This thesis presents the relationship between mussels and fish. Fishes form three basic types of partnerships with bivalves: the mussel serving as an incubator, as a temporary shelter or as a permanent shelter. European bitterlings (Rhodeus amarus) are small cyprinid fishes that form obligatory associations with freshwater mussels. The mussels serve as incubators, providing the eggs and larvae with oxygen and protection thus releasing the fish from any form of parental care. In central and western Europe bitterlings spawn in four species of freshwater mussels: Unio pictorum, U. tumidus, Anodonta anatina and A. cygnea. Female European bitterlings deposit eggs into both inner and outer paired gills of mussels. They display remarkable morphological, physiological and behavioral adaptations for using mussels as incubators for their eggs. Due to their development inside the mussel the bitterlings were relieved from coping early in their life with predators. Many of the bitterling species, place a few eggs into different mussels, thus many offspring still survive even if one or several mussels, in which they laid their sex products, died.