For more than 50 years, people have told strange tales of a tiny fish called the shellear (Parakneria thysi) climbing up waterfalls. Now researchers say they have filmed the behaviour for the first time. Their findings are published in Scientific Reports.
Pacifique Kiwele Mutambala, a researcher at the University of Lubumbashi, and Emmanuel Vreven, an ichthyologist at the Royal Museum for Central Africa in Belgium, and colleagues recorded thousands of the little fish climbing the Luvilombo Falls in the Upemba National Park, Democratic Republic of the Congo in 2018 and 2020.
“The ability of shellears to climb waterfalls has been known anecdotally for over 50 years,” write the authors in the study. “The behaviour is documented here for the first time, with supporting cinematographic and photographic evidence.”
The fish went chasing waterfalls at the end of the rainy season (April to May). They used their pectoral fins, with support from their pelvic fins, to haul themselves up the vertical wall in the splash zone during years of high rainfall.
The fish’s fins have “tiny unicellular hook-like projections” underneath to help them grip onto the wall. “They propel themselves vertically by lateral movements of the posterior part of the body as when swimming,” write the authors. That is, they wiggled their bottoms.
Measuring under five centimetres long, the shellears weren’t moving fast – it took them around 10 hours to climb the 15m waterfall.
As you might imagine, the tiny fish took plenty of breaks during this arduous journey: pausing for less than a minute between certain sections of their upward climb, gripping onto the vertical rock wall, or taking a longer rest when they reached a horizontal ledge. Here, they would usually stop for between 2 and 15 minutes.
“Consequently, large numbers of fish congregate on those ledges before engaging the next climbing phase,” write the authors. “This seems to indicate that these upward movements require a lot of energy and, as observed, the specimens therefore need longer resting periods to recover from the effort.”
On average, a fish can take around nine hours and 45 minutes to scale the waterfall, including 15 minutes of climbing, 30 minutes of short pauses and 9 1-hour rests.
Despite taking so many breaks, the ascent is precarious. “Indeed, some specimens fall abruptly, for example when they are suddenly hit by a jet of water,” write the authors. “Such a fall is also more likely when the fish move upside down when they try to circumvent an overhanging cliff.”
So, why do the animals take this risk? The authors suggest the fish might be trying to return to habitats upstream after floods pushed them downstream, or to avoid food competition or predators.
Although bizarre, this climbing behaviour has previously been observed in other fish species, such as catfish (Rhyacoglanis paranensis), but it isn’t well documented.
The Luvilombo River is impacted by human threats, such as illegal fishing using mosquito nets (with a mesh size small enough to easily catch these little fish) and diverting the water flow to irrigate crops during the dry season. Although this only happens occasionally, it “undermines the ecological continuity of the river,” write the authors, and is a significant threat.
The scientists hope their study highlights the importance of protecting this ecosystem, saying: “This could be made possible by paying more focused attention to these falls and identifying them as a natural monument and/or ecosystem of national interest.”
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