth shark. Adults typically reach in leng ...
s and frilled sharks appear to have even longer gestation periods, but accurate data are lacking.


Oviparity

Some species are oviparous, laying their fertilized eggs in the water. In most oviparous shark species, an egg case with the consistency of
leather Leather is a strong, flexible and durable material obtained from the tanning, or chemical treatment, of animal skins and hides to prevent decay. The most common leathers come from cattle, sheep, goats, equine animals, buffalo, pigs and hog ...
protects the developing embryo(s). These cases may be corkscrewed into crevices for protection. The egg case is commonly called a '' mermaid's purse''. Oviparous sharks include the
horn shark The horn shark (''Heterodontus francisci'') is a species of bullhead shark, in the family Heterodontidae. It is endemic to the coastal waters off the western coast of North America, from California to the Gulf of California. Young sharks are seg ...
,
catshark Catsharks are ground sharks of the family Scyliorhinidae. They are the largest family of sharks with around 160 species placed in 17 genera. Although they are generally known as catsharks, some species can also be called dogfish due to previous ...
,
Port Jackson shark The Port Jackson shark (''Heterodontus portusjacksoni'') is a nocturnal, oviparous (egg laying) type of bullhead shark of the family Heterodontidae, found in the coastal region of southern Australia, including the waters off Port Jackson. It has ...
, and swellshark.


Viviparity

Viviparity is the gestation of young without the use of a traditional egg, and results in live birth. Viviparity in sharks can be placental or aplacental. Young are born fully formed and self-sufficient. Hammerheads, the requiem sharks (such as the
bull A bull is an intact (i.e., not castrated) adult male of the species ''Bos taurus'' (cattle). More muscular and aggressive than the females of the same species (i.e., cows), bulls have long been an important symbol in many religions, includin ...
and blue sharks), and smoothhounds are viviparous.


Behavior

The classic view describes a solitary hunter, ranging the oceans in search of food. However, this applies to only a few species. Most live far more social, sedentary, benthic lives, and appear likely to have their own distinct personalities. Even solitary sharks meet for breeding or at rich hunting grounds, which may lead them to cover thousands of miles in a year. Shark migration patterns may be even more complex than in birds, with many sharks covering entire
ocean basin In hydrology, an oceanic basin (or ocean basin) is anywhere on Earth that is covered by seawater. Geologically, ocean basins are large  geologic basins that are below sea level. Most commonly the ocean is divided into basins fol ...
s. Sharks can be highly social, remaining in large schools. Sometimes more than 100
scalloped hammerhead The scalloped hammerhead (''Sphyrna lewini'') is a species of hammerhead shark in the family Sphyrnidae. It was originally known as ''Zygaena lewini''. The Greek word ''sphyrna'' translates into "hammer" in English, referring to the shape of thi ...
s congregate around seamounts and islands, e.g., in the Gulf of California. Cross-species social hierarchies exist. For example, oceanic whitetip sharks dominate silky sharks of comparable size during feeding. When approached too closely some sharks perform a
threat display Deimatic behaviour or startle display means any pattern of bluffing behaviour in an animal that lacks strong defences, such as suddenly displaying conspicuous eyespots, to scare off or momentarily distract a predator, thus giving the prey anima ...
. This usually consists of exaggerated swimming movements, and can vary in intensity according to the threat level.


Speed

In general, sharks swim ("cruise") at an average speed of , but when feeding or attacking, the average shark can reach speeds upwards of . The
shortfin mako shark The shortfin mako shark (; ; ''Isurus oxyrinchus''), also known as the blue pointer or bonito shark, is a large mackerel shark. It is commonly referred to as the mako shark, as is the longfin mako shark (''Isurus paucus''). The shortfin mako can ...
, the fastest shark and one of the fastest fish, can burst at speeds up to . The great white shark is also capable of speed bursts. These exceptions may be due to the warm-blooded, or
homeothermic Homeothermy, homothermy or homoiothermy is thermoregulation that maintains a stable internal body temperature regardless of external influence. This internal body temperature is often, though not necessarily, higher than the immediate environmen ...
, nature of these sharks' physiology. Sharks can travel 70 to 80 km in a day.


Intelligence

Sharks possess brain-to-body mass ratios that are similar to mammals and birds, and have exhibited apparent curiosity and behavior resembling play in the wild. There is evidence that juvenile lemon sharks can use observational learning in their investigation of novel objects in their environment.


Sleep

All sharks need to keep water flowing over their gills in order for them to breathe; however, not all species need to be moving to do this. Those that are able to breathe while not swimming do so by using their spiracles to force water over their gills, thereby allowing them to extract oxygen from the water. It has been recorded that their eyes remain open while in this state and actively follow the movements of divers swimming around them and as such they are not truly asleep. Species that do need to swim continuously to breathe go through a process known as sleep swimming, in which the shark is essentially unconscious. It is known from experiments conducted on the
spiny dogfish The spiny dogfish (''Squalus acanthias''), spurdog, mud shark, or piked dogfish is one of the best known species of the Squalidae (dogfish) family of sharks, which is part of the Squaliformes order. While these common names may apply to several ...
that its
spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the sp ...
, rather than its brain, coordinates swimming, so
spiny dogfish The spiny dogfish (''Squalus acanthias''), spurdog, mud shark, or piked dogfish is one of the best known species of the Squalidae (dogfish) family of sharks, which is part of the Squaliformes order. While these common names may apply to several ...
can continue to swim while sleeping, and this also may be the case in larger shark species. In 2016 a great white shark was captured on video for the first time in a state researchers believed was sleep swimming.


