Any of several very small, brightly coloured Eurasian freshwater fishes of the carp family (subfamily Acheilognathinae), noted for their unique breeding habit of depositing eggs in the gill chambers of living freshwater mussels.
Most bitterlings rely on mussels both to incubate their eggs and to protect the developing young.
Specifically, the European bitterling, Rhodeus amarus, a common species in slow-moving rivers and ponds across continental Europe.
The European bitterling is often studied for its symbiotic relationship with mussels.