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SHARK, any of some 340 species of cartilaginous fish, belonging to the superorder Selachimorpha. The shark, together with the closely related skate and ray, of the superorder Batidoidimorpha, and the chimaera, of the subclass Holocephali, make up the class Chondrichthyes. These animals are sharply distinguished from the vast number of teleost (bony) fish species, class Osteichtyes, by their cartilaginous skeletons. Sharks are versatile and keen-sensed fish, many species of which are able to hunt and eat nearly all the larger marine animals in both shallow and deep seas. These two features account for their long evolutionary history; many of the shark species living today are quite similar to abundant species that swam in seas of the Cretaceous period more than 100 million years ago. Sharks also reveal great diversity in behavior and size. The whale shark, Rhincodon typus, is the largest shark and also the largest fish in the sea, measuring up to 15 m (49 ft) in length; Squalius laticaudus is the smallest shark, measuring only 15 to 20 cm (6 to 8 in) in length. Sharks are chiefly marine fish found in all seas and are especially abundant in tropical and subtropical waters. Many species migrate up rivers, however, and one, the bull shark, Carcharhinus leucas, reaches Lake Nicaragua in Central America. Sharks are best known as aggressive carnivores that even attack members of their own species, but two of the largest sharks?the basking shark, Cetorhinus maximus, and the whale shark?are docile feeders on plankton, which they strain from the water with sievelike gill rakers in their pharynx. |