Goldfish don't need a teeny-tiny moonshine still. These little guys can handle homebrewing all by themselves, as evidenced by the findings of a recent study published in the journal Scientific Reports. Although they don't distill whiskey, goldfish have long been known for their ability to make their own alcohol. Until now, though, scientists have puzzled over how they manage this phenomenon.
So, why would a fish need to make alcohol? The ability is seen in both goldfish and its wild relative the crucian carp (both of the genus Carassius), which often live in conditions that would be completely unthinkable for other species. Typically, a vertebrate can only survive minutes without oxygen, but these species have evolved to the point where they can last four or five months in oxygen-deprived environments, like the bottoms of rivers, frozen lakes and even your poorly cleaned aquarium at home.
When a creature is left without oxygen it typically experiences a buildup of lactic acid, which can turn toxic. However, these particular fish have two sets of proteins in their muscles, rather than the usual one. The second set of proteins get activated when there isn't any oxygen and changes lactic acid into ethanol. This biochemical adaptation allows the fish to diffuse the substance into the water, helping them prevent the life-threatening condition lactic acidosis.
"During their time in oxygen-free water in ice-covered ponds, which can last for several months in their northern European habitat, blood alcohol concentrations in crucian carp can reach more than 50 mg per 100 millilitres, which is above the drink drive limit in these countries," University of Liverpool evolutionary biologist Michael Berenbrink explained to Phys.org. "However, this is still 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."
This ability has turned these species of fish into quite the Darwinian geniuses. "[T]he evolution of the ethanol-producing pathway has not only made the goldfish one of the arguably most resilient pets under human care, but has also clearly provided Carassius with unique ecological benefits, allowing survival in waters that are uninhabitable for other fish, thereby evading piscine predation and interspecific competition," the researchers explain in the study.