The Bitterling is a small fish of the carp family. Mussels form an essential part of its reproductive system, with bitterling eggs being laid inside them.

Long thought to be symbiotic with the mussels (whose larval phase attaches to fish gills during development), recent research has indicated they are in fact parasitic, with co-evolution being seen in Chinese bitterling and mussel species.

Bitterlings usually reside in areas with dense plant growth. They are a hardy fish, and can survive in water that is not very well oxygenated. They grow to be 3-4 in. long at most. The bitterling’s diet consists of plant material and small larvae of insects.