Some of the most remarkable discoveries about fish in recent years have centered on their clear ability to recognize particular individual fish. Incredible as this may seem, this capacity to distinguish specific individual identities has now been demonstrated in a huge variety of fish.
Can fish recognize their relatives?
The ability to recognize kin is extremely important in many species. In one basic example, guppies have been shown to avoid eating their own offspring while happily munching everyone else's. Parental firemouth cichlids can also recognize their own fry; remarkably, they will kidnap the fry of others and put these on the edge of their own school of offspring so that they, and not their own young, are in harm's way -- a kind of "cichlid shield." Almost all fish are thought to be able to recognize their own siblings, which is important because it prevents the possibility of inbreeding. Female rainbowfish are particularly good at this, avoiding their own brothers for this reason, but forming kin schools with their sisters and half-sisters, which allows the possibility for mutual benefits through co-operation.
What about recognizing unrelated fish?
A large number of fish species choose their "friends," learning to recognize particular individuals and sticking with them over considerable periods of time in a phenomenon known as "familiarity". This can have many benefits for all concerned: familiar fish are less likely to fight over food and are better able to survive predator attacks because they form more cohesive schools. Some fish choose their "friends" extremely carefully; guppies prefer to associate with individuals that co-operate with them in risky circumstances, while sunfish choose to be with individuals with whom they have successfully hunted in the past. Male guppies, ever footloose and fancy free, avoid females that they have met before and with whom they are familiar, preferring to mate with the new girls in town. In doing so, they are able to spread their genes further than would otherwise be possible.
Can fish have "friends" in other species?
The ability of fish to recognize particular individuals is not limited to fish of the same species. Fish that have been tankmates for any length of time repeatedly interact with one another and become familiar. This can occur even when the fish would not be naturally found together in the same habitat. Stories abound of fish that become attached to one another and that even pine if separated.
One such anecdote refers to an African grey knifefish (Xenomystus nign) and climbing perch (Ctenopoma kingsleyae), which not only tolerated one another but seemed to co-operate at feeding time to keep the other tank inhabitants at bay. Another episode concerns an oscar cichlid (Astmnotus ocellatus) and a triangle cichlid (Uaru amphiacanthoides) that were raised together and cohabited peacefully for months. When the oscar was removed to a new tank in an attempt to pair it with another, the triangle cichlid pined and refused to eat for several days until the owner guessing the problem, finally relented and returned the oscar to the tank. The triangle cichlid immediately recovered its former perkiness and its appetite, and the two remained inseparable ever after.