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. Traditional classification divides fish into three extant classes, and with extinct forms sometimes classified within the tree, sometimes as their own classes: Class Agnatha (j
awless fish) Subclass Cyclostomata (hagfish and lampreys) Subclass Ostracodermi (armoured jawless fish) � Class Chondrichthyes (cartilaginous fish) Subclass Elasmobranchii (sharks and rays) Subclass Holocephali (chimaeras and extinct relatives) Class Placodermi (armoured fish) Class Acanthodii ("spiny sharks", sometimes classified under bony fishes) Class Osteichthyes (bony fish) Subclass Actinopterygii (ray finne
d fishes) Subclass Sarcopterygii (fleshy finned fishes, ancestors of tetrapods) The above scheme is the one most commonly encountered in non-specialist and general works. Many of the above groups are paraphyletic, in that they have given rise to successive groups: Agnathans are ancestral to Chondrichthyes, who again have given rise to Acanthodiians, the ancestors of Osteichthyes. With the arrival of phylogenetic nome
nclature, the fishes has been split up into a more detailed scheme, with the following major groups: Class Myxini (hagfish) Class Pteraspidomorphi � (early jawless fish) Class Thelodonti � Class Anaspida � Class Petromyzontida or Hyperoartia Petromyzontidae (lampreys) Class Conodonta (conodonts) Class Cephalaspidomorphi (early jawless fish) (unranked) Galeaspida (unranked) Pituriaspida (unranked) Osteostraci
aclass Teleostei (many orders of common fish) Class Sarcopterygii (lobe-finned fish) Subclass Actinistia (coelacanths) Subclass Dipnoi (lungfish, sister group to the tetrapods) – indicates extinct taxon 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 no
t 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. As of 2006, 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 hagfish 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. Each year, new species are discovered and scientifically described
. As of 2016, there are over 32,000 documented species of bony fish and over 1,100 species of cartilaginous fish. Species are lost through extinction (see biodiversity crisis). Recent examples are the Chinese paddlefish or the smooth handfish. 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 fish, coelacanths, and lungfish. Indeed, lungfish and coelacanths are closer relatives of tetrapods (such as mammals, birds, amphibians, etc.) than of other fish such as ray-finned fish or sharks, so the last common anc
estor of all fish is also an ancestor to tetrapods. As paraphyletic groups are no longer recognised in modern systematic biology, the use of the term "fish" as a biological group must be avoided. 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 to 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 fro
m these other animals. 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 (literally ava).
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