Targeting the invaders – targeted detection of four priority freshwater invasive non-native species using environmental DNA

Early detection is paramount for attempts to remove invasive non-native species (INNS). Traditional methods rely on physical sampling and morphological identification, which can be problematic when species are in low densities and/or are cryptic. The use of environmental DNA (eDNA) as a monitoring tool in freshwater systems is becoming increasingly acceptable and widely used for the detection of single species. Here we demonstrate the development and application of standard PCR primers for the detection of four freshwater invasive species which are high priority for monitoring in the UK and elsewhere: Dreissenid mussels; Dreissena rostriformis bugensis (Andrusov, 1987) and D. polymorpha (Pallas, 1771), and Gammarid shrimps; Dikerogammarus villosus (Sowinsky, 1984) and D. haemobaphes (Eichwald, 1843). We carried out a rigorous validation process for testing the new primers, including DNA detection and degradation rate experiments in mesocosm, and a field comparison with traditional monitoring protocols. We successfully detected all four target species in mesocosms, but success was higher for mussels than shrimps. eDNA from single individuals of both mussel species could be detected within four hours of the start of the experiment. By contrast, shrimp were only consistently detected at higher densities (20 individuals). In field trials, the two mussel species and D. haemobaphes were detected at all sites where the species are known to be present, and eDNA consistently outperformed traditional kick sampling for species detection. However, D. villosus eDNA was only detected in one of five sites where the species was confirmed by kick sampling. These results demonstrate the applicability of standard PCR for eDNA detection of freshwater invasive species, but also highlight the importance of differences between taxa in terms of the detection ability.