Abstract Global warming alters the physiology of ectotherm consumers, potentially making their diets more herbivorous. However, the relevance and generality of these changes remain poorly understood in a multispecies context. To address this critical knowledge gap, we experimentally tested the temperature‐dependent feeding preference of the invasive mysid Limnomysis benedeni on different combinations of autotrophic (phytoplankton) and heterotrophic (zooplankton) prey species varying in body size. Warming increased the ratio of autotrophic‐to‐heterotrophic prey consumption across all treatments, driven by reduced heterotrophic prey consumption and, where larger zooplankton (Brachionus calyciflorus and Daphnia magna) were present, an additional increase in phytoplankton ingestion. Total carbon ingestion did not increase consistently with warming, indicating diet reallocation instead of increased overall feeding intensity at higher temperatures. These findings suggest that warming can change the trophic role of invasive consumers, potentially modulating their effects on community dynamics, and provide a basis for future research in more realistic ecological settings.

After Chelicorophium curvispinum, C. sowinskyi, and C. robustum, recently the fourth Ponto-Caspian filter feeding corophiid (C. maeoticum) has colonized the lower section of the River Rhine in the Netherlands. To test for niche differentiation among the four species along the food particle size spectrum, we measured their filter mesh sizes (i.e., the distance between bristles on the filtering setae). For comparison, we also analyzed a historic sample from the River Tisza in Hungary in which C. maeoticum co-occurred only with C. sowinskyi. We also compared the relative lengths and widths of the second antennae among the species, considering the potential involvement of this appendage in interference competition and deposit feeding. Despite significant interspecific differences in filter mesh sizes in most comparisons, there was a partial overlap between C. maeoticum and C. sowinskyi in the River Rhine but not in the River Tisza. In addition, there was an almost complete overlap between C. curvispinum and C. robustum. Marked interspecific differences could be observed in the relative widths of the second antennae in both sexes, potentially related to performance in competition for microhabitats. Remarkably, the earlier colonists C. curvispinum and C. sowinskyi have thinner second antennae while C. robustum and C. maeoticum with more robust appendages could establish in habitats already occupied by their congeners. Our results suggest that competition for both food and space can shape the coexistence and invasion patterns of Ponto-Caspian corophiids. Nevertheless, more studies are needed to reveal the range of behaviors second antennae are involved in and how their morphological features determine their functionality. The observed differentiation in the measured traits along with differences in body sizes close to the 1.3 ratio are likely to allow the coexistence of the four species and portend the continued spread of C. maeoticum.

Determining the consumptive effects of non-native predators, for which several direct and indirect methods have been applied, is a common goal in invasion biology. Functional responses and stable isotope analysis are among the most widely used approaches; however, they are rarely used in combination. In this study, we used these two complementary methods to compare the predatory impacts of four invasive Ponto-Caspian mysids on zooplankton in a habitat (Danube River) where all four species coexist. The order of the species based on the attack rates of the functional response models matched with their mean δ15N values, indicating a correspondence between their predatory potentials and trophic positions in their invaded habitat. Hemimysis anomala had the highest attack rate on zooplankton presumably due to its higher degree of specialization linked to its pelagic lifestyle. Contrary to our expectations, the largest species, Paramysis lacustris, had an intermediate predatory efficiency and trophic position, similar to those of Limnomysis benedeni but higher than those of the smallest species, Katamysis warpachowskyi. Nevertheless, all of the four species exhibited a considerable predatory potential, suggesting that any of them can contribute significantly to their combined predatory impact depending on their densities. The congruence between the results of the two methods shows that the species can realize their different predatory potentials in their invaded environment, indicating favorable conditions (i.e., food availability and spatial heterogeneity) which allowed dietary differentiation. We recommend the combined use of functional responses and stable isotope analysis, which might allow robust conclusions to be drawn on the trophic ecology of the species and also provide further insights into the studied ecosystem.

