The group of fish we commonly call cyprinids includes many popular aquarium fish: goldfish, minnows, barbs, rasbora, loaches and sharks. Technically, this order of fish is known as the Cypriniformes, from the Greek kyprinos (goldfish) and Latin forma (shape). Taxonomically, the six families are arranged under two superfamilies: the Cyprinioidea contains the carps and minnows (Cyprinidae), and the Cobitioidea contains hillstream loaches (Balitoridae), suckers (Catostomidae), true loaches (Cobitidae), sucking loaches (Gyrinocheilidae), and mountain carps (Psilorhynchidae) in the traditional system. 
Historically, the Cypriniformes included all the forms now placed in the superorder Ostariophysi except that catfish, which were placed in the order Siluriformes. By this definition, the Cypriniformes were paraphyletic [meaning that the group contains its last common ancestor but does not contain all the descendants of that ancestor]. More recently, the orders Gonorhynchiformes, Characiformes (characins and allies), and Gymnotiformes (knifefishes and electric eels) have been separated out to form their own monophyletic orders. 
Cyprinids occur on all the continents except South America, Australia and Antarctica. This wide distribution suggests they had a long time to spread, and it is believed they separated from the Characiformes (characids) at some point very early on. This was unlikely to have occurred earlier than during the early Triassic, about 250 million years ago . The more accepted view is that it took place prior to the breakup of Pangaea, the name given to the united land mass containing all present-day continents, during the Jurassic period some 160 million years ago; the earliest fossil record of a cyprinid comes from the late Jurassic period. The first split of Pangaea resulted in the northern and southern supercontinents, named Laurasia and Gondwanaland respectively, and there is plate tectonic evidence that the Cypriniformes must have been distinct from their Gondwanaland relatives by 110 million years ago [1, 4]. It is widely accepted that the Cypriniformes originated in what is now SE Asia, where there is today the most diversity in the order .
The largest family is Cyprinidae (minnows and carps) with about 275 genera and more than 1500 species. For aquarists, these include goldfish, barbs, rasbora, danios and minnows. The second most popular family for the aquarist is Cobitidae that contains the true loaches. Significantly fewer species from the remaining four families find their way into home aquaria.
In general, fish of the Cyprinidae and Cobitidae must be kept in groups, as they are shoaling by nature. No species has an adipose fin. All are strictly freshwater, with only a few species able to tolerate brackish water. Most are toothless in the jaws, but posess pharyngeal (throat) teeth and/or plates.
Species in Cyprinidae have scaleless heads, no mouth teeth but 1 to 3 rows of pharyngeal teeth. Some have barbels. They occur in a wide range of habitats and these will be discussed under the respective species.
The Cobitidae have flattened stomachs, one row of pharyngeal (throat) teeth and the mouth is inferior (facing downward), indicative of bottom feeders. The body may be covered with small scales or may be partly or completely lacking scales; this is one reason why medications must be used with extreme caution and salt should always be avoided. All species have barbels (usually from 3 to 6 pairs), and one or two thorny spines generally situated between the eyes and that can be erected.
Considerable scientific work is on-going with this group of fishes, and many species have been reassigned to new or different genera over the past decade, and this is not likely to end soon. Similar to the characids, a number of species are scientifically considered incertae sedis (uncertain placement).
 Joseph S. Nelson, Fishes of the World (2006)
 G. Helfman, B. Collette & D. Facey, The Diversity of Fishes (1997)
 Kenji Saitoh, Masaki Miya, Jun G. Inoue, Naoya Ishiguro & Mutsuminame Nishida, Mitochondrial Genomics of Ostariophysan Fishes: Perspectives on Phylogeny and Biogeography (2003)]
 John Briggs, The biogeography of otophysan fishes (Ostariophysi: Otophysi): a new appraisal (2005)
June 14, 2010
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