{{Otheruses}}{{pp-semi-protected|small=yes}}{{Otheruses}}{{pp-semi-protected|small=yes}}
{{Otheruses}}
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{{Taxobox | name = Sharks
| fossil_range = {{Fossil range|Silurian|Recent}}
| trend = unknown
| status_system = iucn2.3
| image = Tiburón.jpg
| image_width = 250px
| image_caption = Grey reef shark
(Carcharhinus amblyrhynchos)
| image_alt = Grey reef shark
| regnum =
AnimalAnimals are a major group of mostly multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously...
ia
| phylum =
ChordataChordates are animals which are either vertebrates or one of several closely related invertebrates. They are united by having, for at least some period of their life cycle, a notochord, a hollow dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal tail...
| classis =
ChondrichthyesChondrichthyes or cartilaginous fishes are jawed fish with paired fins, paired nares, scales, two-chambered hearts, and skeletons made of cartilage rather than bone...
| subclassis =
ElasmobranchiiElasmobranchii is the subclass of cartilaginous fish that includes skates, rays , and sharks .-Evolution:Fossilised shark teeth are known from the early Devonian, around 400 million years ago. During the following Carboniferous period, the sharks underwent a period of diversification, with many...
| superordo =
Selachimorpha
| subdivision_ranks = Orders
| subdivision =
CarcharhiniformesThe ground sharks, order Carcharhiniformes, are the largest order of sharks; they are also called whaler sharks. With over 270 species, carcharhiniforms include a number of common types, such as the blue shark, catsharks, swellsharks, and sandbar shark.Members of the orders are characterized by the...
Heterodontiformes
HexanchiformesHexanchiformes is the order consisting of the most primitive types of sharks, and numbering just five extant species. Fossil sharks that were apparently very similar to modern sevengill species are known from Jurassic specimens....
LamniformesLamniformes is an order of sharks, also known as mackerel sharks...
Orectolobiformes
Pristiophoriformes
SqualiformesSqualiformes is an order of sharks that includes about 80 species in seven families.Members of the order have two dorsal fins, which usually possess spines, no anal fin or nictitating membrane, and five gill slits. In most other respects, however, they are quite variable in form and size...
Squatiniformes
†
SymmoriidaSymmoriida is an extinct order of sharks that contains three families.thumb|left|200px|[[Stethacanthus]]...
†
CladoselachiformesCladoselachiformes is an extinct order of primitive sharks. They were characterized by having an elongated body with a spine in each of the two dorsal fins.-References:*, Dictionary of Zoology .*. Animal Aqua....
†
XenacanthidaXenacanthida is an order of prehistoric sharks that appeared during the Lower Carboniferous period. The family includes the families Xenacanthidae, Diplodoselachidae and Orthacanthidae and the most notable members of the group are the genera Xenacanthus and Orthacanthus. Some Xenacanthus may have...
(Xenacantiformes)
† Iniopterygia
†
EugeneodontidaEugeneodontida is an extinct and poorly known order of bizarre sharks. They possessed a unique "tooth-worl" on the symphysis of the lower jaw as well as pectoral fins supported by long radials. The palatoquadrate was either fused to the skull or reduced...
†
HybodontiformesHybodontiformes was an order of sharks, they were characterised by possessing 1-2 pairs of hooked protrusions on their heads.-External links:* http://www.palaeos.com/Vertebrates/Units/070Chondrichthyes/070.600.html#Hybodontiformes...
}}
Sharks (superorder
Selachimorpha) are a type of
fishA fish is any aquatic vertebrate animal that is typically ectothermic , covered with scales, and equipped with two sets of paired fins and several unpaired fins...
with a full
cartilaginousCartilage is a stiff yet flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes and the intervertebral discs...
skeletonIn biology, a skeleton is a rigid framework that provides protection and structure in many types of animal, particularly those of the phylum Chordata and of the superphylum Ecdysozoa. Exoskeletons are external, as is typical of many invertebrates; they enclose the soft tissues and organs of the...
and a highly
streamlineFluid flow is characterized by a velocity vector field in three-dimensional space, within the framework of continuum mechanics. Streamlines, streaklines and pathlines are field lines resulting from this vector field description of the flow...
d body. The earliest known sharks date from more than 420 million years ago, before the time of the
dinosaur{{Otheruses}}{{pp-semi-protected|small=yes}}{{Otheruses}}{{pp-semi-protected|small=yes}}{{Taxobox|name = Dinosaurs|fossil_range = {{Fossil range|230|65|earliest=230|latest=0|PS=
Descendant taxon Aves survives to present.}}|image = field_dinos_2.jpg...
s.
Since that time, sharks have diversified into
440 species, ranging in size from the small
dwarf lanternsharkThe dwarf lanternshark is a little-known species of dogfish shark in the family Etmopteridae and possibly the smallest shark in the world, reaching a maximum known length of . It is known to be present only on the upper continental slopes off Colombia and Venezuela, at a depth of...
,
Etmopterus perryi, a deep sea species of only {{convert |17 |cm |in |0}} in length, to the
whale sharkThe whale shark, Rhincodon typus, is a slow moving filter feeding shark that is the largest living fish species. The largest confirmed individual had a length of and a weight of more than , but there are unconfirmed claims of considerably larger whale sharks...
,
Rhincodon typus, the largest fish, which reaches approximately {{convert |12 |m |ft |0}} and which feeds only on
planktonPlankton consist of any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. Plankton are defined by their ecological niche rather than their phylogenetic or taxonomic classification...
,
squidSquid are marine cephalopods of the order Teuthida, which comprises around 300 species. Like all other cephalopods, squid have a distinct head, bilateral symmetry, a mantle, and arms. Squid, like cuttlefish, have eight arms and two longer tentacles arranged in pairs...
, and small
fishA fish is any aquatic vertebrate animal that is typically ectothermic , covered with scales, and equipped with two sets of paired fins and several unpaired fins...
through filter feeding. Sharks are found in all seas and are common down to depths of {{convert|2000|m|ft}}, and some live even deeper but they are almost entirely absent below {{convert|3000|m|ft}}. They generally do not live in freshwater, with a few exceptions such as the
bull sharkThe bull shark, Carcharhinus leucas, also known as the whaler shark, Zambezi shark or unofficially known as Zambi in Africa and Nicaragua shark in Nicaragua, is a shark common worldwide in warm, shallow waters along coasts and in rivers. The bull shark is well known for its unpredictable, often...
and the
river sharkThe river sharks are six rare species of shark in the genus Glyphis, although, due to their secretive habits, other species could easily remain undiscovered. The river sharks are members of the family Carcharhinidae, and thus share the basic characteristics of the group...
which can live both in seawater and
freshwaterFreshwater is naturally occurring water on the surface such as bogs, ponds, lakes, rivers and streams, and underground in aquifers and underground rivers. Freshwater is characterized by having low concentrations of dissolved salts...
. They respire with the use of five to seven
gillA gill is an anatomical structure found in many aquatic organisms. It is a respiration organ whose function is the extraction of oxygen from water and the excretion of carbon dioxide...
slits. Sharks have a covering of
dermal denticleDenticles are body surface structures found on some fish and insects. Literally, the term means "small tooth" and refer to the structures' shape....
s that protect their skin from damage and parasites and improve
fluid dynamicsIn physics, fluid dynamics is a sub-discipline of fluid mechanics that deals with fluid flow—the natural science of fluids in motion. It has several subdisciplines itself, including aerodynamics and hydrodynamics...
so the shark can move faster. They have several sets of replaceable teeth.
Well-known species such as the
great whiteThe great white shark, Carcharodon carcharias, also known as great white, white pointer, white shark, or white death, is a large lamniform shark found in coastal surface waters in all major oceans. Reaching lengths of more than and weighing up to , the great white shark is arguably the world's...
and the
hammerheadThe hammerhead sharks are a group of sharks in 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 hammerhead species are placed in the genus Sphyrna; some authorities...
are apex predators at the top of the underwater food chain. Their extraordinary skills as predators fascinate and frighten us, even as their survival is under serious threat from fishing and other human activities.
Etymology
Until the 16th century, sharks were known to mariners as "sea dogs". According to the OED the name "shark" first came into use after Sir
John HawkinsAdmiral Sir John Hawkins was an English shipbuilder, naval administrator and commander, merchant, navigator, and slave trader. As treasurer and controller of the Royal Navy, he rebuilt older ships and helped design the faster ships that withstood the Spanish Armada in 1588...
' sailors exhibited one in London in 1569 and used the word to refer to the large sharks of the
Caribbean SeaThe Caribbean Sea is a sea of the Atlantic Ocean situated in the tropics of the Western hemisphere, bounded to the south and west by the American landmass, with the North Atlantic Ocean proper to the northeast and the Gulf of Mexico to the northwest...
, and later as a general term for all sharks. It has also been suggested to be derived from the Yucatec Maya word for shark,
xok, pronounced [?o?k].
Physical characteristics
{{Main|Physical characteristics of sharks}}
Skeleton
Shark
skeletonIn biology, a skeleton is a rigid framework that provides protection and structure in many types of animal, particularly those of the phylum Chordata and of the superphylum Ecdysozoa. Exoskeletons are external, as is typical of many invertebrates; they enclose the soft tissues and organs of the...
s are very different from those of
bony fishOsteichthyes , also called bony fish, are a taxonomic group of fish that includes the ray-finned fish and lobe-finned fish . The split between these two classes occurred about 450 million years ago....
and
terrestrial vertebratesTetrapods are vertebrate animals having four feet, legs or leglike appendages. Amphibians, reptiles, dinosaurs/birds, and mammals are all tetrapods, and even the limbless snakes are tetrapods by descent...
. Sharks and other
cartilaginous fishChondrichthyes or cartilaginous fishes are jawed fish with paired fins, paired nares, scales, two-chambered hearts, and skeletons made of cartilage rather than bone...
(skates and rays) have skeletons made from
cartilageCartilage is a stiff yet flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes and the intervertebral discs...
and connective tissue. Cartilage is flexible and durable, yet has about half the density of bone. This reduces the skeleton’s weight, saving energy. Sharks have no rib cage and therefore on land a shark's own weight can literally crush it.
Jaw
Like its relatives,
raysBatoidea is a superorder of cartilaginous fish containing more than 500 described species in thirteen families. They are commonly known as rays, but that term is also used specifically for batoids in the order Rajiformes, the "true rays"...
and
skateSkates are cartilaginous fish belonging to the family Rajidae in the superorder Batoidea of rays. There are more than 200 described species in 27 genera.-Description and habitat:...
s, the shark's
jawThe 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 serving to open and close it and is part of the body plan of...
is not attached to the cranium. The jaw's surface, like the shark's vertebrae and gill arches, needs extra support due to its heavier exposure to physical stress and its need for strength. It has a layer of tiny hexagonal plates called "tesserae", which are
crystalA crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is crystallography...
blocks of
calciumCalcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft grey alkaline earth metal, and is the fifth most abundant element by mass in the Earth's crust...
saltA salt, in chemistry, is an ionic compound, and can result from the neutralization reaction of acids and bases. Salts are ionic compounds composed of cations and anions so that the product is electrically neutral...
s arranged as a
mosaicMosaic is the art of creating images with an assemblage of small pieces of colored glass, stone, or other materials. It may be a technique of decorative art, an aspect of interior decoration, or of cultural and spiritual significance as in a cathedral...
