Delrieu-Trottin, E.; Bianic, M.; Miaud, C.; Garcia, M.; Arnal, V.; Hocde, R.; Cochet, C.; Barre, N.; Cornil, L.; Manel, S.

1. Early detection is a crucial tool for identifying and managing the spread of invasive species. 2. In this study, we validated a probe-based quantitative polymerase chain reaction (qPCR) assay for the detection of the invasive Blue Crab, Callinectes sapidus, in the Mediterranean Sea, using 22 initial eDNA environmental samples (eDNA) collected from three coastal lagoons. 3. A subsequent large-scale eDNA sampling campaign (61 samples in 31 sites), conducted in collaboration with local stakeholders, was carried out to map the distribution of Callinectes sapidus along the Occitanie coastline (Western Mediterranean, France). 4. Using eDNA probe-based qPCR, Callinectes sapidus was detected in 32 out the 61 samples (52%), confirming its presence in 24 out of 31 sites surveyed, including the 13 lagoons where its occurrence had already been reported, as well as two additional lagoons and at sea where no prior records existed. 5. Our results demonstrate the utility of eDNA probe-based qPCR for effective monitoring the invasive Blue Crab. The integration of eDNA analysis with citizen science observations enhances the monitoring framework, facilitating early detection and contributing to improved management strategies from a prevention perspective.