<p>Aeglids are among the most threatened freshwater crustaceans worldwide, restricted to South American watersheds and largely confined to lotic environments. Occurrences in artificial canals have not yet been documented. Here, we investigated two sympatric species, <i>Aegla jarai</i> and <i>Aegla muelleri</i>, in Espingarda Creek, a natural lotic stream, and in an adjacent artificial canal. We assessed population structure, abundance and spatial relationships between the species through monthly sampling from June 2001 to July 2002. We hypothesised that their distinct microhabitat preferences would translate into measurable differences in population parameters between the natural stream and the artificial canal. In Espingarda Creek, <i>A. jarai</i> was consistently more abundant and reached larger sizes than <i>A. muelleri</i>. In the canal, only <i>A. jarai</i> appeared regularly, spanning all ontogenetic stages and attaining larger sizes than conspecifics from the creek. Size–frequency distributions of both species were bimodal, and modal comparisons supported competitive asymmetry under sympatry. The contrasting size structure of <i>A. jarai</i> in the absence of congeners reinforces this hypothesis. Our findings suggest that the apparent dominant competitor in natural streams can also be a successful coloniser in artificial habitats. This pattern does not strictly conform to the classical expectation of a strict competition–colonisation trade‑off and is consistent with extensions of the competition–colonisation theory and with ecological plasticity in <i>A. jarai</i>. Overall, this study provides novel insights into coexistence mechanisms in freshwater decapods and highlights the importance of incorporating artificial habitats into conservation strategies for threatened aquatic invertebrates.</p>