Ecology


Feeding

Most sharks are carnivorous.
Basking shark The basking shark (''Cetorhinus maximus'') is the second-largest living shark and fish, after the whale shark, and one of three plankton-eating shark species, along with the whale shark and megamouth shark. Adults typically reach in leng ...
s, whale sharks, and
megamouth shark The megamouth shark (''Megachasma pelagios'') is a species of deepwater shark. It is rarely seen by humans and is the smallest of the three extant filter-feeding sharks alongside the relatively larger whale shark and basking shark. Since its d ...
s have independently evolved different strategies for filter feeding plankton: basking sharks practice ram feeding, whale sharks use suction to take in plankton and small fishes, and megamouth sharks make
suction feeding Aquatic feeding mechanisms face a special difficulty as compared to feeding on land, because the density of water is about the same as that of the prey, so the prey tends to be pushed away when the mouth is closed. This problem was first identifi ...
more efficient by using the
luminescent Luminescence is spontaneous emission of light by a substance not resulting from heat; or "cold light". It is thus a form of cold-body radiation. It can be caused by chemical reactions, electrical energy, subatomic motions or stress on a crystal ...
tissue inside of their mouths to attract prey in the deep ocean. This type of feeding requires gill rakers—long, slender filaments that form a very efficient
sieve A sieve, fine mesh strainer, or sift, is a device for separating wanted elements from unwanted material or for controlling the particle size distribution of a sample, using a screen such as a woven mesh or net or perforated sheet materia ...
—analogous to the baleen plates of the great whales. The shark traps the plankton in these filaments and swallows from time to time in huge mouthfuls. Teeth in these species are comparatively small because they are not needed for feeding. Other highly specialized feeders include
cookiecutter shark The cookiecutter shark (''Isistius brasiliensis''), also called the cigar shark, is a species of small squaliform shark in the family Dalatiidae. This shark occurs in warm, oceanic waters worldwide, particularly near islands, and has been rec ...
s, which feed on flesh sliced out of other larger fish and marine mammals. Cookiecutter teeth are enormous compared to the animal's size. The lower teeth are particularly sharp. Although they have never been observed feeding, they are believed to latch onto their prey and use their thick lips to make a seal, twisting their bodies to rip off flesh. Some seabed–dwelling species are highly effective ambush predators.
Angel shark The angelsharks are a group of sharks in the genus ''Squatina'' of the family Squatinidae. They commonly inhabit sandy seabeds close to in depth. Many species are now classified as critically endangered by the International Union for Conservat ...
s and
wobbegong The wobbegong is the common name given to the 12 species of carpet sharks in the family Orectolobidae. They are found in shallow temperate and tropical waters of the western Pacific Ocean and eastern Indian Ocean, chiefly around Australia and I ...
s use camouflage to lie in wait and suck prey into their mouths. Many benthic sharks feed solely on
crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean group can ...
s which they crush with their flat molariform teeth. Other sharks feed on squid or fish, which they swallow whole. The
viper dogfish The viper dogfish or viper shark (''Trigonognathus kabeyai'') is a rare species of dogfish shark in the family Etmopteridae, and the only extant member of its genus. It has been found in the Pacific Ocean off southern Japan, the Bonin Islands, P ...
has teeth it can point outwards to strike and capture prey that it then swallows intact. The great white and other large predators either swallow small prey whole or take huge bites out of large animals. Thresher sharks use their long tails to stun shoaling fishes, and
sawshark A sawshark or saw shark is a member of a shark order (Pristiophoriformes ) bearing a unique long, saw-like rostrum (snout or bill) edged with sharp teeth, which they use to slash and disable their prey. There are eight species within the Pristi ...
s either stir prey from the seabed or slash at swimming prey with their tooth-studded
rostra The rostra ( it, Rostri, links=no) was a large platform built in the city of Rome that stood during the republican and imperial periods. Speakers would stand on the rostra and face the north side of the comitium towards the senate house and de ...
. Many sharks, including the
whitetip reef shark The whitetip reef shark (''Triaenodon obesus'') is a species of requiem shark, in the family Carcharhinidae, and the only member of its genus. A small shark that does not usually exceed in length, this species is easily recognizable by its slen ...
are cooperative feeders and hunt in packs to herd and capture elusive prey. These social sharks are often migratory, traveling huge distances around
ocean basin In hydrology, an oceanic basin (or ocean basin) is anywhere on Earth that is covered by seawater. Geologically, ocean basins are large  geologic basins that are below sea level. Most commonly the ocean is divided into basins fol ...
s in large schools. These migrations may be partly necessary to find new food sources.


Range and habitat

Sharks are found in all seas. They generally do not live in fresh water, with a few 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 swim both in seawater and freshwater. Sharks are common down to depths of , and some live even deeper, but they are almost entirely absent below . The deepest confirmed report of a shark is a Portuguese dogfish at .


