Freshwater grazers are suitable organisms to investigate the fate of environmental pollutants, such as weathered multi-walled carbon nanotubes (wMWCNTs). One key process is the uptake of ingested materials into digestive or absorptive cells. To address this, we investigated the localization of wMWCNTs in the intestinal tracts of the mud snail Lymnaea stagnalis (L. stagnalis) and the mayfly Rhithrogena semicolorata (R. semicolorata). In L. stagnalis, bundles of wMWCNTs could be detected in the midgut lumen, whereas only single wMWCNTs could be detected in the lumina of the digestive gland. Intracellular uptake of wMWCNTs was detected by transmission electron microscopy (TEM) but was restricted to the cells of the digestive gland. In larvae of R. semicolorata, irritations of the microvilli and damages in the apical parts of the epithelial gut cells were detected after feeding with 1 to 10 mg/L wMWCNTs. In both models, we detected fibrillar structures in close association with the epithelial cells that formed peritrophic membranes (PMs). The PM may cause a reduced transmission of wMWCNT bundles into the epithelium by forming a filter barrier and potentially protecting the cells from the wMWCNTs. As a result, the uptake of wMWCNTs into cells is rare in mud snails and may not occur at all in mayfly larvae. In addition, we monitor physiological markers such as levels of glycogen or triglycerides and the RNA/DNA ratio. This ratio was significantly affected in L. stagnalis after 24 days with 10 mg/L wMWCNTs, but not in R. semicolorata after 28 days and 10 mg/L wMWCNTs. However, significant effects on the energy status of R. semicolorata were analysed after 28 days of exposure to 1 mg/L wMWCNTs. Furthermore, we observed a significant reduction of phagosomes per enterocyte cell in mayfly larvae at a concentration of 10 mg/L wMWCNTs (p < 0.01).

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The invasion of the Ponto–Caspian amphipod Dikerogammarus villosus in European rivers is assumed to reduce macroinvertebrate diversity and to alter ecosystem functions. D. villosus shows an extraordinarily flexible feeding behavior including the ability to use various food sources. On the other hand, its response to predation risk seems to depend on environmental factors. To evaluate the ecological function of D. villosus, we estimated the daily food consumption for different food sources and analyzed potential effects of predator avoidance behavior on feeding. D. villosus consumption of willow leaves or chironomid larvae was quantified in 24-h laboratory experiments with and without kairomones of the European bullhead (Cottus gobio). Consumption rates were estimated based on gut content and gut evacuation rate under semi-natural laboratory conditions enabling the animals to feed over the whole time of the evacuation rate experiment. We observed very high evacuation rates and consequently high consumption rates up to 89% of body weight per day. Consumption rates differed significantly between food sources: D. villosus ingested more leaves than chironomid larvae. In contrast, predator cues did not affect the feeding of D. villosus. This might be explained by its strong refuge affinity and probably benefits its successful invasion. A comparison of the estimated consumption rates with results of an own consumption experiment (and other studies) under more artificial conditions indicated that more natural conditions result in higher consumption rates. Consequently, feeding rates from highly artificial experiments should be used with great caution to assess the ecosystem function of D. villosus.

Invasive species can affect native communities by replacing competitors, overexploiting prey species or altering ecosystem structure. One example is the Ponto-Caspian amphipod Dikerogammarus villosus which has established large populations in European rivers and is widely considered as the main cause for the decline of native benthic invertebrates. This effect has been mainly associated with direct predation, whereas the indirect effects via competition for primary resources are poorly understood and possibly underestimated. To assess the probability of these indirect effects, we performed five outdoor flow-through mesocosm experiments in three European rivers, manipulating the density of D. villosus. We quantified its in-situ food consumption during three 24-h gut content surveys in the mesocosms. Gut evacuation rates for correction were measured in the laboratory for different food sources and under continuous feeding. We analysed the invader's effects on primary resources by quantifying periphyton biomass and community leaf litter decomposition in the mesocosms at different D. villosus densities. The remarkably high food consumption rates (0.38-1.27 mg mg(-1) d(-1), in dry mass/dry body mass) of D. villosus can be attributed to its high gut evacuation rates. The leaf litter decomposition rates indicate that D. villosus is an efficient shredder; however, there was no effect on periphyton biomass. Our results indicate that D. villosus may be a strong competitor with primary consumers in benthic food webs of invaded rivers, with both direct and indirect negative effects on benthic communities. High consumption rates together with opportunistic feeding behaviour probably promote the invasion success of this amphipod.

