Genetic divergences among populations of Aegla longirostri suggest that this group may be formed by cryptic species. In this study, we analysed the carapace shape of six populations of A. longirostri from southern Brazil, using a geometric morphometric approach, to better elucidate the species' systematics. Our results were congruent with the genetic data, meaning that genetically differentiated populations were also morphogeometrically differentiated. The geometric morphometric descriptors showed significant differences in carapace shape among all populations. Geographical distance and isolation may be affecting the gene flow between populations, leading to divergence in both genetic and morphometric traits. Our data indicate the possible existence of cryptic species within A. longirostri, or an incomplete process of speciation. Geometric morphometric methods were efficient in differentiating among populations that do not show divergence in diagnostic characters.

Aegla platensis is a freshwater crustacean species that forms a nonmonophyletic group, which may indicate that the taxon is composed of cryptic species. Our objective was to test if allopatric populations of A. platensis differ in carapace shape, aiming to clarify the species situation. We used a geometric morphometric approach to analyze the carapace shape of 11 populations of A. platensis, four from Argentina and seven from Brazil. Our results showed that all the populations differ significantly in carapace shape, except two populations from Argentina and two from Brazil. The Argentinean and Brazilian populations formed two separate groups and were separated by high values of shape distance. This suggests the existence of reproductive isolation between the populations from the two groups. Shape distances between populations within each country were lower. Allopatric populations of A. platensis may be isolated from each other by the difficulty of migration, leading to interruption of gene flow and differentiation in carapace shape.

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