Relationship with humans


Attacks

In 2006 the
International Shark Attack File The International Shark Attack File' is a global database of shark attacks. It began as an attempt to catalogue shark attacks on servicemen during World War II. The Office of Naval Research funded it from 1958 until 1968. During that time, a p ...
(ISAF) undertook an investigation into 96 alleged shark attacks, confirming 62 of them as unprovoked attacks and 16 as provoked attacks. The average number of fatalities worldwide per year between 2001 and 2006 from unprovoked shark attacks is 4.3. Contrary to popular belief, only a few sharks are dangerous to humans. Out of more than 470 species, only four have been involved in a significant number of fatal, unprovoked attacks on humans: the great white, oceanic whitetip, tiger, and
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 ...
s. These sharks are large, powerful predators, and may sometimes attack and kill people. Despite being responsible for attacks on humans they have all been filmed without using a protective cage. The perception of sharks as dangerous animals has been popularized by publicity given to a few isolated unprovoked attacks, such as the Jersey Shore shark attacks of 1916, and through popular fictional works about shark attacks, such as the '' Jaws'' film series. ''Jaws'' author Peter Benchley, as well as ''Jaws'' director Steven Spielberg, later attempted to dispel the image of sharks as man-eating monsters. To help avoid an unprovoked attack, humans should not wear jewelry or metal that is shiny and refrain from splashing around too much. In general, sharks show little pattern of attacking humans specifically. Research indicates that when humans do become the object of a shark attack, it is possible that the shark has mistaken the human for species that are its normal prey, such as seals.The real reasons why sharks attack humans
By Richard Gray, 8th August 2019.


In captivity

Until recently, only a few benthic species of shark, such as hornsharks,
leopard shark The leopard shark (''Triakis semifasciata'') is a species of houndshark, in the family Triakidae. It is found along the Pacific coast of North America, from the U.S. state of Oregon to Mazatlán in Mexico. Typically measuring 1.2–1.5 m (3.9� ...
s and
catshark Catsharks are ground sharks of the family Scyliorhinidae. They are the largest family of sharks with around 160 species placed in 17 genera. Although they are generally known as catsharks, some species can also be called dogfish due to previous ...
s, had survived in aquarium conditions for a year or more. This gave rise to the belief that sharks, as well as being difficult to capture and transport, were difficult to care for. More knowledge has led to more species (including the large pelagic sharks) living far longer in captivity, along with safer transportation techniques that have enabled long-distance transportation. The great white shark had never been successfully held in captivity for long periods of time until September 2004, when the Monterey Bay Aquarium successfully kept a young female for 198 days before releasing her. Most species are not suitable for home aquaria, and not every species sold by
pet store A pet shop or pet store is a retail business which sells animals and pet care resources to the public. A variety of animal supplies and pet accessories are also sold in pet shops. The products sold include: food, treats, toys, collars, leashes, c ...
s are appropriate. Some species can flourish in home saltwater aquaria. Uninformed or unscrupulous dealers sometimes sell juvenile sharks like the
nurse shark The nurse shark (''Ginglymostoma cirratum'') is an elasmobranch fish in the family Ginglymostomatidae. The conservation status of the nurse shark is globally assessed as Vulnerable in the IUCN List of Threatened Species. They are considered t ...
, which upon reaching adulthood is far too large for typical home aquaria. Public aquaria generally do not accept donated specimens that have outgrown their housing. Some owners have been tempted to release them. Species appropriate to home aquaria represent considerable spatial and financial investments as they generally approach adult lengths of and can live up to 25 years.


In culture


In Hawaii

Sharks figure prominently in
Hawaiian mythology Hawaiian religion refers to the indigenous religious beliefs and practices of native Hawaiians, also known as the kapu system. Hawaiian religion is based largely on the tapu religion common in Polynesia and likely originated among the Tahitian ...
. Stories tell of men with shark jaws on their back who could change between shark and human form. A common theme was that a shark-man would warn beach-goers of sharks in the waters. The beach-goers would laugh and ignore the warnings and get eaten by the shark-man who warned them.
Hawaii Hawaii ( ; haw, Hawaii or ) is a state in the Western United States, located in the Pacific Ocean about from the U.S. mainland. It is the only U.S. state outside North America, the only state that is an archipelago, and the only state ...
an mythology also includes many shark
god In monotheistic thought, God is usually viewed as the supreme being, creator, and principal object of faith. Swinburne, R.G. "God" in Honderich, Ted. (ed)''The Oxford Companion to Philosophy'', Oxford University Press, 1995. God is typically ...
s. Among a fishing people, the most popular of all
aumakua In Hawaiian mythology, an ʻaumakua (; often spelled aumakua, plural, aumākua) is a personal or family god that originated as a deified ancestor, and which takes on physical forms such as spirit vehicles. An 'aumakua may manifest as a shark, ow ...
, or deified ancestor guardians, are shark aumakua. Kamaku describes in detail how to offer a corpse to become a shark. The body transforms gradually until the
kahuna ''Kahuna'' is a Hawaiian language, Hawaiian word that refers to an expert in any field. Historically, it has been used to refer to doctors, surgeons and dentists, as well as priests, ministers, and sorcerers. Background A ''kahuna'' may be ver ...
can point the awe-struck family to the markings on the shark's body that correspond to the clothing in which the beloved's body had been wrapped. Such a shark aumakua becomes the family pet, receiving food, and driving fish into the family net and warding off danger. Like all aumakua it had evil uses such as helping kill enemies. The ruling chiefs typically forbade such sorcery. Many Native Hawaiian families claim such an aumakua, who is known by name to the whole community. Kamohoali'i is the best known and revered of the shark gods, he was the older and favored brother of Pele, and helped and journeyed with her to Hawaii. He was able to assume all human and fish forms. A summit cliff on the crater of Kilauea is one of his most sacred spots. At one point he had a ''
heiau A ''heiau'' () is a Hawaiian temple. Made in different architectural styles depending upon their purpose and location, they range from simple earth terraces, to elaborately constructed stone platforms. There are heiau to treat the sick (''heia ...
'' (temple or shrine) dedicated to him on every piece of land that jutted into the ocean on the island of Molokai. Kamohoali'i was an ancestral god, not a human who became a shark and banned the eating of humans after eating one herself. In Fijian mythology, Dakuwaqa was a shark god who was the eater of lost souls.


