Some sources extend the term "shark" as an informal category including extinct members of Chondrichthyes (cartilaginous fish) with a shark-like morphology, such as hybodonts.

However, the Middle English Dictionary records an isolated occurrence of the word shark (referring to a sea fish) in a letter written by Thomas Beckington in 1442, which rules out a New World etymology.

[13] The oldest confirmed members of Elasmobranchii sensu lato (the group containing all cartilaginous fish more closely related to modern sharks and rays than to chimaeras) appeared during the Devonian.

[19] Batoidea Heterodontiformes Orectolobiformes Carcharhiniformes Lamniformes Hexanchiformes Squatiniformes Pristiophoriformes Squaliformes Sharks belong to the clade Selachimorpha in the subclass Elasmobranchii in the class Chondrichthyes.

[citation needed] Fin skeletons are elongated and supported with soft and unsegmented rays named ceratotrichia, filaments of elastic protein resembling the horny keratin in hair and feathers.

[30] Unlike bony fish, sharks have a complex dermal corset made of flexible collagenous fibers and arranged as a helical network surrounding their body.

Gas exchange takes place in the gills and the reoxygenated blood flows into the efferent branchial arteries, which come together to form the dorsal aorta.

The scalloped hammerhead closes its mouth and gills when diving to depths of around 800 metres, holding its breath until it reaches warmer waters again.

[45] In contrast to bony fish, with the exception of the coelacanth,[46] the blood and other tissue of sharks and Chondrichthyes is generally isotonic to their marine environments because of the high concentration of urea (up to 2.5%[47]) and trimethylamine N-oxide (TMAO), allowing them to be in osmotic balance with the seawater.

A few exceptions exist, such as the bull shark, which has developed a way to change its kidney function to excrete large amounts of urea.

[48][49] Research in 1930 by Homer W. Smith showed that sharks' urine does not contain sufficient sodium to avoid hypernatremia, and it was postulated that there must be an additional mechanism for salt secretion.

The valve provides a long surface area, requiring food to circulate inside the short gut until fully digested, when remaining waste products pass into the cloaca.

[53] 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.

However, some species, including the great white shark (Carcharodon carcharias), do not have this membrane, but instead roll their eyes backwards to protect them when striking prey.

Some believe that electro- and chemoreception are more significant, while others point to the nictating membrane as evidence that sight is important, since presumably the shark would not protect its eyes were they unimportant.

[58] A micro-spectrophotometry study of 17 species of sharks found 10 had only rod photoreceptors and no cone cells in their retinas giving them good night vision while making them colorblind.

[74] The posterior part of a male shark's pelvic fins are modified into a pair of intromittent organs called claspers, analogous to a mammalian penis, of which one is used to deliver sperm into the female.

[88] Sharks possess brain-to-body mass ratios that are similar to mammals and birds,[89] and have exhibited apparent curiosity and behavior resembling play in the wild.

[95] Other highly specialized feeders include cookiecutter sharks, which feed on flesh sliced out of other larger fish and marine mammals.

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.

Out of more than 470 species, only four have been involved in a significant number of fatal, unprovoked attacks on humans: the great white, oceanic whitetip, tiger, and bull sharks.

[112] 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.

[117][118][119] According to the U.S. National Park Service, "Villagers from nearby Vaitogi continue to reenact an important aspect of the legend at Turtle and Shark by performing a ritual song intended to summon the legendary animals to the ocean surface, and visitors are frequently amazed to see one or both of these creatures emerge from the sea in apparent response to this call.

"[117] In contrast to the complex portrayals by Hawaiians and other Pacific Islanders, the European and Western view of sharks has historically been mostly of fear and malevolence.

[121] Sharks are threats in other films such as Deep Blue Sea, The Reef, and others, although they are sometimes used for comedic effect such as in Finding Nemo and the Austin Powers series.

[141] However, the U.S. FDA lists sharks as one of four fish (with swordfish, king mackerel, and tilefish) whose high mercury content is hazardous to children and pregnant women.

[74] Capture-induced parturition is rarely considered in fisheries management despite being shown to occur in at least 12% of live bearing sharks and rays (88 species to date).

"[167] Other threats include habitat alteration, damage and loss from coastal development, pollution and the impact of fisheries on the seabed and prey species.

China, by far the world's largest shark market, and Japan, which battles all attempts to extend the convention to marine species, led the opposition.

It was the first global instrument concluded under CMS and aims at facilitating international coordination for the protection, conservation and management of migratory sharks, through multilateral, intergovernmental discussion and scientific research.