A few years ago, Melanie Stiassny, an ichthyologist at the American Museum of Natural History, plucked a pale, blind, dying fish out of the lower Congo River in Africa.
It wasn’t the first time such a fish had been found floating on the surface, although they are rare.
“It looks very much like a cave fish,” she says of them in general, “but there are no caves. And the weird thing about this fish is we only ever find it dead or dying.”
Then she noticed something new. “As it died in my hand, bubbles formed under its skin and over its gills,” she told the annual meeting of the American Geophysical Union.
These bubbles could mean only one thing. “This animal was dying from catastrophic decompression syndrome” — the fish equivalent of the diver’s bends, a potentially deadly condition that occurs if you rise too rapidly from the depths.
“This fish dying in my hand asked the question, ‘Could there be deep water here?’” she says.
To find out, she teamed up with whitewater kayakers for an exploration that not only discovered the deepest river waters yet found on Earth, but, in the process, helped figure out why the lower Congo has some of the most diverse fish populations on the globe.
There are more than 300 known species along a few hundred kilometeres of river downstream of Kinshasa, the capital of the Democratic Republic of the Congo.
“The lower Congo is truly a hotspot of biological diversity,” she says.
It’s also known for a succession of enormous rapids strung out along 130 kilometres, where the river — second only to the Amazon in the amount of water it carries — narrows and plunges as it begins its final run to the sea.
Some of these rapids, Stiassny says, are “truly incredible”, and in combination, they form “the largest stretch of white water on our planet”.
To map them, she began by having the kayakers instrument their boats with echo sounders.
She also gave them acoustic Doppler current profilers, so they could plot variations in its current. That way, she says, “we get an idea what the water looks like if you’re a fish”.
The first discovery was that the river wasn’t just deep. In places, it’s very deep. In one spot, she measured a depth of 160 metres. In another, it was substantially in excess of 210 metres — more than deep enough for the dark, cave-like waters in which pale, blind fish are normally found.
But that wasn’t all. There were also patterns of strong ascending and descending currents, as well as complex eddies above and below the rapids.
Some of these currents were upwellings strong enough to serve as death traps for bottom-dwelling fish like the one that died in her hands. But that wasn’t all: these currents, she says, are also barriers that very effectively separate fish populations in one part of the river from others, close by.
“They are dividing up populations and preventing them from interbreeding at a very fine geographical scale,” she says. “So you can see why there’s a lot of [diversity].”
All of that, she adds, makes the Lower Congo a great natural laboratory for studying how species evolve, especially how ones in similar niches sometimes wind up with similar traits — a process known as convergent evolution.