A map of geographical ranges of discussed species and the study location; A) range of occurrence of C. microstomus: a polygon with hatched fill -spatial data was provided by IUCN (MfN Berlin, 2021b) and updated by authors to include drainages of Neman and Venta, as no available spatial data for Krasnaya river drainage was found; and C. gobio: a polygon with dotted fill -spatial data provided by IUCN: (MfN Berlin, 2021a); points indicate exact sites of collection of specimens used in the study by Lopes-Lima et al. (2024) to assess U. crassus complex genetic divergence (limited to the map range): black points - U. crassus, white points – U. nanus, grey points – sites of sympatric occurrence of both species; the red diamond indicates the area of this study; B) the sites of collection of animals in this study: 1- P. Phoxinus specimens used during artificial infestation, U. crassus complex specimens used in artificial infestation, U. crassus complex individuals used for genetic reassessment of the population inhabiting the river Warkocz; 2- C. microstomus specimens used during artificial infestation.