Crucian Carp

The crucian carp (Carassius carassius) is a member of the common carp family Cyprinidae.

It is a European species, and its wide range spans from England to Russia; it is found as far north as the Arctic Circle in the Scandinavian countries, and the southern extremities defined by central France and the Black Sea. Its habitat occurs in lakes, ponds, and slow-moving rivers. It has been established that the fish is native to England and not introduced.

The crucian is a medium-sized cyprinid, typically 15 cm in body length, and rarely exceeds in weight over 1.5 kilograms (3.3 pounds), from October 2011">citation needed]. But a maximum total length of 64.0 cm is reported for a male, and the heaviest published weighed 3 kilograms.

They are broadly described as having a body of "golden-green shining color", but a more precise source states that young fish are golden-bronze but darken with maturity, until they gain a dark green back, deep bronze upper flanks, and gold on the lower flanks and belly, and reddish from October 2011">citation needed]or orange fins, although other colour variations exist. One distinguishing characteristic is a convexly rounded fin, as opposed to goldfish (or C. gibelio) hybrids which have concave fins.

The variation in shape of a crucian carp can be very high. When cohabiting waters where predatory such as pike or perch fish are present, there occurs an induced change in the morphology of the population, from a sleeker to a deeper bodied form, into almost perfect disc shape with well-rounded fins, making it difficult for predators to swallow the crucian carp.

The crucian carp is also a type species (generic term) for the entire genus, so this has led to the confusion that this is the sames species as those that are native to East Asia.

There are also reported to be interbred hybrids between the crucian and goldfish (domestic or feral), and a researcher found in laboratory conditions that such cross-breeding are possible, producing viable young. Although the hybrids this researcher produced were sterile or virtually so, genetic contamination into the native population has been levelled as a concern, and even if they don't breed further down the line, the F1 hybrids exhibit hybrid vigour or heterosis, being much more adept at finding food and evading predators than either of their parents, and thus could pose a threat to the native carp population.