The crucian carp (Carassius carassius) is a medium-sized member of the common carp family Cyprinidae. It occurs widely in northern European regions. Its name derives from the Low German karusse or karutze, possibly from Medieval Latin coracinus (a kind of river fish).
Carassius species exhibit some remarkable physiological adaptations to their environment. For example, in entirely anoxic conditions during winter Carassius carassius can survive for considerable periods by anaerobic respiration, with ethanol as the major metabolic end product; a facility that is highly unusual among vertebrates. During summer the fish also may survive anaerobic conditions by this metabolic expedient, though only to a far more limited extent; the winter phenotype can sustain fermentation as a substitute for respiration for several weeks on end. Experimentally the fish have been maintained under anoxic conditions for 140 days. Anoxia can be tolerated longest in the coldest water, even down to 0 °C, because colder conditions lower the metabolic rate. Alcohol production occurs mainly in the muscle tissues, but also in the liver, where the process is thought to have originated. Similarly goldfish can produce alcohol in muscle tissues, but to a much more limited extent.
Experimentally it has been demonstrated that the metabolic process involves the production of pyruvate from lactate, followed by decarboxylation to acetaldehyde which then is hydrogenated to ethanol as the major metabolic end product. In turn the fish largely excretes the ethanol into the water rather than accumulating it to toxic levels in the tissues. Excretion of lactate in significant quantities is not a common nor a desirable metabolic facility, but the excretion of ethanol presents no serious metabolic challenges. This metabolic expedient avoids the fatal accumulation of acid end-products of anaerobic glycolysis.
The crucian carp is a widely distributed European species, its range spanning from England to Russia; it is found as far north as the Arctic Circle in the Scandinavian countries, and as far south as central France and the region of the Black Sea. Its habitat includes lakes, ponds, and slow-moving rivers. It has been established that the fish is native to England and not introduced.
The crucian carp is a medium-sized cyprinid, typically 15 centimetres (5.9 in) in body length, and rarely exceeds in weight over 2 kilograms (4.4 lb), but a maximum total length of 64 centimetres (25 in) has been reported for a male, and the heaviest published weighed 3 kilograms (6.6 lb).
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 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 crucian carp is also the type species for the genus, which has led to confusion in the taxonomy of species native to East Asia.
There are reports of hybridisation between the crucian and domestic or feral goldfish, which has been verified by production of viable hybrids in laboratory conditions. Although the hybrids thus produced were sterile or nearly so, genetic contamination of the native population has been raised as a concern; even if the hybrids cannot continue to propagate, the F1 hybrids exhibit hybrid vigour or heterosis, being much more adept at finding food and evading predators than either of their parents, which has been proposed to constitute a possible threat to the native crucian carp population.
