Koi or more specifically nishikigoi (literally "brocaded carp"), are ornamental varieties of domesticated common carp (Cyprinus carpio) that are kept for decorative purposes in outdoor koi ponds or water gardens.
Koi varieties are distinguished by coloration, patterning, and scalation. Some of the major colors are white, black, red, yellow, blue, and cream. The most popular category of koi is the Gosanke, which is made up of the Kohaku, Taisho Sanshoku, and Showa Sanshoku varieties.
Carp are a large group of fish originally found in Central Europe and Asia. Various carp species were originally domesticated in East Asia, where they were used as food fish. The ability of carp to survive and adapt to many climates and water conditions allowed the domesticated species to be propagated to many new locations, including Japan. Natural color mutations of these carp would have occurred across all populations. Carp were first bred for color mutations in China more than a thousand years ago, where selective breeding of the Prussian carp (Carassius gibelio) led to the development of the goldfish.
The common carp was aquacultured as a food fish at least as far back as the fifth century in China, and in the Roman Empire during the spread of Christianity in Europe. Common carp were first bred for color in Japan in the 1820s, initially in the town of Ojiya in the Niigata prefecture on the northeastern coast of Honshu island. By the 20th century, a number of color patterns had been established, most notably the red-and-white Kohaku. The outside world was not aware of the development of color variations in koi until 1914, when the Niigata koi were exhibited in the annual exposition in Tokyo. At that point, interest in koi exploded throughout Japan. The hobby of keeping koi eventually spread worldwide. They are now commonly sold in most pet stores, with higher-quality fish available from specialist dealers.
Extensive hybridization between different populations has muddled the historical zoogeography of the common carp. However, scientific consensus is that there are at least two subspecies of the common carp, one from Western Eurasia (Cyprinus carpio carpio) and another from East Asia (Cyprinus carpio haematopterus). One recent study on the mitochondrial DNA of various common carp indicate that koi are of the East Asian subspecies. However, another recent study on the mitochondrial DNA of koi have found that koi are descended from multiple lineages of common carp from both Western Eurasian and East Asian varieties. This could be the result of koi being bred from a mix of East Asian and Western Eurasian carp varieties, or being bred exclusively from East Asian varieties and being subsequently hybridized with Western Eurasian varieties (the butterfly koi is one known product of such a cross). Which is true has not been resolved.
Koi varieties are distinguished by coloration, patterning, and scalation. Some of the major colors are white, black, red, yellow, blue, and cream. While the possible colors are virtually limitless, breeders have identified and named a number of specific categories. The most popular category is Gosanke, which is made up of the Kohaku, Taisho Sanshoku, and Showa Sanshoku varieties.
New koi varieties are still being actively developed. Ghost koi developed in the 1980s have become very popular in the United Kingdom; they are a hybrid of wild carp and Ogon koi, and are distinguished by their metallic scales. Butterfly koi (also known as longfin koi, or dragon carp), also developed in the 1980s, are notable for their long and flowing fins. They are hybrids of koi with Asian carp. Butterfly koi and ghost koi are considered by some to be not true nishikigoi