Shark superorder Selachimorpha fish cartilaginous skeleton streamlined body respire five to seven gill slits. Sharks have a cov
Sharks superorder Selachimorpha fish cartilaginous skeleton streamlined body respire five to seven gill slits. Sharks have a covering of dermal denticles that protect their skin from damage and parasites and improve fluid dynamics. They have several sets of replaceable teeth. Sharks range 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 grows to a length of approximately 12 metres (39 ft) and which feeds only on plankton, squid, and small fish through filter feeding. The bull shark, Carcharhinus leucas, is the best known of several species that swim in both seawater and freshwater, as well as in deltas. Unlike bony fish, sharks do not have gas-filled swim bladders for buoyancy. Instead, sharks rely on a large liver, filled with oil that contains squalene. The buoyant liver may constitute up to 30% of their body mass. Its effectiveness is limited, so sharks employ dynamic lift to maintain depth and sink when they stop swimming. Sandtiger sharks are also known to gulp air from the surface and store it in their stomachs, using the stomach as a swim bladder. Because of this, 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 shark, have spiracles that force water across their gills allowing them to remain stationary at rest on the ocean bottom. Some sharks, if inverted or stroked on the nose, enter a natural state of tonic immobility. Researchers have used this condition to handle sharks safely.
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