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1. Although freshwater ecosystems contain high biodiversity and provide valuable ecosystem services, they are severely threatened worldwide. Many broad-scale approaches to conserve freshwater environments have been proposed, but as headwaters tend to host higher levels of biodiversity, identifying priority areas for conservation at a local scale is essential.2. An integrative three-step framework is proposed to recognize such areas, using anomuran crabs of the family Aeglidae as biodiversity indicator taxa, within four freshwater ecoregions in South America. First, the knowledge on the distribution of species occurring within the study area was updated and the species conservation status assessed. Second, species distribution modelling was used to identify areas of high richness and endemism. Third, these areas were compared with the existing protected area network, in addition to land-use and urban information, to indicate priority areas for conservation within four freshwater ecoregions (Laguna dos Patos, Upper Uruguay, Lower Uruguay, Tramandaí and Mampituba) studied at a local scale.3. Analysis of the distribution of species of the genus Aegla showed that the present protected area network poorly covers freshwater habitats within the study region.Three priority areas for conservation were identified, with high levels of endemism and richness of aeglid species. These areas are mainly located close to current protected areas, and we suggest, therefore, that the enlargement of some protected areas, through the establishment of corridors, would better protect several species with distributions restricted to headwaters, such as macroinvertebrates and fishes, some of which are threatened with extinction.

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Species with similar niches may exhibit adaptations to diminish competitive pressure and allow sympatry; freshwater decapods are interesting models for the investigations of these strategies. We studied the behavior of two co-occurring decapod species: the anomuran Aegla longirostri and the crayfish Parastacus pilimanus, and investigated the following: (1) whether the species use the stream similarly; (2) if they present zonation within it; (3) if they have different activity levels; and (4) whether their activity patterns overlap. For this, we radiotracked individuals of both species in their natural environment. Aeglids had larger home ranges than crayfish when considering only the stream. However, crayfish remained in burrows more than in the stream, and the total home ranges did not differ significantly from aeglids. Stream sections used by both species highly overlap, indicating no zonation. Aeglids used the stream more often and more intensively than crayfish, but their diel activity patterns were similar. Despite the lack of zonation in the stream and of temporal segregation, there seems to be some degree of spatial segregation. Differential habitat use seems to be the one of the mechanisms allowing crayfish to coexist with aeglids.

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