ORIGINAL ARTICLE Morphological variation of Transcaucasian chub, Squalius turcicus in southern Caspian Sea basin using geometric morphometric technique Atta Mouludi-Saleh 1 & Soheil Eagderi 1 & Erdogan Cicek 2 & Sevil Sungur 3 Received: 23 May 2019 /Accepted: 22 December 2019 # Institute of Zoology, Slovak Academy of Sciences 2020 Abstract This work was conducted to study the patterns of morphological differences in body shape in Squalius turcicus De Filippi, 1865 from the most east (Atrak River) to the most west (Aras River) populations of the southern Caspian Sea basin using landmark- based geometric morphometric method to understand how the body shape alters in relation to geographical alternations or environmental characters of their habitats. A total of 102 specimens were collected from the Aras, Tajan, Atrak, Ghareh-Su, Khyer and Sefid rivers and after anesthesia, their left faces were photographed. To extract shape data, 14 homologous landmark- points were digitized and data after generalized procrustes analysis, analysed using principal component analysis, canonical variate analysis, cluster analysis, and Mantel test. The results showed significant differences between the body shape of six studied populations, mostly related to snout, base of the dorsal and anal-fins, body depth and pectoral fin position as result of phenotypic plasticity to the environmental features of their habitats. The Tajan and Atrak populations have a higher and lower body depth, and the Kheyr population a longer snout. Keywords Landmark . Morphometrics . Morphological difference . Freshwater Introduction Analysis of the morphological variation is commonly used to delimit population of fishes (Cadrin and Silva 2005). Fishes are well-known vertebrates with high abil- ity to adapt to various aquatic habitats. Morphological plasticity in response to environmental variability is generally found in many fishes, predominantly in fresh- water species (Stearns 1983; Scheiner 1993; Gelsvartas 2005; Mouludi-Saleh et al. 2018a). Hence, morphologi- cal adaptations to environmental conditions along with geographical isolation can provide a crucial information on their evolutionary trend (Smith and Skúlason 1996). The genus Squalius Bonaparte, 1837 comprises medium-sized midwater fishes widely distributed in Europe and West Asia (Vatandoust et al. 2016; Coad 2018 ). The genus Squalius includes four species, S. berak Heckel, 1843, S. lepidus Heckel, 1843, S. namak Khaefi, Esmaeili, Sayyadzadeh, Geiger & Freyhof, 2016 and S. turcicus De Filippi, 1865, reported from Iranian inland waters (Esmaeili et al. 2018 ). Transcaucasian chub, S. turcicus , is found in the Caspian Sea and Urmia Lake basins, living in the mid- dle and upper courses of rivers (Mouludi-Saleh et al. 2018b; Esmaeili et al. 2018). The present work aimed to study the morphological variations of S. turcicus from the most east (Atrak River) to the most west (Aras River) populations in the Caspian Sea basin. Geometric morphometric tech- nique was used to address the question if the body shape of this species follows a morphological pattern based on geographical alternations to consider it as a cline or as an adaptation to environmental characters of its habitats. * Soheil Eagderi soheil.eagderi@ut.ac.ir 1 Department of Fisheries, Faculty of Natural Resources, University of Tehran, Karaj, Iran 2 Department of Biology, Faculty of Art and Science, Nevsehir Hacı Bektas Veli University, Nevsehir, Turkey 3 Vocational School of Health Services, Nevsehir Hacı Bektas Veli University, Nevsehir, Turkey Biologia https://doi.org/10.2478/s11756-019-00409-6