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

Diversity of fish

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. 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.
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. 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. 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. 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).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.The catfish Phreatobius cisternarum lives in underground, phreatic habitats, and a relative lives in waterlogged leaf litter

Fish range in size from the huge 16-metre (52 ft) whale shark to the tiny 8-millimetre (0.3 in) stout infantfish.
Many types of aquatic animals commonly referred to as "fish" are not fish in the sense given above; examples include shellfish, cuttlefish, starfish, crayfish and jellyfish. In earlier times, even biologists did not make a distinction – sixteenth century natural historians classified also seals, whales, amphibians, crocodiles, even hippopotamuses, as well as a host of aquatic invertebrates, as fish. However, according the definition above, all mammals, including Cetaceans like Whales and Dolphins, are not fish. In some contexts, especially in aquaculture, the true fish are referred to as finfish (or fin fish) to distinguish them from these other animals.

Taxonomy

Fish are a paraphyletic group: that is, any clade containing all fish also contains the tetrapods, which are not fish. For this reason, groups such as the "Class Pisces" seen in older reference works are no longer used in formal classifications.
Fish are classified into the following major groups:

• Subclass Pteraspidomorphi (early jawless fish)
• Class Thelodonti
• Class Anaspida
• (unranked) Cephalaspidomorphi (early jawless fish)
  • (unranked) Hyperoartia or Petromyzontida
    • Petromyzontidae (lampreys)
  • Class Galeaspida
  • Class Pituriaspida
  • Class Osteostraci
• Infraphylum Gnathostomata (jawed vertebrates)
  • Class Placodermi (armoured fishes, extinct)
  • Class Chondrichthyes (cartilaginous fish)
  • Class Acanthodii (spiny sharks, extinct)
  • Superclass Osteichthyes (bony fish)
    • Class Actinopterygii (ray-finned fish)
      • Subclass Chondrostei
        • Order Acipenseriformes (sturgeons and paddlefishes)
        • Order Polypteriformes (reedfishes and bichirs).
      • Subclass Neopterygii
        • Infraclass Holostei (gars and bowfins)
        • Infraclass Teleostei (many orders of common fishes)
    • Class Sarcopterygii (lobe-finned fish)
      • Subclass Coelacanthimorpha (coelacanths)
      • Subclass Dipnoi (lungfish)

Some palaeontologists contend that because Conodonta are chordates, they are primitive fish. For a fuller treatment of this taxonomy, see the vertebrate article.
The position of hagfish in the phylum chordata is not settled. Phylogenetic research in 1998 and 1999 supported the idea that the hagfish and the lampreys form a natural group, the Cyclostomata, that is a sister group of the Gnathostomata.
The various fish groups account for more than half of vertebrate species. There are almost 28,000 known extant species, of which almost 27,000 are bony fish, with 970 sharks, rays, and chimeras and about 108 hagfishes and lampreys.A third of these species fall within the nine largest families; from largest to smallest, these families are Cyprinidae, Gobiidae, Cichlidae, Characidae, Loricariidae, Balitoridae, Serranidae, Labridae, and Scorpaenidae. About 64 families are monotypic, containing only one species. The final total of extant species may grow to exceed 32,500.
















http://en.wikipedia.org/wiki/Fish