Despite its status as a proverbial fact, a goldfish’s memory isn’t a few seconds long. Research demonstrated beyond reasonable doubt that goldfish have a memory-span of at least three months and can distinguish between different shapes, colours and sounds. They were trained to push a lever to earn a food reward; when the lever was fixed to work only for an hour a day, the fish soon learned to activate it at the correct time. A number of similar studies have shown that farmed fish can easily be trained to feed at particular times and places in response to an audible signal.

Goldfish don’t swim into the side of the bowl, not because they can see it, but because they are using a pressure-sensing system called the lateral line. Certain species of blind cave fish are able to navigate perfectly well in their lightless environment by using their lateral line system alone.