Evolution on Steroids: Bizarre Fish of the Congo Featured in New Video
Tuesday, December 22 10:23 am
Africa’s Lower Congo River is home not only to the world’s most extreme rapids but also to an extremely diverse assortment of fish. At last count, 320 species—some with bizarre features like long snouts, tiny eyes, and colorless skin—swim in the river’s 350 kilometers of roiling brown water. “I call it evolution on steroids,” says Melanie Stiassny, curator of ichthyology at the American Museum of Natural History. (You can hear Stiassny discuss her team’s adventures and discoveries in the Congo River at the January 6 SciCafe, the Museum’s new after-hours series).
What’s driving the rapid fish evolution in Lower Congo? The Congo Project, an ongoing research effort led by Stiassny and Museum ichthyologist Bob Schelly, is converging on an answer: the river’s turbulence itself. Their work is featured in a new production by Science Bulletins, the Museum’s high-definition current-science video program.
Click to Play Video
After several field seasons identifying new species, Stiassny realized her team was only scratching the surface of the powerful forces that can steer fish evolution. “We hadn’t been looking at the river the way a fish would look at the river,” she says. “We didn’t really know anything about what was below the surface.” So in a recent field season, the fish scientists teamed with water scientists as well as with world-class kayakers to make a historic riverbed survey. The researchers rigged one of the kayaks with a GPS logger and an echo sounder, a device that directs pulses of sound waves toward the river bottom to measure its depth.
The kayakers battled whitewater at the river’s head for 130 kilometers over five days, taking depth readings of the river bottom all the while. The data was astounding. “We couldn’t believe it,” says Stiassny. “It’s almost like you’re boating over a mountain range. There are huge peaks and enormous troughs. This is the deepest river in the world, there’s no question about it.” In one stretch, the echo sounder registered depths of 220 meters—much deeper than the previous record-holder, the Yangtze River.
The Lower Congo’s extraordinary riverbed topography results its extreme rapids, whirlpools, and underwater waterfalls. Stiassny and Shelly’s data are revealing that these water movements act like brick walls to fish: they separate populations so they can’t breed together, which ramps up fish evolution.
One piece of evidence comes from a high-velocity spot just downstream from where the kayakers had paddled. At the calmer edges of the river, Stiassny and her team sampled two groups of a slender species called Teleogramma brichardi. One group was netted from one side and the other from the opposite side. Then Stiassny and Schelly scrutinized the fish back at the Congo Project’s genetics facilities at the Museum. Although the two groups looked nearly identical to each other, at the genetic level the differences were astonishing. Five percent of their DNA units differed. (The DNA of humans and chimpanzees differs by only 1.2 percent.)
This degree of genetic separation indicates that mutations occurring in fish near one riverbank are not being passed to fish near the other bank. That’s a strong clue that the two populations aren’t interbreeding, likely because they can’t cross the powerful current in the center of the channel. Thus, each group is adapting on its own, and evolving in different directions. T. brichardi is well on its way to splitting into two species.
Presumably, the same mechanisms have been at work among the myriad other fish that swim in the Lower Congo for tens of thousands of years. It’s left a legacy of biodiversity that continues to yield surprises for the scientists of the Congo Project—a wild evolutionary ride that shows no signs of slowing.
To read the full version of this article, visit the Science Bulletins story Evolution in Action and scroll to the bottom of the page. To see the HD video on the big screen, visit AMNH’s Hall of Biodiversity, where it will be showing through June 2010.
