Koi are domesticated varieties of which common freshwater fish?

Koi are domesticated varieties of the common carp.

The common carp, Cyprinus carpio, has been bred for food and ornamental traits for centuries. In Japan, selective breeding produced brightly colored carp known as nishikigoi, or koi. Their patterns can include white, red, black, yellow and metallic scales.

Koi breeding became especially important in Japan’s Niigata Prefecture during the nineteenth century. Farmers originally raised carp in rice-growing areas, and unusual color mutations were selected and developed into recognized varieties. Modern koi shows judge features such as pattern, color quality, body shape and skin condition.

Koi are not the same species as goldfish, although both are popular ornamental fish and can sometimes hybridize. Common carp can grow much larger than typical pond koi, and wild or feral populations often have subdued brown or bronze coloration rather than the vivid patterns prized in ornamental strains.