Deep in the upper Congo Basin, thousands of tiny fish are doing something scientists never expected to see on the African continent: scaling a vertical waterfall taller than a four-story building.
A new study published in Scientific Reports documents how Parakneria thysi, a species of shellear fish barely two inches long, climbs the 15-meter (50-foot) Luvilombo Falls in the Democratic Republic of Congo. The behavior, observed during seasonal floods at the end of the rainy season, represents the first confirmed case of waterfall-climbing fish in Africa.
A Slow, Determined Ascent
The fish don't leap or swim up the falls. Instead, they cling to wet rock faces in the splash zone — areas kept moist by spray rather than direct water flow — and inch their way upward using specialized fins and tiny hook-like projections on their bodies. Researchers estimate a single fish may take nearly 10 hours to complete the demanding climb, moving in short bursts and resting frequently between efforts.
Only small to medium-sized individuals, roughly 3.7 to 4.8 centimeters long, were observed making the journey. Larger members of the species appeared too heavy for their fins to support the climb.
Decades of Hidden Behavior
Lead researcher Pacifique Kiwele, an ichthyologist at the Université de Lubumbashi, documented the behavior across four separate observations in 2018 and 2020. "Who would have believed it without being close enough to check, and document it with photographic and film material, that indeed some fish are able to climb waterfalls?" Kiwele said.
While waterfall-climbing fish have been documented in other parts of the world — including gobies in Hawaii and cave fish in Thailand — the Congo discovery is significant because it shows this remarkable adaptation evolved independently in Africa. The fish use a different climbing mechanism than their counterparts elsewhere, relying on their pectoral fins and body hooks rather than suction-cup-like mouth parts.
A Call for Conservation
The discovery also highlights the urgent need to protect the Congo Basin's freshwater ecosystems. Illegal fishing nets stretched across the falls and increasing water diversion for irrigation threaten to disrupt the migration route these fish have likely used for thousands of years.
"This discovery highlights the importance of maintaining the continuity of watercourses, particularly in the context of the Congo Basin, where studies on fish behaviour are virtually nonexistent," Kiwele said. "It illustrates that there are wonders out there that surpass our imagination."
The Congo River system, the world's second-largest by discharge volume, remains one of the least-studied major river systems on Earth. Scientists say the climbing fish are likely just one of many remarkable adaptations waiting to be discovered in its waters.
