Advertisement
Browse Subject Areas
?

Click through the PLOS taxonomy to find articles in your field.

For more information about PLOS Subject Areas, click here.

  • Loading metrics

Getting What Is Served? Feeding Ecology Influencing Parasite-Host Interactions in Invasive Round Goby Neogobius melanostomus

  • Sebastian Emde,

    Affiliation Institute for Ecology, Evolution and Diversity, Goethe-University, Frankfurt am Main, Hesse, Germany

  • Judith Kochmann ,

    judith.kochmann@senckenberg.de

    Affiliation Senckenberg Gesellschaft für Naturforschung, Biodiversity and Climate Research Centre, Frankfurt am Main, Hesse, Germany

  • Thomas Kuhn,

    Affiliation Institute for Ecology, Evolution and Diversity, Goethe-University, Frankfurt am Main, Hesse, Germany

  • Martin Plath,

    Affiliation College of Animal Science and Technology, Northwest Agriculture & Forestry University, Yangling, Shaanxi Province, P. R. China

  • Sven Klimpel

    Affiliations Institute for Ecology, Evolution and Diversity, Goethe-University, Frankfurt am Main, Hesse, Germany, Senckenberg Gesellschaft für Naturforschung, Biodiversity and Climate Research Centre, Frankfurt am Main, Hesse, Germany

Getting What Is Served? Feeding Ecology Influencing Parasite-Host Interactions in Invasive Round Goby Neogobius melanostomus

  • Sebastian Emde, 
  • Judith Kochmann, 
  • Thomas Kuhn, 
  • Martin Plath, 
  • Sven Klimpel
PLOS
x

Abstract

Freshwater ecosystems are increasingly impacted by alien invasive species which have the potential to alter various ecological interactions like predator-prey and host-parasite relationships. Here, we simultaneously examined predator-prey interactions and parasitization patterns of the highly invasive round goby (Neogobius melanostomus) in the rivers Rhine and Main in Germany. A total of 350 N. melanostomus were sampled between June and October 2011. Gut content analysis revealed a broad prey spectrum, partly reflecting temporal and local differences in prey availability. For the major food type (amphipods), species compositions were determined. Amphipod fauna consisted entirely of non-native species and was dominated by Dikerogammarus villosus in the Main and Echinogammarus trichiatus in the Rhine. However, the availability of amphipod species in the field did not reflect their relative abundance in gut contents of N. melanostomus. Only two metazoan parasites, the nematode Raphidascaris acus and the acanthocephalan Pomphorhynchus sp., were isolated from N. melanostomus in all months, whereas unionid glochidia were only detected in June and October in fish from the Main. To analyse infection pathways, we examined 17,356 amphipods and found Pomphorhynchus sp. larvae only in D. villosus in the river Rhine at a prevalence of 0.15%. Dikerogammarus villosus represented the most important amphipod prey for N. melanostomus in both rivers but parasite intensities differed between rivers, suggesting that final hosts (large predatory fishes) may influence host-parasite dynamics of N. melanostomus in its introduced range.

Introduction

Biological invasions have increased exponentially in recent years due to human activities, especially shipping, along with the adverse effects of environmental changes such as global warming [1][3]. Although brackish waters have the highest risk for species introductions, freshwater ecosystems are also strongly affected, especially by the introduction of non-indigenous fishes [4], [5]. Once established in their new environment, invasive non-indigenous species can have tremendous effects on local populations of indigenous species, e.g., through competitive [6], predator-prey [7][9], or host-parasite interactions [10], [11], all of which have the potential to result in altered ecosystem functioning (see review by Strayer [12]).

To date, several studies in aquatic ecosystems have considered the question of how invasive predators can affect native prey populations [13][15], or how invasive prey populations can alter indigenous prey communities [16], [17], and whether or not non-indigenous prey species become integrated into the prey spectrum of indigenous predators [18]. Furthermore, studies have started to concentrate on parasitization patterns of native and invasive species, and several different scenarios are possible: (i) invasive hosts may lose their original parasite load (‘enemy release hypothesis’), providing invasive species with an initial benefit in their novel range [10], [19], [20]. (ii) Introduced hosts may carry new parasite species (parasite spill-over), which may adversely affect native host species [21]. (iii) Invasive hosts may serve as intermediate hosts or vectors for local parasites or diseases (parasite spillback) [21]. (iv) Finally, shift and/or loss of local parasite species would be predicted if the invader is replacing local host species but cannot function as intermediate or definitive host in the parasite life cycles (dilution effect) [22], [23]. Few studies, however, have simultaneously considered predator-prey interactions and parasitization patterns of different trophic levels in ecosystems that are heavily influenced by invasive species [24][26]. This is surprising, given that many parasites with indirect life-cycles rely on the ingestion of their intermediate hosts by further (intermediate or final) host species to successfully complete their life cycles [27], [28]. Biological invasions could provide large numbers of host specimens within a very short time-span (e.g. [29]) that could affect parasite transmission patterns in entire fish communities.

The round goby Neogobius melanostomus (Pallas, 1814) is a frequent invader of brackish and freshwater habitats worldwide, reaching enormous population densities and causing changes of food web dynamics at different trophic levels, e.g., in the North American Great Lakes [30] and in large European rivers, e.g. the Danube [29]. Round gobies nowadays make up app. 80% of fish catches in the Rhine [31], and so an alteration of ecological interactions is also expected for the Rhine. For example, it is known that round gobies act as competitors of spawning or foraging sites with native species [30]. Feeding patterns of N. melanostomus vary in different distribution areas. While dreissenid mussels play an important role in the feeding ecology of N. melanostomus in the Great Lakes and in the Baltic Sea [32], [33], amphipods seem to be their main forage in German rivers [24], [25], [31]. In the Rhine, the Ponto-Caspian amphipod Dikerogammarus villosus (Sowinsky, 1894) has been described as dominating communities of macroinvertebrates and as an important prey species of N. melanostomus [24], [25], [31], [34], [35]. Both species, D. villosus and N. melanostomus function as intermediate hosts for different parasites (e.g., Pomphorhynchus spp. and Raphidascaris spp.) and may be responsible for the spread of these parasites, which could increasingly affect native vertebrate and invertebrate hosts as well [24], [36].

Studies on N. melanostomus that combine the analysis of their feeding habits with parasitological analyses are rare and have focused on the Danube [25], [29] and Rhine [24], [31]. To analyse the role of different amphipod species for metazoan fish parasite transmission as well as temporal variation of diet compositions in invasive N. melanostomus, samples from the rivers Main and Rhine were compared in this study. We hypothesized that (a) N. melanostomus will mainly feed on amphipods throughout the course of our repeated monthly sampling and in both rivers, and accordingly, (b) the availability of amphipod species in a given river will reflect their relative contribution to gut contents of N. melanostomus. Moreover, we expected that (c) monthly infestation rates of amphipods with parasite species and monthly feeding rates of amphipods by N. melanostomus should reflect parasite infestation rates in N. melanostomus. Finally, a detailed description of parasite fauna for two sampling locations in the rivers Main and Rhine was intended to complement current parasite diversity estimates of N. melanostomus in its introduced range.

Materials and Methods

Sampling

A total of n = 350 N. melanostomus were collected from June to October 2011 in the rivers Rhine (49°51′54.7″N 8°21′40.2″E) and Main (50°04′48.9″N 8°31′19.6″E) in Germany. Both sites were similar in habitat structure with rip-rap embanked shorelines (technolithal) that led into bottom substrate of sand and gravel. In contrast to the Rhine, the river bank of the Main had little more vegetation with roots partly reaching into the water.

35 N. melanostomus specimens per site were caught randomly on top of and around rip-raps (depths of ∼40–200 cm) during one day at the end of each month (between ∼9 am–2 pm) using a hook and line technique. Since standardized angling is known to yield an equilibrated sex ratio and homogenously distributed, relatively large-sized specimens in N. melanostomus [37], a fishing rod equipped with an anti-tangle bottom rig consisting of a special sinker (Tiroler Hölzl, 80 g) was used to avoid entanglement between rip-rap interstices. A small, round hook (Owner, barb special, size 14, FRL-044) was baited with 1–3 fly maggots. All hooked fish were used for subsequent examination in the laboratory without any size or sex selection. Each fish was carefully hooked off with a special hook removal tool and was humanely killed inside a plastic bag in order to avoid losing gut contents or parasites. To prevent further digestion or migration of parasites to other organs, fishes were kept separately in plastic bags in a cooling box filled with ice and stored afterwards at −20°C for later examination.

Amphipods were also collected monthly at the same sampling sites turning around large stones and using the ‘kick-sampling’ method after Storey et al. [38]. A small fishing net (15×20 cm, mesh size ∼1 mm) was used to catch as many amphipods as possible within 30 minutes along a 10 m stretch at a depth of up to 50 cm. Amphipods were kept together with organic material and some stones in plastic bags. Entire samples were frozen at −20°C and later separated from sediment to identify amphipods to species level.

Parasitological examination and feeding ecology of N. melanostomus

Gobies were measured for total length (cm) and weight (g), condition factors (CF) were calculated according to Schäperclaus [39]. These measures are key parameters in studies on fish biology and were reported in (Text S1, Table S1) to facilitate comparisons with other studies.

Fish were then examined for their metazoan parasite fauna and stomach content using a stereomicroscope (Olympus SZ 61, magnification x 6.7–45). At first, skin, fins and gills were inspected for ectoparasites. Afterwards, the body cavity was opened to separate the inner organs. Body cavity, rinsed with 0.9% NaCl, gastrointestinal tract, gonads, kidney, liver, mesenteries, spleen and eyes were dissected and examined for endoparasites. Isolated parasites were freed from host tissue and preserved in 70% ethanol (with 4% glycerol) for morphological identification. To this end, glycerine preparations were made according to Riemann [40]. Determination under a microscope (Leitz Dialux 22, magnification x 15.75–630) was aided by original descriptions and descriptions of Golvan [41] and Špakulová et al. [42] for acanthocephalans, and Moravec [43] for nematodes. Subsamples were stored in 100% ethanol for genetic analysis (see Text S2).

Since gobies have no clearly separated stomach and a very short gut, the entire gastrointestinal tract was carefully cut lengthwise with a small pair of scissors. The weights of full and empty stomachs and the weights of each food item were recorded to the nearest 0.001 g after pat-drying on absorbent paper. Very small, as well as almost digested and defragmented parts of one prey group that could not be identified to species level were referred to as ‘not determined’ (indet.) and weighted as a pooled subsample. Only specimens that could clearly be identified, e.g. using assignable parts like eyes or telson, were identified and counted. Other components, mainly mucus and sand, but also undeterminable items were neglected. Prey organisms were sorted and identified to the lowest possible taxon and grouped into the following categories: amphipods, molluscs, insects and ‘others’ (plants, vertebrates, Acari). Isolated food organisms and parasites were preserved in 70% ethanol (with 4% glycerol) for morphological identification.

Amphipods were identified to species-level following Eggers & Martens [44], [45] and preserved in 70% ethanol. For parasitological examination, all amphipods were dissected and carefully screened under a stereomicroscope. Isolated parasites were stored in 100% ethanol. From each monthly sampling, fifty amphipods of each species were randomly taken to determine sex, body size and weight using an ocular micrometer and a micro-balance. Size was measured from the anterior rostrum to the base of the telson while animals were stretched in a straight position [46]. Data are reported in (Text S3, Figure S1).

Statistical analyses

We first tested if the relative abundance of amphipods on site (covariate, arcsine(square root)-transformed percentages relative to the highest monthly abundance value observed for the respective site) determines the proportion of amphipods in N. melanostomus gut contents (monthly mean values were treated as the dependent variable) using analysis of covariance (ANCOVA using SPSS vs. 22), in which ‘site’ was a fixed factor. A Chi2 goodness-of-fit test (using R; R Development Core Team [47]) was then applied to test whether amphipod species compositions as encountered on site are reflected in gut contents.

Gut content analyses comprised calculations of the numerical percentage of prey (N%), the weight percentage of prey (W%), and the frequency of occurrence of prey (F%) [48], [49]. On the basis of these three indices, the index of relative importance (IRI) of different food items was calculated [50]. Differences in gut content assemblage structure between months and rivers were also assessed using two-factorial permutation ANOVA (PERMANOVA; 999 permutations) on Bray-Curtis dissimilarities of 4th-root transformed weights (mg) of the different species in each fish gut using the PRIMER v6 and PERMANOVA+ add-on package (PRIMER-e, Plymouth, UK). The SIMPER procedure [51] was used for post hoc identification of the source of variation.

Parasitological analyses comprised calculations of standard parameters: the prevalence (P), mean intensity (mI), intensity (I) and mean abundance (mA) for each parasite species according to Bush et al. [52]. High mean intensities of Pomphorhynchus sp. infections were found (see results), and previous studies suggested transmission pathways into N. melanostomus via amphipods, especially D. villosus [24]. Therefore, we used a repeated-measures General Linear Model (rmGLM using SPSS vs. 22) to test if mean intensities of Pomphorhynchus sp. in round gobies (dependent variable) differed between sexes (rm) and sites (fixed factor), and if the proportion of amphipods in the gut contents (arcsine(square root)-transformed numerical percentages, covariate) had an effect. The nematode R. acus was also relatively abundant in fish samples, but we restricted our analysis to non-parametric Wilcoxon signed-rank test (using SPSS vs. 22) to test whether differences in infection rates existed between the two rivers.

Results

Amphipod communities

717 to 3,758 amphipods were collected during the monthly samplings, with a total of n = 9,820 in the Rhine and n = 7,536 in the Main (see Table S2). Five invasive but no native amphipod species were found in both rivers, namely D. villosus, Echinogammarus trichiatus (Martynov, 1932), Echinogammarus ischnus (Stebbing, 1899), Chelicorophium curvispinum (Sars, 1895) and Chelicorophium robustum (Sars, 1895). Cryptorchestia cavimana (Heller, 1865) occurred only in samples from the Main. Dikerogammarus villosus was dominating in all samples from the Main (total n = 5,346; 69%), except for September (Figure 1). In contrast, E. trichiatus was the dominant species in all samples from the Rhine (total n = 8,463; 86%; Figure 1). In both rivers a more balanced sex ratio was found for D. villosus (males:females, Rhine: 1∶1.03; Main: 1∶1.29) than for E. trichiatus (Rhine: 1∶2.36; Main: 1∶3.10).

thumbnail
Figure 1. Dominant amphipod species.

Fraction of the two dominant amphipod species (D. villosus = light grey, E. trichiatus = dark grey) in samples collected at our two study sites and numerical percentages of D. villosus in gut contents of N. melanostomus (black squares). Chi2 goodness-of-fit tests were used to compare the availability of different amphipod species on site (expected values) with observed compositions in gut contents. For total numbers of individuals and amphipod species see Table S2.

https://doi.org/10.1371/journal.pone.0109971.g001

General feeding ecology of N. melanostomus

18 (Rhine) and 16 (Main) different prey items were identified in N. melanostomus guts (Table S3, Table S4). The index of relative importance (IRI) found amphipods to be the main diet component of N. melanostomus, with an overall contribution of 71% in the Rhine and 46% in the Main (Figure 2). In the Rhine, amphipods contributed with at least 30% in each monthly sample (Figure 2). The second most important group was molluscs, which contributed with 7–38% to the overall gut content. The widespread and common species Bithynia tentaculata, Potamopyrgus antipodarum and P. antipodarum f. carinata were distinguishable, but, due to a high degree of fragmentation, were combined into ‘Gastropoda indet.’. Insects were rarely consumed, except for July where the IRI for Chironomidae rose to 2,288.83 (Table S3) when very little gut content was found overall. In the Main, highest proportions of amphipods (over 80%) occurred in September and October (Figure 2). Insects were consumed more often than in the Rhine, especially in June (79%) and August (36%). Fish diet was based on molluscs with 50% and 45% in July and August, respectively. Fishes, plants and Acari were rarely consumed in both rivers.

thumbnail
Figure 2. Gut contents of Neogobius melanostomus.

Relative compositions (index of relative importance, IRI) of gut contents of N. melanostomus in two rivers from June until October 2011 as well as the total mean. Bar plot, from bottom to top: Amphipoda (black), Mollusca (medium grey), Insecta (light grey), others (dark grey).

https://doi.org/10.1371/journal.pone.0109971.g002

Gut content assemblage structure showed strong fluctuations between months and rivers. They differed significantly between June and July and June and August in the Rhine, whereas June and August were different from all other months in the Main (PERMANOVA: pseudo-F = 8.64, df = 4, p = 0.001 for the interaction ‘river × month’; for post hoc results see Table 1). Amphipods mostly accounted for the highest average dissimilarity between different monthly samples in the Rhine, whereas amphipods and insects accounted for the highest average dissimilarity between months in the Main (SIMPER procedure).

Amphipod prey preference of N. melanostomus

Few individuals of C. curvispinum were found in N. melanostomus guts, and the dominating amphipod species was D. villosus, especially in the Main, but to a lesser degree also in the Rhine. This was reflected in the ANCOVA, which detected a significant interaction between ‘site’ and ‘relative abundance of amphipods on site’ (Table 2).

Dikerogammarus villosus was disproportionally frequent in gut contents given its availability relative to that of other amphipod species on site (Chi2 goodness-of-fit tests, p<0.001; except for the July sampling in the Main when D. villosus overall was highly abundant in the field; Figure 1). Therefore, an additional ANCOVA with similar model structure was run using percentages of D. villosus in the gut content of N. melanostomus as the dependent variable (Table 3). Whereas a decrease (not increase) of numerical percentages of D. villosus in the gut content of N. melanostomus with increasing availability of D. villosus on site was found in the Main (driving a significant main effect of the covariate; Table 3), this pattern was not observed in the river Rhine (see significant interaction effect in Table 3; Figure 3).

thumbnail
Figure 3. Dikerogammarus villosus in fish guts and in the field.

Numerical percentage of D. villosus in gut contents of N. melanostomus in relation to the relative abundance of D. villosus at the Main (black) and Rhine (grey) between June and October 2011.

https://doi.org/10.1371/journal.pone.0109971.g003

Fish parasites: species identity and general biology

In total, three metazoan parasite species, two in the Rhine and three in the Main, could be isolated from N. melanostomus. The following taxa were identified morphologically: Pomphorhynchus sp., Raphidascaris acus, and Glochidia indet. (Table 4). As noted by Špakulová et al. [42] and Emde et al. [24], morphological identification of species within the acanthocephalan genus Pomphorhynchus can be difficult. Therefore, molecular barcoding was conducted on a subset of n = 3 specimens that were morphologically identified as P. tereticollis. Sequence data for ITS-1/5.8S/ITS-2 (Genbank accession numbers KJ756498–KJ756500) were almost identical (99.0% similarity, e-value: 0.00) to a sequence from P. laevis isolated from the cyprinid Leuciscus cephalus from the Czech Republic (Genbank accession number AY135415), suggesting that all acanthocephalan individuals in this study may belong to the same species. Due to a mismatch between the morphological identification characteristics and genetic information, acanthocephalan specimens were referred to as Pomphorhynchus sp. in this study.

thumbnail
Table 4. Parasitological parameters for the parasite fauna of N. melanostomus.

https://doi.org/10.1371/journal.pone.0109971.t004

All parasites were larval stages (Table 4). Pomphorhynchus sp. occurred only in the cystacanth stage. In the Rhine 91% of specimens were encysted in the mesenteries and liver and 9% were living freely in the body cavity. A similar pattern was found in the Main with 96% encysted in mesenteries and liver and 4% freely in the body cavity. The body cavity also harboured encysted R. acus, which occurred predominantly as L2-larvae (91% in the Main, 88% in the Rhine), and L3-larvae.

Fish parasites: faunal composition

The most prevalent metazoan parasite type was Pomphorhynchus sp. with 100% prevalence in August and September in fish caught in the Rhine (Table 4). Maximum intensity reached 118 specimens per fish. Highest prevalence of Pomphorhynchus sp. in the Main was recorded in June with 74.3%. Mean intensity of Pomphorhynchus sp. was an order of magnitude larger in fishes sampled from the Rhine (maximum mI = 34.6) than from the Main (maximum mI = 3.48) and always greater in female than in male N. melanostomus (rmGLM, significant interaction of ‘sex × site’; Table 5; Figure 4). The nematode R. acus occurred with significantly lower prevalence in the Rhine (min. 28.57%, max. 57.14%) than in the Main (74.29% and 91.43%; Wilcoxon signed-rank test, z = –2.023, p = 0.043; Table 4). A maximum intensity of specimens of R. acus per fish was detected. Undetermined glochidia, i.e., parasitic larvae of unionid bivalves were detected on fish gills only in June (P = 54.3%) and October (P = 38.1%) in the Main.

thumbnail
Figure 4. Amphipod prey and infections with Pomphorhynchus sp.

Relationship between numerical percentages of D. villosus (grey) and Amphipoda indet. (white) in the gut content of N. melanostomus and mean intensities (mI, black line) of Pomphorhynchus sp. in male (grey dashed line) and female (black dashed line) N. melanostomus. For numbers of individuals please refer to Table S3 and Table S4.

https://doi.org/10.1371/journal.pone.0109971.g004

Parasites retrieved from amphipods

Pomphorhynchus sp. was the only parasite species that could be detected in amphipod samples. Two individuals were retrieved from D. villosus in the Rhine; the first was detected in samples from August (157 amphipods screened, P = 0.64%), the second in samples from October (671 amphipods screened, P = 0.15%). Overall, Pomphorhynchus sp. occurred at a prevalence of 0.15% in D. villosus in the river Rhine (two out of 1,350 specimens). The total number of D. villosus was four times larger in the Main than in the Rhine (i.e., n = 5,346), still, no parasites were detected. Low overall abundance precluded an analysis of potential temporal fluctuation in parasite infections of amphipods. Numerical percentages of amphipods in fish gut contents did not predict mean intensities of acanthocephalan parasites in N. melanostomus (Table 5).

Discussion

Feeding ecology of N. melanostomus

Co-evolved trophic relationships can facilitate biological invasions, as exemplified by communities of coexisting invasive N. melanostomus, dreissenid mussels and E. ischnus in the North American Great Lakes [53], [54]. Presence of co-evolved prey, however, appears not to be a prerequisite for N. melanostomus in German rivers, since N. melanostomus was characterized by an opportunistic and broad feeding strategy [see also 30,31]. Opportunistic feeding might also provide a plausible explanation for why we detected no positive correlation between the abundance of D. villosus in the field (generally a preferred type among amphipod prey) and their proportional contribution to gut contents. This was obvious especially during early summer, when prey species other than amphipods became more relevant (higher index of relative importance), especially in the Main, where insects and molluscs became the main food sources. Similarly, the importance of amphipod prey (D. villosus and others) for N. melanostomus in the Danube increased from early to late summer while the importance of chironomid larvae decreased [25]. Ingested insects in our present study were mostly nematoceran larvae, which are generally abundant in slow-flowing waterways like the Main. Non-biting midges (Chironomidae) no longer dominate the invertebrate community of the navigable main channel of the upper Rhine [55], which may explain why insects, overall, were barely ingested. While N. melanostomus is commonly regarded as a predator of fish eggs and fry (e.g. [56]), these were only rarely retrieved from gut contents.

An ontogenetic size dependent diet shift from amphipods and insects to a diet dominated mainly by molluscs is known for round gobies (e.g. [25]), however, fish lengths where shifts seem to occur vary substantially between study regions and most likely depend on availability and abundance of prey organisms [57], [58] as well as on time since invasion [29]. In our present study, the genus Dreissena seems to play a subordinate role compared to the Great Lakes and the Baltic Sea, which may be attributable to more readily available food sources, like insect larvae and amphipods. Generally, a tendency of increasing absolute numbers with increasing fish size was observed for D. villosus and nematocerans. In this context, Emde et al. [24] already demonstrated a size-dependent increase in acanthocephalan infections, which was inter alia explained by a correlation between goby and amphipod (D. villosus) prey body size, as it seems likely that the development of acanthocephalan larvae might only grow in amphipods above a certain size threshold. Thus, smaller gobies, feeding on smaller D. villosus, are less infected by acanthocephalans.

All amphipods found during monthly sampling were non-indigenous species from the Ponto-Caspian region (i.e., Black and Caspian Seas), corroborating studies in several European watersheds [24], [44], [59]. The most common non-indigenous amphipod species were D. villosus und E. trichiatus. Dikerogammarus villosus was dominant in samples from the Main, whereas E. trichiatus was dominant in Rhine samples, suggesting that faunal compositions of invasive amphipods may be more stable temporally and to a lesser degree spatially within the Rhine drainage (see also [24], [60]). Dikerogammarus villosus was detected six years earlier than E. trichiatus in the Rhine and is known for its strong predation on other gammarids [7], [61]. However, the total number of individuals caught in the Rhine was an order of magnitude lower than that of E. trichiatus. Whether higher predation on D. villosus by N. melanostomus in the Rhine compared to the Main could explain this pattern remains uncertain, since no fish densities at both sites were recorded herein.

Regardless of the high numbers of E. trichiatus in the Rhine, N. melanostomus primarily fed on D. villosus. How can this pattern be explained? Sih and Christensen [62] argued that variation in prey behaviour is more likely to affect the direction of predator-prey interactions than active prey choice of predators. Qualitatively, we noted that E. trichiatus at our study sites occurred closer to riverbanks, while D. villosus were found in both shallow and deeper waters, and so E. trichiatus could avoid fish predation in shallower habitats or by hiding between rip-rap interstices. Spatial niche segregation between E. trichiatus and D. villosus was previously observed in the Netherlands where the former seems to occur on soft substrates whereas the latter is most abundant on hard substrates [63]. Thus, different microhabitat use or different activity patterns in D. villosus are likely explanations for their dominance among amphipod prey in N. melanostomus.

Parasites can manipulate the predator avoidance of freshwater amphipods, rendering them more vulnerable to their fish predators (for Gammarus pulex see [64], [65]). Whether infections by Pomphorhynchus sp. affect the predator avoidance of D. villosus is currently not known, but if infected individuals were indeed more prone to predation, this would provide a striking explanation for our finding that gobies were highly infected by Pomphorhynchus sp., yet infectious stages were barely found in their amphipod prey (i.e., D. villosus), and were even completely absent in the Main. It seems reasonable to argue that infected D. villosus were ingested at an accelerated rate compared to uninfected specimens. Generally, infection rates of invertebrate intermediate hosts, especially crustaceans, tend to be low, often ranging between 0.01 and 0.1% prevalence [23], [28]. A possible reason for the higher infestation rates of D. villosus in the Rhine might be the presence of more final hosts (like common barbel Barbus barbus and European chub Squalius cephalus, however this assumption is not based on quantitative data but on personal observations only (S. Emde, personal observation).

Pomphorhynchus sp. is known to include a variety of different first intermediate hosts in its life cycle, such as D. villosus [24], G. pulex [64] and C. curvispinum [65]. Gammarus pulex seems to be completely displaced by invasive species in the Rhine and Main [24] and was not part of the gobies’ diet at both sampling sites. Following a massive decrease since 1995, C. curvispinum currently also plays a negligible role in the gobies’ diet [65]. In the light of the decrease of other amphipod species and the observed dominance of D. villosus in the gobies’ diet, we suggest that D. villosus currently represents the most relevant intermediate host for Pomphorhynchus sp. Still, future studies could investigate additional invertebrate groups and might uncover additional first intermediate hosts for the opportunistic parasites of the genus Pomphorhynchus.

Parasite fauna of N. melanostomus

More than 94 parasites of N. melanostomus have been recorded worldwide [66], and in its introduced range in Europe, 35 metazoan parasite species have been detected so far (e.g., [66][69]). Neogobius melanostomus usually carries more than ten different parasite species per population in its native range [70]. Herein, only three parasite species could be detected in 350 round gobies examined, suggesting that the diversity of N. melanostomus parasites in the Rhine did not change over the past four years ([24], S. Emde personal observation). In other regions, the parasite fauna of invasive N. melanostomus increased rapidly, e.g., in the Gulf of Dansk, where numbers rose from 4 to 12 parasite species within two years [68]. Only 6 to 10 years have passed since round gobies were first recorded in German inland waterways, while the first report of round gobies at our sampling sites was in 2007 [71], [72]. Our results support the ‘enemy release hypothesis’ [19], and release from natural parasites could be one reason promoting the fast spread of round gobies worldwide. This advantage over indigenous fishes, however, will likely be lost if the diversity of the parasite fauna of N. melanostomus increases with time [73]. Whether or not such an increase of parasite diversity will occur in the future requires further monitoring.

All parasites detected in N. melanostomus were larval stages, and so we tentatively argue that currently no native parasite species is able to use N. melanostomus as its final but only as a paratenic host. A higher parasitization of N. melanostomus was observed in the Rhine, where fishes were also smaller and had a lower condition factor than in the Main (Text S1, Table S1). A high parasite load can lead to decreased growth in their fish hosts [74], however, infection studies in controlled environments would be needed to further address this hypothesis.

Pomphorhynchus sp. (Acanthocephala) and Raphidascaris acus (Nematoda) have been detected before in N. melanostomus caught in the Rhine, with similar infection rates for Pomphorhynchus sp. [24]. Latest data of the Danube River also described high abundances of this parasite but detected highest abundances in more recently invaded areas [29]. Similarly high prevalences of R. acus as found in our current study (up to 91.43%) are known from studies in other sections of the Rhine (56%) [75] and the Danube (P = 57%) [67]. Generally, differences in infection rates (prevalence/intensities) among studies could be related to the presence/absence as well as abundance of the parasites’ final hosts. For adult R. acus the European pike (Esox lucius) and brown trout (Salmo trutta fario) are known as principal final hosts [43], whereas it is barbel (Barbus barbus) and chub (Squalius cephalus) for Pomphorhynchus sp. [24]. However, N. melanostomus seems to represent a new, additional intermediate host for these parasites and thus, bridges the trophic level towards new potential, predatory final hosts. Other potential definitive hosts in the rivers Rhine and Main are trout (Salmo trutta) and catfish (Silurus glanis) for Pomphorhynchus [76], [77] and the European eel (Anguilla anguilla), European perch (Perca fluviatilis) and pike-perch (Sander lucioperca) for R. acus [78]. Infection studies need to show whether the female parasite attains gravidity in the potential definitive host or whether these predatory fishes may only act as para-definitive hosts in which the parasite matures but is unable to produce eggs [78]. If they do not act as definitive hosts, the large number of parasite larvae in N. melanostomus will be transmitted to these predatory fishes, however, not be able to complete their life cycle. This would lead to a dilution effect, resulting in a continued loss of infection within the system as has been described for different parasite-host communities before [79], [80] and would therefore be an alternative plausible explanation for the lower infection rates in the Main than in the Rhine.

Parasitic larval stages (Glochidia) of freshwater mussels of the family Unionidae were found in samples from the river Main, which confirms a former report of N. melanostomus serving as a host for unionid glochidia in the Danube [67]. Glochidia could be detected only during some months, because river mussels (Unio sp.) spawn in early summer and swan mussels (Anodonta sp.) in late summer, and glochidia attach to fish gills for only a few weeks [81]. Although unionid mussels are known to occur in the Rhine [82], no glochidia were detected on the gills of N. melanostomus, which could suggest an abundance-correlated effect. Alternatively, N. melanostomus might be a bad host for unionids [83]. Authors infected gobies with Glochidia of which 98% were lost within 16 days. Based on that study, our findings of Glochidia attached to gills of N. melanostomus could therefore be a finding that was the result of a very recent infection.

We initially hypothesized monthly infestation rates of D. villosus with Pomphorhynchus sp. potentially reflecting infestation rates in N. melanostomus. Due to overall low abundances of Pomphorhynchus sp. in D. villosus a statistical analysis in this direction was not possible. We also tested whether the numerical percentage of D. villosus in gut contents predicts mean intensities of Pomphorhynchus sp. but found no such effect. The timing of the parasite’s life cycle, however, has not yet been examined, and so our analysis (that was based on monthly sampling) may not have been appropriate to capture such potential effect.

Sex-related differences in parasite infections are common and can be ascribed to sex-specific behavioural, physiological or morphological differences [84], [85]. In this study, mean intensity of Pomphorhynchus sp. was significantly higher in females than males in the Rhine, supporting the finding of Brandner et al. [29] from the Danube River. No significant sex differences were observed in the Main, but Pomphorhynchus sp. mean intensities were low in the Main overall. Males can allocate much less time to feeding than females (for poeciliid fishes see [86], [87]) lowering their risk to take up parasites from food. Indeed, Charlebois et al. [88] found N. melanostomus males to cease feeding during brood care, while females producing eggs should have increased energy demands.

Our study confirmed that D. villosus functions as the main amphipod prey species for N. melanostomus in German rivers, however, parasite intensities in N. melanostomus differed between sampling locations of Rhine and Main independently of amphipod abundances. We suggest that a characterization of new final fish hosts, especially for Pomphorhynchus sp., at the sites investigated herein could provide important new insight into the ecological causes of variation in parasitization patterns of N. melanostomus in its introduced range.

Supporting Information

Figure S1.

Box–plots of total length and total weight of two amphipod species.

https://doi.org/10.1371/journal.pone.0109971.s001

(TIF)

Table S1.

Biological parameters of Neogobius melanostomus.

https://doi.org/10.1371/journal.pone.0109971.s002

(DOCX)

Table S3.

Gut contents and parameters of Neogobius melanostomus for the river Rhine.

https://doi.org/10.1371/journal.pone.0109971.s004

(DOC)

Table S4.

Gut contents and parameters of Neogobius melanostomus for the river Main.

https://doi.org/10.1371/journal.pone.0109971.s005

(DOC)

Text S1.

Size measurements and condition factors of N. melanostomus.

https://doi.org/10.1371/journal.pone.0109971.s006

(DOCX)

Text S2.

Genetic identification of parasites.

https://doi.org/10.1371/journal.pone.0109971.s007

(DOCX)

Text S3.

Size measurements of D. villosus and E. trichiatus.

https://doi.org/10.1371/journal.pone.0109971.s008

(DOCX)

Acknowledgments

We thank J. Schneider (Office for fish ecological studies – BFS, Frankfurt), S. Gallus and S. Schierz (Goethe University, Frankfurt) for their support with data assessment. We further wish to thank C. Koehler (Dezernat V 51.1 Landwirtschaft-Landschaftspflege-Fischerei, Regierungspräsidium Darmstadt) for providing a fishing license to catch gobies. We are grateful to D. Green who helped with statistics. Finally, we thank the reviewers of this article for their helpful suggestions. The authors declare no conflict of interest.

Ethics Statement

Approval of our present study by a review board institution or ethics committee was not necessary because all fish were caught by a person (S. Emde) holding a valid local fishing license (No. 06258) for the river Main, issued by the ‘Höchster Fischereigenossenschaft’, 65830 Kriftel, Germany. For the river Rhine a special permit (F4/Di-Zi) was issued by the ‘Hessische Landesgesellschaft mbH’, 34121 Kassel, Germany. No living animals were used. In Germany, the fishing license permits the holder to capture and sacrifice the fish, which can be used not only for consumption but also for research purposes. All fish were stunned by a blow on the head and expertly killed immediately by cervical dislocation and a cardiac stab according to the German Animal Protection Law (§ 4) and the ordinance of slaughter and killing of animals (Tierschlachtverordnung § 13). Because of public accessibility no permissions were required to enter the sampling sites.

Author Contributions

Conceived and designed the experiments: SE SK. Performed the experiments: SE TK. Analyzed the data: SE JK MP SK. Contributed reagents/materials/analysis tools: SK MP. Contributed to the writing of the manuscript: SE JK TK MP SK.

References

  1. 1. Carlton JT, Geller JB (1993) Ecological roulette: the global transport of nonindigenous marine organisms. Science 261: 78–82.
  2. 2. Walther G-R, Post E, Convey P, Menzel A, Parmesan C, et al. (2002) Ecological responses to recent climate change. Nature 416: 389–395.
  3. 3. Leprieur F, Beauchard O, Blanchet S, Oberdorff T, Brosse S (2008) Fish invasions in the world’s river systems: when natural processes are blurred by human activities. Plos Biol 6: e28.
  4. 4. Vitule JRS, Freire CA, Simberloff D (2009) Introduction of non-native freshwater fish can certainly be bad. Fish Fish 10: 98–108.
  5. 5. Ricciardi A, MacIsaac HJ (2011) Impacts of biological invasions on freshwater ecosystems. In: Fifty Years of Invasion Ecology: The Legacy of Charles Elton (Ed. Richardson D. M.), 211–224, Blackwell Publishing.
  6. 6. Martin CW, Valentine MM, Valentine JF (2010) Competitive interactions between invasive Nile Tilapia and native fish: the potential for altered trophic exchange and modification of food webs. Plos One 5: e14395
  7. 7. Dick JTA, Platvoet D (2000) Invading predatory crustacean Dikerogammarus villosus eliminates both native and exotic species. P Roy Soc Lond B Bio 267: 977–983.
  8. 8. Salo P, Korpimaki E, Banks PB, Nordstrom M, Dickman CR (2007) Alien predators are more dangerous than native predators to prey populations. P Roy Soc B-Biol Sci 274: 1237–1243.
  9. 9. Paolucci EM, MacIsaac HJ, Ricciardi A (2013) Origin matters: alien consumers inflict greater damage on prey populations than do native consumers. Divers and Distrib 19: 988–995.
  10. 10. Prenter J, MacNeil C, Dick JTA, Dunn AM (2004) Roles of parasites in animal invasions. Trends Ecol Evol 19: 385–390.
  11. 11. Douda K, Lopes-Lima M, Hinzmann M, Machado J, Varandas S, et al. (2013) Biotic homogenization as a threat to native affiliate species: fish introductions dilute freshwater mussel’s host resources. Divers Distrib 19: 933–943.
  12. 12. Strayer DL (2012) Eight questions about invasions and ecosystem functioning. Ecol Lett 15: 1199–1210.
  13. 13. Witte F, Goldschmidt T, Wanink J, van Oijen M, Goudswaard K, et al. (1992) The destruction of an endemic species flock: quantitative data on the decline of the haplochromine cichlids of Lake Victoria. Environ Biol Fish 34: 1–28.
  14. 14. Kats LB, Ferrer RP (2003) Alien predators and amphibian declines Review of two decades of science and the transition to conservation. Divers and Distrib 9: 99–110.
  15. 15. Machida Y, Akiyama YB (2013) Impacts of invasive crayfish (Pacifastacus leniusculus) on endangered freshwater pearl mussels (Margaritifera laevis and M. togakushiensis) in Japan. Hydrobiologia 720: 145–151.
  16. 16. Ricciardi A, Whoriskey FG, Rasmussen JB (1996) Impact of Dreissena polymorpha on native unionid bivalves in the upper St. Lawrence River. Can J Fish Aquat Sci 53: 1434–1444.
  17. 17. Dick JTA, Platvoet D, Kelly DW (2002) Predatory impact of the freshwater invader Dikerogammarus villosus (Crustacea: Amphipoda). Can J Fish Aquat Sci 59: 1078–1084.
  18. 18. Carlsson NOL, Sarnelle O, Strayer DL (2009) Native predators and exotic prey – an acquired taste? Front Ecol Environ 7: 525–532.
  19. 19. Crawley MJ (1987) What makes a community invasible? In: Colonization, succession, and stability (Eds. Gray A.J., Crawley M.J., Edwards P.J.) 429–453, Blackwell, Oxford.
  20. 20. Torchin ME, Lafferty KD, Dobson AP, McKenzie VJ, Kuris AM (2003) Introduced species and their missing parasites. Nature 421: 628–630.
  21. 21. Kelly DW, Paterson RA, Townsend CR, Poulin R, Tompkins DM (2009) Parasite spillback: A neglected concept in invasion ecology? Ecology 90: 2047–2056.
  22. 22. Kopp K, Jokela J (2007) Resistant invaders can convey benefits to native species. Oikos 116: 295–301.
  23. 23. Paterson RA, Townsend CR, Poulin R, Tompkins DM (2011) Introduced brown trout alternative acanthocephalan infections in native fish. J Anim Ecol 80: 990–998.
  24. 24. Emde S, Rueckert S, Palm HW, Klimpel S (2012) Invasive Ponto-Caspian amphipods and fish increase the distribution range of the acanthocephalan Pomphorhynchus tereticollis in the river Rhine. Plos One 7: e53218
  25. 25. Brandner J, Auerswald K, Cerwenka AF, Schliewen UK, Geist J (2013) Comparative feeding ecology of invasive Ponto-Caspian gobies. Hydrobiologia 703: 113–131.
  26. 26. Locke SA, Bulté G, Marcogliese DJ, Forbes MR (2014) Altered trophic pathway and parasitism in a native predator (Lepomis gibbosus) feeding on introduced prey (Dreissena polymorpha). Oecologia 175: 315–24.
  27. 27. Rohde K (2005) Marine parasitology. CSIRO Publishing.
  28. 28. Busch MW, Kuhn T, Münster J, Klimpel S (2012) Marine crustaceans as potential hosts and vectors for metazoan parasites. In: Arthropods as vectors of emerging diseases (Ed. H. Mehlhorn), 329–360, Parasitol Res Monographs 3, Springer, Berlin Heidelberg.
  29. 29. Brandner J, Cerwenka AF, Schliewen UK, Geist J (2013) Bigger is better: Characteristics of round gobies forming an invasion front in the Danube River. PLoS ONE 8(9): e73036.
  30. 30. Kornis MS, Mercado-Silva N, van der Zanden MJ (2012) Twenty years of invasion: a review of round goby Neogobius melanostomus biology, spread and ecological implications. J Fish Biol 80: 235–285.
  31. 31. Borcherding J, Dolina M, Heermann L, Knutzen P, Krüger S, et al. (2013) Feeding and niche differentiation in three invasive gobies in the Lower Rhine, Germany. Limnologica 43: 49–58.
  32. 32. Skóra KE, Rzeznik J (2001) Observations on diet composition of Neogobius melanostomus Pallas 1811 (Gobiidae, Pisces) in the Gulf of Gdansk (Baltic Sea). J Great Lakes Res 27: 290–299.
  33. 33. Rakauskas V, Bacevičius E, Pūtys Ž, Ložys L, Arbačiauskas K (2008) Expansion, feeding and parasites of the round goby, Neogobius melanostomus (Pallas, 1811), a recent invader in the Curonian Lagoon, Lithuania. Acta Zoologica Lituanica 18 3: 180–190.
  34. 34. Haas G, Brunke M, Streit B (2002) Fast turnover in dominance of exotic species in the Rhine river determines biodiversity and ecosystem function: an affair between amphipods and mussels. In: Invasive aquatic species of Europe: distribution, impacts, and management (eds. Leppäkoski E, Gollasch S, Olenin S), 426–432, Dordrecht.
  35. 35. Bernauer D, Jansen W (2006) Recent invasions of alien macroinvertebrates and loss of native species in the upper Rhine River, Germany. Aquatic Invasions 1 2: 55–71 Available: http://aquaticinvasions.net/2006/AI_2006_1_2_Bernauer_Jansen.pdf Accessed 2012 June 5..
  36. 36. Ondračková M, Francová K, Dávidová M, Polačik M, Jurajda P (2010) Condition status and parasite infection of Neogobius kessleri and N. melanostomus (Gobiidae) in their native and non-native area of distribution of the Danube River. Ecol Res 25: 857–866.
  37. 37. Brandner J, Pander J, Mueller M, Cerwenka AF, Geist J (2013) Effects of sampling techniques on population assessment of invasive round goby Neogobius melanostomus. J Fish Biol 82: 2063–2079.
  38. 38. Storey AW, Edward DHD, Gazey P (1991) Surber and kick sampling: a comparison for the assessment of macroinvertebrate community structure in streams of south-western Australia. Hydrobiologia 211: 111–121.
  39. 39. Schäperclaus W (1991) Fish Diseases. Volume 1. (Ed. Kothekar V.S.), Akademie-Verlag, Berlin.
