Experiments suggests archerfish can differentiate between numbers
The researchers placed the fish in a tank of water and then placed a pair of disks over the top of it. The disks were marked with different numbers of dots. The researchers taught the fish to spit a stream of water at disks with a certain number of dots on them, such as six. They then showed the fish a series of disk pairs with different numbers of dots on them. They found that the fish would only spit at the disks with the number of dots that they had been trained to recognize. The researchers also tried changing the number of dots the fish had to choose from and found that the fish were still able to target the right disk.