1. Invasive species are a major threat to the structure and functioning of freshwater ecosystems, yet their functional roles in the invaded habitats are often poorly understood, especially in omnivores. Their impact on the invaded ecosystems may be strongly dependent on local species composition, but so far there is little evidence to support this. 2. We ran a mesocosm experiment to test whether the top-down effect of the widespread omnivorous mysid shrimp of Ponto-Caspian origin, Limnomysis benedeni, is the function of community composition of the invaded community. Zooplankton communities with different initial composition, dominated by either Cladocera or Copepoda, were crossed with the presence/absence of L. benedeni. 3. We found that the effect of L. benedeni depended on community composition by suppressing zooplankton biomass only in communities dominated by Cladocera. This is presumably related to the differences in predator avoidance abilities, specifically that the dominant Daphnia species were less efficient in escaping predation by L. benedeni compared to Copepoda. While the elimination of such large planktonic grazers may cascade down to the level of primary producers, we did not find clear evidence for this, as phytoplankton biomass was mostly unaffected during the 2-week experimental duration. 4. Based on our results, we urge further proof-of-principle experiments coupled with field observations to be able to better predict the effects of invaders on different local communities.

Although the first study on filter feeding in mysids was published almost a hundred years ago, our knowledge on the mechanism is still uncertain mainly due to the inherent difficulty of observing live specimens undisturbed, especially in pelagic species. In this study, we aimed at clarifying the issue using a species more amenable for live observations, the successful Ponto-Caspian invader Limnomysis benedeni. Based on slow-motion video recordings of specimens resting on the wall of the aquarium, we were able to demonstrate that the filtration currents are generated synergistically by the maxillae, the thoracic exopods, and possibly also by the epipods of the first thoracopods and the oostegites in adult females. We could confirm the previously questioned existence of a forwardly directed ventral current between the two rows of thoracopods; however, food particles can approach the mouth from a relatively wide angle. The rows of bristled setae on the proximal lobe of the maxillae can be regarded as the main filters present in most mysids. The studied species also bears accessory filtering setae on the basis of the second thoracopods indicating specialization for filter feeding. It is also likely that some even more specialized pelagic species with bristled setae on their thoracic endopods can use their 'feeding basket' for filtering. Clarifying the filtering mechanism represents a major contribution to our understanding of the func-tional morphology in mysids which might serve as a basis for further studies (e.g., comparisons of filter mesh sizes).

Aim Invasive alien species are a growing problem worldwide due to their ecological, economic and human health impacts. The "killer shrimp" Dikerogammarus villosus is a notorious invasive alien amphipod from the Ponto-Caspian region that has invaded many fresh and brackish waters across Europe. Understandings of large-scale population dynamics of highly impactful invaders such as D. villosus are lacking, inhibiting predictions of impact and efficient timing of management strategies. Hence, our aim was to assess trends and dynamics of D. villosus as well as its impacts in freshwater rivers and streams. Location Europe. Methods We analysed 96 European time series between 1994 and 2019 and identified trends in the relative abundance (i.e. dominance %) of D. villosus in invaded time series, as well as a set of site-specific characteristics to identify drivers and determinants of population changes and invasion dynamics using meta-regression modelling. We also looked at the spread over space and time to estimate the invasion speed (km/year) of D. villosus in Europe. We investigated the impact of D. villosus abundance on recipient community metrics (i.e. abundance, taxa richness, temporal turnover, Shannon diversity and Pielou evenness) using generalized linear models. Results Population trends varied across the time series. Nevertheless, community dominance of D. villosus increased over time across all time series. The frequency of occurrences (used as a proxy for invader spread) was well described by a Pareto distribution, whereby we estimated a lag phase (i.e. the time between introduction and spatial expansion) of approximately 28 years, followed by a gradual increase before new occurrences declined rapidly in the long term. D. villosus population change was associated with decreased taxa richness, community turnover and Shannon diversity. Main Conclusion Our results show that D. villosus is well-established in European waters and its abundance significantly alters ecological communities. However, the multidecadal lag phase prior to observed spatial expansion suggests that initial introductions by D. villosus are cryptic, thus signalling the need for more effective early detection methods.

The degree of dietary specialization has a fundamental impact on the ecological function and interactions of suspension feeders. While niche differentiation by food particle size is common among obligate suspension feeders, its role is not evident in facultative ones. In this study, we aimed at providing new insights on the matter by focusing on sympatric mysid species. As mysids use different mechanisms for raptorial and filter-feeding, they represent a more adequate model system than for example, the more extensively studied copepods. We made morphological measurements on the 4 coexisting invasive Ponto-Caspian mysid species to determine the areas and mesh sizes of their filters. We also quantified their clearance rates on the microalga Cryptomonas sp. in a laboratory experiment to reveal how morphological differences manifest in their overall filtering capacity. We found relatively small but consistent differences in the primary filter area among the species, indicating that morphological constraints due to the enclosed position of the setae might limit the possibility for differentiation. The primary filter mesh sizes were small in all 4 species (0.69-2.73 mu m) with moderate but consistent intraspecific differences, suggesting that the benefit of being able to capture small particles might outweigh the pressure for differentiation. The observed clearance rates were in accordance with the morphological characteristics of the species, highlighting that auxiliary filters (present in one of the species, Limnomysis benedeni) are needed to increase filtering capacity considerably. Our study confirmed that food particle size can contribute to the niche differentiation of facultative filter feeders, but also indicated that they can tolerate a higher overlap than obligate ones. The observed differences were related to the habitat preferences and predatory potentials of the species, suggesting that complementarity among the different niche axes might further facilitate their coexistence.

