But goldfish can switch between the two, depending on the temperature.At low temperatures, when oxygen levels are low, goldfish can switch to anaerobic metabolism, which can still get the job done, but just not as efficiently.And at higher temperatures, when oxygen is more plentiful, they can switch back to the more efficient aerobic metabolism.
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If you had pet fish as a kid, chances are it was a goldfish. Hundreds of millions of them are sold in the US each year. And it’s easy to see why, with their shimmering scales and swirling tail. So calming to watch. And that’s what first led people in China to breed them more than a thousand years ago. But as it turns out, beauty wasn’t the only thing goldfish had going for them. They also had lots of really strange biological traits that made them incredibly resilient. That resilience helped them spread across the planet and into bedrooms everywhere.
And in an unfortunate twist, we’re now breeding away the very qualities that once made the goldfish nearly invincible. Around 265 CE, red, shimmering goldfish first appeared in lakes in southern China. They were a hybrid of the crucian carp and the common carp, both rather standard-looking as far as fish go, with olive or gray coloured bodies. But as evolutionary chance would have it, some of these hybrids had glistening red scales instead. That sheen came from guanine, the same molecule that allows chameleons to change colors and is sometimes used in cosmetics to make them reflect light. Their lustrous scales made goldfish a prized catch. Quite literally. Monks kept goldfish in ponds on the monastery, and they were regarded as sacred because of their beauty.
But by the 14th century goldfish had moved out of the pond and into small porcelain bowls. This was the start of breeding goldfish for their looks. Soon, silver-white, golden-yellow, and tortoiseshell varieties were thriving in their indoor homes. Now one reason there were quickly so many types is that goldfish are what are called allotetraploids. That means their genome has four sets of chromosomes, unlike most animals, which have two sets. Somewhere between 8 and 14 million years ago, the goldfish’s ancestor went through an event called whole genome duplication. Essentially, the ancestor fish inherited the usual one set of chromosomes from each of its parents, and those two sets combined to make its genome. But then that entire genome duplicated. Suddenly, the ancestor fish had an extra copy of its genetic instructions sitting around. And those two copies evolved separately.
That means every time a new mutation occurred in one copy, the old variation of the gene stayed behind in the other. Essentially, goldfish could keep gaining new physical traits, without losing the old ones. By the sixteenth century, several varieties of those selectively bred goldfish were being transported across the ocean from China to Japan. There, goldfish sellers looking for customers would cart the creatures around on summer festival days in wooden tubs. Despite the sweltering temperatures outside, the fish usually managed to keep their cool. And that’s probably thanks to their amazing ability to withstand a wide range of temperatures, anywhere from 0 to 43 degree celsius.
Considering the fact that most freshwater fish are comfy only between 15 and 24 degrees, this made the goldfish an ideal species to lug around in less than ideal conditions. In fact, at the upper end, they had some of the best heat tolerance of all fish. Fish in general are sensitive to water temperatures because they’re poikilotherms. That means their internal body temperature, and therefore all the processes that go on in there, are dictated by the environment. As luck would have it, goldfish were particularly good at adjusting those internal processes to different temperatures.
Take the ability to turn fuel into energy, also known as metabolism. Aerobic metabolism, which requires oxygen, is more effective at turning fuel into energy than metabolism that happens without the gas. But goldfish can switch between the two, depending on the temperature. At low temperatures, when oxygen levels are low, goldfish can switch to anaerobic metabolism, which can still get the job done, but just not as efficiently. And at higher temperatures, when oxygen is more plentiful, they can switch back to the more efficient aerobic metabolism. If temperatures get too far above or below a certain optimum temperature, aerobic metabolism stalls. At high temperatures, this is partly because the heart can’t beat fast enough to pump oxygen around the body and into cells. Goldfish, however, have adapted to be able to beat their hearts at a slower rate under typical circumstances than other cold-water fish, allowing their heart rates to go up when temperatures rise without disrupting the rhythm. Additionally, goldfish can change the balance of fatty acids in their brain, muscle, and intestinal cells to allow more nutrient exchange with their blood when their hearts are pumping slower, particularly at near freezing temperatures. This allows for easier passage of amino acids between the intestines and the blood for growth, repair, or other bodily functions. This adaptation has been useful for goldfish, allowing them to survive long sea voyages and be shipped around the world, as well as survive in low-oxygen environments. Furthermore, their prolific breeding habits have made them appealing to marketers and scientists alike. Goldfish have a wide range of physical features, are easy to breed, and are also sponges, making them particularly useful for studying evolution. Goldfish are known for their unusually large gills and many blood vessels, which allow them to absorb substances easily. This made them a great choice for studying the toxicity of household chemicals in 1917, when zoologist Edwin Powers conducted the first experiments. He placed goldfish in tanks of water containing various chemicals and observed how long they lived and the concentration of the chemicals. By plotting the results on a graph, he was able to determine the threshold of toxicity. Since then, goldfish have been used to study the toxicity of a variety of substances, including growth hormones and insecticides.
Goldfish have also become popular as pets, with breeders around the world creating many different varieties with striking features. However, these so-called “fancy” varieties have lost some of the goldfish’s amazing survival skills, such as the ability to control their direction in water, and are prone to diseases due to low genetic diversity.
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