Dikerogammarus villosus, one of the most successful invaders in European river systems, is commonly regarded as a threat to native biodiversity and a main factor structuring the benthic community of invaded systems. The impact of D. villosus has been intensively studied in small-scale experiments and field observations, but its impact on natural communities on a larger scale remains unclear. Here, we investigated the benthic community structure at ten sites covering a broad range of habitats along the River Rhine (Central Europe) and its tributaries, to determine whether D. villosus is one of the main factors structuring the benthic community. Community composition was analysed using non-metric multidimensional scaling, distance-based redundancy analysis, and correlation analyses. D. villosus was one of nine relevant taxa present that altogether reflected a large part of the variation in the benthic samples, but further analyses indicated that the species might be less important for the community structure than other relevant taxa. Moreover, all nine relevant taxa together can explain only a similar amount of variation in our samples than the five relevant non-faunal environmental factors (water temperature, pH, conductivity, percentage of medium-sized gravel and macrophytes). Overall, our results suggested that rather a combination of non-faunal environmental factors than D. villosus mainly structure the benthic community composition on this larger spatial scale.

The invasive amphipod Dikerogammarus villosus is assumed to threaten native biodiversity in rivers. In spite of small-scale experiments and field observations, its impact on natural communities is largely unknown because it seems to be variable and long-term analyses are rare. We analysed long-term data from the Upper Elbe and Middle Rhine (Germany) for invasion patterns and changes in the community structure. In addition, mesocosm experiments were performed in both rivers to identify density effects of D. villosus on the communities. We assumed that D. villosus is a driver of changes in the macroinvertebrate community and that effects are river-specific due to differing benthic communities. We found two invasion patterns for D. villosus with fast invasion in the River Elbe and slower invasion in the River Rhine. The impact of D. villosus on the species composition was weak in both river communities. Invasion seems to have reduced taxa number and individuals and increased Shannon diversity in the River Rhine, but not in the River Elbe. The correlations between the densities of the invader and other taxa in the long-term data were mostly positive with the exception of two native taxa in the River Rhine, indicating a lack of strong negative species interactions. Also in the mesocosm experiments, the biomass gradient of D. villosus adults did not cause significant changes in the communities. The community in the River Rhine seemed to be more vulnerable to the D. villosus invasion than that in the River Elbe. This might be caused by a dominance of invasive species interacting positively with one another, as suggested by the 'invasional meltdown' theory. The study suggests that community-level effects of invasion may differ between rivers, probably due to differences in the community composition.

The caddisfly species Micropterna lateralis is an abundant representative of limnephilids in intermittent streams. Yet, its basic life history characteristics and adaptations related to environmental factors, such as stream drying, are comparatively understudied. Here, we investigated larval growth and metabolic energy reserves (glycogen, triglycerides) through development in their natural habitat. We concentrated on the larval development because this period represents the important phase of energy accumulation necessary for growth, metamorphosis and embryogenesis. Besides larval physiology, female adults were studied in terms of ovarian maturation. Our results indicate that adult females lack an imaginal diapause, which is otherwise often observed in intermittent stream-inhabiting Limnephilidae. Further, M. lateralis is univoltine and exhibits a relatively fast larval development with five distinct instars, of which four are characterised here (instars II–V). Accrual of biomass occurs in final instars, where a high amount of glycogen is accumulated. Lipid concentrations, on the other hand, are kept constant in final stages and slightly lower than in preceding instars. This dominance of glycogen in final instars found in M. lateralis is highly unusual in insects and of potential adaptive significance for the species’ ability to exploit intermittent habitats.

The Ponto-Caspian amphipod Dikerogammarus villosus is one of the most successful invaders in Central European rivers. Contrary to studies on its ecology, ecophysiological studies comparing the species' physiological traits are scarce. In this context, in particular the metabolic activity of the invasive species has rarely been considered and, moreover, the few existing studies on this species report strongly deviating results. The purpose of this study was to assess the metabolic activity and behavior of D. villosus and other common European amphipod species (Gammarus fossarum, Gammarus roeselii) in relation to temperatures covering the thermal regime of the invaded habitats. Based on direct calorimetric measurements of metabolic heat dissipation at three temperature levels (5°C, 15°C and 25°C), we found the routine metabolic rate of D. villosus to be significantly lower than that of the other studied gammarid species at the medium temperature level. The estimated resting metabolic rate indicated a similar trend. At 5°C and 25°C, both routine and resting metabolic rate did not differ between species. Compared to G. fossarum and G. roeselii, D. villosus exhibited lower locomotor activity at the low and medium temperatures (5°C and 15°C). In contrast, its locomotor activity increased at the high experimental temperature (25°C). G. fossarum and G. roeselii were apparently more active than D. villosus at all studied temperatures. We conclude that D. villosus has both physiological and behavioral adaptations that lead to a reduction in metabolic energy expenditure, which is assumed to be beneficial and might contribute to its invasive success.

