Secrets of The Animal World Elephant

Secrets of The Animal World SharksSharks are a group of fish characterized by a cartilaginous skeleton, 5 to 7 gill slits on the sides of the head, and pectoral fins that are not merged to the head. Acanthodians are frequently referred to as "spiny sharks"; though they are not part of Chondrichthyes proper, they are a paraphyletic assemblage leading to cartilaginous fish as a whole.Ever since, sharks have actually diversified into over 500 species. They range in size from the little dwarf lantern shark (Etmopterus perryi), a deep sea types of only 17 centimeters (6.7 in) in length, to the whale shark (Rhincodon types), the largest fish in the world, which reaches approximately 12 meters (39 ft) in length. Sharks are found in all seas and are common to depths of 2,000 meters (6,600 ft). They typically do not live in freshwater although there are a couple of known exceptions, such as the bull shark and the river shark, which can be and survive found in both seawater and freshwater. They breathe through 5 to 7 gill slits. Sharks have a covering of dermal denticles that safeguards their skin from damage and parasites in addition to improving their fluid dynamics. They have several sets of replaceable teeth.Widely known species such as the excellent white shark, tiger shark, blue shark, mako shark, and the hammerhead shark are pinnacle predators - organisms at the top of their underwater food cycle. Numerous shark populations are threatened by human activities.