goldfish, freshwater fish, genus Carassius, of the family Cyprinidae, popular in aquariums and ponds. Native to China, it was first domesticated centuries ago from the wild form, an olive-colored carplike fish up to 16 in. (40 cm) long. It reverts to this type when it escapes from domestication and has been known to hybridize with the carp. Breeders have developed bizarre varieties with fan, fringe, or veil tails and sometimes with double or triple fins. Some have bulging "telescope" eyes. The majority of those bred commercially are known as scaled goldfish and have a metallic sheen of red, gold, white, silver, or black. The rarer forms, which are called scaleless, actually have transparent scales and appear in bright red, blue, shades of purple, and calico patterns. Marketed goldfish range in length from 1 to 4 in. (2.5–10 cm). In pools, they are beneficial as well as ornamental, since they feed on mosquito larvae; however, their carplike feeding habits make them a nuisance in lakes and streams. Goldfish are classified in the phylum Chordata, subphylum Vertebrata, class Osteichthyes, order Cypriniformes, family Cyprinidae.
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