Sharks tend to be associated with oceans, especially the warmer seas. You may be surprised to know that unlike bony fish, sharks do not have bones at all. They only have cartilage which is a strong fibrous substance, rather than bone. It is softer than bones. The most famous feature of sharks is their teeth, specifically the sharp cutting teeth. They have more than one set of teeth. Some may even have five sets. As teeth worn down, they fall out and are replaced by new ones growing behind. In a single decade, a tiger shark can shed as many as 20,000 teeth. Sharks feel rough to touch. The skin of sharks was once sold as sand paper.
Only 25 out of the 250 species of shark are classified dangerous to man. Sharks have about 5 to 7 openings (gill slits) which allow them to breathe. They are generally known to be restless creature. Unlike most other fish, they do not have a gas-filled swim bladder, which helps control buoyancy. Their lift comes from their side fins and their large, oil filled liver. If a shark were to stop swimming, it would sink. Some species do rest on the ocean bottom.
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