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Sharks are a group (superorder Selachimorpha) of fish with a full cartilaginous skeleton and a streamlined body. They respire with the use of gill slits along the side of, or beginning slightly behind, the head. Sharks have a covering of dermal denticles on their bodies to protect their skin from damage, parasites and to improve fluid dynamics. They have several rows of replaceable teeth.
Please visit
http://en.wikipedia.org/wiki/Shark to see a picture of a shark.
Sharks include species from the hand-sized pygmy shark, Euprotomicrus bispinatus, a deep sea species of only 22 cm in length, to the whale shark, Rhincodon typus, the largest fish, which grows to a length of approximately 12 metres (41 feet) and which, like the great whales, feeds only on plankton through filter feeding. The bull shark, Carcharhinus leucas, is the best known of several species to swim in both salt and fresh water and in deltas. Trawler fisherman Lee Dreyer, from Queensland, Australia, netted 64 aggressive bull sharks in the Logan River, south of Brisbane, in less than three hours last month while fishing about 100m east of the Pacific Motorway bridge. Please visit
http://www.theaustralian.news.com.au/story/0,20867,20426230-5006786,00.html to read about the huge number of bull sharks spotted in Australian rivers and lakes.
Bull sharks are extremely agressive and they have been named as one of the most dangerous in the world, and is linked to numerous attacks that lead to serious injuries and deaths.
Recently, new species of shark have been discovered off the coast of Indonesia, It is a type of small, slender-bodied sharks that "walk" with their fins along coral reefs, researchers announced today. Please visit
http://news.nationalgeographic.com/news/2006/09/060918-walking-shark.html to see a picture of the shark that can walk with their fins.