Fish is a member of the vertebrate poikilotermik (cold blooded) that live in water and breathe with gills. Fish is the most diverse vertebrate species with more than 27,000 worldwide. Taxonomically, fish are a paraphyletic group relationships are much debated; fish usually divided into the jawless fish (class Agnatha, 75 species including lampreys and hagfish), cartilaginous fishes (class Chondrichthyes, 800 species including sharks and rays), and the remaining classified as hard bony fish (class Osteichthyes).
Fish in different sizes, ranging from whale sharks measuring 14 meters (45 ft) up to a stout infantfish measuring only 7 mm (about 1/4 inch). There are some aquatic animals are often regarded as the "fish", such as whales, fish, squid and sea cow, which was not classified as fish.
