It takes three zebrafish to make a school, two are not enough

How many fish does it take to make a school? The answer may vary, but for the much-studied zebrafish, the answer is three. Put three small fish together and they will exhibit in greater numbers the same schooling behavior that could save their lives. Two zebrafish behave very differently.

Some birds are said (possibly incorrectly) to count: ‘One, two, many’, allowing them to steal a fourth or fifth egg from their clutch without any problem, because they believe there are ‘many’ left. Something similar may be going on with zebrafish, except in this case the individual fish that counts is one of three.

For small fish, the safest place is usually a crowd, whose schooling behavior makes it difficult for predators to catch them (although not so much for people with nets). Schooling fish have developed ways of interacting with each other that maintain cohesion, allowing the entire school to quickly adjust in a direction that can keep most of them alive. This behavior has been extensively studied and scientists are interested in the minimum numbers required for this behavior to occur. Strangely enough, it is physicists, not zoologists, who have found the answer.

The researchers had to step outside their usual territory and understand fluid dynamics and pair and triplet correlations from thermodynamics to fish.

After equipping an aquarium with synchronized cameras that can track fish movements in three dimensions, the team placed different numbers of zebrafish in the aquarium to study their behavior.

With 50 zebrafish (Danio rerio) in the tank, training was no surprise, but thanks to the cameras the team was able to capture this in movements that computers can analyze. Sometimes the fish formed a circle, other times they moved in the same direction, in a line or next to each other. It makes sense that the fish have evolved a number of ways of swimming together, because being too predictable would be a gift to predators.

The team discovered that groups of four and even three zebrafish show the same coordination as a school of fifty, but two fish alone in an aquarium do not. Three fish swim side by side, while in two, one follows the other.

The path of three zebrafish shows that they behave as a trio as they do within a larger school

The path of three zebrafish shows that they behave as a trio in the same way as within a larger school.

Image credits: University of Bristol / Yushi Yang

The team also examined the behavior of small fish subpopulations within a larger group and marked them so that the cameras specifically picked up their movements. Three marked fish in a school showed similar movements to three alone in the aquarium. However, when two were flagged, their behavior at school was nothing like that of two with the tank to themselves. “This indicates that fish interact primarily with their closest neighbors, viewing the rest of the group as a fluctuating background,” the researchers write.

Dr. Alexandra Zampetaki of Heinrich Heine University, Düsseldorf, said in a statement: “In practice, three fish form a school, but two are not enough.” Or like Dr. Seuss is said to have said, “One fish, two fish, red fish… school has started.”

Zebrafish are a model animal for zoologists because they are small, produce many embryos and are easy to genetically modify. Senior author Professor C. Patrick Royall from the University of Bristol acknowledged that not all fish have the same minimum, and suggested that goldfish and sardines should also be studied.

Transferring the techniques to flying animals could be more difficult, but Royall hopes the work can also be done in the air, including “flocks of birds like starling noises and swarms of insects like dancing mosquitoes.” If a common pattern is found The team has already noted that the movements of the zebrafish group resemble those of mosquitoes, but not starlings, where the change of one bird can propagate through the flock.

The most ambitious project is studying human behavior during mass gatherings. “We will see whether the simple limit of the number three also applies,” said co-author Professor Hartmut Löwen. Such information could be useful in preventing stampedes that cause deadly crowd crushes.

If you’ve ever seen three suspicious characters moving together and thought something fishy was going on, you might be right.

The research is open access in Nature Communications.

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