Distribution, identification and range expansion of the common Asellidae in Northern Europe, featuring the first record of Proasellus meridianus in the Nordic countries

Two out of the three common Asellidae species in Northern Europe are increasing their ranges, aided by human activities. Here we report the discovery of Proasellus coxalis (Dollfuss 1892) in new areas in Norway and the discovery of Proasellus meridianus (Racovitza 1919) for the first time in the Nordic countries, verified with DNA barcoding. A new, detailed photo-identification guide to Asellus aquaticus Linnaeus 1758, P. coxalis and P. meridianus is presented. In addition to head pattern, attention is drawn to the female pleopods as an easy way to differentiate between the two genera. Then detailed examination of male pleopods 1 and 2 can differentiate between P. coxalis and P. meridianus. The origins, competitive relationships and potential dispersal mechanisms of the two introduced species and the native A. aquaticus are explored. By examining the shipping activity at the small, freshwater port where P. meridianus was found, we highlight the great connectivity between many European brackish and freshwater ports and possible pathways for species transfer. The risk of trans-oceanic freshwater to freshwater (not just brackish and saltwater) species transfer through ballast water needs to be better communicated. Proasellus coxalis may have been introduced to the river system of Lake Stokkalandsvatnet together with fish transported in microaquaria used as live bait for fishing