Goldfish are a really resilient little species of animal. You can basically stick them into any environment and they survive. They may not thrive, but they do survive. Want to stick them in an aquarium with other fish? Go ahead. Want to throw them in a bowl and keep them on your desk at work? Go right ahead, all chances are they may outlast you or even the company you work for. You got sick of them and threw them into a nearby pond? No problem, they just became the biggest predator in the little pond and threw the pond’s entire eco-system out of balance.
Not even icy water can stop these little machines from going forward. If they were ever to find themselves beneath a sheet of ice they just lower their metabolic rate and continue living at a slower pace. The same sheet of ice that should be killing them provides a heat shield and does not let heat escape the pond easily, keeping the water warm enough for the fish to survive. And the lower you go the warmer it becomes, so the Goldfish seeks shelter in the sediment at the bottom of the body of water it finds itself in, and they spend their time there until warmer weather comes around. But as the ice above keeps the heat from getting out, so does it keep the oxygen from coming in. And the deeper you go, the less oxygen there is. The microbes that live at the bottom of the pond consume most of the oxygen available down there when they process food. So this means that the warmest places are also the most anaerobic ones. Without oxygen, cells cannot function. So how exactly do these little fish survive?
A study from 2017 claims that this is due to the fish being alcoholics. Jokes aside, the fish, in fact, use a process that turns carbohydrates into alcohol. And fish are the only vertebrae that we know of so far that can do this.
If the fish finds itself in an area where oxygen is limited, inhalation starts a chain reaction that produces small amounts of energy for the muscles. Something similar happens in humans during exercise. If the muscle requires more oxygen than it can absorb, then the body creates it anaerobically. As a result, lactic acid begins to form. This is why we get sore and feel the burning sensation during exercise.
But instead of producing inordinate amounts of lactic acid in these conditions, Goldfish just turn the lactic acid into ethanol. This “drinking” habit is caused by secondary proteins that control energy production. The primary set of proteins is used in normal conditions, while this secondary one kicks in during anaerobic conditions.
Another reason for this is that lactic acid can potentially poison its host, especially during a five-month period spent underwater with limited oxygen supplies. This is why the fish convert it into ethanol which is much easier to diffuse through the gills.
What is even more interesting is that these double proteins occur due to a genome duplication event. The first set of genomes performs the regular daily activities in the body. This allows the secondary genomes to mutate into something useful like they did in the Goldfish. So, genome duplication was responsible for most of the species we see now adapting to new environments and circumstances.