Freshwater fish Rhodeus sericeus of Northern Europe, introduced to North America. It grows to a length of 90 mm/3.5 in, and has an attractive bluish stripe along each side towards the tail. It is found in lakes and slow-moving rivers.

Having selected a mate, the female bitterling deposits her eggs inside a freshwater mussel via a long tube protruding from her body, and the male then deposits his sperm into the mussel. The eggs are nurtured safe from predators (and unharmed by the mussel) within the mussel's gills for two to three weeks before hatching and swimming away.