In American Samoa

On the island of Tutuila in American Samoa (a U.S. territory), there is a location called Turtle and Shark (''Laumei ma Malie'') which is important in
Samoan culture The traditional culture of Samoa is a communal way of life based on Fa'a Samoa, the unique socio-political culture. In Samoan culture, most activities are done together. The traditional living quarters, or '' fale'' (houses), contain no walls and ...
— the location is the site of a legend called ''O Le Tala I Le Laumei Ma Le Malie'', in which two humans are said to have transformed into a turtle and a shark. According to the U.S. National Park Service, "Villagers from nearby Vaitogi continue to reenact an important aspect of the legend at Turtle and Shark by performing a ritual song intended to summon the legendary animals to the ocean surface, and visitors are frequently amazed to see one or both of these creatures emerge from the sea in apparent response to this call."


In popular culture

In contrast to the complex portrayals by Hawaiians and other Pacific Islanders, the European and Western view of sharks has historically been mostly of fear and malevolence. Sharks are used in popular culture commonly as eating machines, notably in the '' Jaws'' novel and the film of the same name, along with its
sequels A sequel is a work of literature, film, theatre, television, music or video game that continues the story of, or expands upon, some earlier work. In the common context of a narrative work of fiction, a sequel portrays events set in the same ...
. Sharks are threats in other films such as '' Deep Blue Sea'', '' The Reef'', and others, although they are sometimes used for comedic effect such as in '' Finding Nemo'' and the '' Austin Powers'' series. Sharks tend to be seen quite often in cartoons whenever a scene involves the ocean. Such examples include the '' Tom and Jerry'' cartoons, ''
Jabberjaw ''Jabberjaw'' is an American animated television series created by Joe Ruby and Ken Spears and produced by Hanna-Barbera which aired 16 original episodes on ABC from September 11 to December 18, 1976. Reruns continued on ABC until September 3 ...
'', and other shows produced by Hanna-Barbera. They also are used commonly as a clichéd means of killing off a character that is held up by a rope or some similar object as the sharks swim right below them, or the character may be standing on a
plank Plank may refer to: * Plank (wood), flat, elongated, and rectangular timber with parallel faces * Plank (exercise), an isometric exercise for the abdominal muscles *Martins Creek (Kentucky), the location of Plank post office * ''The Plank'' (1967 f ...
above shark infested waters.


Popular misconceptions

A popular myth is that sharks are immune to disease and
cancer Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body. These contrast with benign tumors, which do not spread. Possible signs and symptoms include a lump, abnormal b ...
, but this is not scientifically supported. Sharks have been known to get cancer. Both diseases and
parasites Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson ha ...
affect sharks. The evidence that sharks are at least resistant to cancer and disease is mostly anecdotal and there have been few, if any, scientific or statistical studies that show sharks to have heightened immunity to disease. Other apparently false claims are that fins prevent
cancer Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body. These contrast with benign tumors, which do not spread. Possible signs and symptoms include a lump, abnormal b ...
and treat osteoarthritis. No scientific proof supports these claims; at least one study has shown shark cartilage of no value in cancer treatment.


