Environmental DNA (eDNA) is a rapid, non-invasive method for species detection and distribution assessment using DNA released into an environmental matrix by an organism. eDNA has become a recognised and powerful tool for detecting invasive species in a broad range of ecosystems. However, most studies have focused on vertebrate species while invertebrates have received much less attention. We examined the use of eDNA as a tool for detecting the invasive American signal crayfish, Pacifastacus leniusculus, in Scotland. Species-specific probe and primers were designed for P. leniusculus and a robust real-time quantitative PCR (qPCR) assay and DNA extraction protocol were developed. We investigated the detection capability for P. leniusculus from water samples in a controlled laboratory experiment and determined whether crayfish density (low = 17 or high = 50 crayfish m2) or length of time in tanks (samples taken at 1, 3 and 7 days) influenced DNA detectability. Additionally, the persistence of DNA was investigated after P. leniusculus removal (samples taken at 1, 3 and 7 days post removal). To field test the assay, water samples were taken from sites where crayfish had previously been reported and sites where there was no evidence of crayfish.
P. leniusculus DNA was consistently detected during the entire 7-day experimental period and high density tanks yielded a stronger signal, although all signals were higher at the start of the tank habitation period than at the end. After removal of P. leniusculus, there was a further decrease in tank water P. leniusculus DNA, influenced by density. P. leniusculus DNA could only be detected in high density tanks by the end of the 7-day period, while DNA was no longer detectable in low density tanks 72 hours after P. leniusculus removal. P. leniusculus DNA was detected in water samples at seven of the ten field sites sampled, although the detection rate was low for some sites. Of the three sites with negative results, P. leniusculus had been previously observed at one site but not at the remaining two sites. Our results suggest that refinement of sampling methodology and the eDNA assay are required before the method can be implemented routinely. Nonetheless, eDNA represents a promising technique for early detection and monitoring of invasive P. leniusculus.