[A] group of researchers led by Sandie Millot designed a feeding machine that could be operated with a pull string. If a fish was hungry, it would swim up, pull the string, and the food would be dispensed.
48 of 56 fish figured out how to get the food. Each of the fish was marked with a small tag with a colored bead on it just in front of the dorsal fin.
So far, no surprise. This kind of task is well within the cognitive capabilities of fish.
But then something weird happened – three of the fish made a cognitive leap and joined the hallowed ranks of the tool-using animals of Earth.
Three of the fish figured out that they could use the artificial tags, rather than their mouths, to operate the feeder. They learned to swim past the string and hook it onto their tags so that the food would be released that way.
The fish appeared to accidentally catch their tags on the feeder’s pull-string.
Eventually, after a bit of trial and error and fine-tuning, all three fish were performing the action with apparent intention, using their dorsal tags alone rather than their mouths to operate the feeder.