The crucian carp (Carassius carassius) is a cyprinid fish with its natural distribution in Europe and the western part of Asia. Due to its hardiness and unique ability to survive winter anoxia, it has been translocated to small lakes and ponds, and in Northern Europe since medieval times has been used as a food source. Crucian carp was the only fish in the pond that survived anoxia. Small lakes and ponds with winter anoxia result in dense populations of stunted and slender fish. In lakes with other fish species present, the crucians' numbers and densities are low and they grow to large sizes. In the presence of piscivores such as pike, crucians are deep bodied. The presence of pike-eating crucians, or the pike odors, induce a change in the body form of crucians. The change in body form makes it more difficult for pikes to swallow crucians and the handling time increases. Closely related invasive Carassius species have become a serious threat to crucian carp populations in Central-Eastern Europe and South-Eastern England through competition for space and food resources and hybridization. The crucian carp's close relationship to goldfish (Carassius auratus), the most studied species concerning sex pheromones, has made it possible to demonstrate that sex pheromones are also present in a wild Carassius species and in their natural environment. The results indicate that two species use the same sex hormonal pheromone system. The crucian carp has become an important model for laboratory studies of olfaction and taste.