Multidimensional niche differentiation might increase the stability of coexistence by reducing overall niche overlap which might have implications on the dynamics of biological invasions. The oligohaline Baltic Seais inhabited by three native and one invasive corophiid amphipod species. These filter feeding crustaceans differ in their substrate preferences and salinity optima which could potentially allow their robust coexistence. However, recent theory predicts that competing species must diverge across all non-substitutable resources (e.g., food and space). We have measured the filter mesh size in the four species (i.e., the distance between bristles on the filtering setae), revealing considerable differences among the three natives ( Apocorophium lacustre , Corophium multisetosum and C. volutator ), whereas the invasive Chelicorophium curvispinum showed strong overlap with A. lacustre. Theory suggests that the four species cannot coexist robustly due to their overlap in food particle size irrespective of differences in their salinity optima and substrate preferences which is in accordance with observations of local extinctions of A. lacustre. Nevertheless, the stability ensuing from the multidimensional niche differentiation might delay competitive exclusion; i.e., the spatial separation by salinity and substrate types might decrease the intensity of competition for food. Our data for co-occurring populations indicate that A. lacustre might be able to decrease its filter mesh size overlap with C. curvispinum by character displacement and its broader salinity tolerance also might help the native species to persist in the region. However, the niche shift of the species might increase its overlap with C. multisetosum .

The River Danube has played a pivotal role in the range expansion of Ponto-Caspian faunal elements in recent decades; therefore, the monitoring of its biota is of high scientific and conservation importance. In this publication, the records on peracarid crustaceans yielded by the macrozoobenthos samples for the 4th Joint Danube Survey (2019) are presented. Altogether 21 species (16 Amphipoda, 2 Isopoda, 3 Mysida) were recorded at 44 sites in the Danube (between river km 2581 and 18) and its major tributaries. Invasive Ponto-Caspian species showed the most common occurrence, some of which (Chelicorophium robustum, Chelicorophium sowinskyi, and Paramysis lacustris) have been observed beyond their previously known distribution. Nevertheless, the records of Gammarus spp. in the German Danube section might potentially indicate an improvement in the status of these native species in the region. The survey also confirmed the intensifying colonization of the River Tisza by Ponto-Caspian species, showing the first record of C. robustum in the river as well as additional occurrences of the previous invader, Pontogammarus robustoides. Besides these notable records, the dataset will also serve as a useful reference for potential further range expansions.

Suspension feeders play pivotal roles in the nutrient cycling of almost all aquatic ecosystems. Since sufficiently large differences in the filter mesh size (FMS) can lead to different food web positions, the inter- and intraspecific variability of this trait might be of community-level importance. The aim of this study was to quantify the range of FMS variation within the three invasive Ponto-Caspian Chelicorophium species based on a large material representing various conditions (1,224 specimens from 40 samples across Central Europe), characterize the components of variation within populations, identify the main factors determining intraspecific differences, and reveal how intraspecific variation affects the FMS overlaps among species. The FMS of the most widespread invader, C. curvispinum , varied within the broadest range (between 2.34–8.28 μm, compared to 2.51–5.97 μm in C. robustum and 1.08–3.23 μm in C. sowinskyi ); nevertheless, the contribution of intraspecific plasticity to the invasion success of the species is not evident based on the present study. The within-individual variability of FMS increased with the individual mean of the trait and decreased with body size; however, it showed little differences among samples. The among-individual variation within samples could be partitioned into components related to body size (ontogenetic niche shift/differences among cohorts) and sex (ecological sexual dimorphism) as well as a seemingly random component (individual specialization), varying widely in extent and relative contributions. The FMS of C. curvispinum was significantly larger in the presence of C. sowinskyi than in allopatry, likely reflecting character displacement; however, it did not show further increase when C. robustum was also present. Similar differences could not be observed in C. sowinskyi . The FMS ranges of C. curvispinum and C. robustum never overlapped with that of C. sowinskyi in co-occurrence despite the considerable intraspecific differences among sites, suggesting that their interaction can be seen as a clear case of niche differentiation by food particle size. On the contrary, the strong overlaps observed between C. curvispinum and C. robustum indicate that other factors might play the primary role in their coexistence. The studied species appear to be suitable model organisms for identifying the drivers and mechanisms of FMS variability.

