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Habitat and taxon-specific properties could affect the propensity for cryptic species to be formed. For example, anomurans of the genus Aegla possess characteristics that suggest the existence of cryptic diversity. The widely distributed species Aegla platensis, besides having been considered paraphyletic, shows a considerable amount of morphological variation in the carapace shape among populations. Thus, the aim of this study was to test the hypothesis that A. platensis encompasses a large complex of cryptic species. Seventeen populations of A. platensis from Argentina and Brazil were analysed using three molecular markers. Contrary to our expectations, 16 populations seem to belong to a single species. Only one population of A. platensis might represent an unrecognized new species. These results are intriguing because they do not fit the phylogeographical pattern seen in other aeglids, which usually have narrow distributions. Although intrinsic characteristics and/or historical biogeographical events could be related to these findings, the factors driving the broad distribution of A. platensis still need to be clarified. Finally, we highlight the fact that taxonomic issues in aeglids are far from being fully understood, and the use of a broad population-based sampling can be useful to improve our understanding of the group's systematics and evolution.

The antioxidant defense system (ADS) protects organisms against the potential oxidative stress induced by environmental features, underlying processes of habitat diversification. The anomurans Aegla constitute the most threatened freshwater decapods of South America, occupying pristine habitats with narrow distribution. Using phylogenetic comparative methods, we addressed: Is the variability of habitat physicochemical parameters and tissue ADS phylogenetically structured? How do environmental features correlate with ADS? How do they vary among species? Several physicochemical parameters of water, as well as metals in sediments, were measured in ten aeglid species’ habitats. Additionally, metal accumulation and ADS parameters [metallothionein-like proteins (MTLP), antioxidant capacity against peroxyl radicals (ACAP), and glutathione system (GSH-GSSG)] were evaluated in hepatopancreas. Water conductivity and pH showed phylogenetic signal, while all other physicochemical traits demonstrated plastic variability. Metals were present at natural concentrations, which are corroborated by the relative stable GSH/GSSG ratio, and by their absence of correlation with bioaccumulation levels and MTLP, both phylogenetically structured. However, metal variability across species’ niches is associated with ACAP, a potential biomarker tool. Thus, the physiological sensitivity of aeglids is environmentally driven but also  phylogenetically constrained, unraveling the importance of systematic framework for cross-species investigations and future monitoring strategies of these conspicuous freshwater animals.

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