Ponto-Caspian amphipods (Crustacea, Amphipoda) and their microsporidian parasites recently established in Sweden

The invasive amphipods Dikerogammarus villosus , D. haemobaphes and Pontogammarus robustoides , which originate from the Ponto-Caspian region, are amongst Europe’s most ecologically disruptive amphipods and have recently been detected in Sweden, marking their northernmost record and indicating a concerning expansion of their range. This study employed DNA barcoding to confirm the species identifications, assess population structure and trace the origins of these recently introduced populations. To improve the detection resolution in the case of D. villosus , we designed specific primers targeting two additional mitochondrial markers at genes ND5 and ATP8 , enabling precise identification of the probable source populations, based on known European invasion routes and their phylogeographic structure. Our analysis identified the Western Group of D. villosus as being present in Sweden, with a probable introduction from the large lakes in the foothills of the Alps, possibly via sailing boats that were transported overland. Meanwhile, P. robustoides and D. haemobaphes appear to have invaded from the eastern and southern Baltic regions — including the Baltic States, Vistula Lagoon and Szczecin Lagoon — possibly via ballast water, ship hulls or floating debris. Additionally, we detected microsporidian infections in all three species, identifying Dictyocoela muelleri and D. berillonum as associated parasites. Given the ecological risks posed by these invasive species, coordinated monitoring, particularly along the Baltic coast, is essential for enabling rapid response efforts in the Nordic region.