A substantial range expansion of alien Ponto-Caspian amphipods along the eastern Baltic Sea coast

We report a considerable range expansion of four Ponto-Caspian amphipod species in transitional waters along the southeastern Baltic Sea coast. Chaetogammarus warpachowskyi and Dikerogammarus haemobaphes were found for the first time in Latvia, the former being common along the coast, while the latter was encountered only in the port of Riga. This indicates a 400 to 600 km expansion along the coast, assuming an origin from the Curonian or Vistula lagoons. We also officially document an expansion for Chelicorophium curvispinum in Latvia of ca. 300 km until Riga, confirming recent unpublished records. Moreover, we document a second invasion route of Pontogammarus robustoides into Estonia from nearby Latvian waters by uncovering a population at the port of Pärnu. This species was previously known in Estonia only from the Gulf of Finland (>500 km from Pärnu). With the exception of D. haemobaphes, all newly recorded species were represented by various life-stages and ovigerous females, indicating viable populations. Overall, our findings reveal that Ponto-Caspian amphipods are much more widespread in the Baltic area than previously known and highlight the need for more stringent monitoring