. This gives these areas much of the same strength found in the real bony tissue found in other animals.
Generally there is only one layer of tesserae in sharks, but the jaws of large specimens, such as the
bull sharkThe bull shark, Carcharhinus leucas, also known as the whaler shark, Zambezi shark or unofficially known as Zambi in Africa and Nicaragua shark in Nicaragua, is a shark common worldwide in warm, shallow waters along coasts and in rivers. The bull shark is well known for its unpredictable, often...
,
tiger sharkThe tiger shark, Galeocerdo cuvier is a species of requiem shark and the only member of the genus Galeocerdo. Mature sharks average 3.25 to 4.25 m long and weigh . It can attain a length of over 7.25 m and a weight of 900 kg at maximum...
, and the great white shark, have two to three layers or more, depending on body size. The jaws of a large white shark may have up to five layers. In the rostrum (snout), the cartilage can be spongy and flexible to absorb the power of impacts.
Teeth
{{Main|Shark teeth}}
The teeth of sharks are embedded in the
gumsThe gingiva , or gums, consists of the mucosal tissue that lies over the alveolar bone.-General description:Gingiva are part of the soft tissue lining of the mouth. They surround the teeth and provide a seal around them...
rather than directly fixed to the jaw, and are constantly replaced throughout the shark's life. Multiple rows of replacement teeth are grown in a groove on the inside of the jaw and moved forward in a "conveyor belt"; some sharks can lose some 30,000 teeth in their lifetime. The rate of tooth replacement varies from once every 8–10 days to several months. In most species teeth are replaced one at a time, while in the
cookiecutter sharksIsistius is a genus of dogfish sharks in the family Dalatiidae.-Species:* Cigar shark or cookiecutter shark, Isistius brasiliensis
...
the entire row of teeth is replaced simultaneously.
The shape of a shark's tooth depends on its diet: those that feed on mollusks and crustaceans have dense flattened teeth 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 edges for cutting. The teeth of plankton-feeders such as the
basking sharkThe basking shark, Cetorhinus maximus, is the second largest living shark, after the whale shark. It is a cosmopolitan species — it is found in all the world's temperate oceans. It is a slow moving and generally harmless filter feeder.-Taxonomy:...
are greatly reduced and non-functional.
Fins
The fin skeletons are elongated and supported with soft and unsegmented rays named ceratotrichia, filaments of elastic
proteinProteins are organic compounds made of amino acids arranged in a linear chain and folded into a globular form. The amino acids in a polymer chain are joined together by the peptide bonds between the carboxyl and amino groups of adjacent amino acid residues...
resembling the horny
keratinKeratins are a family of fibrous structural proteins; tough and insoluble, they form the hard but un-mineralized structures found in reptiles, birds, amphibians, and mammals...
in
hairHair is a protein filament that grows through the epidermis from follicles deep within the dermis. The fine, soft hair found on many nonhuman mammals is typically called fur; wool is the characteristically curly hair found on sheep and goats. Found exclusively in mammals, hair is one of the...
and feathers. Sharks can only drift away from objects directly in front of them because their fins do not allow them to swim backwards.
Dermal denticles
{{Main|Dermal denticle}}
Unlike bony fish, sharks have a complex dermal
corsetA corset is a garment worn to mold and shape the torso into a desired shape for aesthetic or medical purposes...
made of flexible collagenous
fiberFiber, also spelled fibre, is a class of materials that are continuous filaments or are in discrete elongated pieces, similar to lengths of thread. They are very important in the biology of both plants and animals, for holding tissues together. Human uses for fibers are diverse...
s 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. In the past, sharkskin has been used as
sandpaperSandpaper is a form of paper where an abrasive material has been fixed to its surface.Sandpaper is part of the "coated abrasives" family of abrasive products. It is used to remove small amounts of material from surfaces, either to make them smoother , to remove a layer of material , or sometimes to...
. Their dermal teeth give them hydrodynamic advantages as they reduce
turbulenceIn fluid dynamics, turbulence or turbulent flow is a fluid regime characterized by chaotic, stochastic property changes. This includes low momentum diffusion, high momentum convection, and rapid variation of pressure and velocity in space and time. Flow that is not turbulent is called laminar flow...
when
swimmingSwimming is biologically propelled motion through a liquid medium. Swimming has evolved a number of times in a range of organisms ranging from arthropods to fish to molluscs.-Basic swimming – jellyfish:...
.
Tails
Sharks have very distinctive tails. The
tailThe tail is the section at the rear end of an animal's body; in general, the term refers to a distinct, flexible appendage to the torso. It is the part of the body that corresponds roughly to the sacrum and coccyx in mammals and birds...
s (caudal fins) of sharks vary considerably between species and are adapted to the lifestyle of the shark. The tail provides thrust and so speed and acceleration are dependent on tail shape. Different tail shapes have evolved in sharks adapted for different environments. Sharks possess a heterocercal caudal fin in which the
dorsalIn anatomy, the dorsum is the upper side of animals that typically run, fly, or swim in a horizontal position, and the back side of animals that walk upright. In vertebrates the dorsum contains the backbone. The term dorsal refers to anatomical structures that are either situated toward or grow...
portion is usually noticeably larger than the ventral portion. This is due to the fact that the shark's vertebral column extends into that dorsal portion, allowing for a greater surface area for
muscleMuscle is the contractile tissue of the body and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
attachment which would then be used for more efficient
locomotionAnimal locomotion, which is the act of self-propulsion by an animal, has many manifestations, including running, jumping and flying. Animals move for a variety of reasons, such as to find food, a mate, or a suitable microhabitat, and to escape predators...
among the negatively
buoyantIn physics, buoyancy is the upward force that keeps things afloat. The net upward buoyancy force is equal to the magnitude of the weight of fluid displaced by the body. This force enables the object to float or at least seem lighter....
cartilaginous fishes. This is in contrast to most bony fishes, which possess a homocercal caudal fin.
The tiger shark's tail has a large upper
lobeIn anatomy, a lobe is a clear anatomical division or extension that can be determined without the use of a microscope This is in contrast to a lobule, which is a clear division only visible histologically....
which delivers the maximum amount of power for slow cruising or 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
porbeagleThe porbeagle, Lamna nasus, is a pelagic predatory shark of the family Lamnidae. The porbeagle is considered vulnerable to extinction , and the European Union has proposed listing the porbeagle under the United Nations Convention on International Trade in Endangered Species . it is a U.S....
, which hunts schooling fish such as
mackerelMackerel is a common name applied to a number of different species of fish, mostly, but not exclusively, from the family Scombridae. They may be found in all tropical and temperate seas. Most live offshore in the oceanic environment but a few, like the Spanish mackerel , enter bays and can be...
and
herringHerring are relatively small oily fish of the genus Clupea found in the shallow, temperate waters of the North Pacific and the North Atlantic oceans, including the Baltic Sea. Two species of Clupea are currently recognized, the Atlantic herring and the Pacific herring , each of which may be...
has a large lower lobe to help it keep pace with its fast-swimming prey. Some tail adaptations have other purposes. The
thresherThresher sharks are large lamniform sharks of the family Alopiidae. Found in all temperate and tropical oceans of the world, the family contains three species all within the genus Alopias.-Taxonomy:...
feeds on fish and
squidSquid are marine cephalopods of the order Teuthida, which comprises around 300 species. Like all other cephalopods, squid have a distinct head, bilateral symmetry, a mantle, and arms. Squid, like cuttlefish, have eight arms and two longer tentacles arranged in pairs...
, which it herds and stuns with its powerful and elongated upper lobe.
Buoyancy
Unlike bony fish, sharks do not have gas-filled swim bladders for buoyancy. Instead, sharks rely on a large
liverThe liver is a vital organ present in vertebrates and some other animals; it has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion...
, filled with oil that contains
squaleneSqualene is a natural organic compound originally obtained for commercial purposes primarily from shark liver oil, though botanic sources are used as well, including amaranth seed, rice bran, wheat germ, and olives. All higher organisms produce squalene, including humans. It is a hydrocarbon and a...
and the fact that cartilage is about half as heavy as bone. The liver constitutes up to 30% of their body mass. The liver's effectiveness is limited, so sharks employ dynamic lift to maintain depth and then sink when they stop swimming. Sand tiger sharks are also known to store air in their stomachs, using the stomach as a swim bladder. Most sharks need to constantly swim in order to breathe and cannot sleep very long, if at all, or they will sink. However certain shark species, like the
nurse sharkThe nurse shark, Ginglymostoma cirratum, is a shark in the nurse sharks family, the only member of its genus Ginglymostoma. Nurse sharks can reach a length of 4.3 m and a weight of 330 lbs .-Taxonomy:...
, are capable of pumping water across their gills, allowing them to rest on the ocean bottom.
Some sharks, if inverted or stroked on the
noseAnatomically, a nose is a protuberance in vertebrates that houses the nostrils, or nares, which admit and expel air for respiration in conjunction with the mouth....
, enter a natural state of
tonic immobilityMany animals, such as sharks, beetles, snakes and the Virginia opossum, are capable of appearing to be dead to an observer, while otherwise alive. This could either be a reflex action, as in tonic immobility, or a defense mechanism for avoiding predators, as in thanatosis, which is probably...
. Researchers can use this condition to handle sharks safely.
Respiration
Like other fish, sharks extract
oxygenOxygen Oxygen Oxygen (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter) is the element with atomic number 8 and represented by the symbol O...
from seawater as it passes over their
gillA gill is an anatomical structure found in many aquatic organisms. It is a respiration organ whose function is the extraction of oxygen from water and the excretion of carbon dioxide...
s. Unlike other fish, shark gill slits are not covered, but lie in a row behind the head. A modified slit called a
spiracleSpiracles are small openings on the surface of some animals that usually lead to respiratory systems.In elasmobranchs , a spiracle is found behind each eye, and is often used to pump water through the gills while the animal is at rest .-Spiracles in insects:Insects and some more advanced spiders...
lies just behind the
eyeEyes are organs that detect light, and send electrical impulses along the optic nerve to the visual and other areas of the brain. Complex optical systems with resolving power have come in ten fundamentally different forms, and 96% of animal species possess a complex optical system...
; the spiracle assists water intake during
respirationAquatic respiration is the process whereby an aquatic animal obtains oxygen from water.Earth's natural bodies of water have a low oxygen concentration--much lower than the level of oxygen in air at the earth's surface. Smaller organisms can obtain sufficient oxygen through the skin Aquatic...
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 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- [Headline text]--75.137.171.242 01:21, 21 October 2009 :bThe circulatory system is an organ system that passes nutrients , gases, hormones, blood cells, nitrogen waste products, etc. to and from cells in the body to help fight diseases and help stabilize body temperature and pHb to maintain...
process begins when deoxygenated
bloodBlood is a specialized bodily fluid that delivers necessary substances to the body's cells — such as nutrients and oxygen — and transports waste products away from those same cells....
travels to the shark's two-chambered
heartThe heart is a muscular organ found in all vertebrates that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...
