Which freshwater fish is famous for laying its eggs inside freshwater mussels?

The European bitterling is famous for laying its eggs inside freshwater mussels.

During breeding, the female European bitterling develops a long ovipositor. She inserts it into the mussel’s inhalant siphon and deposits eggs in the mussel’s gill chamber. The male then releases sperm nearby, and the mussel draws the sperm-containing water inside. This unusual arrangement protects the fish eggs during their early development.

The relationship is not entirely one-sided. As the mussel filters water, it releases larvae called glochidia, which attach temporarily to the bitterling’s gills or skin before dropping away as young mussels. The fish and mussel therefore provide each other with a form of transport or shelter.

The species is native to much of Europe and parts of Asia. Its dependence on suitable freshwater mussels means that pollution, habitat alteration and declining mussel populations can also threaten bitterling populations.