Characteristics of fish

Characteristics of fish
Fish range in size from the 14m (45ft) whale shark to the 7mm (just over 1/4 of an inch) long stout infantfish and the 13mm Philippine goby. Fish can be found in almost all large bodies of water in salt, or brackish, or fresh water, at depths from just below the surface to several thousand meters. However, hyper-saline lakes like the Great Salt Lake of the United States do not support fish. Some species of fish have been specially bred to be kept and displayed in an aquarium, and can survive in the home environment.
Hagfish, while generally classified in Agnatha ("jawless") and as fish, actually lack vertebrae. For this reason, hagfish, which are also commonly known as "slime eels," are sometimes not considered to be fish. The other living member of Agnatha, the lamprey, has primitive vertebrae made of cartilage. Hagfish are a staple food in Korea. They are classified in the order Myxini and the family Myxinidae. Both hagfish and lamprey have slimy skin without scales or plates. They also have a notochord that remains throughout life; circular, jawless mouths; and unpaired fins. Hagfish are found in the oceans and lampreys are found in both freshwater and ocean environments. Most lampreys are parasitic.
Fish belonging to the class Chondrichthyes are distinguished by cartilage skeletons, as opposed to skeletons of bone. They have movable jaws and paired fins. Almost all of the Chondrichthyes—sharks, rays, skates, and chimaeras—are found in ocean environments.

Northern pike, a ray-finned fish

Most fish species (about 95 percent) are placed in the class Osteichthyes (which some taxonomies consider a superclass). They have bony internal skeletons and skins with scales. (As a general rule for the taxon. Not all bony fish have scales, and scales may be absent or present in two closely related species (Nelson 1994); Catfish is an example of an order of fish that lack scales.) The Osteichthyes taxon include coelacanths (lobe-finned fish), lungfish, and ray-finned fish. Coelacanths were thought to have been extinct for millions of years until fishermen caught one in 1938. Lungfish have lungs, as well as gills. Ray-finned fish are what many people refer to when they use the term fish, as these are our most familiar fish, including bass, eels, and many sports fish. Ray-finned fish have fins that are supported by bones (rays).
Many types of aquatic animals named "fish," such as jellyfish, starfish, and cuttlefish, are not true fish. A number of sea dwelling creatures, like dolphins and whales, are actually mammals.
While fish are poikilothermic in that they do not maintain constant internal temperatures and the temperature often mirrors the ambient temperature, certain species of fish maintain elevated body temperatures to varying degrees. These include teleosts (bony fishes) in the suborder Scombroidei and billfishes, tunas, and one species of "primitive" mackerel (Gasterochisma melampus). All sharks in the family Lamnidae—shortfin mako, long fin mako, white, porbeagle, and salmon shark—are known to have this capacity, and evidence suggests the trait exists in family Alopiidae (thresher sharks). The degree of being able to have elevated temperatures varies from the billfish, which warm only their eyes and brain, to bluefin tuna and porbeagle sharks, which can elevate body temperatures in excess of 20 °C above ambient water temperatures. In many cases, this phenomena has been traced to heat exchange, as warmer blood being returned to the gills in small veins runs close to colder, oxygenated blood in narrow arteries leaving the gills. This ability to have elevated temperatures allows fish to be active in colder waters and to have enhanced swimming ability because of the warmer muscles. In general, most fish can survive only at a relatively small range of body temperatures, but may adjust their depth in large bodies of water in order to find preferable ranges.

Classification

Fish are a very diverse assemblage, so much so that the term fish is itself more one of convenience than a taxonomic rank (Nelson 1994). It is used to designate a paraphyletic group, whereby the most recent common ancestor is included but not all descendants, with tetrapods (four-legged vertebrates, or descendants of four-legged vertebrates) being excluded. It is thus not surprising that different taxonomists may classify fish differently.

Latimeria chalumnae (coelacanth, a lobe-finned fish)

Vertebrates are generally classified into two groups, the Agnatha (jawless vertebrates) and the Gnathostomata (jawed vertebrates). The later group includes fish with hinged jaws, but also includes amphibians, reptiles, birds, and mammals (the tetrapods). In most taxonomies, Agnatha and Gnathostomata are each considered a superclass, although sometimes Agnatha is considered a class (see taxonomy).
The Agnatha, in addition to including the modern day lampreys (Petromyzontiformes) and hagfish (Myxiniformes), also includes several extinct orders.
Within Gnathostomata, several classes of fish are recognized. Two of these classes have living representatives, the Chondrichthyes (cartilaginous fish) and Osteichthyes (bony fish). In some taxonomies, Osteichthyes is considered a superclass.
Within the Osteichthyes, two extant subclasses (or classes) are generally recognized, the Sarcopterygii (lobe-finned fish) and the Actinopterygii (ray-finned or spiny-finned fish). The coelacanths are generally placed within the Sacropterygii subclass. The Actinopterygii are generally divided into the Chondrostei and the Neopterygii, the latter of which includes the Teleostei (modern bony fishes), a classification into which most fish fit today.
A general grouping of fish, without reference to the names of ranks of taxa (superclass, class, subclass, etc.) is presented above, in the image box. Below is presented a more detailed taxonomic scheme with the rank names, as derived from that offered by Benton (2004), in his text Vertebrate Paleontology:

• SUBPHYLUM VERTEBRATA
  • Class 'Agnatha'
    • Subclass Myxinoidea (hagfish)
    • Subclass Petromyzontida (lampreys)
    • Subclass †Conodonta
    • Subclass †Pteraspidomorphi
    • Order †Thelodonti
    • Order †Anaspida
    • Subclass unnamed
      • Order †Galeaspida
      • Order †Pituriaspida]
      • Order †Osteostraci
  • Infraphylum Gnathostomata (jawed vertebrates)
    • Class †Placodermi
    • Class Chondrichthyes (cartilaginous fish)
      • Subclass Elasmobrachii (sharks, rays, skates)
      • Subclass Subterbranchialia
        • Superorder Holocephali (chimaeras)
    • Class †Acanthodii
    • Class Osteichthyes (bony fish)
      • Subclass Actinopterygii (ray-finned fish)
        • Superdivision Chondrostei (sturgeons, paddlefish)
        • Superdivision Neopterygii (teleosts—modern bony fish)
      • Subclass Sarcopterygii (lobe-finned fish)
        • Order Dipnoi (lungfish)
        • Infraclass Crossopterygii
          • Order Actinistia (coelacanths)

† = extinct (the group lacks any living members)
Each year, biologists find 200 to 300 species of fish that had not been previously known (Grady 2006).

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