Scientists say the turbulent waters of the lower Congo River divided one fish family so thoroughly that it split into several different species. The researchers published their findings in the journal Molecular Ecology.

One 200-mile stretch of the river has a peculiar claim to fame—it’s become a sort of evolutionary playground, boasting more than 300 different species of fish alone. “In this very short section of the Congo, we find a tremendous diversity of fishes,” co-author Melanie Stiassny of the American Museum of Natural History said in a statement.

At a mere 3 to 5 million years old, Stiassny said, this segment of the river is still relatively young. “So what is it about this system that makes it such a pump for species?”

New species are formed when an existing species is split into two populations, often by some sort of insurmountable physical barrier. Over thousands of years, the different environments and pressures faced by the two populations will be so different that they’ll evolve into two separate species.

But there are currently no major dams in this section of the Congo, nor does the river branch or trickle off into lakes. The fish are all essentially swimming in the same body of water.

Stiassny and her colleagues had a theory: The behavior of the water itself had broken fish families apart. To test their hypothesis, they collected 53 fish, all members of the genus Teleogramma, from different sections of the strange 200-mile stretch. The researchers sequenced the fishes’ DNA and compared their bodies, looking for similarities and differences.

There were plenty of differences. Within those 53 fish the researchers had representatives of all five Teleogramma species [PDF]. But some of those species were practically living on top of one another—sometimes less than a mile apart. But there was always something between them: roiling river rapids.

Alter et al. 2017. Molecular Genetics.

Lead author Elizabeth Alter, of CUNY York College and AMNH, said the rapids are working the same way a wall or a mountain might, keeping the fish populations separate. “What's particularly unique about the lower Congo is that this diversification is happening over extremely small spatial scales,” she said in the statement. “There is no other river like it.”

The fish may be facing even greater barriers in the near future, as the region has been proposed as the site of a new dam—a situation that Stiassny says would "majorly disrupt" this extraordinary ecosystem.