Shark

Sharks are a group of fish designate by a rubbery skeleton, five to seven gill slits on the sides of the top, and pectoral fins that are not amalgamated to the top. Modern sharks ar classified at intervals the {clade|biological cluster} Selachimorpha (or Selachii) and ar the sister group to the rays. However, the term "shark" has also been used for extinct members of the taxonomic group class outside the Selachimorpha, such as Cladoselache and Xenacanthus, as well as other class like the holocephalid eugenedontidans. Under this broader definition, the earliest known sharks date back to quite 420 million years past. Acanthodians are typically referred to as "spiny sharks"; tho' they're not a part of class correct, they are a paraphyletic assemblage resulting in chondrichthian as a full.

Since then, sharks have diversified into over five hundred species. They range in size from the tiny dwarf lanternshark (Etmopterus perryi), a deep sea species of solely seventeen centimetres (6.7 in) in length, to the whale shark (Rhincodon typus), the largest fish within the world, which reaches roughly twelve metres (40 ft) in length. Sharks ar found in all seas and are common to depths of two,000 metres (6,600 ft). They generally don't sleep in fresh though there ar a number of glorious exceptions, such as the cub shark and therefore the river shark, which will survive and be found in each water and fresh. Sharks have a covering of dermal denticles that secure their skin from damage and parasites in addition to up their fluid dynamics. They have numerous sets of interchangeable teeth.

Well-known species such as the good mackerel shark, tiger shark, blue shark, mako shark, and the shark are apex predators—organisms at the highest of their underwater organic phenomenon. Many shark populations ar vulnerable by human activities.