There are two endemic species of Medaka, "Kitano-medaka[キタノメダカ](Oryzias sakaizumii)" and "Minami-medaka[ミナミメダカ](Oryzias latipes)" in Japan. Minami-medaka lives in Honshū, Shikoku, Kyūshū, Seinan Islands(south-west islands in Japan including Okinawa) along the Pacific Ocean and Kitano-medaka lives in Tōhoku, Hokuriku allong the Sea of Japan.
As a difference between the two species, male Medaka has a cut in the dorsal fin and that of Minami-medaka is deeper than that of Kitano-medaka. And generally, Kitano-medaka has a black mesh pattern and Minami-medaka has black long narrow spots on the tail fin.
But the most popular Medaka is Hi-medaka[ヒメダカ]. Kitano-medaka and Minami-medaka have a quiet color but Hi-medaka has orange color. This is because Hi-medaka is a mutant that doesn't have a black pigment. Hi-medaka has been familiar as a pet since long ago.
There is a small pond in my garden and mosquito larvae generated in that. I bought Hi-medakas for eliminating the larvae because Medaka eats them. One day after that, I found that their children were swimming. Now I'm happy they are growing little by little.
Children of Hi-medaka