The bird-infecting acanthocephalan Polymorphus minutus has been suggested to comprise different lineages or even cryptic species using different intermediate hosts. To clarify this open question, we investigated Polymorphus cf. minutus cystacanths originating from amphipod intermediate hosts from 27 sites in Germany and France. Parasites and hosts were identified using integrated datasets (COI and/or morphology for hosts and COI + ITS1-5.8S-ITS2 for parasites).
Mitochondrial and nuclear data (ITS1) strongly support the existence of three cryptic species in Polymorphus cf. minutus (type 1-3). These three types reveal a high degree of intermediate host specificity, with Polymorphus type 1 only encountered in Gammarus fossarum type B, Polymorphus type 2 in Echinogammarus sp. and Echinogammarus berilloni, and Polymorphus type 3 in Gammarus pulex and Gammarus roeselii. Our results point to a so far neglected cryptic diversity of the genus Polymorphus in Central Europe. Furthermore, Polymorphus type 2 is most likely a non-native parasite in Germany that co-invaded with E. berilloni from the Mediterranean area. Potentially, type 3 originates from South-East Europe and migrated to Germany by G. roeselii, where it might have captured G. pulex as an intermediate host. Therefore, our findings can be seen in the context of ecological globalization in terms of the anthropogenic displacement of intermediate hosts and its impact on the genetic divergence of the parasites.