. Here the shark pumps blood to its gills via the ventral aorta artery where it branches off into
afferentAfferent is an anatomical term with the following meanings:*Conveying towards a center, for example the afferent arterioles conveying blood towards the Bowman's capsule in the Kidney. Opposite to Efferent.*Something that so conducts, see Afferent nerve...
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 aortaEach primitive aorta receives anteriorly a vein—the vitelline vein—from the yolk-sac, and is prolonged backward on the lateral aspect of the notochord under the name of the dorsal aorta....
. The blood flows from the dorsal aorta throughout the body. The deoxygenated blood from the body then flows through the posterior
cardinal veinsDuring development of the veins, the first indication of a parietal system consists in the appearance of two short transverse veins, the ducts of Cuvier, which open, one on either side, into the sinus venosus. Each of these ducts receives an ascending and descending vein...
and enters the posterior cardinal sinuses. From there blood enters the heart
ventricleIn the heart, a ventricle is a heart chamber which collects blood from an atrium and pumps it out of the heart...
and the cycle repeats.
Thermoregulation
Most sharks are "cold-blooded", or more precisely
poikilothermA poikilotherm is an animal whose internal temperature varies along with that of the ambient environmental temperature. Most, but not all, ectotherms are poikilothermic...
ic, meaning that their internal body temperature matches that of their ambient environment. Members of the family
LamnidaeLamnidae is a family of sharks, commonly known as mackerel sharks or white sharks. They are large, fast-swimming sharks, found in oceans worldwide....
, such as the
shortfin mako sharkThe shortfin mako shark, Isurus oxyrinchus , is a large mackerel shark. Along with the closely related longfin mako it is commonly referred to as "mako shark".-Anatomy and appearance:...
and the
great white sharkThe great white shark, Carcharodon carcharias, also known as great white, white pointer, white shark, or white death, is a large lamniform shark found in coastal surface waters in all major oceans. Reaching lengths of more than and weighing up to , the great white shark is arguably the world's...
, are homeothermic and maintain a higher body temperature than the surrounding water. In these sharks, a strip of aerobic red muscle located near the center of the body generates the heat, which the body retains via a
countercurrent exchangeCountercurrent exchange along with Concurrent exchange comprise the mechanisms used to transfer some property of a fluid from one flowing current of fluid to another across a semipermeable membrane or thermally-conductive material between them. The property transferred could be heat, concentration...
mechanism by a system of
blood vesselThe blood vessels are the part of the circulatory system that transport blood throughout the body. There are three major types of blood vessels: the arteries, which carry the blood away from the heart, the capillaries, which enable the actual exchange of water and chemicals between the blood and...
s called the
rete mirabileA rete mirabile is a complex of arteries and veins lying very close to each other, found in some vertebrates...
("miraculous net"). The common thresher shark has a similar mechanism for maintaining an elevated body temperature, which is thought to have evolved independently.
Osmoregulation
In contrast to bony fish, with the exception of the
CoelacanthCoelacanth is the common name for an order of fish that includes the oldest living lineage of gnathostomata known to date...
, the blood and other tissue of sharks and
ChondrichthyesChondrichthyes or cartilaginous fishes are jawed fish with paired fins, paired nares, scales, two-chambered hearts, and skeletons made of cartilage rather than bone...
in general is isotonic to their marine environments because of the high concentration of
ureaUrea or carbamide is an organic compound with the chemical formula
2CO. The molecule has two amine residues joined by a carbonyl functional group....
and
trimethylamineTrimethylamine is an organic compound with the formula N
3. This colorless, hygroscopic, and flammable tertiary amine has a strong "fishy" odor in low concentrations and an ammonia-like odor at higher concentrations...
N-oxide (TMAO), allowing them to be in osmotic balance with the seawater. This adaptation prevents most sharks from surviving in fresh water, and they are therefore confined to marine environments. A few exceptions to this rule exist, such as the
bull sharkThe bull shark, Carcharhinus leucas, also known as the whaler shark, Zambezi shark or unofficially known as Zambi in Africa and Nicaragua shark in Nicaragua, is a shark common worldwide in warm, shallow waters along coasts and in rivers. The bull shark is well known for its unpredictable, often...
which has developed a way to change its
kidneyThe kidneys are paired organs, which have the production of urine as their primary function. Kidneys are seen in many types of animals, including vertebrates and some invertebrates. They are part of the urinary system, but have several secondary functions concerned with homeostatic functions. ...
function to excrete large amounts of urea. When a shark dies the urea is broken down to ammonia by bacteria — because of this, the dead body will gradually start to smell strongly of ammonia.
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 of blood in seawater. They are more attracted to the chemicals found in the guts of many species, and as a result often linger near or in
sewageSewage is water-carried wastes, in either solution or suspension, that flow away from a community. Also known as wastewater flows, sewage is the used water supply of the community. It is more than 99.9% pure water and is characterized by its volume or rate of flow, its physical condition, its...
outfalls. Some species, such as
nurse sharkThe nurse shark, Ginglymostoma cirratum, is a shark in the nurse sharks family, the only member of its genus Ginglymostoma. Nurse sharks can reach a length of 4.3 m and a weight of 330 lbs .-Taxonomy:...
s, have external barbels that greatly increase their ability to sense prey.
Sight
Shark
eyeEyes are organs that detect light, and send electrical impulses along the optic nerve to the visual and other areas of the brain. Complex optical systems with resolving power have come in ten fundamentally different forms, and 96% of animal species possess a complex optical system...
s are similar to the eyes of other vertebrates, including similar
lensesThe lens is a transparent, biconvex structure in the eye that, along with the cornea, helps to refract light to be focused on the retina. The lens, by changing shape, functions to change the focal distance of the eye so that it can focus on objects at various distances, thus allowing a sharp real...
,
corneaThe cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts light, accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is approximately 43...
s and
retinaThe vertebrate retina is a light sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
s, though their eyesight is well adapted to the
marineAn ocean is a large body of saline water, and a principal component of the hydrosphere. Approximately 75% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...
environment with the help of a tissue called
tapetum lucidumThe tapetum lucidum is a layer of tissue in the eye of many vertebrate animals, that lies immediately behind or sometimes within the retina. It reflects visible light back through the retina, increasing the light available to the photoreceptors...
. This means that sharks can contract and dilate their
pupilThe pupil is an opening located in the center of the iris of the eye that allows light to enter the retina. It appears black because most of the light entering the pupil is absorbed by the tissues inside the eye. In humans the pupil is round, but other species, such as some cats, have slit pupils...
s, like humans, something no teleost fish can do. This tissue is behind the
retinaThe vertebrate retina is a light sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
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. Sharks have eyelids, but they do not blink because the surrounding water cleans their eyes. To protect their eyes some species have
nictitating membraneThe nictitating membrane is a transparent or translucent third eyelid present in some animals that can be drawn across the eye for protection and to moisten the eye while also keeping visibility...
s. This membrane covers the eyes during predation, and when the shark is being attacked. However, some species, including the
great white sharkThe great white shark, Carcharodon carcharias, also known as great white, white pointer, white shark, or white death, is a large lamniform shark found in coastal surface waters in all major oceans. Reaching lengths of more than and weighing up to , the great white shark is arguably the world's...
(
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-Electroreception, sometimes called electroception, is the biological ability to perceive electrical impulses. It is particularly common among aquatic creatures since salt water is a much more efficient conductor than air. It is used for electrolocation and for electrocommunication...
and chemoreception 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. In effect the shark's field of vision can swap between
monocularA monocular is a modified refracting telescope used to magnify the images of distant objects by passing light through a series of lenses and prisms; the use of prisms results in a lightweight telescope...
and stereoscopic at any time.
Hearing
Although it is hard to test the hearing of sharks, there are indications that they have a sharp sense of hearing and can possibly hear prey many miles away. A small opening on each side of their heads (not to be confused with the
spiracleSpiracles are small openings on the surface of some animals that usually lead to respiratory systems.In elasmobranchs , a spiracle is found behind each eye, and is often used to pump water through the gills while the animal is at rest .-Spiracles in insects:Insects and some more advanced spiders...
) leads directly into the inner ear through a thin channel. The
lateral lineIn aquatic organisms , the lateral line is a sense organ used to detect movement and vibration in the surrounding water. Lateral lines are usually visible as faint lines running lengthwise down each side, from the vicinity of the gill covers to the base of the tail...
shows a similar arrangement, as it 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
tetrapodTetrapods are vertebrate animals having four feet, legs or leglike appendages. Amphibians, reptiles, dinosaurs/birds, and mammals are all tetrapods, and even the limbless snakes are tetrapods by descent...
s the external opening into the inner ear has been lost.
Electroreception
{{Main|Electroreception}}
The
Ampullae of LorenziniThe ampullae of Lorenzini are special sensing organs called electroreceptors, forming a network of jelly-filled canals. They are mostly discussed as being found in cartilaginous fishes ; however, they are also reported to be found in Chondrostei such as Reedfish and sturgeon. Lungfish have also...
are the electroreceptor organs of the shark, and they number in the hundreds to thousands. Sharks use the
Ampullae of LorenziniThe ampullae of Lorenzini are special sensing organs called electroreceptors, forming a network of jelly-filled canals. They are mostly discussed as being found in cartilaginous fishes ; however, they are also reported to be found in Chondrostei such as Reedfish and sturgeon. Lungfish have also...
to detect the electromagnetic fields that all living things produce. This helps sharks (mostly the hammerhead) 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
EarthEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
also generate electric fields that sharks can use for orientation and possibly navigation.
Lateral line
{{Main|Lateral line}}
This system is found in most fish, including sharks. It detects motion or vibrations in water. The shark uses its lateral line to detect the movements of other organisms, especially wounded fish. The shark can sense frequencies in the range of 25 to 50
HzThe hertz is a unit of frequency. It is defined as the number of complete cycles per second. It is the basic unit of frequency in the International System of Units , and is used worldwide in both general-purpose and scientific contexts...
.
Life history
Shark lifespans vary by species. Most live 20 to 30 years. The
spiny dogfishThe spiny dogfish, spurdog, mud shark, or piked dogfish, Squalus acanthias, is one of the best known of the dogfish which are members of the family Squalidae in the order Squaliformes. While these common names may apply to several species, Squalus acanthias is distinguished by having two spines ...
has the longest lifespan at more than 100 years. Whale sharks (
Rhincodon typus) may also live over 100 years.
Reproduction
Sharks practice
internal fertilizationInternal fertilization is a form of fertilization of an egg by within the body of an animal, whether female or hermaphroditic. This is distinct from external fertilization, where the union of the ova and spermatozoa occur outside of the organism....
. The posterior part of a male shark's pelvic fins are modified into a pair of
intromittent organAn intromittent organ is a general term for an external organ of a male organism that is specialized to deliver sperm during copulation. Intromittent organs are found most often in terrestrial species, as most aquatic species fertilize their eggs externally, although there are exceptions.-Species...
s called claspers, analogous to a mammalian
penisThe penis is an external sexual organ of certain biologically male organisms, in both vertebrates and invertebrates....
, of which one is used to deliver
spermThe term sperm is derived from the Greek word sperma and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell...
into the female.
MatingIn biology, mating is the pairing of opposite-sex or hermaphroditic organisms for copulation and, in social animals, also to raise their offspring. For animals, mating methods include random mating, disassortative mating, assortative mating, or a mating pool....
has rarely been observed in sharks. The smaller catsharks 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
oviductIn oviparous animals , the passage from the ovaries to the outside of the body is known as the oviduct. The eggs travel along the oviduct...