Threats to sharks


Fishery

In 2008, it was estimated that nearly 100 million sharks were being killed by people every year, due to commercial and recreational fishing. In 2021, it was estimated that the population of oceanic sharks and rays had dropped by 71% over the previous half-century. Shark finning yields are estimated at for 2000, and for 2010. Based on an analysis of average shark weights, this translates into a total annual mortality estimate of about 100 million sharks in 2000, and about 97 million sharks in 2010, with a total range of possible values between 63 and 273 million sharks per year. Sharks are a common seafood in many places, including Japan and Australia. In the Australian state of
Victoria Victoria most commonly refers to: * Victoria (Australia), a state of the Commonwealth of Australia * Victoria, British Columbia, provincial capital of British Columbia, Canada * Victoria (mythology), Roman goddess of Victory * Victoria, Seychelle ...
, shark is the most commonly used fish in fish and chips, in which fillets are battered and
deep-fried Deep frying (also referred to as deep fat frying) is a cooking method in which food is submerged in hot fat, traditionally lard but today most commonly oil, as opposed to the shallow oil used in conventional frying done in a frying pan. Norma ...
or crumbed and grilled. In fish and chip shops, shark is called flake. In
India India, officially the Republic of India (Hindi: ), is a country in South Asia. It is the seventh-largest country by area, the second-most populous country, and the most populous democracy in the world. Bounded by the Indian Ocean on the so ...
, small sharks or baby sharks (called sora in
Tamil language Tamil (; ' , ) is a Dravidian language natively spoken by the Tamil people of South Asia. Tamil is an official language of the Indian state of Tamil Nadu, the sovereign nations of Sri Lanka and Singapore, and the Indian territory of P ...
,
Telugu language Telugu (; , ) is a Dravidian language spoken by Telugu people predominantly living in the Indian states of Andhra Pradesh and Telangana, where it is also the official language. It is the most widely spoken member of the Dravidian language fam ...
) are sold in local markets. Since the flesh is not developed, cooking the flesh breaks it into powder, which is then fried in oil and spices (called sora puttu/sora poratu). The soft bones can be easily chewed. They are considered a delicacy in coastal
Tamil Nadu Tamil Nadu (; , TN) is a state in southern India. It is the tenth largest Indian state by area and the sixth largest by population. Its capital and largest city is Chennai. Tamil Nadu is the home of the Tamil people, whose Tamil language ...
.
Iceland Iceland ( is, Ísland; ) is a Nordic island country in the North Atlantic Ocean and in the Arctic Ocean. Iceland is the most sparsely populated country in Europe. Iceland's capital and largest city is Reykjavík, which (along with its s ...
ers ferment Greenland sharks to produce a delicacy called hákarl. During a four-year period from 1996 to 2000, an estimated 26 to 73 million sharks were killed and traded annually in commercial markets. Sharks are often killed for shark fin soup. Fishermen capture live sharks, fin them, and dump the finless animal back into the water.
Shark finning Shark finning is the act of removing fins from sharks and discarding the rest of the shark back into the ocean. This act is prohibited in many countries. The sharks are often still alive when discarded, but without their fins.Spiegel, J. (2000 ...
involves removing the fin with a hot metal blade. The resulting immobile shark soon dies from suffocation or predators. Shark fin has become a major trade within black markets all over the world. Fins sell for about $300/lb in 2009.Ask your senator to support the Shark Conservation Act
/ref> Poachers illegally fin millions each year. Few governments enforce laws that protect them. In 2010 Hawaii became the first U.S. state to prohibit the possession, sale, trade or distribution of shark fins. From 1996 to 2000, an estimated 38 million sharks had been killed per year for harvesting shark fins. It is estimated by
TRAFFIC Traffic comprises pedestrians, vehicles, ridden or herded animals, trains, and other conveyances that use public ways (roads) for travel and transportation. Traffic laws govern and regulate traffic, while rules of the road include traffi ...
that over 14,000 tonnes of shark fins were exported into Singapore between 2005–2007 and 2012–2014. Shark fin soup is a status symbol in Asian countries and is erroneously considered healthy and full of nutrients. Scientific research has revealed, however, that high concentrations of BMAA are present in shark fins. Because BMAA is a neurotoxin, consumption of shark fin soup and cartilage pills, therefore, may pose a health risk. BMAA is under study for its pathological role in neurodegegerative diseases such as, ALS, Alzheimer's disease, and
Parkinson's disease Parkinson's disease (PD), or simply Parkinson's, is a long-term degenerative disorder of the central nervous system that mainly affects the motor system. The symptoms usually emerge slowly, and as the disease worsens, non-motor symptoms becom ...
. Sharks are also killed for meat. European diners consume dogfishes, smoothhounds,
catshark Catsharks are ground sharks of the family Scyliorhinidae. They are the largest family of sharks with around 160 species placed in 17 genera. Although they are generally known as catsharks, some species can also be called dogfish due to previous ...
s, makos, porbeagle and also skates and rays. However, the
U.S. The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territori ...
FDA lists sharks as one of four fish (with swordfish,
king mackerel The king mackerel (''Scomberomorus cavalla'') or kingfish, is a migratory species of mackerel of the western Atlantic Ocean and Gulf of Mexico. It is an important species to both the commercial and recreational fishing industries. Description Th ...
, and
tilefish 250px, Blue blanquillo, ''Malacanthus latovittatus'' Tilefishes are mostly small perciform marine fish comprising the family Malacanthidae. They are usually found in sandy areas, especially near coral reefs. Commercial fisheries exist for th ...
) whose high mercury content is hazardous to children and pregnant women. Sharks generally reach
sexual maturity Sexual maturity is the capability of an organism to reproduce. In humans it might be considered synonymous with adulthood, but here puberty is the name for the process of biological sexual maturation, while adulthood is based on cultural definit ...
only after many years and produce few offspring in comparison to other harvested fish. Harvesting sharks before they reproduce severely impacts future populations. Capture induced premature birth and abortion (collectively called capture-induced parturition) occurs frequently in sharks/rays when fished. Capture-induced parturition is rarely considered in fisheries management despite being shown to occur in at least 12% of live bearing sharks and rays (88 species to date). The majority of shark fisheries have little monitoring or management. The rise in demand for shark products increases pressure on fisheries. Major declines in shark stocks have been recorded—some species have been depleted by over 90% over the past 20–30 years with population declines of 70% not unusual. A study by the
International Union for Conservation of Nature The International Union for Conservation of Nature (IUCN; officially International Union for Conservation of Nature and Natural Resources) is an international organization working in the field of nature conservation and sustainable use of nat ...
suggests that one quarter of all known species of sharks and rays are threatened by extinction and 25 species were classified as critically endangered.


