At present, the invasive round and bighead gobies are the most abundant benthic fish species in the near shore zone of the Middle Danube. We compared their diet seasonally in natural and artificial habitats and contrasted it with the food supply. The composition of the macroinvertebrate community was determined mainly by seasonal changes, whereas habitat type had smaller effect. Round gobies followed these changes flexibly. They consumed mainly chironomids in the spring, whilst amphipods and molluscs in the summer and autumn. Bighead gobies relied on amphipods in each season and in both habitats, and consumed fish, too, including round goby (intraguild predation). Diet overlap was determined by the morphological differences of the species allowing a varying degree of differentiation according to the seasonally changing food supply. IntroductionOne of the most spectacular changes in the fauna of Central and Eastern European large river systems in the last decades has been the ongoing spread and proliferation of PontoCaspian fauna elements, primarily macroinvertebrates (crustaceans, molluscs) and gobiid fishes (BIJ DE VAATE et al., 2002;COPP et al., 2005). These aquatic invaders have been reported to cause serious changes in the structure and functioning of their newly invaded habitat including the displacement of their native counterparts or the reconstruction of the food web (RICCIARDI, 2001;MINCHIN et al., 2002;VAN RIEL et al., 2006). Round goby (Apollonia melanostoma (PALLAS, 1814), formerly Neogobius melanostomus) for example, the most successful invasive goby species caused substantial changes in the composition and abundance of benthic macroinvertebrates (BARTON et al., 2005), declines of native fish species (FRENCH and JUDE, 2001;CORKUM et al., 2004) and increased energy transfer to predators in the well studied Laurentian Great Lakes. Gobiid fishes * Corresponding author 610 P. BORZA et al.

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30The River Danube plays a central role in the spread of Ponto-Caspian species as a part of the

Summary Facilitative interactions among co‐evolved representatives of the endemic Ponto‐Caspian fauna are regarded as a major factor of their invasion success. Nevertheless, the most renowned examples represent interactions between different trophic levels or functional groups, while ecologically similar species can be expected to show competition‐based niche partitioning. Here, we test for differences in the realized niche of three invasive Dikerogammarus species (Crustacea: Gammaridae) in their co‐occurring range. We sampled multiple habitats within sites distributed along the River Danube to test whether some environmental variables could reveal spatial niche differentiation among the three species of Dikerogammarus, and if so, to test a predictive model outside the zone of co‐occurrence. Spatial niche differentiation was present among the species, primarily determined by current velocity (and associated substrate preference), likely reflecting a stress tolerance–competitive ability trade‐off. Suspended matter concentration was also relevant, suggesting food resources (through filter feeding) might represent another important niche axis, somewhat loosening the terms of co‐existence between D. haemobaphes and the other two species. Environmental variables could effectively explain the absence of D. bispinosus in the Lower Danube, implying that the co‐existence of the three species is possible only along a sufficiently wide current velocity gradient, and the observed turnovers are the result of niche expansion in the absence of the stronger competitor. Hence, differences in invasion success may be attributed to a stress tolerance–competitive ability trade‐off. Our results suggest the advantage of D. villosus is attributable to its competitive dominance, allowing it to monopolize lentic and/or structured habitats, which represents a fortunate pre‐adaptation to anthropogenic alterations of aquatic ecosystems. The presence of D. villosus does not greatly affect the expansion of D. haemobaphes; however, the exclusion of D. bispinosus from lentic habitats by D. villosus probably strongly limits its potential to spread by active dispersal.

This manuscript is textually identical with the published paper: 1 Borza P, Huber T, Leitner P, Remund N, Graf W (2017)

20The amphipod genus Niphargus comprises hundreds of eyeless and depigmented species with 21 narrow ranges in Western Palaearctic subterranean freshwaters. Two morphologically and 22 ecologically similar species, N. hrabei and N. valachicus, are atypical due to their epigean 23 lifestyle and large ranges. Given their wide and largely sympatric distributions, we explored 24 their potential ecological niche overlap by comparing morphological functional traits, patterns 25 of co-occurrence and habitat selection, and tested for cryptic diversity by examining variation 26 of mitochondrial and nuclear markers in selected populations. Mitochondrial sequences and 27This manuscript is textually identical with the published paper:

The river Danube is the backbone of the ‘southern invasion corridor’, one of the most important passages for the spread of Ponto-Caspian invaders in Europe. However, not all of these species used the passive or active upstream movement in the main channel to reach the upper sections and tributaries, some found detours. Mass occurrences of the Ponto-Caspian peracarid, Pontogammarus robustoides (Sars, 1894) were recorded at 17 sites along the entire Hungarian section of the River Maros, for the first time in the River Tisza catchment and also in Hungary. Those populations are found ca. 707 km upstream from the closest known and confirmed locality in the lower Danube section. We confirmed their identity by DNA barcoding and showed that all individuals fit in with the lower Danube population, thus identifying the source of this introduction. The most likely vector allowing the jump dispersal of the species is fish stocking in the Romanian section of the River Maros, which − combined with downstream drift to the Serbian Danube section and the relatively busy ship traffic between Belgrade and Vienna − might provide the opportunity to bypass the dispersal barrier represented by the unregulated Middle Danube and open the way towards Western Europe.