shark

Grey reef shark (Carcharhinus amblyrhynchos)

Sharks (superorder Selachimorpha) are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago, before the time of the dinosaurs.^[1]

Since that time, sharks have diversified into 440 species, ranging in size from the small dwarf lanternshark, Etmopterus perryi, a deep sea species of only 17 centimetres (7 in) in length, to the whale shark, Rhincodon typus, the largest fish, which reaches approximately 12 metres (39 ft) and which feeds only on plankton, squid, and small fish through filter feeding. Sharks are found in all seas and are common down to depths of 2,000 metres (6,600 ft), and some live even deeper but they are almost entirely absent below 3,000 metres (9,800 ft). They generally do not live in freshwater, with a few exceptions such as the bull shark and the river shark which can live both in seawater and freshwater.^[2] 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 so the shark can move faster. They have several sets of replaceable teeth.^[3]

Well-known species such as the great white and the hammerhead 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.

Contents [hide] 1 Etymology 2 Physical characteristics 2.1 Skeleton 2.1.1 Jaw 2.1.2 Teeth 2.1.3 Fins 2.2 Dermal denticles 2.3 Tails 3 Physiology 3.1 Buoyancy 3.2 Respiration 3.3 Thermoregulation 3.4 Osmoregulation 4 Senses 4.1 Smell 4.2 Sight 4.3 Hearing 4.4 Electroreception 4.5 Lateral line 5 Life history 5.1 Reproduction 5.1.1 Asexual reproduction 6 Behavior 6.1 Feeding 6.2 Speed 6.3 Intelligence 6.4 Sleep 7 Distribution and habitat 8 Evolution 9 Taxonomy 10 Interaction with humans 10.1 Shark fishery 10.2 Shark attacks 10.3 Sharks in captivity 11 Conservation 12 Sharks in cultural tradition 12.1 Sharks in Hawaiian mythology 12.2 Popular misconceptions 13 See also 14 References 15 External links

Etymology

Until the 16th century,^[4] sharks were known to mariners as "sea dogs".^[5] According to the OED the name "shark" first came into use after Sir John Hawkins' sailors exhibited one in London in 1569 and used the word to refer to the large sharks of the Caribbean Sea, 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].^[6]

Physical characteristics

Main article: Physical characteristics of sharks

Skeleton

Shark skeletons are very different from those of bony fish and terrestrial vertebrates. Sharks and other cartilaginous fish (skates and rays) have skeletons made from cartilage and connective tissue. Cartilage is flexible and durable, yet has about half the density of bone. This reduces the skeleton’s weight, saving energy.^[7] Sharks have no rib cage and therefore on land a shark's own weight can literally crush it.^[8]

Jaw

Like its relatives, rays and skates, the shark's jaw 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 crystal blocks of calcium salts arranged as a mosaic.^[9] 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 shark, tiger shark, 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.^[7] In the rostrum (snout), the cartilage can be spongy and flexible to absorb the power of impacts