Shark culling

In 2014, a shark cull in Western Australia killed dozens of sharks (mostly tiger sharks) using drum lines, until it was cancelled after public protests and a decision by the Western Australia EPA; from 2014 to 2017, there was an "imminent threat" policy in Western Australia in which sharks that "threatened" humans in the ocean were shot and killed. This "imminent threat" policy was criticized by senator Rachel Siewart for killing endangered sharks. The "imminent threat" policy was cancelled in March 2017. In August 2018, the Western Australia government announced a plan to re-introduce drum lines (though, this time the drum lines are "SMART" drum lines). From 1962 to the present, the government of Queensland has targeted and killed sharks in large numbers by using drum lines, under a "shark control" program—this program has also inadvertently killed large numbers of other animals such as dolphins; it has also killed endangered
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 ...
s. Queensland's drum line program has been called "outdated, cruel and ineffective". From 2001 to 2018, a total of 10,480 sharks were killed on lethal drum lines in Queensland, including in the Great Barrier Reef. From 1962 to 2018, roughly 50,000 sharks were killed by Queensland authorities. The government of
New South Wales ) , nickname = , image_map = New South Wales in Australia.svg , map_caption = Location of New South Wales in AustraliaCoordinates: , subdivision_type = Country , subdivision_name = Australia , established_title = Before federation , es ...
has a program that deliberately kills sharks using nets. The current net program in New South Wales has been described as being "extremely destructive" to marine life, including sharks. Between 1950 and 2008, 352
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 tempera ...
s and 577 great white sharks were killed in the nets in New South Wales — also during this period, a total of 15,135 marine animals were killed in the nets, including dolphins, whales, turtles, dugongs, and critically endangered
grey nurse shark The sand tiger shark (''Carcharias taurus''), gray nurse shark, spotted ragged-tooth shark or blue-nurse sand tiger, is a species of shark that inhabits subtropical and temperate waters worldwide. It inhabits the continental shelf, from sandy sho ...
s. There has been a very large decrease in the number of sharks in eastern Australia, and the shark-killing programs in Queensland and New South Wales are partly responsible for this decrease. Kwazulu-Natal, an area of
South Africa South Africa, officially the Republic of South Africa (RSA), is the Southern Africa, southernmost country in Africa. It is bounded to the south by of coastline that stretch along the Atlantic Ocean, South Atlantic and Indian Oceans; to the ...
, has a shark-killing program using nets and drum lines—these nets and drum lines have killed turtles and dolphins, and have been criticized for killing wildlife. During a 30-year period, more than 33,000 sharks have been killed in KwaZulu-Natal's shark-killing program — during the same 30-year period, 2,211 turtles, 8,448 rays, and 2,310 dolphins were killed in KwaZulu-Natal. Authorities on the French island of Réunion kill about 100 sharks per year. Killing sharks negatively affects the marine ecosystem. Jessica Morris of
Humane Society International Humane Society International (HSI) is the international division of The Humane Society of the United States. Founded in 1991, HSI has expanded The HSUS's activities into Central and South America, Africa, and Asia. HSI's Asian, Australian, Cana ...
calls shark culling a "knee-jerk reaction" and says, "sharks are top order predators that play an important role in the functioning of marine ecosystems. We need them for healthy oceans." George H. Burgess, the former director of the
International Shark Attack File The International Shark Attack File' is a global database of shark attacks. It began as an attempt to catalogue shark attacks on servicemen during World War II. The Office of Naval Research funded it from 1958 until 1968. During that time, a p ...
, "describes
hark Hark is from a Middle English word "herken", meaning to listen carefully. It may refer to: Geography *Hark, Byzantine name for the Muş Province area of Turkey *"The Hark" (Harkness Commons), a Gropius building on the Harvard Law School campus ...
culling as a form of revenge, satisfying a public demand for blood and little else"; he also said shark culling is a "retro-type move reminiscent of what people would have done in the 1940s and 50s, back when we didn't have an ecological conscience and before we knew the consequences of our actions." Jane Williamson, an associate professor in marine ecology at Macquarie University, says "There is no scientific support for the concept that culling sharks in a particular area will lead to a decrease in shark attacks and increase ocean safety."


Other threats

Other threats include habitat alteration, damage and loss from coastal development, pollution and the impact of fisheries on the seabed and prey species. The 2007 documentary ''
Sharkwater ''Sharkwater'' is a 2006 Canadian documentary film written and directed by Rob Stewart. Helping to protect sharks, changing government policy, and inspiring the creation of shark conservation groups, ''Sharkwater'' is considered one of conservati ...
'' exposed how sharks are being hunted to extinction.


