A Current Biology study tracked ten groups of four clonal Amazon mollies for 55 days and found that coordinated shoaling develops over weeks, with group traits that diverge in ways individual fish can’t explain.
A school of fish moving as one is a classic image of collective behavior. Each fish adjusts to its neighbors, and the group turns, splits and regroups as though guided by a single mind.
But how does that coordination arise, and why do some groups behave differently from others? A new study of genetically identical fish suggests that the answer lies partly in the experiences fish share as they grow up together.
Researchers from the Cluster of Excellence “Science of Intelligence” in Berlin, the Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB), Humboldt University of Berlin and the University of California, Davis report in Current Biology that coordinated group behavior in Amazon mollies develops over weeks rather than being present from birth, and that groups become distinct in ways that cannot be predicted from their members.
“A swarm is more than the sum of its parts,” said David Bierbach, an author of the study. “This study of clonal fish shows that social experiences during development play a key role in the emergence of group behavior.”
A fish that clones itself
The Amazon molly, Poecilia formosa, is a small freshwater fish from Mexico and Texas with an unusual way of life. The species consists only of females, which give birth to genetically identical daughters.
Oddly, Amazon mollies still need males. A female must mate with a male of a related species to trigger the development of her eggs, but the male’s genes are not passed on. The species is named after the Amazons, the female warriors of Greek mythology, not the river.
That makes Amazon mollies a powerful tool for studying behavior. Earlier work showed that even genetically identical mollies raised under the same conditions develop consistent individual differences in behavior, a kind of personality. The new study asked whether groups develop their own characteristic behaviors too.
Watching groups from birth
The researchers reversed the usual question. Rather than studying groups that had already formed, they followed genetically identical fish from the moment their groups were created.
They tracked ten groups of four siblings each, reared together from birth under identical conditions, for 55 days. Automated video tracking recorded the positions of every fish during daylight hours, a design the researchers called the “Big Brother Setup.”
Shoaling takes weeks to learn
In the first two weeks, the young fish were spaced as though they were moving independently. From about three weeks, they began to stay closer together, with major shifts around day 30. Their movements also became more coordinated as they responded more to one another.
That suggests coordinated shoaling is something the fish develop over time rather than something they are born doing.
Groups with their own character
Even so, the groups differed from the very first day, despite identical genes and environments. Some characteristics, such as average swimming speed and how far the fish tended to stay from the center of the tank, remained fairly consistent within each group.
But other aspects of group behavior changed dramatically. A group’s behavior in its first five days was very different from its behavior in its last five days. That was surprising, because individual mollies are known to keep consistent differences over time.
The finding suggests that a group’s behavior cannot be explained simply by averaging the behavior of its members. Instead, the way the fish interact appears to shape what the group becomes.
As the fish became more responsive to one another, social interactions may have increasingly steered each group, amplifying, reducing or redirecting existing differences. Group differences became less predictable during this period of rapid social change. The study cannot identify which specific interactions caused which outcomes, but it suggests that social feedback contributes to groups drifting apart.
Not fixed entities
“The findings add another layer to our understanding of individual differences and collective behavior,” Bierbach said. “Animal groups are often treated as relatively stable entities, with characteristic patterns of movement or social organization. But the results from the Amazon mollies suggest that these group-level characteristics can change substantially during development.”
Separating the effects of individual differences from the effects of interactions has long been a challenge in studying collective behavior, from fish schools and bird flocks to insect colonies. Clonal fish offer a way to hold genetics constant and isolate the role of social experience.
The study involved only ten small groups of a single species in laboratory tanks, and the results may not apply directly to large wild shoals. The release does not list the paper’s full author list or funders.
Notes
- “Group-level phenotypes are idiosyncratic yet unpredictable over development in a clonal fish.” Current Biology, 18 September 2026. DOI: 10.1016/j.cub.2026.08.073.
- Technische Universität Berlin – Science of Intelligence. “A swarm is more than the sum of its parts.” EurekAlert!, 30 September 2026. https://www.eurekalert.org/news-releases/1146041.
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