I am asking because I found the glochidia of Unionidae on gills of European grayling in headwaters of Vishera River (Perm Krai, Russia) with water hardness 10-40 mg/l and Ca CO3 concentration 8-11 mg/l. According to previous studies on macrobenthos distribution along the river, the settled stage of Unionids is typical only for the lower and middle reaches of the river where mineralization is higher. This can indicate the migrations over 200 km for the species within the river.