The invasive amphipod Dikerogammarus villosus has become increasingly dominant in German river ecosystems since the River Rhine invasion in the mid-1990s. Because it is preying on other invertebrate taxa, its appearance is often assumed to be associated with a drastic decrease in species diversity and changes in natural benthic communities. Despite this, the trophic function of D. villosus and its predation potential are rarely studied in natural river communities. Here, we assess the trophic function of D. villosus in two invaded systems, the Elbe River and the River Rhine, using stable carbon and nitrogen isotope analyses. In the two studied river food webs, D. villosus had a generally low trophic position (TP), indicating a likely function as primary consumer or at most as an omnivore. The significantly higher TP in the River Rhine (TP = 2.6) than in the Elbe River (TP = 1.9) suggested a partial use of animal prey in the River Rhine. This was supported by the results of the isotope mixing model SIAR which predicted in addition to a high importance of herbivory in both rivers a likely feeding on other invasive amphipod species in the Rhine community. We conclude that D. villosus has a variable but mostly low predacious behaviour, probably depending on the benthic community structure. To allow more realistic trophic analyses for D. villosus in the field, we determined the trophic enrichment factors (TEFs) for nitrogen and carbon isotopes of two diet types in the tissue of D. villosus in an eight-week laboratory experiment. The carbon isotope enrichment ( 1 3 C) from the chironomid diet was negative in the consumer (-1.75 ‰), whereas the carbon from leaf litter was enriched positively in the tissue (3.27 ‰). The TEFs for nitrogen ( 1 5 N) were more similar for plant and animal diet (leaf litter: 3.56 ‰, chironomids: 2.29 ‰). Because of the high specificity of the enrichment factors with respect to species and food source, we recommend their use in further trophic analyses.

We studied the physiological and behavioral responses of the Ponto-Caspian amphipod Dikerogammarus villosus during exposure to four total ammonia concentrations (NH3+NH4+; TotAmm): 0.003 (control), 0.06, 1.6, and 7.0mmol L−1 (0.042, 0.84, 22.4, and 98.0mg L−1) for a period of up to 12h at 21°C. During the transition period from the control to treatment concentration as well as during the first hour of exposure to 0.06 and 1.6mmol L−1, gammarids increased their locomotor activity, which was manifested in significantly higher routine metabolic rates compared to control conditions. At control conditions, the resting metabolic rate amounted to 0.98±0.26mW g−1 and significantly increased by 19 and 37% at 0.06 and 1.6mmol L−1, respectively. The highest examined [TotAmm] caused a rapid and significant increase in resting metabolic rate by 37% within the first 4h of exposure before gammarids died. The exposure to elevated [TotAmm] also resulted in a significant decreased RNA:DNA ratio and significantly higher glycogen concentrations compared to the control. We conclude that even a short exposure to TotAmm of 0.06mmol L−1, which may occur in natural habitats, disturbs the physiology and behavior of D. villosus and leads to increased metabolic costs of the maintenance and reduced protein synthesis.

Summary 1. We assessed sex‐specific seasonal changes in major energy storage compounds (triglycerides, glycogen) in Gammarus fossarum and Gammarus pulex collected from the field, with respect to their reproductive activity. 2. The dynamics of stored energy followed a seasonal pattern in both species and sexes. Moreover, over a 4‐year period, these changes were independent of the year in which they were investigated. Stored energy reached a peak in late winter, but was depleted in late summer and early autumn, coinciding with the reproductive periods. 3. Triglyceride (annual mean ± SD) accounted for 79.7 ± 11.9% of the total stored energy and was responsible for the seasonal pattern. In contrast, glycogen contributed a lesser percentage (20.3 ± 11.9%). Over the study period, the amount of stored energy ranged between 0.39 and 4.08 kJ g −1 dry mass (triglyceride: 0.19–3.69 kJ g −1 dry mass; glycogen: 0.14–0.80 kJ g −1 dry mass). 4. In both species, the energy reserves of males were drastically depleted shortly before the cessation of precopulatory mate guarding in the field, thus offering a bioenergetic explanation for the reproductive period in these two widespread species.

Physiological indicators of fitness present a measure of an organism’s response to a changing environment. An analysis of how these organisms allocate and store their energy resources provides an understanding of how they cope with such environmental changes. Each individual has to balance the investment necessary to acquire a certain resource with the energy gained by it. This trade-off can be monitored by measuring several physiological indicators of fitness such as energy storage components, metabolic state or RNA/DNA ratio. Because environmental adaptations and ecological strategies of survival are best examined within the natural environment, our research has to rely on the physiological indicators that are easily accessible in the field. The physiological indicators presented here are significant for an individual’s fitness and in turn lead to reliable values in field-collected samples. Based on our own expertise and on a literature survey, the physiological relevance of the presented indicators is explained. Furthermore, some consideration to the analytical methods used to obtain the physiological indicators is given, and possible errors introduced at the sampling site and during the laboratory procedures are discussed. This work demonstrates that the integration of ecological and physiological expertise facilitates the identification of future ecological problems much earlier than separate approaches of both disciplines alone.