Bitterling, minnowlike fish of the family Cyprinidae, found in the fresh waters of Europe and Asia Minor. It is remarkable for its association with freshwater mussels. In the breeding season the female develops a 2-in (5-cm) tube with which she deposits her eggs inside a mussel. The milt (sperm), deposited by the male near the mussel, is drawn in through the mussel's respiratory siphon to fertilize the eggs. Meanwhile the mussel releases its larvae, which cling to the skin of the female bitterling and are carried around before dropping to the bottom.