Metacercariae of <i>Diplostomum</i> spp. from eyes of the Bullhead <i>Cottus koshewnikowi</i> Gratzianov, 1907 from the northern European watershed

Metacercariae of <i>Diplostomum</i> spp. are widespread fish parasites. In this study we obtained the first data on infection of Bullhead <i>Cottus koshewnikowi</i> with these larvae in five rivers of northern Europe (Finland and Russia) using molecular and morphological description. Three <i>Diplostomum</i> spp. were revealed in the eyes of bullheads. <i>Diplostomum spathaceum</i> and <i>D. mergi</i> Lineage 3 sensu Georgieva et al. (2013) were found in the lens, while <i>Diplostomum</i> sp. Lineage 6 sensu Blasco-Costa et al. (2014) was found in the retina. We obtained molecular data on these three species and provided morphological characteristics of the latter two species. Partial sequences of the cytochrome c oxidase subunit 1 (<i>cox1</i>) and ITS1-5.8S-ITS2 were amplified for 20 isolates. Using molecular data, we ascertained the species identification and obtained new information on the life cycles of <i>D. mergi</i> Lineage 3 and <i>Diplostomum</i> sp. Lineage 6. Partial <i>cox1</i> sequences were used to assess the haplotype diversity of <i>D. mergi</i> Lineage 3 and <i>Diplostomum</i> sp. Lineage 6 in the study area. Discriminant analysis showed that <i>D. mergi</i> Lineage 3 was morphometrically close both to the species of the lens complex (<i>D. mergi</i> Lineage 2, <i>D. mergi, D. nordmanni</i>, and <i>D. parviventosum</i>) and to the species from the retina (<i>D. pungiti, D. volvens</i>). Dimensions of <i>Diplostomum gobiorum</i> lay far outside the confidence interval of <i>D. mergi</i> Lineage 3. Our molecular and morphological data and the new information about the hosts and the distribution of these parasites are a crucial step towards elucidating the diversity and life cycles of these important parasites. The data on the infection of bullheads in the River Utsjoki (a tributary of the River Teno, Finland) with metacercariae of <i>Diplostomum</i> spp. offer some insights into the relationships between the introduced host and the native parasites. Infection of bullheads, which are considered invasive in the Teno River system, with metacercariae of <i>Diplostomum</i> spp. may lead to increased infection levels in resident fish.