Conservation

In 1991,
South Africa South Africa, officially the Republic of South Africa (RSA), is the Southern Africa, southernmost country in Africa. It is bounded to the south by of coastline that stretch along the Atlantic Ocean, South Atlantic and Indian Oceans; to the ...
was the first country in the world to declare Great White sharks a legally protected species (however, the KwaZulu-Natal Sharks Board is allowed to kill great white sharks in its " shark control" program in eastern South Africa). Intending to ban the practice of shark finning while at sea, the United States Congress passed the Shark Finning Prohibition Act in 2000. Two years later the Act saw its first legal challenge in ''
United States v. Approximately 64,695 Pounds of Shark Fins ''United States v. Approximately 64,695 Pounds of Shark Fins'' (520 F.3d 976) is a 2008 decision of the United States Court of Appeals for the Ninth Circuit concerning civil forfeiture in admiralty law. Judge Stephen Reinhardt wrote for a three-j ...
''. In 2008 a Federal Appeals Court ruled that a
loophole A loophole is an ambiguity or inadequacy in a system, such as a law or security, which can be used to circumvent or otherwise avoid the purpose, implied or explicitly stated, of the system. Originally, the word meant an arrowslit, a narrow ver ...
in the law allowed non-fishing vessels to ''purchase'' shark fins from fishing vessels while on the high seas.
United States v. Approximately 64,695 Pounds of Shark Fins
'', 520 F.3d 976, ( 9th Cir., 2008).
Seeking to close the loophole, the
Shark Conservation Act The Shark Conservation Act of 2009 (SCA) (, ) was passed by the 111th United States Congress that amended the High Seas Driftnet Fishing Moratorium Protection Act and the Magnuson–Stevens Fishery Conservation and Management Act to improve the con ...
was passed by Congress in December 2010, and it was signed into law in January 2011. In 2003, the European Union introduced a general shark finning ban for all vessels of all nationalities in Union waters and for all vessels flying a flag of one of its member states. This prohibition was amended in June 2013 to close remaining loopholes. In 2009, the
International Union for Conservation of Nature The International Union for Conservation of Nature (IUCN; officially International Union for Conservation of Nature and Natural Resources) is an international organization working in the field of nature conservation and sustainable use of nat ...
's '' IUCN Red List of Endangered Species'' named 64 species, one-third of all oceanic shark species, as being at risk of extinction due to fishing and shark finning. In 2010, the Convention on International Trade in Endangered Species ( CITES) rejected proposals from the
United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territori ...
and
Palau Palau,, officially the Republic of Palau and historically ''Belau'', ''Palaos'' or ''Pelew'', is an island country and microstate in the western Pacific. The nation has approximately 340 islands and connects the western chain of the ...
that would have required countries to strictly regulate trade in several species of
scalloped hammerhead The scalloped hammerhead (''Sphyrna lewini'') is a species of hammerhead shark in the family Sphyrnidae. It was originally known as ''Zygaena lewini''. The Greek word ''sphyrna'' translates into "hammer" in English, referring to the shape of thi ...
, oceanic whitetip and
spiny dogfish shark The spiny dogfish (''Squalus acanthias''), spurdog, mud shark, or piked dogfish is one of the best known species of the Squalidae (dogfish) family of sharks, which is part of the Squaliformes order. While these common names may apply to several ...
s. The majority, but not the required two-thirds of voting delegates, approved the proposal. China, by far the world's largest shark market, and Japan, which battles all attempts to extend the convention to marine species, led the opposition. In March 2013, three endangered commercially valuable sharks, the hammerheads, the oceanic whitetip and porbeagle were added to Appendix 2 of CITES, bringing shark fishing and commerce of these species under licensing and regulation. In 2010, Greenpeace International added the
school shark The school shark (''Galeorhinus galeus'') is a houndshark of the family Triakidae, and the only member of the genus ''Galeorhinus''. Common names also include tope, tope shark, snapper shark, and soupfin shark. It is found worldwide in temperate ...
,
shortfin mako shark The shortfin mako shark (; ; ''Isurus oxyrinchus''), also known as the blue pointer or bonito shark, is a large mackerel shark. It is commonly referred to as the mako shark, as is the longfin mako shark (''Isurus paucus''). The shortfin mako can ...
, mackerel shark,
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 tempera ...
and
spiny dogfish The spiny dogfish (''Squalus acanthias''), spurdog, mud shark, or piked dogfish is one of the best known species of the Squalidae (dogfish) family of sharks, which is part of the Squaliformes order. While these common names may apply to several ...
to its seafood red list, a list of common supermarket fish that are often sourced from unsustainable fisheries. Advocacy group Shark Trust campaigns to limit shark fishing. Advocacy group
Seafood Watch __NOTOC__ Seafood Watch is a sustainable seafood advisory list, and has influenced similar programs around the world. It is best known for developing science-based seafood recommendations that consumers, chefs, and business professionals use to i ...
directs American consumers to not eat sharks. Under the auspices of the
Convention on the Conservation of Migratory Species of Wild Animals The Convention on the Conservation of Migratory Species of Wild Animals, also known as the Convention on Migratory Species (CMS) or the Bonn Convention, is an international agreement that aims to conserve migratory species throughout their r ...
(CMS), also known as the Bonn Convention, the
Memorandum of Understanding on the Conservation of Migratory Sharks The Memorandum of Understanding on the Conservation of Migratory Sharks is an international instrument for the conservation of migratory species of sharks. It was founded under the auspices of the Convention on the Conservation of Migratory Speci ...
was concluded and came into effect in March 2010. It was the first global instrument concluded under CMS and aims at facilitating international coordination for the protection, conservation and management of migratory sharks, through multilateral, intergovernmental discussion and scientific research. In July 2013, New York state, a major market and entry point for shark fins, banned the shark fin trade joining seven other states of the United States and the three Pacific U.S territories in providing legal protection to sharks. In the
