New Co‐Introduced Trematode, <i>Maritrema dikerogammari</i> sp. nov., From Invasive Amphipods in the Danube: Insights Into <i>Maritrema</i> Taxonomy and Phylogeny

Biological invasions are among the most disruptive forces affecting global biodiversity. The "killer shrimp" Dikerogammarus villosus, a pervasive Ponto-Caspian invader across Europe, serves as a key model for understanding the ecological consequences of such expansions. However, despite its extensive spread, the helminth fauna associated with this invader remains poorly characterized, obscuring potential spillover effects and the evolutionary history of its symbionts. To address this gap, we surveyed 54 European freshwater localities (2020-2025) and screened 1271 D. villosus specimens from 10 positive stations. These sites span the Danube (Slovakia) and Baltic (Poland) drainage basins, representing the southern and northern invasion corridors. Using an integrative taxonomic approach combining detailed morphology (12 fixed adults and 4 live metacercariae) and multi-locus phylogenetics (18S, 28S, ITS, and COI), we formally describe Maritrema dikerogammari sp. nov. infecting D. villosus. Our analyses place the new species within a distinct freshwater clade as the sister taxon to the Carpathian endemic Maritrema neomi. Historical records of Maritrema inusitatum from the Dnieper River were misidentifications, and we reassign them to the new species, confirming the parasite's longstanding presence in both the Danube and Dnieper basins. This study documents a clear case of "hidden co-invasion," where a cryptic parasite established itself in the invasion corridor undetected. Phylogenetic reconstruction reveals that the evolution of this group is driven by a fundamental marine-to-freshwater transition, shaped by ancient vicariance and recent host-facilitated dispersal. We refine the taxonomy of the genus Maritrema and underscore the critical role of invasion routes in the spread of associated fauna.