First report of Saccocoelioides nanii (Digenea: Haploporidae) infecting an extremophile host, Jenynsia sulfurica (Cyprinodontiformes: Anablepidae).

A species of Saccocoelioides (Haploporidae) is reported from the intestine of the livebearer fish Jenynsia sulfurica (Anablepidae), which inhabits an extremophile environment in northwestern Argentina. This host species lives in a sulfide-rich thermal stream characterized by several extreme environmental conditions, including high temperatures, low oxygen levels, moderate pH, and elevated total dissolved solids. Morphological and molecular analyses indicate that the parasite under study is consistent with Saccocoelioides nanii. Morphological comparisons revealed minor variations in body size and proportions, while molecular analysis based on partial sequences of 28S rDNA, COI and ITS2 regions confirmed its genetic similarity to previously described S. nanii populations. This discovery represents the first record of S. nanii parasitizing a member of the family Anablepidae, and the first occurrence under extremophile conditions. This finding expands the known host range and geographic distribution of the species and highlights the importance of integrative morphological and molecular approaches in understanding the diversity of this group.