Morphological differentiation of endemic water frogs (Ranidae: Pelophylax) from the southwestern Balkans

The southwestern Balkans is inhabited by three endemic water frog species, Pelophylax shqipericus, P. epeiroticus and P. kurtmuelleri. Although these species are genetically and bioacoustically distinct from each other, their morphological differentiation is less pronounced as is generally observed in the genus Pelophylax. In this study, we analyzed morphological data of 215 individuals of these Balkan endemics in order to evaluate their morphological variability and facilitate their identification in the field. Moreover, since P. kurtmuelleri is a disputable taxon, we compared it with closely related P. ridibundus from Central Europe. The most pronounced morphological differences expressed by the analysis of variance (ANOVA) and the discriminant analysis of the principal components (DAPC) were found between P. kurtmuelleri and the two other Balkan species. Pelophylax kurtmuelleri and P. ridibundus, as well as P. shqipericus and P. epeiroticus were only slightly differentiated by morphology, which demonstrates that the morphological characters measured do not reflect the genetic divergence of the species and are weak indicators of their taxonomic status. Morphological characters suitable for species identification include a combination of indices L/CINT, T/CINT, L/T, and DP/CINT (L - snout-vent length, CINT-length of the metatarsal tubercle, T - length of the tibia, DP - length of the first toe), and qualitative traits like the pattern and shape of the lateral spots, presence/absence of yellow pigment in the flanks and thighs, foot webbing colouration, size and shape of the metatarsal tubercle, and colouration of the male vocal sacs.