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The Complete Guide on Fish
A fish is a poikilothermic (cold-blooded) * water-dwelling vertebrate with gills. There are over 27,000 species of fish, making them the most diverse group of vertebrates. Taxonomically, fish are a paraphyletic group whose exact relationships are much debated; a common division is into the jawless fishes (class Agnatha, 75 species including lampreys and hagfish), the cartilaginous fishes (class Chondrichthyes, 800 species including sharks and rays), with the remainder classed as bony fishes (class Osteichthyes).
Fishes come in different sizes, from the 14-meter (45 ft) whale shark to a 7-mm (just over 1/4 of an inch) long stout infantfish. Many types of aquatic animals named "fish", such as jellyfish and cuttlefish, are not true fish. Certain species of tuna maintain an elevated core temperature, so they are not, strictly speaking, poikilothermic. The Great White Shark is the only known truly endothermic (warm-blooded) fish.
