Diversity of Diplostomum spp. (Digenea: Diplostomidae) metacercariae in parrs of Salmo salar Linnaeus, 1758 and Salmo trutta Linnaeus, 1758 from the European Russian Far North.

Metacercariae of Diplostomum spp. are globally distributed pathogens that infect the eyes and brains of fish and lampreys, including populations in northern latitudes. This study investigated the species diversity and distribution of Diplostomum in juvenile Atlantic salmon Salmo salar and brown trout Salmo trutta from 11 rivers in northwestern Russia. Phylogenetic analyses revealed two diplostomid species: 'D. mergi' Lineage 3 and D. numericum. It is the first record of these species in the Russian part of the Arctic. These species, previously recorded in Western/Central Europe and Russian Karelia, likely spread via major migratory flyways of their avian definitive hosts. The species composition of Diplostomum metacercariae in riverine salmonids of the studied region was notably poorer than in lacustrine salmonids from other Arctic localities. The haplotype distributions of recorded Diplostomum species showed no spatial structuring, consistent with broad-scale dispersal processes.