United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territori ...
, and as of January 16, 2019, 12 states including (
Massachusetts Massachusetts (Massachusett: ''Muhsachuweesut Massachusett_writing_systems.html" ;"title="nowiki/> məhswatʃəwiːsət.html" ;"title="Massachusett writing systems">məhswatʃəwiːsət">Massachusett writing systems">məhswatʃəwiːsət'' En ...
,
Maryland Maryland ( ) is a state in the Mid-Atlantic region of the United States. It shares borders with Virginia, West Virginia, and the District of Columbia to its south and west; Pennsylvania to its north; and Delaware and the Atlantic Ocean to ...
, Delaware,
California California is a state in the Western United States, located along the Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the most populous U.S. state and the 3rd largest by area. It is also the m ...
,
Illinois Illinois ( ) is a state in the Midwestern United States. Its largest metropolitan areas include the Chicago metropolitan area, and the Metro East section, of Greater St. Louis. Other smaller metropolitan areas include, Peoria and Rockf ...
,
Hawaii Hawaii ( ; haw, Hawaii or ) is a state in the Western United States, located in the Pacific Ocean about from the U.S. mainland. It is the only U.S. state outside North America, the only state that is an archipelago, and the only state ...
,
Oregon Oregon () is a state in the Pacific Northwest region of the Western United States. The Columbia River delineates much of Oregon's northern boundary with Washington, while the Snake River delineates much of its eastern boundary with Idaho. T ...
,
Nevada Nevada ( ; ) is a state in the Western region of the United States. It is bordered by Oregon to the northwest, Idaho to the northeast, California to the west, Arizona to the southeast, and Utah to the east. Nevada is the 7th-most extensive, ...
, Rhode Island, Washington, New York and
Texas Texas (, ; Spanish: ''Texas'', ''Tejas'') is a state in the South Central region of the United States. At 268,596 square miles (695,662 km2), and with more than 29.1 million residents in 2020, it is the second-largest U.S. state by ...
) along with 3
U.S. territories Territories of the United States are sub-national administrative divisions overseen by the federal government of the United States. The various American territories differ from the U.S. states and tribal reservations as they are not sover ...
( American Samoa, Guam and the
Northern Mariana Islands The Northern Mariana Islands, officially the Commonwealth of the Northern Mariana Islands (CNMI; ch, Sankattan Siha Na Islas Mariånas; cal, Commonwealth Téél Falúw kka Efáng llól Marianas), is an unincorporated territory and commonw ...
) have passed laws against the sale or possession of shark fins. Several regions now have shark sanctuaries or have banned shark fishing — these regions include American Samoa, the Bahamas, the
Cook Islands ) , image_map = Cook Islands on the globe (small islands magnified) (Polynesia centered).svg , capital = Avarua , coordinates = , largest_city = Avarua , official_languages = , lan ...
, French Polynesia, Guam, the Maldives, the Marshall Islands, Micronesia, the
Northern Mariana Islands The Northern Mariana Islands, officially the Commonwealth of the Northern Mariana Islands (CNMI; ch, Sankattan Siha Na Islas Mariånas; cal, Commonwealth Téél Falúw kka Efáng llól Marianas), is an unincorporated territory and commonw ...
, and
Palau Palau,, officially the Republic of Palau and historically ''Belau'', ''Palaos'' or ''Pelew'', is an island country and microstate in the western Pacific. The nation has approximately 340 islands and connects the western chain of the ...
. In April 2020 researchers reported to have traced the origins of shark fins of endangered
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 ...
s from a retail market in Hong Kong back to their source populations and therefore the approximate locations where the sharks were first caught using DNA analysis. In July 2020 scientists reported results of a survey of 371 reefs in 58 nations estimating the conservation status of reef sharks globally. No sharks have been observed on almost 20% of the surveyed reefs and shark depletion was strongly associated with both socio-economic conditions and conservation measures. Sharks are considered to be a vital part of the ocean
ecosystem An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syste ...
. According to a 2021 study in ''
Nature Nature, in the broadest sense, is the physical world or universe. "Nature" can refer to the phenomena of the physical world, and also to life in general. The study of nature is a large, if not the only, part of science. Although humans are ...
'', overfishing has resulted in a 71% global decline in the number of oceanic sharks and
rays Ray may refer to: Fish * Ray (fish), any cartilaginous fish of the superorder Batoidea * Ray (fish fin anatomy), a bony or horny spine on a fin Science and mathematics * Ray (geometry), half of a line proceeding from an initial point * Ray (gra ...
over the preceding 50 years. The oceanic whitetip, and both the scalloped hammerhead and
great hammerhead The great hammerhead (''Sphyrna mokarran'') or great hammerhead shark is the largest species of hammerhead shark, belonging to the family Sphyrnidae, attaining an average length of and reaching a maximum length of . It is found in tropical and w ...
s are now classified as critically endangered. Sharks in tropical waters have declined more rapidly than those in temperate zones during the period studied. A 2021 study published in '' Current Biology'' found that overfishing is currently driving over one-third of sharks and rays to
extinction Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...
.


See also

* List of sharks * List of prehistoric cartilaginous fish genera * Osteichthyes * Marine vertebrate * Outline of sharks * Shark meat


Further reading

*
Sharks ‘critical’ to restoring damaged ecosystems, finds study
''
The Guardian ''The Guardian'' is a British daily newspaper. It was founded in 1821 as ''The Manchester Guardian'', and changed its name in 1959. Along with its sister papers ''The Observer'' and ''The Guardian Weekly'', ''The Guardian'' is part of the Gu ...
'', 22 March 2021 * Musick, John A and Musick, Susanna (2011
"Sharks"
In: ''Review of the state of world marine fishery resources'', pages 245–254, FAO Fisheries technical paper 569, FAO, Rome. .

''National Geographic'', 28 October 2010.


References

;General references * * * * * {{Good article Sharks Commercial fish Elasmobranchii Electroreceptive animals Ichthyology Predators Extant Silurian first appearances