Kingdom: Animalia

Phylum: Chordata,  Craniata

Fish, or sometimes plural fishes, are any member of a paraphyletic (having a common ancestor) group of organisms that consist of all gill-bearing aquatic craniate(having a skull) animals that lack limbs with digits. 


A Barracuda fish swims near coral in a strong current in the ocean. image: wikimedia

Included in this definition are:

  •  the living hagfish, 
  • lampreys, and 
  • cartilaginous (Chondrichthyes, such as sharks) and bony fish, 
  • as well as various extinct related groups.

Most fish are ectothermic ("cold-blooded"), allowing their body temperatures to vary as ambient temperatures change, though some of the large active swimmers like white shark and tuna can hold a higher core temperature.

Fish are abundant in most bodies of water. They can be found in nearly all aquatic environments, from high mountain streams (e.g., char and gudgeon) to the abyssal and even hadal depths of the deepest oceans (e.g., gulpers and anglerfish). 


A school of fish in an aquarium. image: R.Conan-Davies

At 32,000 species, fish exhibit greater species diversity than any other group of vertebrates.

Fish and humans

Fish are an important resource for humans worldwide, especially as food. Commercial and subsistence fishers hunt fish in wild fisheries (see fishing) or farm them in ponds or in cages in the ocean (see aquaculture). 

They are also caught by recreational fishers, kept as pets, raised by fishkeepers, and exhibited in public aquaria. Fish have had a role in culture through the ages, serving as deities, religious symbols, and as the subjects of art, books and movies.

Evolution of fish

Because the term "fish" is defined negatively, and excludes the tetrapods (i.e., the amphibians, reptiles, birds and mammals) which descend from within the same ancestry, it is paraphyletic, and is not considered a proper grouping in systematic biology. The traditional term pisces (also ichthyes) is considered a typological, but not a phylogenetic classification.

The earliest organisms that can be classified as fish were soft-bodied chordates that first appeared during the Cambrian period. Although they lacked a true spine, they possessed notochords which allowed them to be more agile than their invertebrate counterparts. 

Fish would continue to evolve through the Paleozoic era, diversifying into a wide variety of forms. Many fish of the Paleozoic developed external armour that protected them from predators. 


Coccosteus decipiens, a placoderm from the Devonian ( Age of fishes)  of Europe and North America, pencil drawing. image: Nobu Tamura, wikipedia

The first fish with jaws appeared in the Silurian period, after which many (such as sharks) became formidable marine predators rather than just the prey of arthropods.

see also: