Surveillance of European green crab (Carcinus maenas) and monitoring of three native crab species using different environmental DNA collection methods

European green crab ( Carcinus maenas ) is an invasive species of global concern due to its ecological and economic impacts. The present study explores the use of environmental DNA (eDNA) for early surveillance of European green crab on the coast of Southern Vancouver Island, British Columbia, Canada. Two field sites were selected: Sooke Basin, a known invasion site for European green crab, and Sidney Spit, located within the Gulf Islands National Park Reserve, where the species has not been reported and where Parks Canada conducts regular monitoring. We assessed three eDNA sampling approaches: filtered water, FloppE-Dip (sediment), and a 3D-printed metaprobe passive sampler that was deployed alongside baited crab traps at 15 stations. Targeted qPCR-based eDNA assays were developed and validated for C. maenas and three native crabs: red rock crab ( Cancer productus ), graceful crab ( Metacarcinus gracilis ), and Dungeness crab ( M. magister ). Among the collection methods, the metaprobe passive sampler consistently yielded the highest crab-specific eDNA concentrations and detection frequencies. At Sidney Spit, only red rock and graceful crabs were captured in traps, where eDNA using metaprobe samples yielded 100% detection frequency. In Sooke Basin, metaprobe and filtered water samples detected all four species at all stations, including high eDNA concentrations for European green crab. These results demonstrate the effectiveness of eDNA, particularly passive sampling using a 3D-printed metaprobe, for early detection of invasive crabs, supporting its integration into routine monitoring to inform timely management interventions.