Koi or nishikigoi are ornamental domesticated varieties of the common carp Cyprinus carpio that are most often kept for decorative purposes in outdoor ponds and water gardens. They are sometimes also called Japanese carp.
Koi were developed from common carp in Japan in the 1820s, and are still very popular there as they are a symbol of love and friendship. Many different colors and color patterns have since been developed; common colors include white, black, red, yellow, blue, and cream.
The carp is 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 led to the development of the goldfish.
It is believed that common carp were first introduced into Japan by way of China between 400 to 600 years ago. Common carp were first bred for color in Japan in the 1820s, initially in the town of Ojiya in the Niigata prefecture on the north eastern 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.