"These are elephant fish," Stiassny says. "Their jaws are at the end of their snouts so they can pick food from the gravel."
The evolutionary adaptations are apparent. Each individual was caught in a different location, and each snout is specialized to the character of the river floor in which it fed. Long and thin snouts allow fish to probe for food in deep and small-grained gravel; short and fat snouts allow them to feed on algae-caked bedrock. "Darwin's fishes," Stiassny says.
A series of mud-colored minnows caught in different locations that look identical to me excite Stiassny. "That's really where we see evolution in action," Stiassny says. "In 50 or 100 years, the fish that look the same today may well look different. We can see the start of that genetic drift."
That night, Gardiner plugs a data card in his laptop. Winged insects flock to the glowing screen, their buzzing mostly drowned out by the steady drone of the river and the occasional whoosh of its surge breaking on the beach. The computer hums while processing data. Eventually Gardiner pulls up a graph profiling the river's bed. It looks like a U—as smooth as a mountain valley carved by a glacier. The current just beneath the surface is traveling at 30 miles per hour, and the channel is 640 feet deep.
"That's the deepest point measured on a river in the world," Gardiner says. "There's no question about that."
Shelton is peering over Gardiner's shoulder, shaking his head and deciphering blue and red lines on the computer screen that represent water movement and velocity.
"Just like we thought," he says. "Fabulous stuff." He nudges a moth off the screen and points to a place in the riverbed where a long blue line indicates the current dropping vertically from a ledge into the canyon's trough.
"It's an underwater waterfall," he says, slapping Gardiner's shoulder. It's falling at 40 feet per second. Upstream of the waterfall is an eddy, the water relatively still. This point is likely habitat for the blind cichlid: calm pockets where sheering currents have trapped the fish at great depths. Deep-river specimens, like the one found today, surface only when the river surges and flushes individuals into the harsh environment of the main flow. In terms of Stiassny's hypothesis, the finding suggests that the Congo's currents partition habitat from side to side and from top to bottom—just like a mountain range.
"It shows water can be an evolutionary barrier, even for fish," Gardiner says.