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Sharks are a group (superorder Selachimorpha) of fish, with a full that cartilaginous skeleton, a streamlined body plan, with normally 5, but up to 7 (depending on species) gill slits along the side of, or beginning slightly behind, the head (in some species, a modified slit called a spiracle, is located just behind the eye), dermal denticles covering the body to protect from damage, parasites and improve fluid dynamics, and rows of replaceable teeth in the mouth. Sharks have keen olfactory senses, with some species able to detect as little as one part per million of blood in seawater. They are even more attracted to the chemicals found in the gut of many species, and often linger near or in sewage outfalls. Some species, such as Nurse sharks, have external barbels that greatly increase their ability to sense prey. The short duct between the anterior and posterior nasal openings are not fused like in bony fish. Shark eyes are similar to the eyes of other vertebrates, including similar lenses, corneas and retinas, though their eyesight is well adapted to their marine environment with the help of a tissue called tapetum lucidum. This tissue is behind the retina and reflects light back to the retina, thereby increasing visibility in the dark waters. The effectiveness of the tissue varies, with some sharks having stronger nocturnal adaptations. Sharks have eyelids, but they do not blink because the surrounding water cleans their eyes. To protect their eyes some have nictitating membranes. This membrane covers the eyes during predation, and when it is being attacked. However, some species, including the Great White, do not have this membrane, but instead roll their eyes backwards to protect them. Sharks generally rely on their superior sense of smell to find prey, but once they are close they also use the lateral lines running along their sides to sense movement in the water and also employ special sensory pores on their heads to detect electrical fields created by prey and the ambient electric fields of the ocean. Their teeth are not attached to the jaw, but embedded in their flesh, and in many species are constantly replaced throughout the shark's life. The lower teeth are primarily used for holding prey, while the top are used for cutting into it. Sharks also have a sharp sense of hearing and can hear prey many miles away. A small opening on each side of their head (not to be confused with the spiracle) leads directly into the inner ear through a thin channel. The lateral line shows a similar arrangement as it is open to the environment via a series of openings called lateral line pores. This is a reminder of the common origin of these two vibration and sound detecting organs that is grouped together as the acoustico-lateralis system. In bony fish and tetrapods the external opening into the inner ear has been lost. There are exceptions to the "large", "marine" (as in 'ocean-going') and "predatory" portions of the characterization. Sharks include everything from the hand-sized Pygmy Shark, a deep sea species, to the Whale Shark, the largest fish (although sharks are not closely related to bony fish) which is known to grow to a maximum length of approximately 15 metres (49 feet) and which, like the great whales, feeds only on plankton. Although not unique among sharks, the Bull Shark is the better known of several species to regularly swim in both salt and fresh water environments (most famously in Lake Nicaragua, in Central America) and in most deltas. A few of the larger species, the Mako and White Shark, are mildly homeothermic, able to maintain their body temperature at a level above the ocean's temperature. This is possible because of the presence of the rete mirabile, a counter current exchange mechanism that reduces the loss of body heat. Like other fish, sharks extract their oxygen from seawater as it passes over their gills. Due to their size and the nature of their metabolism, sharks have a higher demand for oxygen than most fish and they cannot rely on ambient water current to provide an adequate supply of oxygenated water. If a shark were to stop swimming, the necessary water circulation for respiration would become too low and the animal could suffocate, although some sharks have been known to "nap" on the bottom. Some sharks, like the Blacktip Reef Shark and Nurse Shark, can pump water over their gills as they rest. There are also known instances, such as in certain caves along the Yucatan coast, where sharks of varying species rest on the cave floors and allow the fresh water outflow to pass over them. The outflow is strong enough to allow for respiration, and it is believed that the reason for this behaviour is that the fresh water helps remove certain parasites. Also, unlike other fish, sharks do not have gas-filled swim bladders, but rather rely on an oil-filled liver for (limited) buoyancy, so they sink when they stop swimming; a resting shark always sinks to the sea bed. Sand tiger sharks are known to gulp air at the surface and store it in their stomach to provide buoyancy. Unlike bony fish, the sharks have a complex dermal corset arranged as a helical network and made of flexible collagenous fibres surrounding their body. This works as an outer skeleton, providing attachment for their swimming muscles. The sharks saves more energy while swimming this way than if they didn't have their collagenous corset. A similar arrangement of collagen fibres has been discovered in dolphins and squids. Their dermal teeth gives them hydrodynamic advantages as they are reducing the turbulence when swimming. A shark, if inverted, enters a natural state of paralysis. Researchers use this condition for handling sharks safely. A popular myth is that sharks are immune to disease and cancer, however, this is untrue and there are both diseases and parasites that affect sharks. The evidence that sharks are at least resistant to cancer and disease is mostly anecdotal and there have been few, if any, scientific or statistical studies that have shown sharks to have heightened immunity to disease. Urine in sharks accumulates in the blood and is then secreted through the skin. Their upper jaws are not attached to the skull


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