Araz Rashydov

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.

The host diversity, complex life cycle and distribution of digeneans underline their value as bioindicators of ecosystem health. We examined the intestinal digenean trematode Cephalotrema elasticum of insectivorous mammals, sampled from Gammarus balcanicus (Crustacea: Amphipoda) as second intermediate hosts. The species is particularly relevant as part of the helminth species complex associated with protected mammals. Our study is based on an intensive survey of helminths in 8920 gammarids collected from 38 localities in Slovakia, Poland, and France. DNA sequences for three markers (18S rDNA, 28S rDNA, and CO1 mtDNA) were analysed using Bayesian inference. Molecular data suggest that C. elasticum represents a distinct genetic lineage closely related to Cryptotropidae. Rare anatomical traits, such as the marginal or submarginal genital pore arrangement, may influence mating success and reproductive isolation within Microphalloidea. Genetic and morphological evidence supports establishing a new family, Cephalotrematidae, within Microphalloidea, to include Cephalotrema, Pseudocephalotrema, and Opisthioparorchis. A family diagnosis and a key to the genera are provided to facilitate further taxonomic studies. The geographic distribution of C. elasticum highlights its utility as a biological indicator, reflecting the decline of water shrew populations due to climate-induced ecological changes, particularly in the French Pyrenees.