In their day-to-day social lives, humans decide who other persons are by looking at their faces. Primates including chimpanzees and monkeys, too, distinguish other individuals by looking at their faces. Cows and sheep, which form a herd and live in a group, also distinguish other individuals based on their faces. These animals live in a society where they meet and interact each other in a certain number of the same individuals repeatedly for a long period of time. In such societies, to be able to distinguish others individually is one of the most important requisites to live in their respective societies. Without that ability, they can’t form a society with tactical interactions like humans. And Fish have this ability.