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.

This study was carried out to identify the relations between macroinvertebrate communities and river basin specific (RBS) pollutants in the Danube River. The investigation was performed at 68 sites along 2,500 km of the Danube. Forward selection (FS), canonical correspondence analyses (CCA), the Spearman correlation coefficient (SC) and BIO-ENV analysis (to detect synergistic effects) were used to identify the relations between the macroinvertebrate dataset and selected biological metrics with RBS pollutants. Of the 20 analysed pollutants (preselected based on NORMAN network methodology), seven (2,4-dinitrophenol, chloroxuron, bromacil, dimefuron, amoxicillin, bentazon and fluoranthene) were found to significantly correlate with macroinvertebrate communities. BIO-ENV analysis revealed 3 subsets of environmental variables that were in high correlation with the biota resemblance matrix, consisting mainly of a combination of the above-mentioned pollutants. Our results indicate that there are significant correlations between chemical determinants and aquatic biota. Moreover, this study contributes to the validation of the methodology used for prioritization of RBS pollutants proposed by the NORMAN network.

In 2017, the mysid Paramysis lacustris (Czerniavsky, 1882) was found for the first time in the Hungarian Danube section, representing the first psammo-pelophilous Ponto-Caspian peracarid colonizing the Middle Danube. In 2018, a brief survey focusing on this species revealed its presence in a more than 500-km-long river section spanning from Austria (Vienna, river km 1926) to Croatia (Batina, river km 1425). The largest populations of P. lacustris might be formed in reservoirs and slow-flowing stretches, where the appearance of the species might imply a considerable impact in connection with its zooplanktivorous feeding and important role in the diet of fish. Similar to all the other Ponto-Caspian peracarids that have crossed the Middle Danube, P. lacustris can reasonably be expected to continue its spread toward Western Europe in the future.

We comparatively examined the role of littoral and deep water sampling methods in assessing macroinvertebrate assemblages and in characterizing longitudinal changes in assemblage structure along >2,500‐km–long course of the Danube River, Europe. The effectiveness of detecting taxa corresponded well with an inshore–offshore gradient in sampling (i.e., distance from shore). Nevertheless, each method (i.e., littoral multihabitat sampling, kick and sweep sampling, and deep water dredging) contributed to some degree to overall taxa richness and species composition. Sampling in different depth zones characterized different assemblages, and consequently, inshore–offshore position was at least as important determinant of assemblage structure as longitudinal position of sampling sites in the river. Although we found significant congruency in the spatial variability of assemblages among the sampling methods, the relationships were only moderate. Our study on the large Danube River confirms studies from smaller rivers in other geographic regions that littoral monitoring provides higher taxa richness and more responsive changes to longitudinal gradients than deep water samples. Nevertheless, it also shows that sampling in different depth zones provides supplementary information on assemblage structure. Understanding changes in macroinvertebrate assemblages related to differences in sampling method is crucial to improve the bioassessment and environmental management of large rivers.

Gammarus roeselii is a common European amphipod, characterized by dorsal spines that defend against fish predation. Despite significant variability in spine length and number, the extent of this variation has never been quantified. Furthermore, widespread freshwater Gammarus taxa, including G. roeselii, are diverse species complexes, comprising numerous narrowly-endemic cryptic or overlooked species. As such, we hypothesized that the morphological variation among Central European populations of G. roeselii reflects the presence of multiple unrecognized species. We measured body and spine length from phenotypically-diverse populations and sequenced a fragment of the mitochondrial gene for the cytochrome c oxidase subunit I to link phenotypes and lineages. Morphological analyses revealed extensive variation and indicated 2 groups: morph A bearing 3 blunt, short spines and morph B with 3 to 4 long, pointy spines. Phylogenetic analyses indicated conspecificity of populations and morphs due to unexpectedly-small genetic divergence and haplotype sharing, thus refuting our hypothesis. All studied populations belong to a widespread Central and Western European clade, which is nestled among the Southeast European lineages of the species complex. We conclude that the anti-predator defense of G. roeselii in Central Europe is extremely variable despite little inter-population genetic differentiation. We presume that this variation is linked to predation intensity, reflecting either phenotypic plasticity or local adaptations of this successful colonizer, and could also shed more light into the complex evolution of spines in the Amphipoda.