. 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
matingIn biology, mating is the pairing of opposite-sex or hermaphroditic organisms for copulation and, in social animals, also to raise their offspring. For animals, mating methods include random mating, disassortative mating, assortative mating, or a mating pool....
. 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.
Unlike most bony fishes, sharks are
K-selectedIn ecology, r/K selection theory relates to the selection of combinations of traits that trade off the quantity and quality of offspring to promote success in particular environments. The terminology of r/K-selection was coined by the ecologists Robert MacArthur and E. O...
reproducers, meaning that they produce a small number of well-developed young as opposed to a large number of poorly-developed young.
FecundityFecundity, derived from the word fecund, generally refers to the ability to reproduce. In biology and demography, fecundity is the potential reproductive capacity of an organism or population, measured by the number of gametes , seed set or asexual propagules. Fecundity is under both genetic and...
in sharks ranges from 2 to over 100 young per reproductive cycle. Sharks display three ways to bear their young, varying by species, oviparity, viviparity and
ovoviviparityOvoviviparity, ovovivipary, or ovivipary, is a mode of reproduction in animals in which embryos develop inside eggs that are retained within the mother's body until they are ready to hatch...
.
Most sharks are ovoviviparous, this means that the egg's yolk and fluids secreted by glands in the walls of the oviduct nourishes the embryos. The eggs hatch within the oviduct, and 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. Some species practice
oophagyOophagy is the practice of embryos feeding on eggs produced by the ovary while still inside the mother's uterus.Oophagy is thought to occur in all sharks in the order Lamniformes and has been recorded in the bigeye thresher , the pelagic thresher Oophagy (egg eating) is the practice of embryos...
, where the first embryos to hatch eat the remaining eggs in the oviduct. This practice is believed to be present in all lamniforme sharks, while the developing pups of the
grey nurse sharkThe grey nurse shark , spotted ragged-tooth shark or sand tiger shark , Carcharias taurus, is a large shark inhabiting coastal waters worldwide, with many different names in different countries in the world. Despite a fearsome appearance and strong swimming abilities, it is a relatively placid and...
take this a stage further and consume other developing embryos (intrauterine cannibalism). The survival strategy for the species that are ovoviviparous is that the young are able to grow to a comparatively large size before birth. The whale shark is now considered to be ovoviviparous after long having been classified as oviparous. Extrauterine
whale sharkThe whale shark, Rhincodon typus, is a slow moving filter feeding shark that is the largest living fish species. The largest confirmed individual had a length of and a weight of more than , but there are unconfirmed claims of considerably larger whale sharks...
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.
DogfishDogfish can have many meanings, but usually refers to a type of shark:* A shark belonging to the order Squaliformes, or to one of its constituent families:* Family Centrophoridae * Family Dalatiidae...
have the longest known gestation period of any shark, at 18 to 24 months.
Basking sharkThe basking shark, Cetorhinus maximus, is the second largest living shark, after the whale shark. It is a cosmopolitan species — it is found in all the world's temperate oceans. It is a slow moving and generally harmless filter feeder.-Taxonomy:...
s and frilled sharks appear to have even longer gestation periods, but accurate data is lacking.
Some sharks lay eggs, this is called oviparity. In most of these species, the developing embryo is protected by an egg case with the consistency of leather. Sometimes these cases are corkscrewed into crevices for protection. The
mermaid's purseMermaid's purses are the egg cases of skates, shark and rays. They are among the common objects which are washed up by the sea. Because they are lightweight, they are often found at the strandline, the furthest point of the high tide...
, which can wash up on shore, is an empty egg case. Oviparous sharks include the
horn sharkThe horn shark, Heterodontus francisci, is a species of bullhead shark, family Heterodontidae. It is endemic to the coastal waters off the western coast North America, from California to the Gulf of California. Young sharks are segregated spatially from the adults, with the former preferring deeper...
,
catsharkThe cat sharks or catsharks are a family of sharks, with over 110 species recorded. Paradoxically perhaps, while the group is called the cat shark family, many species are commonly called dogfish...
,
Port Jackson sharkThe Port Jackson shark, Heterodontus portusjacksoni, is a nocturnal, oviparous type of bullhead shark of the family Heterodontidae, found in the coastal region of southern Australia, including the waters off Port Jackson. It has a large head with prominent forehead ridges and dark brown...
, and
swellsharkThe swellshark, Cephaloscyllium ventriosum, is a catshark of the family Scyliorhinidae. It is found in the subtropical eastern Pacific Ocean between latitudes 40° N and 37° S, from the surface to 460 m....
.
Finally some sharks maintain a
placentaThe placenta is an organ unique to mammals that connects the developing fetus to the uterine wall. The placenta supplies the fetus with oxygen and food, and allows fetal waste to be disposed of via the maternal kidneys...
l link to the developing young, this method is called viviparity. This is more analogous to mammalian gestation than that of other fishes. The young are born alive and fully functional.
HammerheadsThe hammerhead sharks are a group of sharks in 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 hammerhead species are placed in the genus Sphyrna; some authorities...
, the requiem sharks (such as the
bullThe bull shark, Carcharhinus leucas, also known as the whaler shark, Zambezi shark or unofficially known as Zambi in Africa and Nicaragua shark in Nicaragua, is a shark common worldwide in warm, shallow waters along coasts and in rivers. The bull shark is well known for its unpredictable, often...
and
blueThe blue shark, Prionace glauca, is a carcharhinid shark which is found in the deep waters of the world's temperate and tropical oceans. They prefer cooler waters and are not found, for example, in the Yellow Sea or in the Red Sea. Blue sharks are known to migrate long distances, from New England...
sharks), and smoothhounds are viviparous.
Asexual reproduction
There are two documented cases in which a female shark who has not been in contact with a male has conceived a pup on her own through a process known as
parthenogenesisParthenogenesis is an asexual form of reproduction found in females where growth and development of embryos occurs without fertilization by a male. In plants, parthenogenesis means development of an embryo from an unfertilized egg cell, and is a component process of apomixis...
. The details of this process are not well understood but
genetic fingerprintingDNA profiling is a technique employed by forensic scientists to assist in the identification of individuals on the basis of their respective DNA profiles. DNA profiles are encrypted sets of numbers that reflect a person's DNA makeup, which can also be used as the person's identifier...
showed that the pups had no paternal genetic contribution, ruling out
spermThe term sperm is derived from the Greek word sperma and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell...
storage. The extent of this behavior in the wild is unknown, as are whether other species have this capability. Mammals are now the only major vertebrate group in which
asexual reproductionAsexual reproduction is reproduction which does not involve meiosis, ploidy reduction, or fertilization. Only one parent is involved in asexual reproduction. A more stringent definition is agamogenesis which refers to reproduction without the fusion of gametes...
has not been observed.
Scientists assert that asexual reproduction in the wild is rare, and probably a last ditch effort to reproduce when a mate isn't present. Asexual reproduction diminishes
genetic diversityGenetic diversity is a level of biodiversity that refers to the total number of genetic characteristics in the genetic makeup of a species. It is distinguished from genetic variability, which describes the tendency of genetic characteristics to vary....
, which helps build defenses against natural threats. Species that rely solely on it are probably on the road to extinction. Asexual reproduction may have contributed to the
blue sharkThe blue shark, Prionace glauca, is a carcharhinid shark which is found in the deep waters of the world's temperate and tropical oceans. They prefer cooler waters and are not found, for example, in the Yellow Sea or in the Red Sea. Blue sharks are known to migrate long distances, from New England...
's decline off the
IrishIreland is the third-largest island in Europe and the twentieth-largest island in the world. It lies to the north-west of continental Europe and is surrounded by hundreds of islands and islets. To the east of Ireland, separated by the Irish Sea, is the island of Great Britain...
coast.
Behavior
The classic view describes a solitary hunter, ranging the oceans in search of food. However, this applies to only a few species, with most living far more sedentary, benthic lives. Even solitary sharks meet for breeding or at rich hunting grounds, which may lead them to cover thousands of miles in a year. Migration patterns in sharks may be even more complex than in birds, with many sharks covering entire ocean basins. However, shark behavior has only begun to be formally studied, so there is much more to learn.
Sharks can be highly social, remaining in large schools; sometimes over 100 individuals of
scalloped hammerheadThe scalloped hammerhead, Sphyrna lewini, is a hammerhead shark of the family Sphyrnidae. Originally Zygaena lewini, it was later moved to its current name. The Greek word sphyrna translates into "hammer" in English, referring to the shape of this shark's head.This shark is also known as the...
s congregate around
seamountA seamount is a mountain rising from the ocean seafloor that does not reach to the water's surface , and thus is not an island. These are typically formed from extinct volcanoes, that rise abruptly and are usually found rising from a seafloor of 1,000–4,000 meters depth...
s and islands, e.g., in the
Gulf of CaliforniaThe Gulf of California is a body of water that separates the Baja California Peninsula from the Mexican mainland...
. Cross-species social hierarchies exist with
oceanic whitetip sharkThe oceanic whitetip shark, Carcharhinus longimanus, is a large pelagic shark of tropical and warm temperate seas. It is a stocky shark, most notable for its long, white-tipped, rounded fins....
s dominating
silky sharkThe silky shark, Carcharhinus falciformis, is a species of requiem shark, family Carcharhinidae, named for the smooth texture of its skin. One of the most abundant sharks in the pelagic zone, the highly mobile silky shark can be found around the world in tropical waters. It is most often found near...
s of comparable size when feeding.
When approached too closely some sharks will perform a
threat displayShark threat display, a type of agonistic display, is a behaviour observed in some sharks when they feel threatened or protective. It consists of a contorting of the body into a series of "ritualized" postures coupled with an exaggerated swimming style...
to warn off the prospective predators. This usually consists of exaggerated swimming movements, and can vary in intensity according to the level of threat.
Feeding
{{otheruses-section|shark feeding|the sport of shark feeding|Shark baiting}}
Most sharks are carnivorous. Some species, including
tiger sharkThe tiger shark, Galeocerdo cuvier is a species of requiem shark and the only member of the genus Galeocerdo. Mature sharks average 3.25 to 4.25 m long and weigh . It can attain a length of over 7.25 m and a weight of 900 kg at maximum...
s, will eat just about anything. The vast majority seek particular prey, and rarely stray from these.
Sharks such as
whaleThe whale shark, Rhincodon typus, is a slow moving filter feeding shark that is the largest living fish species. The largest confirmed individual had a length of and a weight of more than , but there are unconfirmed claims of considerably larger whale sharks...
