For many years Gambusia affinis was thought to be the same species as the Eastern Mosquitofish, G. holbrooki, but has since been given distinct species status. They can be distinguished by several means. In G. holbrooki the leading ray of the males' gonopodium has a serrated edge, the dorsal fin usually has 7 rays and males also generally have a higher degree of dark spotting on the body than Gambusia affinis males. Male Gambusia affinis also lack the serrated leading ray of the gonopodium and this species usually has only 6 dorsal rays.