36 37 After Chelicorophium curvispinum , two other Ponto-Caspian tube-dwelling, filter feeding amphipod 38 species ( C. robustum and C. sowinskyi ) have colonized several catchments in Central and Western 39 Europe in recent decades. To reveal the mechanism of niche differentiation among them, we measured 40 the mesh sizes of their filtering apparatus and analyzed multi-habitat sampling data from the River 41 Danube using RDA-based variance partitioning between environmental and spatial explanatory 42 variables. Morphometric data showed clear differentiation among the species by filter mesh size ( C. 43 curvispinum > C. robustum > C. sowinskyi ). Field data also indicated the relevance of suspended 44 matter; however, the mere quantity of suspended solids included in the analysis could not explain the 45 abundance patterns effectively. Current velocity, substrate types, and total nitrogen content also had a 46 non-negligible effect; however, their role in the niche differentiation of the species is not evident. In 47 summary, differences in their filter mesh sizes indicate a niche differentiation by food particle size 48 among the invasive Chelicorophium species, allowing their stable coexistence given sufficient size 49 variability in their food source. Consequently, the two recent invaders increase the effectiveness of 50 resource utilization, resulting in a more intensive benthic-pelagic coupling in the colonized 51 ecosystems.

After Chelicorophium curvispinum , two other Ponto-Caspian tube-dwelling, filter-feeding amphipod species ( Chelicorophium robustum and Chelicorophium sowinskyi ) have colonized several catchments in Central and Western Europe in recent decades. To reveal the mechanism of niche differentiation among them, we measured the mesh sizes of their filtering apparatus and analyzed multi-habitat sampling data from the River Danube using RDA-based variance partitioning between environmental and spatial explanatory variables. Morphometric data showed a clear differentiation among the species by filter mesh size ( C. curvispinum > C. robustum > C. sowinskyi ). Field data also indicated the relevance of suspended matter; however, the mere quantity of suspended solids included in the analysis could not explain the abundance patterns effectively. Current velocity, substrate types, and total nitrogen content also had a non-negligible effect; however, their role in the niche differentiation of the species is not evident. In summary, differences in their filter mesh sizes indicate a niche differentiation by food particle size among the invasive Chelicorophium species, allowing their stable coexistence given sufficient size variability in their food source. Consequently, the two recent invaders increase the effectiveness of resource utilization, resulting in a more intensive benthic–pelagic coupling in the colonized ecosystems.

Studying interactions among co-evolved invaders might help us in understanding, predicting, and perhaps mitigating the impact of the invading species on the native biota. The factors of spatial niche differentiation were investigated among invasive Ponto-Caspian peracarids with the aim of revealing how co-evolved species can coexist with the 'killer shrimp' Dikerogammarus villosus, an invasive gammarid replacing non-Ponto-Caspian species throughout Europe. Multi-habitat samples from the third Joint Danube Survey were analysed by partitioning the variation in species density data between environmental and spatial explanatory variable sets. Relevant predictors were identified by forward selection and their role was interpreted based on the redundancy analysis (RDA) triplot. The effect of substrate types was further analysed in certain species using generalized linear models. The analysis revealed characteristic differences in habitat preference (i.e. spatial niche differentiation) among the species, allowing coexistence with D. villosus at different spatial scales. The relatively small and lean body of Chaetogammarus ischnus and Jaera sarsi might allow the avoidance of interference with large Dikerogammarus specimens by using narrow interstices among pebbles and stones (microhabitat-scale differentiation). The remaining Ponto-Caspian species included in the analysis showed an affinity for substrate types (Obesogammarus obesus) or current velocity intervals (Dikerogammarus bispinosus) that differed from those preferred by D. villosus (mesohabitat-scale differentiation), presumably in connection with feeding preferences in some cases (Dikerogammarus haemobaphes and Trichogammarus trichiatus). These results provide a framework for a preliminary risk assessment concerning the continuing high potential for range expansion of D. villosus: i.e. the identification of the most vulnerable species in those regions of the world not invaded at present but potentially colonizable, based on their habitat preference and morphology. The lessons learned from Ponto-Caspian peracarids can be applied to the whole macroinvertebrate fauna, as the same principles (i.e. the avoidance of interference) can be expected to determine their coexistence with D. villosus.