,
baskingThe basking shark, Cetorhinus maximus, is the second largest living shark, after the whale shark. It is a cosmopolitan species — it is found in all the world's temperate oceans. It is a slow moving and generally harmless filter feeder.-Taxonomy:...
and
megamouthThe megamouth shark, Megachasma pelagios, is an extremely rare species of deepwater shark. Since being discovered in 1976, only a few megamouth sharks have been seen with 47 specimens known to have been caught or sighted as of 2009 and three recordings on film...
sharks use filter feeding. These three species evolved
planktonPlankton consist of any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. Plankton are defined by their ecological niche rather than their phylogenetic or taxonomic classification...
feeding independently and use different strategies. Whale sharks feed using suction to take in large concentrations of plankton and small fishes. Basking sharks are ram-feeders, swimming through plankton blooms with their mouth wide open. Megamouth sharks make suction feeding more efficient, using luminescent tissue inside the mouth to attract prey in the deep ocean. This type of feeding requires gill rakers, long slender filaments that form a very efficient
sieveIn general, a sieve separates wanted/desired/needed elements from unwanted material using a tool such as a mesh, net or other filtration or distillation methods, but it is also used for classification of powders by particle size, or for size measurement as an analytical technique. The word "sift"...
, analogous to the
baleenBaleen or whalebone enables baleen whales to feed. These whales do not have teeth, but instead have rows of baleen plates in the upper jaw – flat, flexible plates with frayed edges, arranged in two parallel rows, looking like combs of thick hair. Baleen is not bone, but is composed of...
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 the
cookiecutter sharkThe cookiecutter shark, Isistius brasiliensis, also known as the cigar shark or luminous shark, is a small, rarely-seen dogfish shark.-Anatomy and morphology:...
s, which feed on flesh sliced out of other larger fish and marine mammals. The teeth in these sharks are enormous, compared to their size. The lower jaw’s 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 sharkThe angel sharks are an unusual genus of sharks with flattened bodies and broad pectoral fins that give them a strong resemblance to skates and rays. The 16+ known species are in the genus Squatina, the only genus in its family, Squatinidae, and order Squatiniformes. They occur worldwide in...
s and
wobbegongWobbegong is the common name given to the 11 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 Indonesia, although one species occurs as far north as Japan...
s use camouflage to lie in wait and suck prey into their mouths. Many benthic sharks feed solely on crustaceans which they crush with their flat molariform teeth.
Other sharks feed on squid or fishes, which the swallow whole. The
viper dogfishThe viper dogfish, Trigonognathus kabeyai, is a dogfish, the only species in the genus Trigonognathus, found off Wakayama and Tokushima, Japan, in the northwest Pacific Ocean at depths of between 330 and 360 m. Its maximum length is 47 cm....
has teeth it can point outwards to strike and capture prey that it then swallows intact. The
great whiteThe great white shark, Carcharodon carcharias, also known as great white, white pointer, white shark, or white death, is a large lamniform shark found in coastal surface waters in all major oceans. Reaching lengths of more than and weighing up to , the great white shark is arguably the world's...
and other large predators can either swallow small prey whole or take huge bites out of large animals.
Thresher sharkThresher sharks are large lamniform sharks of the family Alopiidae. Found in all temperate and tropical oceans of the world, the family contains three species all within the genus Alopias.-Taxonomy:...
s use their long tails to stun shoaling fishes, and
sawsharkThe sawsharks or saw sharks are an order of sharks bearing long blade-like snouts edged with teeth, which they use to slash and disable their prey...
s may either stir prey from the seabed or slash at swimming prey with their tooth-studded
rostraThe Rostra 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 deliver orations to those assemble in between...
.
Many sharks, including the
whitetip reef sharkThe whitetip reef shark, Triaenodon obesus, is a species of requiem shark, family Carcharhinidae, and the only member of its genus. A small shark usually not exceeding in length, this species is easily recognizable by its slender body and short but broad head, as well as tubular skin flaps beside...
are cooperative feeders and hunt in packs to herd and capture elusive prey. These social sharks are often highly migratory, travelling huge distances around ocean basins in large schools. These migrations may be partly necessary to find new food sources.
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 either vomit or turn their stomachs inside out and evert unwanted items from their mouths.
One of the biggest differences between shark and mammalian digestion is sharks’ extremely short intestine. This short length is achieved by the
spiral valveA spiral valve is the lower portion of the intestine of some sharks, rays, skates and bichirs. A modification of the ileum, the spiral valve is internally twisted or coiled to increase the surface area of the intestine, to increase nutrient absorption....
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
cloacaIn zoological anatomy, a cloaca is the posterior opening that serves as the only such opening for the intestinal, reproductive and urinary tracts of certain animal species. The word comes from Latin, and means sewer...
.
Speed
In general, sharks swim ("cruise") at an average speed of {{convert |8|km/h|mph}} but when feeding or attacking, the average shark can reach speeds upwards of {{convert |19|km/h|mph}}. The
shortfin makoThe shortfin mako shark, Isurus oxyrinchus , is a large mackerel shark. Along with the closely related longfin mako it is commonly referred to as "mako shark".-Anatomy and appearance:...
may range upwards of {{convert |50|km/h|mph}}. The
shortfin mako sharkThe shortfin mako shark, Isurus oxyrinchus , is a large mackerel shark. Along with the closely related longfin mako it is commonly referred to as "mako shark".-Anatomy and appearance:...
is the fastest shark and one of the fastest fish. The
great white sharkThe great white shark, Carcharodon carcharias, also known as great white, white pointer, white shark, or white death, is a large lamniform shark found in coastal surface waters in all major oceans. Reaching lengths of more than and weighing up to , the great white shark is arguably the world's...
is also capable of bursts of
speedSpeed is the rate of motion, or equivalently the rate of change of distance.Speed is a scalar quantity with dimensions length/time; the equivalent vector quantity to speed is velocity. Speed is measured in the same physical units of measurement as velocity, but does not contain the element of...
. These exceptions may be due to the "
warm-bloodedIn biology, a warm-blooded animal species is one whose members maintain thermal homeostasis; that is, they keep their body temperature at a roughly constant level, regardless of the ambient temperature. This involves the ability to cool down or produce more body heat. Warm-blooded animals mainly...
", or homeothermic, nature of these sharks' physiology.
Intelligence
Contrary to the common wisdom that sharks are instinct-driven "eating machines", recent studies have indicated that many species possess powerful problem-solving skills, social skills and curiosity. The brain- to body-mass ratios of sharks are similar to mammals and birds. In 1987, near Smitswinkle Bay,
South AfricaThe Republic of South Africa is a country located at the southern tip of Africa, with a coastline on the Atlantic and Indian Oceans. To the north lie Namibia, Botswana and Zimbabwe, to the east are Mozambique and Swaziland, while Lesotho is an independent country surrounded by South Africa.Modern...
, a group of up to seven
great white sharkThe great white shark, Carcharodon carcharias, also known as great white, white pointer, white shark, or white death, is a large lamniform shark found in coastal surface waters in all major oceans. Reaching lengths of more than and weighing up to , the great white shark is arguably the world's...
s worked together to move a partially beached dead whale to deeper waters to feed. Sharks can engage in playful activities.
PorbeagleThe porbeagle, Lamna nasus, is a pelagic predatory shark of the family Lamnidae. The porbeagle is considered vulnerable to extinction , and the European Union has proposed listing the porbeagle under the United Nations Convention on International Trade in Endangered Species . it is a U.S....
sharks have been seen repeatedly rolling in kelp and chasing an individual who trailed a piece of kelp behind it.
Sleep
Some sharks can lie on the bottom while actively pumping water over their gills, but their eyes remain open and actively follow divers. When a shark is resting, it does not use its nares, but rather its
spiracleSpiracles are small openings on the surface of some animals that usually lead to respiratory systems.In elasmobranchs , a spiracle is found behind each eye, and is often used to pump water through the gills while the animal is at rest .-Spiracles in insects:Insects and some more advanced spiders...
s. If a shark tried to use its nares while resting on the ocean floor, it would be sucking up sand rather than water. Many scientists believe this is one of the reasons sharks have spiracles. The
spiny dogfishThe spiny dogfish, spurdog, mud shark, or piked dogfish, Squalus acanthias, is one of the best known of the dogfish which are members of the family Squalidae in the order Squaliformes. While these common names may apply to several species, Squalus acanthias is distinguished by having two spines ...
's
spinal cordThe spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain. The brain and spinal cord together make up the central nervous system. It is around 45 cm long in men and around 43 cm long in women. The length of the spinal cord is much shorter than...
rather than its brain, coordinates swimming, so it is possible for
spiny dogfishThe spiny dogfish, spurdog, mud shark, or piked dogfish, Squalus acanthias, is one of the best known of the dogfish which are members of the family Squalidae in the order Squaliformes. While these common names may apply to several species, Squalus acanthias is distinguished by having two spines ...
to continue to swim while sleeping. It is also possible that sharks sleep in a manner similar to
dolphinDolphins are marine mammals that are closely related to whales and porpoises. There are almost forty species of dolphin in seventeen genera. They vary in size from and , up to and . They are found worldwide, mostly in the shallower seas of the continental shelves, and are carnivores, mostly...
s, one cerebral hemisphere at a time, thus maintaining some consciousness and cerebral activity at all times.
{{-}}
Distribution and habitat
Sharks are found in all seas. They generally do not live in freshwater, with a few exceptions such as the
bull sharkThe bull shark, Carcharhinus leucas, also known as the whaler shark, Zambezi shark or unofficially known as Zambi in Africa and Nicaragua shark in Nicaragua, is a shark common worldwide in warm, shallow waters along coasts and in rivers. The bull shark is well known for its unpredictable, often...
and the
river sharkThe river sharks are six rare species of shark in the genus Glyphis, although, due to their secretive habits, other species could easily remain undiscovered. The river sharks are members of the family Carcharhinidae, and thus share the basic characteristics of the group...
which can swim both in seawater and freshwater. Sharks are common down to depths of {{convert |2000|m|ft|-3}}, and some live even deeper, but they are almost entirely absent below {{convert |3000|m|ft|-3}}. The deepest confirmed report of a shark is a
Portuguese dogfishThe Portuguese dogfish, Centroscymnus coelolepis, is a sleeper shark of the family Dalatiidae, found around the world on continental slopes and abyssal plains between latitudes 64°N and 48°S, at depths of between 150 and 3,700 m. It reaches a length of 1.2 m....
at {{convert|3700|m}}.
Evolution
Evidence for the existence of sharks extends back over 450–420 million years, into the
OrdovicianThe Ordovician is a geologic period and system, the second of six of the Paleozoic era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian period and is followed by the Silurian period...
period, before land vertebrates existed and before many plants had colonized the continents. Only scales have been recovered from the first sharks and not all paleontologists agree that these are from true sharks. The oldest generally accepted shark scales are from about 420 million years ago, in the
SilurianThe Silurian is a geologic period and system that extends from the end of the Ordovician period, about 443.7 ± 1.5 Ma , to the beginning of the Devonian period, about 416.0 ± 2.8 Ma . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
period. The first sharks looked very different from modern sharks. The majority of modern sharks can be traced back to around 100 million years ago. Most fossils are of teeth, often in large numbers. In some cases pieces of the skeleton and even complete fossilized remains have been discovered. Estimates suggest that a shark may grow tens of thousands of teeth, which explains the abundance of fossils. The teeth consist of easily fossilized
calcium phosphateCalcium phosphate is the name given to a family of minerals containing calcium ions together with orthophosphates , metaphosphates or pyrophosphates and occasionally hydrogen or hydroxide ions.It is the principal form of calcium found in bovine milk...
, an
apatiteApatite is a group of phosphate minerals, usually referring to hydroxyapatite, fluorapatite, chlorapatite and bromapatite, named for high concentrations of OH
−, F
−, Cl
− or...
