Assessing the genuine identity of alien Vallisneria (Hydrocharitaceae) species in Europe

•The identity of alien Vallisneria species was investigated in European populations.•Based on a molecular analysis V. australis and V. neotropicalis were detected.•A key is provided for the identification of Vallisneria species in Europe. The genus Vallisneria consists of linear-leaved aquatic plants within the Hydrocharitaceae family. Only one indigenous member (V. spiralis) occurs in Europe. Populations of alien species appeared in some European countries in recent decades and these were named in various ways. To clarify the identity of alien Vallisneria we collected samples for a molecular phylogenetic analysis from six populations in five European countries. We investigated the nuclear ribosomal Internal Transcribed Spacer (ITS) region. Based on our results we found that samples from Belgium, Germany, Hungary and Italy belong to the recently described V. australis, a species native to Australia, while a sample from Russia is identical with V. neotropicalis. We provide a key to distinguish these from other similar species. All populations probably derived from dumped plant parts in the drainage water from aquariums. Most populations of V. australis are established and this species might therefore become invasive in the future. The investigated V. australis populations proved to be not uniform genetically.