Fish on the Waterfall: How Parakneria thysi Overcomes the Challenge of Vertical Waterfalls

In riverine environments, many fish species need to migrate between different habitats in order to feed, reproduce, or escape unfavorable conditions. When they encounter obstacles created by changes in riverbed elevation, some fishes can use powerful bursts of acceleration to jump over relatively low barriers. If the stream gradient is gentler, they may simply swim upstream against the current and gradually reach their upstream habitats.

But what happens when fishes face waterfalls that are nearly vertical—or even completely vertical? For most species, such obstacles are virtually impassable. Nevertheless, a few fishes have been observed apparently climbing along rock faces and eventually reaching the top of waterfalls. This seemingly improbable behavior has fascinated researchers for many years.

Until recently, however, most reports of waterfall-climbing fishes were based largely on eyewitness accounts and local folklore, with little direct scientific evidence, and were therefore often regarded as anecdotal. A study published in 2026 provided the first direct evidence that the shellear, Parakneria thysi, inhabiting the Luvilombo River, is capable of climbing nearly vertical waterfalls.

The Shellear, Parakneria thysi, Its Distribution, and the Waterfalls It Encounters（Image source：Mutambala, P. K et al. (2026).， CC BY-NC-ND 4.0 。）

In fact, this remarkable ability has been mentioned repeatedly in the family Kneriidae, to which P. thysi belongs. The genera Kneria and Parakneria are primarily distributed in the high-elevation regions of southeastern Congo Basin in Africa, where rivers are characterized by numerous rapids and fast-flowing waters. Consequently, these fishes have long been considered highly adapted to torrential environments.

Previously, only species of Kneria had been reported to possess waterfall-climbing abilities. For example, the large-eared kneria, Kneria auriculata, has been observed climbing the wall of a dam in Mozambique, and some researchers even suggested that the distribution of certain highland populations may have resulted from their ability to cross waterfalls. However, these claims lacked direct evidence, and some scientists questioned whether these fishes were truly capable of overcoming waterfalls several meters high.

Nevertheless, both Kneria and Parakneria exhibit morphological adaptations to fast-flowing habitats. Their pectoral fins are held horizontally, and their ventral surfaces provide a relatively large contact area with the substrate. In some species, the pectoral and pelvic fins also possess thickened skin pads that may facilitate attachment to rock surfaces. However, whether these structures evolved specifically to enhance adhesion during climbing, or simply to reduce abrasion against rocks, remained unclear.

The discovery of P. thysi climbing nearly vertical rock faces at the Luvilombo Falls, together with direct observational evidence, therefore provided the first unequivocal proof that members of the Kneriidae are indeed capable of climbing waterfalls, rather than the phenomenon being merely a collection of scattered observations and speculation.

Researchers observed large numbers of P. thysi in the rapids of the Luvilombo River, where the fish frequently attached themselves to rocks using their paired pectoral and pelvic fins. This behavior is so conspicuous that local communities named the species using the Sanga word kulumba, meaning “to adhere,” suggesting that local people had long recognized the fish's unusual behavior.

During surveys conducted in 2009, 2018, and 2020, researchers repeatedly observed P. thysi climbing the approximately 15-meter-high Luvilombo Falls. This behavior occurred primarily at the end of the rainy season and the beginning of the dry season. The fishes generally moved upward along wet rock faces beside the waterfall where water flow was weaker, although during periods of high discharge they instead climbed along the rocky margins on either side of the falls.

Most of the climbing individuals were small to medium-sized fishes, and their numbers sometimes reached several thousand. The fishes first attached themselves to the vertical rock face using their pectoral fins and then employed their pelvic fins to increase stability. During climbing, both pairs of fins were fully expanded and pressed firmly against the substrate. Once securely attached, the fish slowly advanced upward by undulating the posterior half of the body from side to side while adjusting their direction with the pectoral and pelvic fins.

To understand how P. thysi adheres to vertical surfaces, researchers examined the pectoral and pelvic fins in detail. They discovered that the ventral surfaces of both fin pairs possess thickened skin pads densely covered with microscopic hook-like projections. These projections are longer and more densely packed on the pectoral fins than on the pelvic fins. In contrast, the ventral surface of the body lacks comparable adhesive structures, indicating that climbing depends primarily on the pectoral and pelvic fins rather than the abdomen.

