Goliath tiger fish, such as the one seen above, are among the uniquely adapted "monster fish" of the Congo River, which winds through several African countries.
A recent, unprecedented river run on the Congo yielded a raft of new discoveries, including different species--some potentially new--in nearly every nook and cranny, scientists announced this week.
The river was also found to be possibly the world's deepest, and its extraordinary changes in depths and currents help explain why it's such a hotbed of fish diversity.
"What we're seeing here is kind of evolution on steroids," said team leader Melanie Stiassny, a fish biologist at the American Museum of Natural History. Stiassny, a member of the National Geographic Society's Conservation Trust, was among the marine and evolutionary biologists, hydrologists, and kayakers who conducted the exhaustive research in summer 2008. (The National Geographic Society owns National Geographic News.)
Despite calmer stretches like the one photographed above in summer 2008, the Congo is the world's highest-volume white-water river, according to Trip Jennings, a National Geographic Expeditions Council grantee.
As part of a 2008 expedition, Jennings and his team of kayakers navigated the churning river's powerful whirlpools.
"The six of us were tossed about like pool toys in a hurricane," Jennings said. "Giant whirlpools threatening to suck us under spawned without warning, and oceanlike breaking waves crashed on us from all sides."
The kayakers started in the Democratic Republic of the Congo's capital city of Kinshasa and ended 85 miles (137 kilometers) downstream in Pioka--marking the first time this stretch of the river had been run successfully.
Their boats were equipped with echo sounders to collect data on the river's depth, revealing that the Congo may be the world's deepest river--one section reached a record 755 feet (230 meters).
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.
The Goliath tiger fish, said to grow up to 150 pounds (68 kilograms), may be able to overcome barriers in the tumultuous and turbid Congo River that smaller fish cannot. During a 2008 expedition on the Congo, scientists looked for the "monster" fish upstream and down, to see how far it could travel.
The DNA samples the team collected will help determine if tiger fish found in different parts of the river have morphed into separate species.
The Congo's fish diversity thrilled the scientists. "What we're talking about here is like a species factory--it's a species pump," team leader Melanie Stiassny told National Geographic Television & Film.
"If Darwin had come here, I think we would have had the theory of evolution published a lot sooner."
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