Contrasting Patterns of Genetic Variability in Pet‐Traded Red Swamp Crayfish <i>Procambarus clarkii</i> and Its Feral Populations

ABSTRACT The ornamental pet trade has emerged as one of the most important pathways for the introduction of non‐native aquatic species. In the last two decades, freshwater crayfish have become a popular ornamental group, with numerous non‐native populations established through this way, including some populations of the North American red swamp crayfish Procambarus clarkii (Girard, 1852). Whilst the introduction routes of aquaculture‐derived populations have been identified through molecular methods and historical documents, obtaining traceability in the pet trade market is often more challenging. We analysed 283 pet‐traded and feral P. clarkii individuals, originating from Europe and Southeast Asia, for variation of a fragment of the mitochondrial gene for cytochrome c oxidase subunit I (COI) to determine the genetic variation amongst 11 pet shops in various countries, outdoor aquaculture in Indonesia and 27 established feral populations in Europe likely originating from releases of ornamental animals. We found 13 haplotypes, four of which had not been detected in P. clarkii before. Overall, higher haplotype diversity was found in established feral populations (Hd = 0.555) and outdoor aquaculture (Hd = 0.667) compared to pet shops (Hd = 0.141). We observed up to three haplotypes shared between the Asian pet trade and Europe. Whilst one of them appeared in a pet shop in Indonesia and open waters in Hungary, another was observed in the pet trade in Cambodia, outdoor aquaculture in Indonesia and in the stock of European wholesalers. The third was the most common haplotype and provided little information regarding the species' introduction routes. Additionally, one haplotype present in the European pet trade was commonly found in European waters. Particularly noteworthy was the high haplotype variability of Hungarian populations collected in the metropolitan area of Budapest, which is considered a European hotspot of ornamental crayfish introductions. The findings of our study support the low haplotype diversity in pet shops, likely stemming from a limited number of founders of most P. clarkii ornamental stocks. In contrast, higher genetic variability in feral populations may be due to recurrent introduction events from different sources. Furthermore, the results support the hypothesis that the crayfish occurring in the European pet trade originate from the Asian ornamental trade rather than from the North American native range of P. clarkii . Legally binding regulations on listing non‐native species do not seem to curb the importation of ornamental non‐native species. However, the implementation of molecular methods conducted at border controls (by veterinarians) and post‐border (by wholesalers and retailers) in cooperation with research institutions can efficiently serve as preventive measures to halt species importations. We emphasise using molecular analyses for both species identification and tracking introduction routes of biological invasions.