. When a shark dies, the decomposing skeleton breaks up, scattering the apatite prisms. Skeletal preservation requires rapid burial in bottom sediments.
Among the most ancient and primitive sharks is
CladoselacheCladoselache is a genus of extinct shark. It appeared in the Devonian period.This primitive shark grew to be up to long and roamed the oceans of North America. It is known to be a fast moving and fairly agile predator due to its streamline body and deep forked tail...
, from about 370 million years ago, which has been found within
PaleozoicThe Paleozoic or Palaeozoic Era is the earliest of three geologic eras of the Phanerozoic eon...
strata in
OhioOhio is a Midwestern state of the United States. The thirty-fourth largest state by area in the U.S., it is the seventh-most populous with nearly 11.5 million residents...
,
KentuckyThe Commonwealth of Kentucky is a state located in the East Central United States of America. Kentucky is a Southern state situated in the Upland South, although the state is infrequently placed, geographically and culturally, in the Midwest. Kentucky is one of four U.S. states constituted as a...
and
TennesseeTennessee is a state located in the Southeastern United States. According to the 2008 census, it has a population of 6,214,888, an increase of nearly 9.5% since 2000. Tennessee is the 14th fastest growing state in the US and is ranked 17th by population. It is ranked 36th by total land area. In...
. At that point in
EarthEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
's history these rocks made up the soft sediment of the bottom of a large, shallow ocean, which stretched across much of
North AmericaNorth America is the northern continent of the Americas, situated in the Earth's northern hemisphere and in the western hemisphere. It is bordered on the north by the Arctic Ocean, on the east by the North Atlantic Ocean, on the southeast by the Caribbean Sea, and on the west by the North Pacific...
.
Cladoselache was only about {{convert|1|m|ft}} long with stiff triangular fins and slender jaws. Its teeth had several pointed cusps, which wore down by 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 sharkThe great white shark, Carcharodon carcharias, also known as great white, white pointer, white shark, or white death, is a large lamniform shark found in coastal surface waters in all major oceans. Reaching lengths of more than and weighing up to , the great white shark is arguably the world's...
s and the pelagic
shortfinThe shortfin mako shark, Isurus oxyrinchus , is a large mackerel shark. Along with the closely related longfin mako it is commonly referred to as "mako shark".-Anatomy and appearance:...
and longfin makos. The presence of whole fish arranged tail-first in their stomachs suggest that they were fast swimmers with great agility.
From about 300 to 150 million years ago, most fossil sharks can be assigned to one of two groups. The
XenacanthidaXenacanthida is an order of prehistoric sharks that appeared during the Lower Carboniferous period. The family includes the families Xenacanthidae, Diplodoselachidae and Orthacanthidae and the most notable members of the group are the genera Xenacanthus and Orthacanthus. Some Xenacanthus may have...
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, appeared about 320 million years ago and lived mostly in the oceans, but also in freshwater.
Modern sharks began to appear about 100 million years ago. Fossil mackerel shark teeth date to the Lower Cretaceous. One of the most recently evolved families is the
hammerhead sharkThe hammerhead sharks are a group of sharks in 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 hammerhead species are placed in the genus Sphyrna; some authorities...
(family Sphyrnidae), which emerged in the
EoceneThe Eocene epoch, lasting from 55.8 ± 0.2 to 33.9 ± 0.1 Ma , is a major division of the geologic timescale and the second epoch of the Palaeogene period in the Cenozoic era. The Eocene spans the time from the end of the Paleocene epoch to the beginning of the Oligocene epoch. The start of the...
. The oldest white shark teeth date from 60 to 65 million years ago, around the time of the extinction of the dinosaurs. In early white shark evolution there are at least two lineages: one with coarsely serrated teeth that probably gave rise to the modern great white shark, and another with finely serrated teeth that grew to gigantic size. This group includes the extinct
MegalodonThe megalodon , Carcharodon megalodon or Carcharocles megalodon , was a giant shark that lived in prehistoric times during the Neogene period, and was a super-predator....
,
Carcharodon megalodon, which like most extinct sharks is only known from its teeth and a few vertebrae. This shark could grow to more than {{convert |16 |m |ft |0}} long and is the biggest known carnivorous fish to have ever existed. Fossils reveal that this shark preyed upon whales and other large marine
mammalMammals are a class of vertebrate animals whose females are characterized by the possession of mammary glands while both males and females are characterized by sweat glands, hair, three middle ear bones used in hearing, and a neocortex region in the brain.Mammals are divided into three main...
s.
Taxonomy
Sharks belong to the superorder Selachimorpha in the subclass
ElasmobranchiiElasmobranchii is the subclass of cartilaginous fish that includes skates, rays , and sharks .-Evolution:Fossilised shark teeth are known from the early Devonian, around 400 million years ago. During the following Carboniferous period, the sharks underwent a period of diversification, with many...
in the class
ChondrichthyesChondrichthyes or cartilaginous fishes are jawed fish with paired fins, paired nares, scales, two-chambered hearts, and skeletons made of cartilage rather than bone...
. The Elasmobranchii also include
raysBatoidea is a superorder of cartilaginous fish containing more than 500 described species in thirteen families. They are commonly known as rays, but that term is also used specifically for batoids in the order Rajiformes, the "true rays"...
and skates; the Chondrichthyes also include
ChimaeraChimaeras are cartilaginous fish in the order Chimaeriformes, known informally as ghost sharks, ratfish , or rabbitfishes...
s. It is currently thought that the sharks form a
polyphyleticA polyphyletic group is one whose members' last common ancestor is not a member of the group.For example, the group consisting of warm-blooded animals is polyphyletic, because it contains both mammals and birds, but the most recent common ancestor of mammals and birds was cold-blooded...
group: some sharks are more closely related to rays than they are to some other sharks.
The superorder Selachimorpha is divided into Galea (or Galeomorphii), and Squalea. The Galeans are the Heterodontiformes, Orectolobiformes,
LamniformesLamniformes is an order of sharks, also known as mackerel sharks...
, and
CarcharhiniformesThe ground sharks, order Carcharhiniformes, are the largest order of sharks; they are also called whaler sharks. With over 270 species, carcharhiniforms include a number of common types, such as the blue shark, catsharks, swellsharks, and sandbar shark.Members of the orders are characterized by the...
. Lamnoids and Carcharhinoids are usually placed in one
cladeA clade is a term used in modern alpha taxonomy, the scientific classification of living and fossil organisms, to describe a monophyletic group, defined as a group consisting of a single common ancestor and all its descendants.[The term "monophyletic group" is used in this article...]
, but recent studies show the Lamnoids and Orectoloboids are a clade. Some scientists now think that Heterodontoids may be Squalean. The Squalea is divided into Hexanchoidei and Squalomorpha. The Hexanchoidei includes the
HexanchiformesHexanchiformes is the order consisting of the most primitive types of sharks, and numbering just five extant species. Fossil sharks that were apparently very similar to modern sevengill species are known from Jurassic specimens....
and Chlamydoselachiformes. The Squalomorpha contains the
SqualiformesSqualiformes is an order of sharks that includes about 80 species in seven families.Members of the order have two dorsal fins, which usually possess spines, no anal fin or nictitating membrane, and five gill slits. In most other respects, however, they are quite variable in form and size...
and the Hypnosqualea. The Hypnosqualea is now sometimes considered invalid. It includes the Squatiniformes, and the Pristorajea, which may also be invalid, includes the Pristiophoriformes and the
BatoideaBatoidea is a superorder of cartilaginous fish containing more than 500 described species in thirteen families. They are commonly known as rays, but that term is also used specifically for batoids in the order Rajiformes, the "true rays"...
.
There are more than 440 species of sharks split across eight
ordersIn scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
, listed below in roughly their evolutionary relationship from ancient to modern:
- Hexanchiformes
Hexanchiformes is the order consisting of the most primitive types of sharks, and numbering just five extant species. Fossil sharks that were apparently very similar to modern sevengill species are known from Jurassic specimens....
: Examples from this group include the cow sharkCow sharks, or the Hexanchidae, are a family of sharks characterized by extra pairs of gill slits. There are probably only two seven-gilled genera, Heptranchias and Notoryhncus....
s, frilled shark and even a shark that resembles a marine snake.
- Squaliformes
Squaliformes is an order of sharks that includes about 80 species in seven families.Members of the order have two dorsal fins, which usually possess spines, no anal fin or nictitating membrane, and five gill slits. In most other respects, however, they are quite variable in form and size...
: This group includes the bramble sharkThe bramble shark, Echinorhinus brucus, is covered with large, thorn-like denticles, and hence the name "bramble" shark.-Physical characteristics:...
s, dogfishDogfish can have many meanings, but usually refers to a type of shark:* A shark belonging to the order Squaliformes, or to one of its constituent families:* Family Centrophoridae * Family Dalatiidae...
and roughsharkSqualidae is the family of dogfish sharks. They are found in the Atlantic, Pacific, and Indian oceans, from tropical equatorial climates to the Arctic and Antarctic....
s, and prickly sharkThe prickly shark, Echinorhinus cookei, is a species of large bottom dwelling shark from the Pacific Ocean.-Physical characteristics:The prickly shark has no anal fin and two small spineless dorsal fins set far on the back by the tail...
.
- Pristiophoriformes: These are the sawshark
The sawsharks or saw sharks are an order of sharks bearing long blade-like snouts edged with teeth, which they use to slash and disable their prey...
s, with an elongated, toothed snout that they use for slashing their prey.
- Squatiniformes: Also known as angel shark
The angel sharks are an unusual genus of sharks with flattened bodies and broad pectoral fins that give them a strong resemblance to skates and rays. The 16+ known species are in the genus Squatina, the only genus in its family, Squatinidae, and order Squatiniformes. They occur worldwide in...
s, they are flattened sharks with a strong resemblance to stingrays and skateSkates are cartilaginous fish belonging to the family Rajidae in the superorder Batoidea of rays. There are more than 200 described species in 27 genera.-Description and habitat:...
s.
- Heterodontiformes: They are generally referred to as the bullhead
The bullhead sharks are a small order of basal modern sharks . There are nine living species in a single genus, Heterodontus, in the family Heterodontidae. All are relatively small, with the largest species being just in adult length...
or horn sharkThe horn shark, Heterodontus francisci, is a species of bullhead shark, family Heterodontidae. It is endemic to the coastal waters off the western coast North America, from California to the Gulf of California. Young sharks are segregated spatially from the adults, with the former preferring deeper...
s.
- Orectolobiformes: They are commonly referred to as the carpet shark
The order Orectolobiformes, also collectively known as the carpet sharks because many members have carpet-like patterned markings, includes a number of familiar types of sharks, such as the nurse sharks and whale shark, as well as some unusual species, such as the blind shark.Carpet sharks have two...
s, including zebra sharkThe zebra shark, Stegostoma fasciatum , is a species of carpet shark and the sole member of the family Stegostomatidae. It is found throughout the tropical Indo-Pacific, frequenting coral reefs and sandy flats to a depth of 62 m...
s, nurse sharkThe nurse shark, Ginglymostoma cirratum, is a shark in the nurse sharks family, the only member of its genus Ginglymostoma. Nurse sharks can reach a length of 4.3 m and a weight of 330 lbs .-Taxonomy:...
s, wobbegongWobbegong is the common name given to the 11 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 Indonesia, although one species occurs as far north as Japan...
s and the whale sharkThe whale shark, Rhincodon typus, is a slow moving filter feeding shark that is the largest living fish species. The largest confirmed individual had a length of and a weight of more than , but there are unconfirmed claims of considerably larger whale sharks...
