Brought more than twice: the complex introduction history of the red swamp crayfish into Europe

One of the biggest challenges in understanding and managing biological invasions is the identification of the routes of introduction. This information is often incomplete because of unnoticed, unreported and, sometimes, illegal translocations. Reports on the introduction of the red swamp crayfish (Procambarus clarkii) into Europe describe that it was introduced for the first time to southern Spain (1973 and 1974) from Louisiana; from there, it rapidly spread throughout several European countries. While other importation events and pathways of introduction have been suggested in the literature, there is no evidence that these led to wild populations in Europe. Our present study suggests two additional introduction routes into Europe from non-European areas where the red swamp crayfish had previously been reported. By using mtDNA, we found a shared haplotype between the Lake Naivasha in Kenya and Western Europe, as well as another between either western United States or Asia, and Malta. These findings support historical reports found in the literature for the former case and also point towards pet trade as driver of new introductions for the latter, thus highlighting the complex introduction history of the red swamp crayfish populations in Europe