An integrative taxonomic characterization of extremophile fish Aphanius almiriensis (Teleostei: Cyprinodontiformes) in a hypersaline geothermal system (NW Anatolia).

The small killifish Aphanius almiriensis (Kottelat, Barbieri & Stoumboudi, 2007) inhabits from brackish springs to estuaries and coastal lagoons in the eastern Mediterranean basin. This study focused on the detailed characterization of Aphanius almiriensis from a hypersaline geothermal spring (Tuzla, Çanakkale, NW Anatolia), where extreme salinity (61.14 ppt in winter, January 2023) and temperature (42.7°C in summer, July 2023) conditions were observed. The obtained specimens of killifish in the sampling site, with a slightly convex dorsal head profile and a more angular ventral head profile, are similar in this respect to specimens from Greece. In most individuals (25 out of 27), the two frontal scales do not contact each other. Genetic analyses indicate that the species exhibits a minimum p-distance of 3.1% from Aphanius fasciatus in the mitochondrial cytochrome c oxidase subunit I (COI) region. Although previously reported, this population had not been thoroughly characterized up to date; however, based on the present study, it exhibits genetic and morphological characteristics consistent with A. almiriensis specimens from Greece.