Earth’s deepest river conceals an evolutionary mystery

In 2007, fishermen living along the lower Congo, the deepest river on Earth, brought Melanie Stiassny a fish. It was 6 inches long, ghostly white and eyeless. Like most fish held out of water, it was dying. What surprised her was what was killing it.

“There were nitrogen bubbles forming under its skin and gills,” said Stiassny, an ichthyologist with the American Museum of Natural History in New York. It was a clear sign of decompression sickness, an often-fatal ailment that results when animals are rapidly depressurised. In humans, the buildup of nitrogen bubbles in the blood during a rapid ascent from deep water is called the bends.

“I thought, could this thing really be dying of the bends?” she said. “And if that was the case, how deep is the water here?”

The Congo River runs for 2,500 lazy miles through Africa’s equatorial basin, coiling like a snake through the jungles of the Democratic Republic of Congo. Then, 186 miles from the ocean, it drops into a dangerous series of gorges, sinking 12 feet every mile. The swirling waters of the Kongo Central province, bordering the Republic of Congo and Angola, contain 30% of the river’s fish diversity and have long attracted ichthyologists.

Stiassny works with The Congo Project, a partnership among the University of Marien Ngouabi in the Republic of Congo, University of Kinshasa, in the DRC, and the AMNH that studies the unique fish and environment of the lower river. Their research, presented in January at the American Geophysical Union, has revealed a river landscape more bizarre than previously imagined. Its hostile currents and depths of more than 700 feet are an incredible natural laboratory for studying convergent evolution, or how diverse species develop similar environmental adaptations.

The initial clue to the river’s depths came from the dying fish. According to Victor Mamonekene, an ichthyologist with the University of Marien Ngouabi, the fish — known locally as mondeli bureau, or “white man in an office” — had long been known to the residents of Bulu village, where the scientists first saw it. The species was originally described in 1976 as a cichlid, part of the family that includes tilapia, bluegill and peacock bass. But the animal is quite different from its relatives: very small, eyeless, and pale as a cave fish.

To find out where a potential deep water fish might have come from, Stiassny said, the Congo Project partnered with the National Geographic Society and the US Geological Survey. In 2008 and 2009, they dispatched a team of “crazy whitewater kayaker dudes,” she said, to run the rapids during low-water season. Their kayaks carried echo-sounders and depth measurement equipment.

The river gorge is inaccessible and sparsely populated. Its rapids carry five times the amount of water of the Mississippi River down a channel less than a mile wide, and it is practically nonnavigable by boat.

“The strong current that reigns there makes it a very dangerous area for working on water,” Mamonekene said.

Luckily, all of the kayakers returned in one piece. The data gathered by the whitewater team revealed a harsh underwater landscape: huge towers of rock, stripped bare of sediments and plants by fast currents that run both upstream and downstream.

“Almost as if you’ve got two rivers in the same channel,” Stiassny said.

The complexity and power of the currents are the key to the evolutionary richness of the lower Congo, she said. This hostile environment keeps breeding populations of fish separated in isolated pockets of water, and resulted in the evolution of new species.

“We’d get populations diverging on either side of a rapid,” Stiassny said. “Maybe they’re only separated by a kilometre, yet it’s as if they’re 50,000 miles apart.”

Interspersed with shallow rapids are canyons deep enough that the team had to use measuring equipment designed for the deep ocean to study them. Parts of the Congo River measured around 720 feet deep, almost as far down as the twilight zone of the ocean. Immense vortices of water plunge up and down the canyon walls like underwater waterfalls.

Stiassny’s team suspects that the mondeli bureau live in these deep water canyons, and are occasionally plucked up by the currents that churn along the rock walls. They are rocketed hundreds of feet up to the surface, the nitrogen bubbling out of their blood. The same bends that kill them make their diets a mystery, bursting their swim bladders and pushing out the contents of their guts.

Still, the team’s research suggests that the deep waters are nutrient poor: Mondeli bureau have lost a gene that suppresses appetite, suggesting that the species binges when it can. “Food is probably sporadically being brought down into some of these currents, and when it is these guys just eat like mad,” Stiassny said.

The mondeli bureau is one of six fish in the lower Congo to develop elongated forms, lose their pigments and shrink their eyes. The others — from families as diverse as catfish, elephant fish and spiny eels — are found in the shallower waters along the riverbanks. That suggests that the mondeli bureau’s strange adaptations may not be dictated just by living in the depths, but by the currents of the lower Congo itself.

Unrelated species sometimes arrive at remarkably similar anatomies through a process called convergent evolution. Many species of fish and salamanders have independently slowed their metabolisms and lost their eyes to survive in deep caves.

But while such convergence has long been recognised as a phenomenon, researchers are still trying to discover more about how it works in shaping animals. That’s why this project is significant, according to Kirk Winemiller of Texas A&M University, an evolutionary biologist who was not involved in the research.

“It seems like there are certain optimal solutions to problems,” he said. “Melanie’s work shows multiple species adapting to an extreme environment with fairly predictable patterns.”

The next step, Stiassny said, is to find the fish elsewhere in Congo most closely related to the mondeli bureau, so they can compare them with their more anatomically specialised cousins. They’re also investigating the genetics and physiology of other blind fish in the river, to work out whether they’re using similar genetic mutations to adapt to the river currents.

“There are many questions,” Mamonekene said. “Almost everything remains to be discovered about these species, hence the need to continue this research.”

© 2020 New York Times News Service