LAUSR

Abstract In this study we examined morphological variation in two macrophyte species (Najas major All., N. marina L. and their hybrids) obtained from German fresh water systems. Clear-cut delimitation of these two taxa is notoriously difficult but important as they are used as indicator organisms for water quality within the European Water Framework Directive (WFD) and have recently been revealed as two genetically separate species. To reliably identify both taxa and their hybrids, we used an integrative approach testing six discrete and two ratio-based morphological leaf and seed characteristics against restriction fragment-length polymorphism patterns (RFLP) based on PCR of rDNA internal transcribed spacer (ITS) sequences. Morphometric data of 475 plant individuals from 25 different German lakes showed basic correlation with the species delimitation suggested by molecular data, but revealed considerable overlap for characteristic state ranges, which can lead to misidentification of species if a low number of observations is made on these traits. Hybrids showed a mosaic of both parental and intermediate morphological traits. Notably, the traditionally employed feature “number of teeth along the margin on the leaf sheaths” proved to be of low diagnostic value. Leaf dimensions, especially leaf widths, were shown to be more reliable characteristics for distinguishing parental taxa. In practice, further use of both Najas species within the implementation of the WFD should be accompanied by molecular genetic testing to detect both cryptic co-occurrence and hybridization. This study points out the importance of thorough sampling and molecular screening in widespread and taxonomically difficult groups.