.
- Carcharhiniformes
The ground sharks, order Carcharhiniformes, are the largest order of sharks; they are also called whaler sharks. With over 270 species, carcharhiniforms include a number of common types, such as the blue shark, catsharks, swellsharks, and sandbar shark.Members of the orders are characterized by the...
: Commonly known as groundsharks, the species include the blueThe blue shark, Prionace glauca, is a carcharhinid shark which is found in the deep waters of the world's temperate and tropical oceans. They prefer cooler waters and are not found, for example, in the Yellow Sea or in the Red Sea. Blue sharks are known to migrate long distances, from New England...
, tigerThe tiger shark, Galeocerdo cuvier is a species of requiem shark and the only member of the genus Galeocerdo. Mature sharks average 3.25 to 4.25 m long and weigh . It can attain a length of over 7.25 m and a weight of 900 kg at maximum...
, bullThe bull shark, Carcharhinus leucas, also known as the whaler shark, Zambezi shark or unofficially known as Zambi in Africa and Nicaragua shark in Nicaragua, is a shark common worldwide in warm, shallow waters along coasts and in rivers. The bull shark is well known for its unpredictable, often...
, grey reef, blacktip reefThe blacktip reef shark, Carcharhinus melanopterus, is a species of requiem shark, family Carcharhinidae, easily identified by the prominent black tips on its fins...
, Caribbean reefThe Caribbean reef shark, Carcharhinus perezi is a species of requiem shark, family Carcharhinidae. It is found in the tropical waters of the western Atlantic Ocean from Florida to Brazil, and is the most commonly encountered reef shark in the Caribbean Sea...
, blacktail reef, whitetip reefThe whitetip reef shark, Triaenodon obesus, is a species of requiem shark, family Carcharhinidae, and the only member of its genus. A small shark usually not exceeding in length, this species is easily recognizable by its slender body and short but broad head, as well as tubular skin flaps beside...
and oceanic whitetip sharkThe oceanic whitetip shark, Carcharhinus longimanus, is a large pelagic shark of tropical and warm temperate seas. It is a stocky shark, most notable for its long, white-tipped, rounded fins....
s (collectively called the requiem sharkRequiem sharks refers to all members of the family Carcharhinidae: this includes migratory, live-bearing sharks of warm seas such as the tiger shark, the blue shark, the bull shark, and the milk shark. The name comes from the French word for shark, "requin". Family members have the usual...
s) along with the houndsharks, catsharkThe cat sharks or catsharks are a family of sharks, with over 110 species recorded. Paradoxically perhaps, while the group is called the cat shark family, many species are commonly called dogfish...
s and hammerhead sharkThe hammerhead sharks are a group of sharks in 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 hammerhead species are placed in the genus Sphyrna; some authorities...
s. They are distinguished by an elongated snout and a nictitating membraneThe nictitating membrane is a transparent or translucent third eyelid present in some animals that can be drawn across the eye for protection and to moisten the eye while also keeping visibility...
which protects the eyes during an attack.
- Lamniformes
Lamniformes is an order of sharks, also known as mackerel sharks...
: They are commonly known as the mackerel sharks. They include the goblin sharkThe goblin shark, Mitsukurina owstoni, is a deep-sea shark, the sole living species in the family Mitsukurinidae. The most distinctive characteristic of the goblin shark is the unorthodox shape of its head. It has a long, trowel-shaped, beak-like rostrum or snout, much longer than other sharks'...
, basking sharkThe basking shark, Cetorhinus maximus, is the second largest living shark, after the whale shark. It is a cosmopolitan species — it is found in all the world's temperate oceans. It is a slow moving and generally harmless filter feeder.-Taxonomy:...
, megamouth sharkThe megamouth shark, Megachasma pelagios, is an extremely rare species of deepwater shark. Since being discovered in 1976, only a few megamouth sharks have been seen with 47 specimens known to have been caught or sighted as of 2009 and three recordings on film...
, the thresher sharkThresher sharks are large lamniform sharks of the family Alopiidae. Found in all temperate and tropical oceans of the world, the family contains three species all within the genus Alopias.-Taxonomy:...
s, shortfinThe shortfin mako shark, Isurus oxyrinchus , is a large mackerel shark. Along with the closely related longfin mako it is commonly referred to as "mako shark".-Anatomy and appearance:...
and longfin mako sharks, and great white sharkThe great white shark, Carcharodon carcharias, also known as great white, white pointer, white shark, or white death, is a large lamniform shark found in coastal surface waters in all major oceans. Reaching lengths of more than and weighing up to , the great white shark is arguably the world's...
. They are distinguished by their large jaws and ovoviviparousOvoviviparity, ovovivipary, or ovivipary, is a mode of reproduction in animals in which embryos develop inside eggs that are retained within the mother's body until they are ready to hatch...
reproduction. The Lamniformes include the extinct megalodonThe megalodon , Carcharodon megalodon or Carcharocles megalodon , was a giant shark that lived in prehistoric times during the Neogene period, and was a super-predator....
, Carcharodon megalodon.
Shark fishery
It is estimated that 100 million sharks are killed by people every year, due to commercial and recreational fishing. Sharks are a common seafood in many places around the world, including
Japanis an island country in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, People's Republic of China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
and
AustraliaAustralia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the continental mainland , the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans...
. In the Australian State of
VictoriaVictoria is the second most populous state in Australia. Geographically smallest mainland state, Victoria is bordered by New South Wales to the north, South Australia to the west, and Tasmania to the south, across the Bass Strait. Victoria is the most densely populated state, with over 70% of...
shark is the most commonly used fish in
fish and chipsFish and chips is a popular take-away food which originated in the United Kingdom. It consists of deep-fried fish in batter or breadcrumbs with deep-fried chipped potatoes.Popular tradition associates the dish with the United Kingdom; and fish and chips remains very popular in the UK and in...
, in which fillets are battered and deep-fried or crumbed and grilled and served alongside chips. In fish and chip shops, shark is called
flakeFlake is a term used in Australia to indicate the flesh of any of several species of small shark, particularly Gummy shark. The term probably arose in the late 1920s when the large-scale commercial shark fishery off the coast of Victoria was established. Until that time, shark was generally an...
. In
IndiaIndia, officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country, and the most populous democracy in the world. Bounded by the Indian Ocean on the south, the Arabian Sea on the west, and the Bay of Bengal...
small sharks or baby sharks (called sora in
Tamil languageTamil is a Dravidian language spoken predominantly by Tamil people of the Indian subcontinent. It has official status in India, Sri Lanka and Singapore. Tamil is also spoken by significant minorities in Malaysia, Mauritius and Réunion as well as emigrant communities around the world...
,
Telugu languageTelugu is a Dravidian language native to the Indian subcontinent. It is the official language of Andhra Pradesh, one of the largest states of India. It is also one of the twenty-two scheduled languages of the Republic of India and was conferred the status of a Classical language by the Government...
) are sold in local markets. Since the flesh is not developed completely boiling breaks it into powder which is then fried in oil and spices (called sora puttu). The soft bones can be easily chewed. They are considered a delicacy in coastal
Tamil NaduTamil Nadu is one of the 28 states of India. Its capital and largest city is Chennai . Tamil Nadu lies in the southernmost part of the Indian Peninsula and is bordered by Puducherry , Kerala, Karnataka and Andhra Pradesh...
. In
IcelandThe Republic of Iceland is a European island country located in the North Atlantic Ocean. It has a population of about 320,000 and a total area of 103,000 km². Its capital and largest city is Reykjavík, whose surrounding area is home to approximately two thirds of the national population...
,
Greenland sharkThe Greenland shark, Somniosus microcephalus, also known as the sleeper shark, gurry shark, ground shark, grey shark, or by the Inuit Eqalussuaq, is a large shark native to the waters of the North Atlantic Ocean around Greenland and Iceland. These sharks live further north than any other shark...
s are fished to produce
hákarlHákarl or kæstur hákarl is a food from Iceland. It is a Greenland or basking shark which has been cured with a particular fermentation process and hung to dry for 4-5 months. Hákarl has a very particular ammonia-rich smell and fishy taste, similar to Jewish deli whitefish or very strong cheese...
or fermented shark, which is widely regarded as a
national dishA national dish is a dish, food or a drink that represents a particular country, nation or region. It is usually something that is naturally made or popular in that country.-Overview:...
.
Sharks are often killed for
shark fin soupShark fin soup is a Chinese soup that has been a popular item of Chinese cuisine since the Ming Dynasty, usually served at special occasions such as weddings and banquets, or as a luxury item in Chinese culture....
: the finning process involves the removal of the fin with a hot metal blade. Fishermen capture live sharks, fin them, and release the finless animal back into the water. The now immobile shark soon dies from suffocation or predators. Shark finning has become a major trade within black markets all over the world with shark fins going at about $300/lb in 2009. Millions of sharks are illegally poached each year for their fins and too few governments enforce laws that protect them.{{Citation needed|date=September 2009}} The dish is considered a status symbol in Asian countries, and is considered healthy and full of nutrients, with some even claiming they prevent cancer and other ailments. No scientific proof supports these claims; at least one study has shown shark cartilage of no value in cancer treatment. The shark fin trade is a major problem and has gained international attention. Conservationists have campaigned to make finning illegal in the U.S.
Sharks are also killed for meat. The meat of dogfishes, smoothhounds, catsharks, makos, porbeagle and also skates and rays are in high demand by European consumers. However, the
U.S.The United States of America is a federal constitutional republic comprising fifty states and a federal district...
FDAThe Food and Drug Administration is a Government agency of the United States Department of Health and Human Services and is responsible for regulating and supervising the safety of foods, tobacco products, dietary supplements, Medication drugs, vaccines, Biopharmaceutical, blood transfusion,...
lists sharks as one of four fish (with
swordfishSwordfish , also known as Broadbill in some countries, are large, highly migratory, predatory fish characterized by a long, flat bill. They are a popular sport fish, though elusive. Swordfish are elongated, round-bodied, and lose all teeth and scales by adulthood. They reach a maximum size of 177 in...
,
king mackerelThe king mackerel 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:...
, and
tilefishTilefishes, also known as blanquillo, are mostly small perciform marine fish comprising the family Malacanthidae.Commercial fisheries exist for the largest species, making them important food fish, although the American Food and Drug Administration warns pregnant or breastfeeding women against...
) that children and women who are or may be pregnant should refrain from eating.
For details see
mercury in fishFish and shellfish have a natural tendency to concentrate mercury in their bodies, often in the form of methylmercury, a highly toxic organic compound of mercury. Fish products have been shown to contain varying amounts of heavy metals, particularly mercury and fat-soluble pollutants from water...
.
