The Chao Phraya basin is home to around 280 species of fish, including about 30 endemics. By far the most diverse family is Cyprinidae with 108 species. The mainstream of the Chao Phraya River has about 190 native fish species. In general, the aquatic fauna of Chao Phraya and Mae Klong show clear similarities, and they are sometimes combined in a single ecoregion with 328 fish species. Despite their similarities, there are also differences between the aquatic fauna of Chao Phraya and Mae Klong; the latter (but not the former) is home to a few taxa otherwise only known in major Burmese rivers: the Irrawaddy, Salween, and Tenasserim. The aquatic fauna in Chao Phraya–Mae Klong also show clear similarities with that of the middle Mekong (the lower Mekong fauna more closely resembles that of the eastern Malay Peninsula). It is believed that the upper Mekong was connected to Chao Phraya (rather than present-day lower Mekong) until the Quaternary, which explains the similarities in their river faunas. This included the Nan River basin, a tributary of the Chao Phraya, which is home to a number of taxa (for example, "Ambastaia nigrolineata" and "Sectoria") otherwise only known from Mekong. Of the fish species known from the Chao Phraya–Mae Klong, only about 50 are absent from the Mekong.