Climbing Parakneria thysi and the Hook-like Projections on Its Pectoral and Pelvic Fins（Image source：Mutambala, P. K et al. (2026).， CC BY-NC-ND 4.0 。）

The researchers also found that, as body size increases, the relative area of these adhesive pads does not increase substantially, whereas body mass continues to rise rapidly. Consequently, larger individuals experience greater loads per unit attachment area, potentially reducing their climbing performance. This may explain why large individuals were rarely observed on the waterfall.

In addition to their adhesive structures, the pectoral girdle and fin morphology of P. thysi exhibit specialized adaptations to fast-flowing environments, enabling the fins to support the body more effectively and resist water currents. By comparison, species of Kneria also inhabit rapids but show fewer of these specializations, suggesting that members of Parakneria may be even better adapted to torrential habitats and rock attachment.

Climbing is not a continuous process. During ascent, the fishes frequently pause for several tens of seconds while remaining firmly attached to the rock surface. Upon reaching horizontal rock ledges within the waterfall, large numbers of individuals gather and rest before continuing upward, indicating that climbing is energetically demanding.

Based on their observations, the researchers estimated that P. thysi requires approximately 30 to 60 seconds of actual movement to climb one meter of vertical rock, corresponding to a speed of about 1.5 to 3 centimeters per second. However, this movement is interrupted by numerous resting periods. Calculations suggest that a single fish requires roughly 15 minutes of climbing time and an additional 30 minutes of resting time to reach the top of the waterfall. Furthermore, the Luvilombo Falls contain nine major horizontal rock ledges where the fishes remain for approximately one hour each. As a result, the total ascent may take nearly 9 hours and 45 minutes, implying that P. thysi may need almost an entire night to complete its remarkable climb.

A Parakneria thysi Resuming Its Ascent After Resting on a Horizontal Rock Ledge（Image source：Mutambala, P. K et al. (2026).， CC BY-NC-ND 4.0 。）

The ascent is also risky. Some individuals suddenly lose their grip and fall due to the force of the water, particularly when encountering overhanging rock faces or attempting to navigate around protruding boulders.

To explain this behavior, the researchers proposed several hypotheses. First, during years of abundant rainfall, floods create temporary lake-like habitats downstream of the waterfall, causing fishes to aggregate and initiate upstream migration. In years with lower rainfall, such behavior is far less apparent, suggesting that the migration is strongly influenced by environmental conditions.

Second, because the migrating individuals are primarily small to medium-sized fishes, the researchers interpreted this phenomenon as an example of partial migration, in which only a portion of the population participates in upstream movements. These migrants may include individuals that were swept downstream by floods as well as young fishes born downstream during the same year, both of which move upstream to recolonize rapid habitats.

Third, the behavior may also be associated with predator avoidance and reduced competition for food, since large numbers of fishes and predators congregate downstream during floods. Finally, human activities, including fishing and water diversion, may further influence the migration and distribution of these fishes.

Taken together, the climbing behavior of P. thysi is unlikely to result from a single factor and is probably driven by a combination of hydrological conditions, population structure, and ecological pressures. In years of high rainfall, floods alter downstream habitats and concentrate fishes, triggering upstream migration. This movement is dominated by small and medium-sized individuals and represents a clear example of partial migration. To accomplish their ascent, the fishes have evolved specialized pectoral and pelvic fins that allow them to adhere to rocks and slowly advance upward while coping with frequent resting periods and substantial energetic costs.

Overall, the waterfall-climbing ability of P. thysi provides a rare and fascinating example of how fishes can evolve complex behavioral and morphological adaptations to survive in extreme riverine environments.

（Author: Bai Leng）

Reference：

Mutambala, P. K., Kalumba, L. N., Cerwenka, A. F., Brecko, J., Vanden Spiegel, D., Schedel, F. D. B., Bragança, P. H. N., da Costa, L. M., Manda, B. K., Abwe, E., Mulelenu, C. M., Mathys, A., Chakona, A., Manda, A. C., & Vreven, E. J. M. W. N. (2026). Fish climbing in the upper Congo Basin (Central Africa), first report for the shellear Parakneria thysi on the Luvilombo Falls. Scientific Reports.