Shark Attacks and Invades Mississippi River - World Documentaries

Sharks are a group of fish distinguisheded by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not integrated to the head. Modern sharks are classified within the clade Selachimorpha (or Selachii) and are the sis group to the rays. Nevertheless, the term "shark" has also been used for extinct members of the subdivision Elasmobranchii outside the Selachimorpha, such as Cladoselache and Xenacanthus. Under this more comprehensive definition, the earliest known sharks go back to greater than 420 million years ago.
They vary in dimension from the little dwarf lanternshark (Etmopterus perryi), a deep sea species of just 17 centimetres (6.7 in) in length, to the whale shark (Rhincodon typus), the largest fish in the world, which reaches around 12 metres (39 ft) in length. Sharks are found in all seas and are usual to midsts of 2,000 metres (6,600 ft). They generally do not live in freshwater although there are a few known exceptions, such as the bull shark and the stream shark, which can be and make it through found in both salt water and freshwater.
Popular species such as the great white shark, tiger shark, blue shark, mako shark, and the hammerhead shark are peak predators-- microorganisms on top of their underwater food chain. Many shark populaces are intimidated by human tasks.