Fish

A fish is any aquatic vertebrate animal that is covered with scales, and equipped with two sets of paired fins and several unpaired fins. Most fish are "cold-blooded", or ectothermic, allowing their body temperatures to vary as ambient temperatures change. 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 . At 31,500 species, fish exhibit greater species diversity than any other class of vertebrates.

Food prepared from animals classified as fish is also referred to as fish, and is an important human food source. 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 and raised by fishkeepers, and are 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.

The term "fish" most precisely describes any non-tetrapod craniate i.e. an animal with a skull and in most cases a backbone that has gills throughout life and whose limbs, if any, are in the shape of fins. 2 Unlike groupings such as birds or mammals, fish are not a single clade but a paraphyletic collection of taxa, including hagfishes, lampreys, sharks and rays, ray-finned fishes, coelacanths, and lungfishes. 3

A typical fish is ectothermic, has a streamlined body for rapid swimming, extracts oxygen from water using gills or uses an accessory breathing organ to breathe atmospheric oxygen, has two sets of paired fins, usually one or two rarely three dorsal fins, an anal fin, and a tail fin, has jaws, has skin that is usually covered with scales, and lays eggs.

Each criterion has exceptions. Tuna, swordfish, and some species of sharks show some warm-blooded adaptations they can heat their bodies significantly above ambient water temperature. 3 Streamlining and swimming performance varies from fish such as tuna, salmon, and jacks that can cover 10 20 body-lengths per second to species such as eels and rays that swim no more than 0.5 body-lengths per second. 3 Many groups of freshwater fish extract oxygen from the air as well as from the water using a variety of different structures. Lungfish have paired lungs similar to those of tetrapods, gouramis have a structure called the labyrinth organ that performs a similar function, while many catfish, such as Corydoras extract oxygen via the intestine or stomach. 5 Body shape and the arrangement of the fins is highly variable, covering such seemingly un-fishlike forms as seahorses, pufferfish, anglerfish, and gulpers. Similarly, the surface of the skin may be naked as in moray eels , or covered with scales of a variety of different types usually defined as placoid typical of sharks and rays , cosmoid fossil lungfishes and coelacanths , ganoid various fossil fishes but also living gars and bichirs , cycloid, and ctenoid these last two are found on most bony fish . 6 There are even fishes that live mostly on land. Mudskippers feed and interact with one another on mudflats and go underwater to hide in their burrows. 7 The catfish Phreatobius cisternarum lives in underground, phreatic habitats, and a relative lives in waterlogged leaf litter. 8