First watch the awesome video below, depicting how a school of fish reacts to hunting behavior of blacktip sharks (Carcharhinus limbatus) off the Maldive Islands.
The accompanying explanation, via Why Evolution is True:
Animals form large groups for many reasons, from reproduction to migration to avoidance of predators. Some of the largest groups of vertebrates are seen when birds flock and fish school as an anti-predator defense.
This amazing video illustrates the apparent coordinated movement of individuals in a large school, although in fact the movement of each fish is thought to be independent. How the fish do this remains somewhat of a mystery. Obviously, vision is critical (fish don’t or can’t school after dark, and fish that have been blinded also don’t form schools) but fish also often have prominent markings on their shoulders or tails (schooling marks) which appear to serve as reference marks indicating their movement.
Other possible cues include pheromones, sound, and the sensitivity of a fish’s lateral line. Fish that have had their lateral line removed swim closer together, suggesting that the lateral line keeps fish at a minimum distance from each other; fish appear to be able to ‘feel’ when another fish comes close because the lateral line is sensitive to pressure. In contrast to the fish avoiding the sharks in this video, the movements of the sharks are clearly coordinated as it is in many predators.
How most people think a school of fish, or other group of animals (that have awesome names by the way), operates is by moving together because of some external forcing or “group think.” This however is not the case. The fish in this case move independently, reacting to the movement of the fish around it.
If we were to come up with some simple rules, like “move in the same direction as the fish next to you” and “keep a certain distance between you and the fish next to you”, these rules extrapolated to a large group would produce the amazing schooling that we see in the video. Then we can say that this movement can ultimately be caused by the reaction of a single individual fish.
Evolution is awesome.
[Via Why Evolution is True]