The Congo's freshwater elephant fish, such as the one seen above, use their long snouts to sift through river bottom sediments for food--another example of the Congo as a hotbed of evolution.
The fast currents and raucous rapids of the lower river create physical barriers that smaller fish cannot cross, said fish biologist Melanie Stiassny, who led a 2008 expedition on the Congo. At some points, the river flows more than a million cubic feet (28,000 cubic meters) a second--enough to fill more than 800 Olympic-size swimming pools every minute.
Such barriers isolate fish populations, and over time they become more and more distinct from one another, eventually evolving into new species.
During a survey of Congo fish, Stiassny found seemingly different species just a few hundred meters apart. Some had developed flatter bodies to avoid being pulled by the current, for example. Likewise, living in a low-visibility, sediment-filled stretch of the river, the African electric catfish had evolved the ability to stun nearby prey with up to 350 volts of electricity.
Stiassny and her colleagues took DNA samples, which should allow them to determine how many new and different species they have found in the Congo River.
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Photograph courtesy Todd Wendel/National Geographic Television