Abstract
We describe one new eyeless Hyalella species, H. cuyana n. sp., from the hyporheic zone of Calingasta River, San Juan Province, Argentina. Hyalella cuyana n. sp. differs morphologically from its stygobiont congeners by characters of antennae (both antennae subequal in size; aesthetascs elongated); coxal plate 4 (posteriorly without excavation); epimeral plates 1-3 (rounded posterodistally); male gnathopod 2 propodus (almond-shaped, ovate, palm slope strongly oblique); and pleopods (inner ramus reduced to 2-3 articles). We provide a detailed morphological comparison between stygobiont and stygophile Hyalella species. We comment on the sympatry and new report of stygobiont Malacostraca species Parastygocaris andina and Cuyojanira sp. Severe environmental changes and significant decline of habitat quality recorded along the Calingasta River increase the risk of extinction for this vulnerable and endemic fauna. We recommend that the new Hyalella species should be listed as Critically Endangered (CR) in the IUCN categorization according to criteria B1a as it is only known from the type locality; and criteria B1b iii (continuing decline, observed, inferred in the quality of habitat).
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