Estimating the distributional expansion of the invasive crayfish Procambarus clarkii (Girard, 1852) (Decapoda: Astacidea: Cambaridae) in South Korea using environmental DNA analysis

The red swamp crayfish Procambarus clarkii (Girard, 1852) is a globally invasive species that poses serious threats to native biodiversity and ecosystem functioning. In South Korea, established populations of P. clarkii have been documented from the Jiseokcheon River in the Yeongsangang River basin since 2018; however, information on its current nationwide distribution remains limited. We conducted environmental DNA (eDNA) surveys at 19 sites across 11 river systems in South Korea, including sites with no prior distributional records, to evaluate the current distribution and potential range expansion of P. clarkii. Water samples collected during July-August 2024 were analyzed using quantitative real-time PCR (qPCR) with two species-specific primer-probe sets targeting the mitochondrial COI gene. Procambarus clarkii eDNA was detected at three sites in the Yeongsangang River basin, whereas no eDNA was detected in the other surveyed river systems. Notably, P. clarkii eDNA was not detected at a site in the Mangyeonggang River system despite previous reports of individuals and eDNA data sampled there, indicating a possible local population decline or very low abundance. By contrast, detections within the Yeongsangang River basin, together with elevated eDNA concentrations at some sites, are consistent with localized upstream expansion and increased local abundance. The concordant results from the two primer-probe sets, together with Sterivex-based on-site filtration, support the reliability of the applied eDNA framework. These findings identify the Yeongsangang River basin as the current primary focus of the P. clarkii invasion in South Korea, with evidence consistent with localized upstream expansion and increasing abundance, and provide baseline data for early detection and long-term monitoring, as well as for developing management strategies.