eDNA-based detection as an early warning tool for detecting established and emerging invasive amphipods

Introductions of invasive non-native species often have severe effects on environments and can lead to the collapse of local populations. Freshwater systems are highly biodiverse habitats that are particularly sensitive to the introduction of non-native species, which is recognized as one of the leading causes of animal extinctions worldwide. Effective management firstly depends on the early detection of incipient invasions and subsequent rapid response. Here, we focus on the cryptic and difficult-to-detect amphipod, Crangonyx pseudogracilis, which is already established in several countries in Europe, and C. floridanus, which was recently found in the UK and Ireland. Their exact distribution is relatively unknown, and their morphological similarities to each other makes them taxonomically difficult to distinguish. To obtain better insights on the actual distribution of both species, we developed and validated species-specific assays for the detection of environmental DNA traces from both C. floridanus and C. pseudogracilis. Next, we analysed a large number of eDNA samples collected from several freshwater systems in Ireland where the presence of both alien species is known, and from unknown sites in Belgium, Netherlands, France, and Norway (countries where only C. pseudogracilis has previously been recorded). Using qPCR analyses, we successfully detected eDNA from both C. pseudogracilis and C. floridanus in the known locations in Ireland. Sites investigated in Belgium, Netherlands, France, and Norway were negative for the presence of both species. The qPCR based detection of freshwater amphipods from aquatic eDNA is a cost-effective and sensitive method to monitor establishment and spread of invasive species as well as species of conservation priority. Based on our results, we recommend that eDNA surveys can be used as a powerful tool for stakeholders, including ecologists, especially in at-risk areas to detect potential early invasion of these species.