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.Shoubridge, E. A. & Hochachka, P. W. (1980). Ethanol end product of vertebrate anaerobic metabolism. Science, N.Y. 209,307-308.
