Geometric morphometric techniques were applied for a better comprehension of inter- and intra-specific morphological variability of freshwater aeglid crabs. Carapace morphological patterns were used to address hypothesis regarding 1) the simple existence of local adaptations or 2) actual stable evolutionary features within the lineages studied. Two clades were included in this analysis: the former encompassing the closely related species Aegla castro, A. parana, A. schmitti, and the latter including the closely-related species A. ligulata, A. longirostri and A. inconspicua. Overall, distinct carapace shapes were found not only between species but also among different populations of same species. In some cases, species belonging to distinct, distantly related clades were more similar in carapace morphology than to closely related species of the same clade. This meant that there was no stable carapace morphology pattern for each major lineage. Results suggest that carapace of these crabs is plastic within lineages, although has a stable, unchangeable component readily recognizable by the geometric morphometric analysis at the species level, plus a more plastic component that may change according to the environment in which they inhabit.