Is Groenlandia densa (Potamogeonaceae) a native species to Lithuania?

The lack of well-documented records on rare and endangered species often poses the problem of determining whether a species is native to the area. The aim of this study was to review and evaluate all existing data on the occurrence of Groenlandia densa (L.) Fourr. (Potamogeonaceae), a species thought to be extinct in Lithuania. Literature sources from the 18th century onwards and herbarium specimens kept in Lithuanian herbaria (WI, BILAS), were studied and critically evaluated. The analysis of the literature data showed that the site in the Nemunas River is not related to the recent territory of Lithuania. The only herbarium specimen of this species, considered a collection of plants growing naturally in “a stream near the village of Melnyčėlė”, was misidentified. The herbarium specimens collected from the fishponds represented introduced plants. Therefore, Groenlandia densa should be treated as a casual alien species in Lithuania, and there is no confirmed information on the former natural populations of the species.