Maximal Swimming Speed Explains Differences in Habitat Use Among Invasive Ponto‐Caspian <i>Dikerogammarus</i> Species (Crustacea, Amphipoda)

ABSTRACT Gammarid amphipods are among the fastest swimming macroinvertebrates; yet, the ecological aspects of their swimming ability have received little attention so far. A previous study revealed that the three invasive Ponto‐Caspian Dikerogammarus species ( D. bispinosus , D. haemobaphes , and D. villosus ) show niche differentiation along the current velocity gradient. In this study, we aimed to link swimming speed to ecological patterns by testing whether differences in their maximal swimming speeds coincide with their tolerances for currents. Using specimens of the three species collected in the Hungarian section of the River Danube, we conducted an aquarium experiment where the animals were induced to perform an escape behavior. The maximal swimming speeds were calculated based on video recordings of the events. The maximal swimming speeds of the species matched their positions along the current velocity gradient ( D. bispinosus &gt; D. haemobaphes &gt; D. villosus ); however, interspecific differences were significant only between D. bispinosus and the other two species, and only if their body lengths were considered. A higher maximal swimming speed might give a better chance to return to the bottom if washed away by strong currents before getting eaten by predators, allowing the use of more exposed microhabitats. Although larger D. villosus specimens also have high maximal swimming speeds, they need wider crevices to hide from predators, available only under stones or larger pebbles. Our results provide a deeper understanding of the mechanisms underlying the niche differentiation of the species, which can help formulate new hypotheses on how microhabitat availability, predation pressure, or body size distribution modulate their interactions.