The Ponto-Caspian region (the lower basins of the Black, Azov, and Caspian seas) is an evolutionary hotspot that harbors an unusual aquatic fauna particularly adapted to significant salinity fluctuations. Many of these adaptable species of crustaceans, fishes, mollusks, and annelids have invaded inland waters of the Northern Hemisphere over the past century, causing local extinctions of native species and restructuring of ecological communities. Of the invasive Ponto-Caspian species, amphipod crustaceans are the most numerous. Despite their noted negative impacts and the ever-increasing threat due to global warming, the factors that promote invasion success are incompletely understood, with increased fecundity and aggressivity thought to be important. It is also not known whether the aliens occupy the same ecological niche as the native species – if not, the replacement of native species with invasive ones might have a significant effect that would ripple throughout the ecosystem. The main aim of the proposed topic is to compare native and invasive Ponto-Caspian amphipods from Lithuanian inland waters in a multidisciplinary framework that would combine field research and laboratory experiments with functional morphology. Specifically, it seeks to understand (1) how amphipod morphology (e. g. body shape, size, and appendage length) is related to habitat preference, (2) how it influences the outcome of interspecific agonistic interactions and (3) predation by fish, and use it to test (4) if invasive species can trigger relatively rapid adaptation of native species. The results would significantly improve our understanding of the ecological niches of both the native and alien species and could also help predict the effects of the invaders that are expected to arrive in the future.