A mysterious case involving the death of some fish led researchers to discover the deepest river on the planet: the Congo River. This is a place where raging rapids, powerful currents and even submerged “waterfalls” divide water, just as mountain ranges separate habitats on the planet.

According to research presented on December 12, 2019, at the annual meeting of the American Geophysical Union (AGU), these impractical river barriers isolate fish populations and have shaped the birth of hundreds of species. Scientists’ first glimpse into the depths of the Congo River began more than a decade ago when they spotted pale and blind fish that only appeared when they died. A death caused by decompression syndrome, in which air bubbles form in the blood and body tissues.

In an effort to understand how this was possible, the researchers found that parts of the river were several hundred meters below the surface, deeper than any river on Earth. More than 300 species of fish are found only in the lower part of the river. The rapids are so powerful that they physically separate fish populations, prompting new species to evolve even when there is not much physical distance that separates animals from their close relatives.

” In one place, we found this fish particularly strange, ” says Melanie Stiassny, curator of the Department of Ichthyology at the American Museum of Natural History in New York. ” He is a blind, depigmented cichlid. He looks a lot like a cavefish, but there are no caves in the river. When he died in my hand, bubbles formed under his skin and over his gills “, a sure sign of decompression syndrome, says the woman. This gave rise to a question that the researchers had not previously considered: could there be very deep waters in the lower part of the Congo River?

” The results we have achieved have been quite surprising: it is profound. Very profound, ” says Stiassny. The bottom of the river in the lower part of the Congo is over 200 meters below the surface. Although the mystery of the dying fish may have been solved, there is still much to discover on this river and the animals that live there. Curiously, some of the fish populations that are isolated from each other have nevertheless developed similar traits, in a process known as convergent evolution.


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