Goldfish are a bit "meh", aren't they? They're not as popular a pet as cats or dogs, presumably due to the deplorable lack of cuddles. Really they just swim around, opening and closing their mouths, endlessly tormenting your cat with the promise of a tasty snack. That's about it.
But there is more to your fishy pet than meets the eye. Here are four things you probably didn't know about goldfish. Warning: may contain alcohol.
Goldfish were originally kept for meat
The goldfish didn't start out as a pet. It was dinner.
Modern goldfish (Carassius auratus auratus) are a domesticated version of a wild carp from east Asia. Their wild ancestor was silver-grey. Known as "chi", it was at one time the most common fish eaten in China.
Every so often, a genetic mishap would produce a fish that was a brilliant red, yellow, or orange. In the wild, such fish stood out and were gobbled up quickly by predators. But in the ninth century Chinese people – mainly Buddhist monks – began to keep chi in ponds, where they were safe from predators.
According to legend, Governor Ting Yen-tsan discovered both golden and yellow chi in a pond outside the city of Jiaxing. The pond then became a "pond of mercy".
In the Buddhist tradition, it is a good deed to set an animal free, especially if the animal is rare. So it became common practice throughout China for the rare coloured chi to be spared the stewpot and released into ponds. Official records document an accumulation of colourful chi in ponds around 975 CE.
But for at least 100 years, they were no different from wild chi. Unlike domesticated animals, the proto-goldfish hid from humans and did not eat the food that was given to them. "They were captives exploited for religious purposes," according to E. K. Balon of the University of Guelph in Ontario, Canada.
The goldfish bowl was a disruptive technology
By around 1240 CE, goldfish were domesticated and distinct from their chi ancestors. They were tame and would readily eat the food they were given. In public ponds of mercy, goldfish lived alongside chi, turtles, and other fish. But those who could afford to build their own ponds on private land tended to stock them only with the beautiful, colourful goldfish.
With plenty of goldfish on hand, it became easier for their keepers to crossbreed individual fish, to make individuals with desired appearances. According to Balon, this began in earnest in 1163 in the goldfish pond at Te Shou Palace in Hangchow city.
Between then and the 1500s, it became popular to keep goldfish in bowls. That made the keeping of goldfish far more affordable, as nearly everyone had earthen vessels.
The ensuing frenzy of artificial breeding produced the 250-ish varieties of goldfish we see today, described as "freaks" and "monstrosities" by the 1969 Purnell's Encyclopedia of Animal Life. "To recite their names is enough to make this point: veiltail, eggfish, telescope, calico, celestial, lionhead, tumbler, comet or meteor and pearl scale. There are also the water bubble eye, blue fish, brown fish, brocade, pompon and fantail and many others."
Certainly these modifications don't help them survive in the wild. While the different goldfish breeds have features that "satisfied human preference and curiosity", their ornate tail fins are "fancy but uncontrollable" and their bodies are "unfittingly fat", according to a 2009 paper by Tomoyoshi Komiyama of the Tokai University School of Medicine in Isehara, Japan and colleagues.
Goldfish are an invasive species
That being said, some goldfish breeds are hardier than others, and they can be real pests. One study in the UK found at least five invasive varieties doing quite well in ponds: golden, fairground, brown, shubunkin, and lionhead.
While chi are native to the rivers and lakes of eastern and central Asia, goldfish are now found throughout Europe, South Africa, Madagascar, and the Americas, as well as islands in Oceania and the Caribbean. Most populations began either with deliberate releases of unwanted pets or escapes from breeding or distribution facilities.
In Europe, they pose a threat by interbreeding with the native Crucian carp, and in Nevada, they outcompete the Pahrump poolfish. They can wipe out aquatic plants by over-foraging. One study also suggests they kick up so much mud that other species struggle to find food.
A 2001 study found that introduced goldfish eat the eggs and larvae of the long-toed salamander. While they don't normally eat eggs, goldfish are fast learners. If they see other fish feeding on them, they're liable to start, and if one goldfish figures it out, it could rapidly spread through a population thanks to social learning. The goldfish ate the salamander eggs and larvae with such fervour that they "alone are capable of eliminating northern Idaho amphibians from inhabiting suitable breeding sites," say the researchers.
They help us understand our eyesight, and booze
Goldfish have become commonplace in laboratories, perhaps because they're simple to train and easily obtainable.
They are one of the most studied animals in the field of visual perception and cognition. They can perceive the same colours we do, which not even all primates can do, making them an ideal study animal. Juvenile goldfish are even bad at seeing the colour blue, but improve as they age, a pattern repeated in human infants. In one respect they are unlike us: whereas humans have three types of colour-sensing cells in our eyes, goldfish also have a fourth type of colour receptor that allows them to perceive ultraviolet light. But despite this, their other three colour sensors are an excellent analogue for our own.
Goldfish are also particularly useful for understanding the effects of alcohol on the brain and body. That's because "the concentration of alcohol in their blood rapidly approximates the concentration of alcohol in the water in which they swim", according to Donald Goodwin of Washington University in St Louis, Missouri and his colleagues in 1971. That means you can gauge the inebriation of a goldfish non-invasively, just by knowing how much alcohol is in their fishbowl.
In 1969 Ralph Ryback of Boston City Hospital in Massachusetts used that fact to see how different kinds of alcohol affected goldfishes’ ability to learn. It turns out fish swimming in a bourbon solution are more impaired than those splashing around in vodka. So now you know.