Koi
Middle Ages

In the Japanese history book Nihon Shoki (Chronicles of Japan, 720), it is written that Emperor Keikō released carp in a pond for viewing when he visited Mino Province (present Gifu Prefecture) in the fourth year of his reign (74 AD). In Cui Bao's Gǔjīnzhù (古今注, Annotations on the Ancient and Modern Period) from the Western Jin Dynasty (4th century A.D.) in China, carp of the following colors are described: red horse (赤驥), blue horse (青馬), black horse (玄駒), white horse (白騏), and yellow pheasant (黄雉). In China in those days, carp were called horses because they were believed to be the vehicles of hermits and to run in the sky.

Japan's oldest drug dictionary, Fukane Sukehito's Honzō Wamyō (本草和名, 918) mentions red carp (赤鯉), blue carp (青鯉), black carp (黒鯉), white carp (白鯉), and yellow carp (黄鯉) as Japanese names corresponding to the above Chinese names, suggesting that carp of these colors existed in China and Japan in those days. In addition, Hitomi Hitsudai's drug dictionary Honchō Shokkan (本朝食鑑, Japanese Medicine Encyclopedia, 1697) states that red, yellow, and white carp of the three colors were in Japan at that time.

However, it is believed that these single-colored carp were not a variety created by artificial selection, as is the case with today's koi, but rather a mutation-induced color change. In ancient times, carp was farmed primarily for food. Mutational color variation in carp is relatively common in nature, but is not suitable for development alongside farming for food in poor rural communities; color inheritance is unstable and selection to maintain color variation is costly. For example, in current-day farming of koi as ornamental fish, the percentage of superior colored fish to the number of spawn is less than 1%.

The Amur carp (Cyprinus rubrofuscus) is a member of the cyprinid family species complex native to East Asia. Amur carp were previously identified as a subspecies of the common carp (as C. c. haematopterus), but recent authorities treat it as a separate species under the name C. rubrofuscus. Amur carp have been aquacultured as a food fish at least as long ago as the fifth century BC in China.
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