Zebra mbuna fish and stingrays have been found to be capable of adding and subtracting numbers. The experiment tasked the fishes with swimming through chambers where they’d only receive a food reward if they got their mental math right.

Their numerical skills are detailed in a new paper published in Scientific Reports. It reveals that under experimental conditions, the bony fish, zebra mbuna (a type of cichlid), and stingrays (who sit with sharks in the elasmobranchs) could add and subtract one from the numbers one to five in an effort to secure a food reward.

The discovery comes from a team including Professor Vera Schluessel from the University of Bonn, Germany, who have been training fish in numerical tasks for a few years. If you’re curious, math lessons for fish go something like this:

• Fish swims into a compartment separated by two screens
• Researchers present different things on each screen, for example, five dots on one and three on the other
• Fish will choose a side to swim through randomly, and either receive a reward or get nothing
• With time, they learn to associate the visual stimuli on the screens with the desired outcome

Once the cichlids and stingrays had the basics down, the researchers could make things more complicated. They then used a projector to show geometric shapes in certain colors, which acted like a question.

The color of those shapes represented whether the answer (displayed on the two screens) involved subtraction or addition by a factor of one. For example, blue was the signal to add, so if shown three blue triangles the fishes were looking for a screen option showing four blue triangles.

Yellow was the signal to subtract by a factor of one, so the correct answer to three yellow triangles was two yellow triangles. As the fishes learned from trial and error, some individuals began to make sense of the numerical tasks presented to them driven by the motivation of a food reward.

The brain fuel for the cichlids was dry pellets, while stingrays are apparently more driven by earthworms, squid, and fish.

Of those tested, six of the zebra mbuna and three of the stingrays were able to work out the marine math, getting it right 78 and 94 percent of the time in addition tasks respectively. Both were slightly less reliable with subtraction, getting the right answer 69 and 89 percent of the time, respectively.

Individual variation existed in both groups, something which is perhaps more surprising to non-aquarium folk than those used to working with fish in science.

“Most people don’t really know that fish also have personalities,” Schluessel told IFLScience. “Not every fish is like the other fish, so some are really good at something, and some are not. So, you have a fairly wide individual variation.”

Promising test results so far, but Schluessel says the team hopes to further the research by making it a bit harder. They also hope to continue working with elasmobranchs (like the stingrays) whose numerical skills, to date, have been less well studied compared to bony fish.

Next stop: Aquarium Countdown.