This YouTube video, sent in by a reader, shows how a school of fish reacts to hunting behavior of blacktip sharks (Carcharhinus limbatus) off the Maldive Islands.
Notice how the fish seem to move in a coordinated fashion, almost as one, and how they tend to group behind the sharks, where they’re less liable to be nommed. Such “coordinated” group movement is not unusual in flocking animals; we’ve seen it before in the amazing behavior of flocks of starlings (see the videos here). The thing is, biologists don’t really understand what cues animals use when groups of them appear to move as one.
It hasn’t escaped my notice that the sharks seem to be driving the fish toward the wharf, perhaps to either trap them or stun them against the pilings. (Dolphins, by the way, sometimes stun prey by whacking them with their tail.)
I asked my colleague Steve Pruett-Jones to watch it and, as an animal behaviorist, send me his take. Here it is:
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.
By “independent” above, Steve means that the fish are not all responding to a single external cue (which may in fact be what the sharks are doing when they make their “hunting rush” in this video), but to the presence of surrounding fish. This suggests two things: first, that this “coordinated” behavior is really a response to the movement of a single individual, who sets off a wave that propagates through the group. Second, the speed of propagation seems much faster than can be explained by the sum of the reaction times of all the individuals. The fact is that we simply don’t yet understand how this type of group movement works. That seems like a simple question, but it’s a simple question that’s hard to answer.