CASE STUDY | The anoxia-tolerant crucian carp

The crucian carp typically lives in ponds where ice cover in the winter prevents oxygen exchange with the atmosphere, leading to progressive aquatic hypoxia and anoxia. Oxygen uptake during hypoxia is aided by an extremely high oxygen-binding affinity of hemoglobin in the blood and remodeling of the gill respiratory exchange surfaces. But the crucian carp can also survive complete anoxia at cold temperatures for several months. In anoxia, anaerobic metabolism is supported by the largest glycogen stores found in any vertebrate, and the major anaerobic end product is ethanol, whereby excessive lactate acid accumulation, resulting from prolonged anaerobic metabolism, is avoided. Energy consumption by the brain is reduced by metabolic depression, mediated by inhibitory neurotransmitters and neuromodulators, and modalities such as vision and hearing are suppressed until normoxic conditions return. However, cardiac output is maintained in anoxia, probably to facilitate transport of lactate and glucose, and ethanol release to the water so that self-intoxication is avoided. Depolarization of mitochondria is prevented by fumarate acting as an electron acceptor at complex II, and mitochondrial function is thus maintained in anoxia and in re-oxygenation.