The shark is the name given to a group of elasmobranch fish that are characterised by having a cartilaginous skeleton, between five and seven gills, and pectoral fins. In modern classification, sharks are considered to be a sister species to rays, although the term shark has also been applied to a number of extinct species.
Under the broadest definition of shark, the species has been dated to more than 400 million years since their arrival as a distinct species. Currently, we know of approximately 500 species of shark, ranging from the smallest dwarf lantern shark, a species that exists in the deep sea and reaches approximately 17 cm in length, through to the whale shark, the largest fish in the world that can reach 12 metres in length. Sharks are present in all seas of the world and are most commonly found down to depths of 2,000 metres, although some species are never found above this depth. Sharks generally do not live in freshwater, although the bull shark and river shark can commonly be seen in both freshwater and seawater. In order to protect themselves from skin parasites, sharks are covered with dermal denticles that also help to improve their fluid dynamics. Sharks are perhaps best known as a species for their numerous sets of replaceable teeth.
The most well-known species of shark, such as the mako shark, hammerhead shark, blue shark, and tiger shark, are categorised as apex predators, although almost all shark populations are threatened by human activity. Sharks are commonly fished as their fins are considered to be a delicacy in many countries when turned into a soup. They are also regularly caught in nets as a bycatch, or even by tuna fishers by accident, although the latter are required to release the shark back in the ocean rather than killing them.
The hammerhead shark is one of the more distinct species in the shark family, categorised and named for the unique shape of their head which resembles a hammer. The shark’s eyes are on either side of the hammer and it is speculated that this gives them an advantage while hunting in their native waters.
Sharks are of great interest to marine biologists are routinely selected for study, including attempts to track their migratory patterns, sometimes with a goal of preventing them from being killed by humans. Many experts have theorised that some species of shark will become extinct if plastic pollution of the ocean continues.