High genetic variation of invasive signal crayfish in Europe reflects multiple introductions and secondary translocations

The signal crayfish Pacifastacus leniusculus, native to northwestern North America, has become the most widespread invasive crayfish species in Europe. It has been introduced repeatedly since 1959, and altogether >60,000 individuals were imported. Secondary introductions across the continent followed, resulting in its current presence in >= 26 European countries. Recent studies indicate that multiple highly divergent lineages of signal crayfish exist in its North American range, and previous investigators have suggested that >1 signal crayfish subspecies are present in Europe. We investigated its genetic variation by sequencing a fragment of the mitochondrial gene for cytochrome c oxidase subunit I (COI) to clarify the diversity of this invasive species in its European range. We analyzed 348 signal crayfish from 68 European populations and compared the resulting patterns with reference sequences from North America. All European individuals studied fell within 1 clade of P. leniusculus, but we observed substantial variation at the analyzed marker. Altogether, we recorded 27 COI haplotypes (4 very widespread) in Europe, but no clear distributional pattern of these haplotypes corresponds to numerous secondary introductions across Europe. Maximum pairwise divergence at COI among haplotypes detected in Europe was up to 4%. Such extent of genetic variation should be considered when developing and validating species-specific DNA-based probes for environmental detection of this invasive species because mitochondrial genes (and COI in particular) are often the markers of choice for this purpose.