, Clupea harengus,
one of the most abundant species in the world]]
A fish is a 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 rayss), with the remainder classed as bony fishes (class Osteichthyes).
Fishes come in different sizes, from the 14-meter (45 ft) whale shark to a 8-mm (3/8 inch) long dwarf goby. 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.
"Fishes" is the proper English plural form of "fish" that biologists use when speaking about two or more fish species, as in "There are over 25,000 fishes in the world" (meaning that there are over 25,000 fish species in the world). When speaking of two or more individual fish organisms, then the word "fish" is used, as in "There are several million fish in the species Gadus morhua" (meaning that G. morhua comprises several million individuals). To see both in action, consider the statement "There are twelve fish in this aquarium, representing five fishes" (meaning that the aquarium contains twelve individuals, some of the same species and some of different species, for a total of five species).
Fish are an important source of food. Other water-dwelling animals such as mollusks and crustaceans (commonly called shellfish) are often considered as fish when used as food. Catching fish for the purpose of food or sport is known as fishing. The annual yield from all fisheries worldwide is about 100 milliontonnes.
Overfishing is a threat to many species of fish. On May 15, 2003, the journal Nature reported that all large oceanic fish species worldwide had been so systematically overcaught that fewer than 10% of 1950 levels remained.  Particularly imperilled were sharks, Atlantic cod, and Pacific sardines. The authors recommended immediate, drastic cutbacks in fish catches and reservation of ocean habitats worldwide.