Shark cartilageShark cartilage is a dietary supplement made from the dried and powdered cartilage of a shark; that is, from the tough material that composes a shark's skeleton. Shark cartilage is claimed to combat and/or prevent a variety of illnesses, most notably cancer. It is often marketed under the names...
has been advocated as effective against
cancerCancer is a class of diseases in which a group of cells display uncontrolled growth , invasion , and sometimes metastasis...
and for treatment of
osteoarthritisOsteoarthritis , is a group of diseases and mechanical abnormalities involving degradation of joints, including articular cartilage and the subchondral bone next to it...
since many people falsely believe that sharks cannot get cancer and that taking it will prevent people from getting these diseases. Sharks generally reach sexual maturity slowly and produce very few offspring in comparison to other harvested fish. Harvesting sharks before they reproduce has severe impacts on future populations. Organizations such as the
Shark TrustShark Trust is a charitable organization founded in the UK in 1997"dedicated to promoting the study, management, and conservation of sharks,skates and rays in the UK and internationally."....
campaign to limit shark fishing. According to
Seafood WatchSeafood Watch is one of the best known sustainable seafood advisory lists, and has influenced similar programs around the world. It is a program designed to raise consumer awareness about the importance of buying seafood from sustainable sources...
, sharks are currently on the list of fish that American consumers, who are sustainability minded, should avoid.
Shark attacks
{{Main|Shark attack}}
In 2006 the
International Shark Attack FileThe 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 panel of shark experts developed a standard system for...
(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 360 species, only four have been involved in a significant number of fatal, unprovoked attacks on humans: the
great whiteThe great white shark, Carcharodon carcharias, also known as great white, white pointer, white shark, or white death, is a large lamniform shark found in coastal surface waters in all major oceans. Reaching lengths of more than and weighing up to , the great white shark is arguably the world's...
,
oceanic whitetipThe oceanic whitetip shark, Carcharhinus longimanus, is a large pelagic shark of tropical and warm temperate seas. It is a stocky shark, most notable for its long, white-tipped, rounded fins....
,
tigerThe tiger shark, Galeocerdo cuvier is a species of requiem shark and the only member of the genus Galeocerdo. Mature sharks average 3.25 to 4.25 m long and weigh . It can attain a length of over 7.25 m and a weight of 900 kg at maximum...
, and
bull sharkThe bull shark, Carcharhinus leucas, also known as the whaler shark, Zambezi shark or unofficially known as Zambi in Africa and Nicaragua shark in Nicaragua, is a shark common worldwide in warm, shallow waters along coasts and in rivers. The bull shark is well known for its unpredictable, often...
s.
These sharks, are large, powerful predators, and may sometimes attack and kill people. Interestingly, 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 1916The Jersey Shore shark attacks of 1916 were a series of shark attacks along the coast of New Jersey between July 1 and July 12, 1916, in which four people were killed and one injured. Since 1916, scholars have debated which shark species was responsible and the number of animals involved, with the...
, and through popular fictional works about shark attacks, such as the
JawsJaws is a 1975 American horror/thriller film directed by Steven Spielberg and based on Peter Benchley's novel Jaws. The police chief of Amity Island, a fictional summer resort town, tries to protect beachgoers from a giant great white shark by closing the beach, only to be overruled by the town...
film series.
Jaws, author
Peter BenchleyPeter Bradford Benchley was an American author, best known for his novel Jaws and its subsequent film adaptation, the latter co-written by Benchley and directed by Steven Spielberg...
in his later years attempted to dispel the image of sharks as man-eating monsters.
Sharks in captivity
{{Main|Sharks in captivity}}
Until recently only a few benthic species of shark, such as
hornsharksThe horn shark, Heterodontus francisci, is a species of bullhead shark, family Heterodontidae. It is endemic to the coastal waters off the western coast North America, from California to the Gulf of California. Young sharks are segregated spatially from the adults, with the former preferring deeper...
, leopard sharks and
catsharkThe cat sharks or catsharks are a family of sharks, with over 110 species recorded. Paradoxically perhaps, while the group is called the cat shark family, many species are commonly called dogfish...
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. At the same time, transportation techniques have improved and long distance movement of sharks is becoming easier. One shark that never had been successfully held in captivity for long was the great white. But in September 2004 the Monterey Bay Aquarium successfully kept a young female great white shark for 198 days before releasing her back into the wild.
Most species of shark are not suitable for domestic aquaria and not every species of shark sold by pet stores make good inhabitants for personal aquaria. Some species can be kept well in home saltwater aquaria. Uninformed or unscrupulous dealers sometimes sell juvenile sharks like the
nurse sharkThe nurse shark, Ginglymostoma cirratum, is a shark in the nurse sharks family, the only member of its genus Ginglymostoma. Nurse sharks can reach a length of 4.3 m and a weight of 330 lbs .-Taxonomy:...
, which upon reaching adulthood will have far outgrown typical home aquaria. Public aquaria are generally not interested in accepting donated specimens that have overgrown their housing and some shark owners have been tempted to
releaseAn introduced, alien, exotic, non-indigenous, or non-native species, or simply an introduction, is a species living outside its native distributional range, which has arrived there by human activity, either deliberate or accidental...
them into the wild. Species appropriate to home aquaria represent considerable spatial and financial investments as they generally approach adult lengths of 3 feet and can live up to 25 years.
Conservation
The majority of shark fisheries around the globe have little monitoring or management. With the rise in demand for shark products there is a greater pressure on fisheries. Stocks decline and collapse because it is difficult for sharks to breed rapidly enough to maintain population levels, given the long interval between birth and sexual maturity. Major declines in shark stocks have been recorded—some species have been depleted by over 90% over the past 20–30 years with a population decline of 70% not unusual. Many governments and the UN have acknowledged the need for shark fisheries management, but due to the low economic value of shark fisheries, the small volumes of products produced and the poor public image of sharks, little progress has been made.
Other threats to sharks include habitat alteration, damage and loss from coastal development, pollution and the impact of fisheries on the seabed and prey species. The practice of
shark finningShark finning refers to the removal and retention of shark fins and the discard at sea of the carcass. The shark is most often still alive when it is tossed back into the water. The finless sharks are unable to swim and sink to the ocean bottom and die. Shark finning takes place at sea so the...
, attracts much controversy and regulations are being enacted to prevent it from occurring. The acclaimed 2007 documentary,
Sharkwater exposed how sharks are being hunted to extinction, in part due to the massive Asian demand for
shark fin soupShark fin soup is a Chinese soup that has been a popular item of Chinese cuisine since the Ming Dynasty, usually served at special occasions such as weddings and banquets, or as a luxury item in Chinese culture....
.
In 2009, representative
Madeleine BordalloMadeleine Z. Bordallo is the Delegate from Guam to the United States House of Representatives.-Biography:Bordallo was born in Graceville, Minnesota to a military family and grew up on Guam after her father was stationed there. She attended St. Mary’s College in South Bend, Indiana and the College...
of
GuamGuam is an island in the western Pacific Ocean and is an organized, unincorporated territory of the United States. It is one of five U.S. territories with an established civilian government. The island's capital is Hagåtña...
introduced the
Shark Conservation Act of 2009The Shark Conservation Act of 2009 is a bill in the 111th United States Congress that would amend the High Seas Driftnet Fishing Moratorium Protection Act and the Magnuson-Stevens Fishery Conservation and Management Act to improve the conservation of sharks. The bill was approved by the House of...
, which passed the house. After passing the house, Senator
John KerryJohn Forbes Kerry is the senior United States Senator from Massachusetts, and is chairman of the Senate Foreign Relations Committee....
introduced the bill into the Senate, where it is currently pending. The bill would make already existing shark finning laws stronger.
Sharks in Hawaiian mythology
Sharks figure prominently in
Hawaiian mythologyHawaiian mythology refers to the legends, historical tales and sayings of the ancient Hawaiian people. It is considered a variant of a more general Polynesian mythology, developing its own unique character for several centuries before about 1800. It is associated with the Hawaiian religion, the...
. Stories tell of men with shark jaws on their back who could change between shark and human form. A common theme in the stories was that a shark-men 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.
HawaiiHawaii is the newest of the 50 U.S. states, and is the only state made up entirely of islands. It is located on an archipelago in the central Pacific Ocean, southwest of the continental United States, southeast of Japan, and northeast of Australia. The state was admitted to the Union on August...
an mythology also includes many shark
godGod is a deity in theistic and deistic religions and other belief systems, representing either the sole deity in monotheism, or a principal deity in polytheism....
s. Among a fishing people, the most popular of all
aumakuaIn Hawaiian mythology, an aumakua is a family god, often a deified ancestor. The Hawaiian plural of aumakua is nā aumākua , although in English the plural is usually aumakuas. Nā aumākua frequently manifested as animals such as sharks or owls...
, 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
kahunaKahuna is a Hawaiian word, defined in the as a "Priest, sorcerer, magician, wizard, minister, expert in any profession." Forty different types of kahuna are listed in the book, Tales from the Night Rainbow...
can point the awe-struck family to the markings on the shark's body that correspond to the clothing in which the body of their beloved 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'iIn Hawaiian mythology, Ka-moho-alii is a shark god and a brother of Kāne Milohai, Pele, Kapo, Nāmaka and Hiiaka.Ka-moho-ali'i swam in the area around Maui and Kahoolawe. When a ship was lost at sea, Ka-moho-alii shook his tail in front of the fleet and the kahuna would feed him "awa" , and...
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
HawaiiHawaii is the newest of the 50 U.S. states, and is the only state made up entirely of islands. It is located on an archipelago in the central Pacific Ocean, southwest of the continental United States, southeast of Japan, and northeast of Australia. The state was admitted to the Union on August...
. He was able to assume all human and fish forms. A summit cliff on the crater of K?lauea is one of his most sacred spots. At one point he had a
heiauA heiau is a Hawaiian temple. Many types of heiau existed, including heiau to treat the sick , offer first fruits, offer first catch, start rain, stop rain, increase the population, ensure health of the nation, achieve success in distant voyaging, reach peace, and achieve success in war...
(temple or shrine) dedicated to him on every piece of land that jutted into the ocean on the island of Moloka'i. 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 mytholog,
Dakuwanga was a shark god who was the eater of lost souls.
Popular misconceptions
A popular myth is that sharks are immune to disease and
cancerCancer is a class of diseases in which a group of cells display uncontrolled growth , invasion , and sometimes metastasis...
; however, this remains to be proven. Sharks may get cancer. Both diseases and parasites 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.
See also
- Outline of sharks
Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. They respire with the use of five to seven gill slits. Sharks have a covering of dermal denticles that protect their skin from damage and parasites and improve fluid dynamics...
- List of sharks
- List of prehistoric cartilaginous fish
- Marine vertebrates
- Shark finning
Shark finning refers to the removal and retention of shark fins and the discard at sea of the carcass. The shark is most often still alive when it is tossed back into the water. The finless sharks are unable to swim and sink to the ocean bottom and die. Shark finning takes place at sea so the...
- Shark sanctuary
New article name is Shark SanctuaryShark sanctuaries forbid commercial fishing operations from catching sharks. As of late 2009, the world's only shark sanctuary surrounds the small island nation of Palau....
External links
{{Sisterlinks|Shark}}
{{Wikispecies|Selachimorpha}}
{{Wikibooks|Dichotomous Key|Selachimorpha}}
{{Shark nav}}
{{Chondrichthyes}}
{{Selachimorpha}}