Freshwater crustaceans of the genus Aegla are endemic to southern South America, and present greater richness in the river basins of the state of Rio Grande do Sul, Brazil. However, recent studies have shown the enormous potential for discovering new species of Aegla in the state of Paraná. In the last six year the number of species recorded has almost doubled, increasing from 9 to 16. In the present work, we expand the current knowledge about the diversity of Aegla in a set of poorly explored drainage basins in the western region of the state and describe a new species, Aegla urussanga n. sp., based on morphological and molecular evidence. A combination of morphological characters supports the new species: orbital spine absent, non-elevated epigastric prominences, robust protogastric lobes with scales, anterior dorsal margin of epimeron 2 armed and with pronounced concavity, sub-rectangular palmar crest of chelipeds, internal margin of the ventral face of the ischium of the cheliped with a proximal stout spine, a distal stout spine, and up to three tubercles with spiniform scales. The molecular data, based on partial COI sequences, also support the distinction of the new species from others that occur in adjacent river basins. The species with the smallest genetic distance from A. urussanga n. sp. is Aegla castro Schmitt, 1942 which occurs in the Iguaçu River basin. The new species occurs in distinct streams, probably interconnected in the past and currently isolated by the Itaipu reservoir and is the second species of Aegla described from the complex of drainage basins called Paraná 3 basin.

Anomuran crabs of the genus Aegla Leach, 1821 are endemic to temperate and subtropical regions of South America and are commonly found in streams and rivers, where they are generally restricted to healthy environments because they are sensitive to environmental changes. The genus presents worrying data regarding its conservation, like the many Brazilian freshwater species classified as being at some level of threat of extinction. We inventoried unexplored watersheds, estimated the population density of Aegla jacutinga Marçal & Teixeira, 2020 in its type locality, updated information on the distribution of the species, and assessed its conservation status using IUCN criteria.

The decapod family Trichodactylidae is widely distributed in river systems of South America. However, the first record of Sylviocarcinus australis in the state of Paraná, Brazil, is reported herein, suggesting a possible human introduction, or lack of previous records, for a species whose known occurrences to date are in the Paraguay-middle Paraná River basins. The presence of S. australis in the upper Paraná River basin raises questions about its origin, extension of its occurrence and any possible ecological impacts. As this is a single record of adult males, with no females or juveniles recorded so far in the sampled area, it is not possible to conclusively trace the effective establishment of the species in this area. However, the proximity of the area to Iguaçu National Park, an important conservation area, highlights the possible presence of the species in a sensitive conservation area, with an imminent need to monitor local biodiversity. Examples of introductions of other species of freshwater crabs have shown that there are problems in coexistence with local, native species, and so understanding the potential ecological impacts of this species is crucial to ensuring the preservation of these local species.

Historically, Aegla schmitti has been considered one of the most widely distributed aeglid species in Brazil and classified as “Least Concern” according to the IUCN criteria. Given that recent studies have shown that many aeglids possess restricted distributions, some limited to their type localities, we investigated the taxonomic status of A. schmitti using historical DNA (hDNA) extracted from paratypes. We compared these sequences with historical sequences deposited in GenBank derived from distinct river basins (Ribeira do Iguape and Iguaçu), which have been systematically used as the standard molecular reference for the species by various authors. Phylogenetic via Bayesian inference revealed that the paratypes represent a distinct, monophyletic lineage (Aegla schmitti sensu stricto), whereas sequences historically deposited in GenBank under this name belong to distantly related clades because of taxonomic misidentification. Crucially, the sequences previously attributed to this species represent three distinct, undescribed candidate cryptic species. These results highlight the need for caution when working with groups that potentially form species complexes and when using sequences deposited in GenBank. Furthermore, the conservation status of A. schmitti requires re-evaluation, as the present existence of the topotypic population remains a testable hypothesis requiring urgent field surveys; moreover, the populations currently attributed to A. schmitti, and now recognized as newly discovered cryptic lineages, might be restricted to smaller drainage areas, highlighting the need for extensive field surveys to assess their actual distribution and potential vulnerability to local anthropogenic pressures.

The genus Aegla, comprising 105 known species, showcases remarkable adaptation and diversity in freshwater ecosystems. This study documents the aeglid fauna of the Itararé River basin and describes a new species, thereby expanding the known diversity of the genus. The inventory of the basin indicates the occurrence of at least two species and reveals areas of apparent absence, which may be associated with environmental degradation related to agricultural activities in the lower portions of the basin. The new species was compared with all described congeners based on diagnostic morphological characters and examined in detail using morphological and molecular data in comparison with nine species from adjacent basins, confirming its distinct status. It is genetically closest to Aegla lancinhas, from which it can be distinguished morphologically based on characters related to the extent of the anterolateral spine and the dorsal margin of the cheliped dactylus.