In the vernacular, a person who is over-fond of alcohol is sometimes said to drink like a fish.
It turns out that if the reference is to a goldfish then it is remarkably apt. The world’s most ubiquitous aquarium residents, along with their cousin the crucian carp (Carassius carassius), have evolved the ability to survive harsh winters in oxygen-free waters by converting lactic acid into ethanol.
Because of this ability, the fish can survive for weeks, even months, in conditions that would kill pretty much anything else more complex that a bacterium. It also means their blood alcohol level rises so high that if they attempted to drive a car they would risk a hefty fine and instant loss of licence.
The ability of goldfish and crucian carp to generate life-saving alcohol – sometimes up to 50 milligrams per 100 millilitres of blood – has been known for a while, but how they do it has remained a mystery.
Looking for similar talents in other fish species has drawn a blank. Indeed, pretty much the only organisms known to be able to pull off the same trick are several species of yeast.
Now, researchers at the Universities of Oslo and Liverpool have uncovered the molecular mechanism by which the conversion works.
Fagernes and her colleagues discovered that the muscles of goldfish and crucian carp contain two sets of the proteins associated with channeling carbohydrates within cells towards mitochondria, where it is broken down.
Other species have just a single set. In the goldfish, one set works in exactly the same way as it does for all other fish. The second, however, springs into action when oxygen levels in the water decrease. It contains a mutation that catalyses the production of ethanol outside the mitochondria.
Genetic analysis reveals that the altered gene coding for ethanol production was present at the time goldfish and crucian carp last shared a common ancestor – about eight million years ago.
The mutation allows both species to convert what in other animals is a lethal end product of oxygen deprivation into a way of sustaining life.
“This is a much better situation than filling up with lactic acid, which is the metabolic end product for other vertebrates, including humans, when devoid of oxygen,” says Liverpool evolutionary physiologist Michael Berenbrink.
Fagernes adds: “The ethanol production allows the crucian carp to be the only fish species surviving and exploiting these harsh environments, thereby avoiding competition and escaping predation by other fish species with which they normally interact in better oxygenated waters.”
The ethanol is quickly excreted into the water before it reaches toxic levels itself. While this might seem to be a waste of energy, it avoids the lethal consequences of too much ethanol or lactic acid in the blood, and the fish compensate for the energy loss by using by downregulating non-essential processes and using glycogen stored in the liver. As a result, the fish can survive oxygen deprivation for the long months of winter.