Beyond the Rapids: Ecological Plasticity, Habitat Connectivity, and Predatory Specialization in Hydrocynus goliath

Abstract

Hydrocynus goliath is one of the largest freshwater predatory fishes of the Congo River Basin and is characterized by pronounced piscivorous specialization, large body size, and occurrence across a heterogeneous riverine and lacustrine landscape. Despite its ecological prominence, important aspects of its habitat use, movement ecology, life history, and population dynamics remain poorly resolved. Here, we synthesize available knowledge on the geographic distribution, habitat use, hydrodynamic environment, trophic ecology, movement and connectivity, ontogenetic ecology, anthropogenic pressures, and conservation of H. goliath, with particular emphasis on the relationship between predatory specialization and environmental plasticity. The available evidence indicates that the species occupies a broad range of freshwater environments within the Congo Basin, including major rivers, rapids, deep channels, tributaries, and lakes, while retaining a strongly specialized piscivorous trophic niche. Phylogeographic evidence further indicates that the distribution and diversification of Hydrocynus have been influenced by historical drainage connectivity and geological reconfiguration. However, broad geographic occurrence should not be interpreted as direct evidence of individual-level ecological plasticity. Species-specific information on contemporary movement, habitat selection, seasonal displacement, reproduction, recruitment, and ontogenetic habitat shifts remains limited. We therefore distinguish environmental breadth at the species level from demonstrated behavioral or physiological plasticity at the individual level. Hydrocynus goliath is best regarded as a highly specialized piscivore occupying an environmentally heterogeneous freshwater system, with substantial unresolved variation in movement and habitat use. Future research integrating telemetry, population genomics, trophic analyses, and seasonal ecological sampling is needed to determine the mechanisms underlying its persistence across the Congo River continuum.