Improving environmental DNA (eDNA) methods for the detection and monitoring of invasive Dreissenid mussels

Dreissenid bivalves, zebra mussel (Dreissena polymorpha, Pallas, 1771) and quagga mussel (Dreissena rostriformis bugensis, Andrusov, 1897), are small freshwater molluscs native to the Ponto-Caspian region in Eastern Europe. Increasing globalization has facilitated their spread and establishment outside of their native range, and they are now widespread in Europe and North America. They are responsible for significant environmental impacts and economic losses, which are more pronounced in water-related companies. The most recent estimates suggest a cost of 5 million pounds for UK water industries. The use of sensitive tools for early detection and monitoring of dreissenid mussels is thus essential. Environmental DNA (eDNA) is described as genetic material that can be obtained from environmental samples. Targeted eDNA assays must meet specific criteria outlined in a 5-level validation scale to ensure reliability of results and be considered operational for use in routine monitoring. In this thesis, I aimed to increase the validation level of two targeted eDNA assays for dreissenid mussels. For zebra mussels, this included assessing the seasonal and spatial variation of eDNA detectability and estimating detection probabilities from statistical modelling. eDNA concentration and detection rates peaked in the summer, consistent with the increase in temperature which triggers mussel spawning. We also observed differences in eDNA concentration between waterbody types, being higher in reservoirs. A new targeted assay for quagga mussels was optimised and used to demonstrate they are more widespread than previously thought, with positive detections in several rivers, recreational lakes and the canal system, highlighting the need to implement urgent biosecurity measures to control their spread. We were able to increase the validation level of both assays tested in this thesis, thus increasing the reliability of their results. The work developed here constitutes therefore an important step towards the operational use of eDNA for routine monitoring of dreissenid mussels