Round goby Neogobius melanostomus is one of the three invasive Ponto-Caspian gobies found in the Sava River basin in Croatia. It has a negative effect on the native ecosystems due to its high invasive potential that enables its rapid spread upstream in inland watercourses. The objective of this study was to determine the differences in sagittal otolith and scale shape of the frontal and established populations of N. melanostomus in two different habitat types. At two sampling sites in the main course of the Sava (Babina Greda, Slavonski Brod) and in a modified habitat (Sava-Odra canal), 20 individuals of N. melanostomus were collected and analysed. The geometric morphometric method was used together with Procrustes analysis of variance, canonical variate analysis and discriminant function analysis to analyse the otolith and scale shape. Significant differences in both otolith and scale shapes were found between settled (Babina Greda) and frontal (Slavonski Brod) populations from the main course of the Sava River. The main differences were the wider anterior part of the scales and a more pronounced open indentation on the otolith of the Babina Greda population. The settled population in the Sava-Odra canal was significantly different in otolith shape from the Babina Greda population but not from the Slavonski Brod population, suggesting that the upstream migration consisted of individuals with specific phenotype and genotype. Otoliths generally expressed greater shape variability than scales and thus may be more useful for differentiating between closely related populations on a finer scale. The results of this study suggest that the shape of otoliths and scales can be used to distinguish between closely related populations of N. melanostomus from different habitats but more comprehensive studies are needed due to the small number of samples.
