抄録
Medaka or ricefishes, adrianichthyids of the order Beloniformes have been believed a single species by its superficial body shape for more than 160 years, since 1846 when “Fauna Japonica” had published by Temminck and Schlegel. In the 1980s, allozyme studies have shown that Japanese wild population of Medaka, could be divided into two genetic groups, the Northern and the Southern populations. Oryzias latipes（Temminck and Schlegel, 1846）was originally described as Poecilia latipes from Siebold’s collection now at the NCB Naturalis. However, considering the lectotype and paralectotypes of O. latipes, it is showed that O. latipes is only identical with the Southern population in the body shape. The Southern population is found in freshwaters the Pacific coast of eastern, western and southern Japan. Oryzias sakaizumii Asai, Senou and Hosoya, 2011, it has been generally regarded as the genetic variation, the Northern population distributed in the northern coast of the Sea of Japan, within a single species O. latipes. However, the Northern and Southern populations parapatrically coexist the same river drainage, which is the distributional boundary zone for these populations. Both have also been reported to exhibit different schooling behaviors. This implies the ecological diversification between both populations. In addition, frequencies of mitochondrial cytochrome b gene mitotype there have been showed an establishment of reproductive isolation. Furthermore, this species is distinguished from O. latipes by having some diagnostic characters such as notched dorsal-fin or melanophore network, confirming that the Northern population is a distinct species, O. sakaizumii. Therefore, it should be recognized that the Northern and the Southern populations differentiate in a specific level.
