Researchers have solved the mystery of how crucian carp manage to survive for months at a time in water without oxygen.

Carp are highly vulnerable to predation and one survival strategy they employ is to inhabit seasonally anoxic (low oxygen) ponds where predators are unable to survive.

A drop in water temperature prompts the carp to store vast amounts of glycogen in their brains, the researchers found. This enables them to make the switch to anaerobic metabolism – which does not require oxygen – from February to April in the northern hemisphere.

“Carp use carbohydrates stored as glycogen as a source of energy during anoxic conditions,” says principal researcher Matti Vornanen at the University of Joensuu, Finland. They break down the stored glycogen into glucose and ethanol, providing the energy they need to survive, he explains.


15-fold increase

Carp mainly use glycogen stores from the liver to survive these harsh conditions, but by studying the fish over different seasons, researchers discovered that brain glycogen stores increased 15-fold during February, when ponds are most depleted of oxygen.

The study also showed that the carps’ sodium-potassium pump – responsible for keeping brain cells functioning in extreme conditions – was intact, but reduced its activity by 10-fold in lower water temperatures. This would help to reduce the amount of energy the carp need for continued brain activity in anoxic conditions, the researchers say.

Although some turtles and frogs also survive without oxygen for long periods, they do so by going into “suspended animation”. Carp are unique in that they have evolved physiological mechanisms to cope with oxygen shortage whilst remaining active.

journal reference: American Journal of Physiology – Regulatory, Integrative and Comparative Physiology (DOI: 10.1152/ajpregu.00172.2006)