Phenotypic differentiation of Ponto-Caspian gobies during a contemporary invasion of the upper Danube River

Evolution is known to act on contemporary timescales and invasive organisms are often used to study rapid evolutionary changes of geno- and phenotypes under natural conditions. The ability and speed of local adaptation is discussed as a key character triggering successful invasions. Variation of body shape among populations of two highly invasive, sympatric Ponto-Caspian goby species (Teleostei: Gobiidae) with a comparable invasion history in the upper Danube River, i.e. Neogobius melanostomus and Ponticola kessleri, was assessed using geometric morphometric methods. Phenotypic variation established within less than 15 generations was evident in both species. It was mainly correlated with geographical location, but in N. melanostomus also with substrate type, an ecological variable reflecting habitat quality. The two species differed in their degree of intraspecific variation which was more pronounced in N. melanostomus, the numerically dominant invader in the upper Danube. Body shape variation significantly correlating with geographical rather than ecological variables suggests a heritable component and renders phenotypic plasticity as a lone explanation unlikely. Patterns of body shape similarity among upper Danubian goby populations suggest a stepping-stone rather than a continuous expansion model for both species, where multiple introductions, possibly from various origins, may have shaped differentiation.