Since the 1980s, scientists have been baffled by the incredible abilities of goldfish and their wild relatives, crucian carp, to survive in ice-covered ponds and lakes for months on end. The secret to their survival? Alcohol.

The majority of vertebrates, including humans, die after a few minutes without oxygen. While most animals have the ability to metabolise carbohydrates without oxygen, this generates a build-up of lactic acid that eventually becomes toxic. Goldfish and crucian carp have evolved a set of enzymes which convert the lactic acid to alcohol, which can then be easily removed from the body via their gills. “Usually, other species die long before the decrease in oxygen availability is even a problem for the crucian carp,” says Cathrine Fagernes at the University of Oslo. “By using this method, the fish gets rid of the dangerous end products.”

The longer the fish is in freezing conditions without oxygen, the higher the levels of alcohol in their system get. Dr Berenbrink of the University of Liverpool has recorded blood alcohol levels in crucian carp exceeding 50mg per 100 millilitres, which is the same as the drink-drive limit in Scotland, and more than double the drink-drive limit in Sweden, Poland, and Cyprus. Despite the high levels of alcohol in the bloodstream, it is not the drink that kills them. If the winter lasts too long, the fish run out of fuel stored in their livers and die.

Biology and Medicine applicants might wish to look at what lessons can be learned from evolutionary adaptations, including the creation of a duplicate set of genes (such as those found in the crucian carp). Anthropology students could consider the popularity of goldfish as pets, including the possibility that their resilience makes them especially suited to this.

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