The Crucian carp (Carassius carassius) is a medium sized cyprinid fish which mainly inhabits
small still waters. There is now a strong, but as yet unsubstantiated belief that this native species is under increasing threat from hybridisation, competition and disease from other closely related nonnative fishes such as common carp (Cyprinus carpio) and goldfish (Carassius auratus). These threats are considered to be Europe-wide and are compounded by difficulties in identification of pure-bred C. carassius and hybrids by external morphological investigation and by some unresolved taxonomic problems. Of particular note is the species in Europe commonly known as the “gibel” or “prussian” carp. It has long been discussed but is still not known whether this is a species in its own right, a subspecies of the goldfish or whether it may be of hybrid origin.