Tracing the quagga mussel invasion along the Rhine river system using eDNA markers: early detection and surveillance of invasive zebra and quagga mussels

Early detection and monitoring of invasive species is important for the development of effective measures directed at minimising the negative effects of invaders. In the early stages of invasion, aquatic invasive species are typically rare and their detection using costly and time-consuming field surveys is often challenging. Environmental DNA (eDNA) methods are increasingly applied in freshwater systems to detect and quantify target species with relative ease over large geographic scales and across the invasion fronts. In this study we test eDNA detection and quantification methods for invasive zebra and quagga mussels. Both mussel species have invaded widely in North America and Europe and show strong negative ecosystem-wide impacts. We extracted DNA from filtered water samples which we collected along the Rhine catchment in Switzerland, Germany and The Netherlands, including the known invasion area of the zebra mussel and the invasion front of the quagga mussel. Standard PCR and quantitative PCR (qPCR) methods were compared for detection and qPCR was used to quantify the eDNA signal for each species. Our results show that the invasion front of the quagga mussel has moved southwards, including areas where this species had not been detected previously with traditional benthic invertebrate sampling methods. Standard PCR and qPCR showed a similar performance in detecting both mussel species. Moreover, the eDNA quantification of the two species showed low variance within sampling site and matched with expected densities of zebra and quagga mussels based on previous field survey studies. The tested eDNA methods are cost effective and have the potential to be widely applied for the surveillance of zebra and quagga mussels in the future.