Ruppia is a genus of submerged monocots that exhibits a nearly worldwide distribution and inhabits mainly brackish and saline waters. This genus has important ecological values and is significant for environment monitoring and plant systematic study. Since Linnaeus's time, there existed confusion at both specific and family level about Ruppia. The number of taxa within Ruppia, whether there is a single cosmopolitan species or more than one species, has caused much disagreement among taxonomists. At present, five species, R. maritima, R. cirrhosa, R. tuberose, R. megacarpa and R. polycarpa are generally accepted. As to the systematic position of the genus, there are two main opinions: including Ruppia in the family Potamogetonaceae or keeping it in its own family Ruppiaceae. Some systems of plant taxonomy accepted Ruppiaceae, but comparison of diagnostic characters between Potamogeton and Ruppia tended to retain Ruppia within the Potamogetonaceae, although the molecular analyses in recent years show that Ruppia has a close relationship with Cymodoceaceae and Posidoniaceae. In addition, the evolution of Ruppia is discussed based on fossil records and fruit characters of the related fossil taxa. The fossil genera Limnocarpus and Midravalva are regarded as the ancestors of Ruppia and it is suggested that its present brackish habitat developed from a fresh water ancestor. Finally, some questions which should be paid much attention on further study of this genus are discussed.
