Distribution of Nymphaea pygmaea and N. tetragona (Nymphaeaceae) in Hokkaido, Sakhalin Island, and the Kuril Islands based on herbarium specimen records

To determine the distributional area of Nymphaea pygmaea (Salisb.) W.T.Aiton and N. tetragona Georgi (Nymphaeaceae) and to identify the boundary between the two species, we conducted an examination of 199 specimens obtained from four herbaria (NGU, OSA, SAPS, TNS). Our analysis revealed that the specimen collection localities of N. pygmaea and N. tetragona were distinctly separated from east to west on the Ishikari Lowland (42°–44°N, 141°–142°E). Furthermore, we found that the distributional boundary between these two species was located somewhat south of the 50° N latitude, which has been identified in North America and Europe. We also discussed several factors that could be responsible for the clear geographic division of the two species in Hokkaido, such as differences in geological factors, habitats among the species, and potential reproductive interference.