Sharks

Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachimorpha (or Selachii) and are the sister group to the rays. However, the term "shark" has also been used for extinct members of the subclass Elasmobranchii outside the Selachimorpha, such as Cladoselache and Xenacanthus, as well as other Chondrichthyessuch as the holocephalid eugenedontidans.

Under this broader definition, the earliest known sharks date back to more than 420 million years ago.^[1] Acanthodians are often referred to as "spiny sharks"; though they are not part of Chondrichthyes proper, they are a paraphyletic assemblage leading to cartilaginous fish as a whole. Since then, sharks have diversified into over 500 species. They range in size from the small dwarf lanternshark (Etmopterus perryi), a deep sea species of only 17 centimetres (6.7 in) in length, to the whale shark (Rhincodon typus), the largest fish in the world, which reaches approximately 12 metres (40 ft) in length. Sharks are found in all seas and are common to depths of 2,000 metres (6,600 ft). They generally do not live in freshwater although there are a few known exceptions, such as the bull shark and the river shark, which can survive and be found in both seawater and freshwater.^[2]Sharks have a covering of dermal

denticles that protects their skin from damage and parasites in addition to improving their fluid dynamics. They have numerous sets of replaceable teeth.^[3]

Well-known species such as the great white shark, tiger shark, blue shark, mako shark, thresher shark, and hammerhead sharkare apex predators—organisms at the top of their underwater food chain. Many shark populations are threatened by human activities.

Taxonomy

Sharks belong to the superorder Selachimorpha in the subclass Elasmobranchii in the classChondrichthyes. The Elasmobranchii also include rays and skates; the Chondrichthyes also include Chimaeras. It is currently thought that the sharks form a polyphyletic group: some sharks are more closely related to rays than they are to some other sharks.^[19]

The superorder Selachimorpha is divided into Galea (or Galeomorphii), and Squalea (or Squalomorphii). The Galeans are the Heterodontiformes, Orectolobiformes, Lamniformes, and Carcharhiniformes. Lamnoids and Carcharhinoids are usually placed in one clade, but recent studies show the Lamnoids and Orectoloboids are a clade. Some scientists now think that Heterodontoids may be Squalean. The Squaleans are divided into Hexanchoidei and Squalomorpha. The Hexanchoidei includes the Hexanchiformes and Chlamydoselachiformes. The Squalomorpha contains the Squaliformes and the Hypnosqualea. The Hypnosqualea may be invalid. It includes the Squatiniformes, and the Pristorajea, which may also be invalid, but includes the Pristiophoriformesand the Batoidea.^[19][20]

There are more than 470 species of sharks split across thirteen orders, including four orders of sharks that have gone extinct:^[20]

• Carcharhiniformes: Commonly known as ground sharks, the order includes the blue, tiger, bull, grey reef, blacktip reef, Caribbean reef, blacktail reef, whitetip reef, and oceanic whitetip sharks (collectively called the requiem sharks) along with the houndsharks, catsharks, and hammerhead sharks. They are distinguished by an elongated snout and a nictitating membrane which protects the eyes during an attack.
• Heterodontiformes: They are generally referred to as the bullhead or horn sharks.
• Hexanchiformes: Examples from this group include the cow sharks and frilled sharks, which somewhat resembles a marine snake.
• Lamniformes: They are commonly known as the mackerel sharks. They include the goblin shark, basking shark, megamouth shark, the thresher sharks, shortfinand longfin mako sharks, and great white shark. They are distinguished by their large jaws and ovoviviparous reproduction. The Lamniformes also include the extinct megalodon, Carcharodon megalodon.
• Orectolobiformes: They are commonly referred to as the carpet sharks, including zebra sharks, nurse sharks, wobbegongs, and the whale shark.
• Pristiophoriformes: These are the sawsharks, with an elongated, toothed snout that they use for slashing their prey.
• Squaliformes: This group includes the dogfish sharks and roughsharks.
• Squatiniformes: Also known as angel sharks, they are flattened sharks with a strong resemblance to stingrays and skates.
• † Cladoselachiformes
• † Hybodontiformes
• † Symmoriida
• † Xenacanthida (Xenacantiformes)

In captivity

Until recently, only a few benthic species of shark, such as hornsharks, leopard sharks and catsharks, had survived in aquarium conditions for a year or more. This gave rise to the belief that sharks, as well as being difficult to capture and transport, were difficult to care for. More knowledge has led to more species (including the large pelagic sharks) living far longer in captivity, along with safer transportation techniques that have enabled long distance transportation.^[ For a long time, the great white shark had never been successfully held in captivity for long, but in September 2004, the Monterey Bay Aquarium successfully kept a young female for 198 days before releasing her.

Most species are not suitable for home aquaria, and not every species sold by pet stores are appropriate. Some species can flourish in home saltwater aquaria.^[89] Uninformed or unscrupulous dealers sometimes sell juvenile sharks like the nurse shark, which upon reaching adulthood is far too large for typical home aquaria.^[89] Public aquaria generally do not accept donated specimens that have outgrown their housing. Some owners have been tempted to release them.^[89] Species appropriate to home aquaria represent considerable spatial and financial investments as they generally approach adult lengths of 3 feet (90 cm) and can live up to 25 years.

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