One of the most arresting sights in the whole animal kingdom is that of a school of fish turning this way and that in perfect harmony with their schoolmates. It seems almost impossible to believe that an animal supposedly as simple as a fish could execute such a well-choreographed routine so beautifully, and yet they do. Of course, this impression is slightly exaggerated. When it comes down to the nuts and bolts of schooling behavior, the fish are not choreographed at all, but simply responding to localized cues. Nonetheless, the effect it produces is beautiful but much easier to understand.
So how exactly do fish in schools stay in such perfect synchrony with each other?
No matter how large the school, each fish in it has a relatively small number of near neighbors. The behavior of these nearby fishes exerts a strong influence on each individual fish to the extent that it simply copies what they do. When a school turns, say, when it meets an underwater obstacle or encounters a danger, the movement almost always begins with a single fish or at most a small number of fish. Those nearest the turning fish will, in the absence of any other information, replicate that movement. Then those next to them also turn and in this fashion the signal transmits across the whole school.
Slowed right down using video footage, you can see a wave of activity passing through the school. This phenomenon is sometimes known as the "Trafalgar effect," presumably because of the way that the firing of the cannons on the flagship brought about the firing of guns in the rest of the fleet like a domino topple. The most amazing thing about a school turning like this is the speed with which the signal to turn is conveyed throughout the school. This rapid response is brought about in part by the need of each individual fish not to stand out from the crowd. Those that do may be picked out by a predator.
Does the school act more like a single, large animal than just a collection of individuals?
In many respects, yes. Although each fish has its own identity, the school sometimes seems to operate as if it has a life of its own. This is particularly apparent when a school "senses" its environment. Schools of fish are excellent at sensing the various stimuli in their surroundings, far better than a similar-sized, simple, loose collection of individual fish. If different members of the school detect the odor of food and each turns towards it, not all will be exactly accurate in their course, but the average direction that all the detecting fish set a course for -- the direction that eventually the school will head in -- is usually extremely precise.
This effect is sometimes referred to as the "wisdom of crowds" and is also seen in humans. For example, if 100 people at a fairground stall are asked to guess the number of sweets in a jar, their individual guesses may be wide of the mark but the average of their guesses is often remarkably accurate.
The overall shape of a fish school changes according to what the fish are doing. When the fish are searching for food, for example, the school spreads out to form a wide front like a search party. In contrast, when the school is under threat, the fish tend to move