This has reportedly spread from South America to eutrophic waters of southern states during recent centuries, partly due to its use in aquaria (FNA 22, W). There are only a few records from Ky., all dated after 1960. The species has recently been found for the first time in Ind. (Weakley at al. 2023). The species is easily confused with the larger species, Spirodela polyrhiza, but there is some justification for its placement in Landoltia as a monotypic genus; 2n = 40, 46, 50. Landoltia differs from Spirodela as follows (W): roots 1-12 per frond (versus 7-21), all perforating the scalelike leaflet borne on the underside (versus only some perforating); fronds with 3-7 nerves (versus 7-21); fronds 1.5-3× as long as wide (versus1-1.5×).