Goldfish and their wild crucian carp relatives can survive for five months without breathing oxygen – and now we know how. The fish have evolved a set of enzymes that, when oxygen levels drop, ultimately helps convert carbohydrates into alcohol that can then be released through the gills.

For most animals – including humans – a lack of oxygen can be fatal within minutes. We can metabolise carbohydrates without oxygen, but the process generates toxic lactic acid that quickly builds up in our bodies, reports New Scientist.

On the face of it, this should pose a big problem for crucian carp. They live in ponds and lakes in northern Europe and Asia that freeze over in winter, so the fish have to survive for months without oxygen until the ice melts in spring.

But the carp – and their close relative the goldfish – have developed a workaround. When they metabolise carbohydrates anaerobically, the end product is not lactic acid but alcohol, which is easier to remove from their bodies.

‘The adaptation is very rare among animals,’ says Michael Berenbrink at the University of Liverpool in the UK.

Berenbrink is a member of a team led by Cathrine Fagernes at the University of Oslo, Norway, that probed the carps’ biology to work out how they achieve the feat.

They discovered that the fish have their own specialised alcohol-production system. This comprises a modification of a set of the enzymes that channel energy-rich carbohydrates into mitochondria, the energy-producing parts of a cell. During their evolution, the fish gained a second set of the enzymes, which helps turn the metabolic products into alcohol when oxygen levels drop. The enzymes act in essentially the same way as brewer’s yeast.

‘Usually, other species die long before the decrease in oxygen availability is even a problem for the crucian carp,’ says Fagernes. ‘By using this method, the fish gets rid of the dangerous end products.’

Oxygen-free living

‘This shows how important it is to understand simple mechanisms for surviving anoxia,’ says William Martin at the University of Düsseldorf in Germany.

The study suggests this adaptation evolved 8 million years ago in the common ancestor of carp and goldfish, through a process known as whole-genome duplication. This is when an organism ends up by chance with an extra set of its genes, which can then be repurposed to take on new functions.

By making alcohol, crucian carp and goldfish can survive where no other fish can, meaning they can avoid predators or competitors. But their adaptation does mean that the fish spend most of the winter with blood alcohol levels of roughly 55 milligrams per 100 millilitres – which Berenbrink points out exceeds the drink-driving limit in some northern European countries.

Perhaps understandably, then, the fish are a bit slower than normal, says Fagernes.