The Crucian carp (Carassius carassius) is a member of the family Cyprinidae, which includes many other fish such as the Common carp, or the smaller minnows. They inhabit lakes, ponds, and slow-moving rivers throughout Europe and Asia. The crucian is a medium-sized cyprinid, which rarely exceeds a weight of over 3.3 lb (1.5 kg). They usually have a dark green back, golden sides, and reddish fins, although other color variations exist. It is known for its ability to survive without oxygen (up to 5.5 months in winter).
They are often caught as a sport fish: the British rod-caught record for largest crucian is 4 lb, 9 oz caught by Martin Bowler in a lake in southern England in 2003. There have been various bids for a breakage of this record since, but they have been passed off as the specimens have not been said to have been “true” crucians, but hybrids between the carp and one of its relatives, such as the goldfish, which are not native to the British Isles. These hybrids often exhibit hybrid vigor or heterosis, being much more adept at finding food and evading predators than either of their parents, and thus pose somewhat of a threat to the native carp population, and to other native aquatic animals.
These carp are also occasionally kept as freshwater aquarium fish, as well as in water gardens, although they are not commonly available commercially, mainly because they are not in particularly high demand due to the presence of more colorful fish such as the koi or orfe.
Many sources will claim that crucian carp are the wild version of the goldfish Carassius auratus auratus). While they are certainly very closely related, they are different species.