The introduction history of the non-native shrimp Palaemon macrodactylus in the Baltic Sea indicates repeated anthropogenic dispersal

The shrimp species Palaemon macrodactylus, Rathbun 1902 is native to the Pacific coast of Japan, East China, and Korea. It has successfully spread to non-native habitats in North and South America, Australia, and the entire European coastline. Recently, this species was also found in the Baltic Sea near the Port of Gdańsk and the Kiel Canal. However, the latter population could not be considered established. To reconstruct the recent colonization of the Baltic Sea and determine the most probable pathways and vectors of this expansion, molecular analysis and ecological surveys were conducted. A total of 78 sequences of a fragment of the cytochrome oxidase subunit 1 (COI) from 10 different populations from the Baltic Sea and North Sea were generated and complemented with publicly available sequences. This resulted in the estimation of a haplotype network and the calculation of genetic differentiation and diversity. The results provide the first record of an established population of P. macrodactylus in the Kiel Canal area and reveal a complex expansion of this species with multiple introductions into the Gdansk Marina. Additionally, the distribution of this species in the Baltic Sea is mainly limited to port areas, indicating that shipping, particularly biofouling on boat hulls, is the primary dispersal vector.