  40. 40. Riemann F (1988) Introduction to the study of meiofauna. Higgins RP und Thiel H (Eds.). Smithsonian Institution Press: 293–301.
  41. 41. Golvan YJ (1969) Systematique des acanthocephales (Acanthocephala, Rudolphi 1801). L’ordre des Palaeacanthocephala Meyer 1931. La superfamille des Echinorhynchoidea (Cobbold 1876) Golvan et Houin, 1963. Mémoires du Museum National d’Histoire Naturelle, Série A, Zoologie Band 57, Paris: 373 p.
  42. 42. Špakulová M, Perrot-Minnot M-J, Neuhaus B (2011) Resurrection of Pomphorhynchus tereticollis (Rudolphi, 1809) (Acanthocephala: Pomphorhynchidae) based on new morphological and molecular data. Helminthologia 48 3: 268–277.
  43. 43. Moravec F (1994) Parasitic nematodes of freshwater fishes of Europe. Academy of Sciences of the Czech Republic, Academia.
  44. 44. Eggers TO, Martens A (2001) Bestimmungsschlüssel der Süßwasser–Amphipoda (Crustacea) Deutschlands. Lauterbornia 42: 1–68.
  45. 45. Eggers TO, Martens A (2004) Ergänzungen und Korrekturen zum “Bestimmungsschlüssel der Süßwasser-Amphipoda (Crustacea) Deutschlands”. Lauterbornia 50: 1–13.
  46. 46. Quigley MA, Lang GA (1989) Measurement of amphipod body length using a digitizer. Hydrobiologia 171: 255–258.
  47. 47. R Development Core Team (2010) R: A language and environment for statistical computing. Foundation for Statistical Computing, Vienna, Austria.
  48. 48. Hyslop EJ (1980) Stomach content analysis - a review of methods and their application. J Fish Biol 17: 411–429.
  49. 49. Amundsen PA, Gabler HM, Staldvik FJ (1996) A new approach to graphical analysis of feeding strategy from stomach contents data – modification of the Costello (1990) method. J Fish Biol 48: 607–614.
  50. 50. Pinkas L, Oliphant MD, Iverson ILK (1971) Food habits of albacore, bluefin tuna and bonito in Californian waters. Calif Fish Game 152: 1–105.
  51. 51. Clarke KR (1993) Non-parametric multivariate analyses of changes in community structure. Aust J Ecol 18: 117–143.
  52. 52. Bush O, Lafferty AD, Lotz JM, Shostak AW (1997) Parasitology meets ecology on his own terms: Margolis, et al. revisited. J Parasitol 83: 575–583.
  53. 53. McKinney ML, Lockwood JL (1999) Biotic homogenizaton: a few winners replacing many losers in the next mass extinction. Trends Ecol Evol 14: 450–453.
  54. 54. Vanderploeg HA, Nalepa TF, Jude DJ, Mills EL, Holeck KT, et al. (2002) Dispersal and emerging ecological impacts of Ponto-Caspian species in the Laurentian Great Lakes. Can J Fish Aquat Sci 59: 1209–1228.
  55. 55. IKSR (2002) Das Makrozoobenthos des Rheins 2000, Internationale Kommission zum Schutz des Rheins (IKSR), Bericht Nr. 128-d.doc; Koblenz.
  56. 56. Corkum LD, Sapota MR, Skora KE (2004) The round goby, Neogobius melanostomus, a fish invader on both sides of the Atlantic Ocean. Biol Invasions 6: 173–181.
  57. 57. Karlson AML, Almqvist G, Skóra KE, Appelberg M (2007) Indications of competition between non-indigenous round goby and native flounder in the Baltic Sea. ICES J Mar Sci 64: 479–486.
  58. 58. Campbell LM, Thacker R, Barton D, Muir DCG, Greenwood D, et al. (2009) Re-engineering the eastern Lake Erie littoral food web: the trophic function of non-indigenous Ponto-Caspian species. J Great Lakes Res 35: 224–231.
  59. 59. Grabowski M, Jaždžewski K, Konopacka A (2007) Alien Crustacea in Polish waters. Aquatic Invasions 2: 25–38.
  60. 60. Chen W, Bierbach D, Plath M, Streit B, Klaus S (2012) Distribution of amphipod communities in the Middle to Upper Rhine and five tributaries. BioInvasions Rec 1: 263–271.
  61. 61. Podraza P, Ehlert T, Roos P (2001) Erstnachweis von Echinogammarus trichiatus (Crustacea: Amphipoda) im Rhein. Lauterbornia 41: 129–133.
  62. 62. Sih A, Christensen B (2001) Optimal diet theory: when does it work, and when and why does it fail? Anim Behav 61: 379–390.
  63. 63. Boets P, Lock K, Tempelman D, van Haaren T, Platvoet D, et al. (2012) First occurrence of the Ponto-Caspian amphipod Echinogammarus trichiatus (Martynov, 1932) (Crustacea: Gammaridae) in Belgium. BioInvasions Rec 1: 115–120.
  64. 64. Baldauf SA, Thünken T, Frommen JG, Bakker TC, Heupel O, et al. (2007) Infection with an acanthocephalan manipulates an amphipod’s reaction to a fish predator’s odours. Int J Parasitol 37: 61–65.
  65. 65. Van Riel MC, Van der Velde G, Bij de Vaate A (2003) Pomphorhynchus spec. (Acanthocephala) uses the invasive amphipod Chelicorophium curvispinum (G.O. Sars, 1895) as an intermediate host in the river Rhine. Crustaceana 7: 241–246.
  66. 66. Kvach J, Stepien CA (2008) Metazoan parasites of introduced Round and Tubenose Gobies in the Great Lakes: Support for the “Enemy Release Hypothesis”. J Great Lakes Res 34: 23–35.
  67. 67. Ondračková M, Dávidová M, Pečínková M, Blažek R, Gelnar M, et al. (2005) Metazoan parasites of Neogobius fishes in the Slovak section of the River Danube. J Appl Ichthyol 21: 345–349.
  68. 68. Kvach J, Skóra KE (2007) Metazoa parasites of the invasive round goby Apollonia melanostoma (Neogobius melanostomus) (Pallas) (Gobiidae: Osteichthyes) in the Gulf of Gdańsk, Baltic Sea, Poland: a comparison with the Black Sea. Parasitol Res 100: 767–774.
  69. 69. Francová K, Ondračová M, Polačik M, Jurajda P (2011) Parasite fauna of native and non-native populations of Neogobius melanostomus (Pallas, 1814) (Gobiidae) in the longitudinal profile of the Danube River. J Appl Ichthyol 27: 879–886.
  70. 70. Kvach Y (2005) A comparative analysis of helminth faunas and infection of ten species of gobiid fishes (Actinopterigii: Gobiidae) from the North-Western Black Sea. Acta Ichthyol Piscat 35: 103–110.
  71. 71. Borcherding J, Staas S, Krüger S, Ondračková M, Ślapansky L, et al. (2011) Non-native Gobiid species in the lower River Rhine (Germany): recent range extensions and densities. A review of Gobiid expansion along the Danube-Rhine corridor – geopolitical change as a driver for invasion. J Appl Ichthyol 27: 153–155.
  72. 72. Roche KF, Janač M, Jurajda P (2013) A review of Gobiid expansion along the Danube-Rhine corridor – geopolitical change as a driver for invasion. Knowl Manag Aquat Ec 411: 01.
  73. 73. Gendron AD, Marcogliese DJ, Thomas M (2012) Invasive species are less parasitized than native competitors, but for how long? The case of the round goby in the Great Lakes-St. Lawrence Basin. Biol Invasions 14: 367–384.
  74. 74. Woo PTK, Buchmann K (2012). Fish Parasites. Pathobiology and Protection. CABI.
  75. 75. Nachev M, Ondračková M, Severin S, Ercan F, Sures B (2010) The impact of invasive gobies on the local parasite fauna of the family percidae and the gudgeon (Gobio gobio) in the Rhine River. In: Tagungsband der Deutschen Gesellschaft für Protozoologie und Parasitologie 2010.
  76. 76. Hine PM, Kennedy CR (1974) Observations on the distribution, specificity and pathogenicity of the acanthocephalan Pomphorhynchus laevis (Müller). J Fish Biol 6: 521–535.
  77. 77. Dezfuli BS, Castaldelly G, Bo T, Lorenzoni M, Giari L (2011) Intestinal immune response of Silurus glanis and Barbus barbus naturally infected with Pomphorhynchus laevis (Acanthocephala). Parasite Immunol 33: 116–123.
  78. 78. Moravec F (2013) Parasitic Nematodes of Freshwater fishes of Europe. Academia Praha, 264–284.
  79. 79. Kopp K, Jokela J (2007) Resistant invaders can convey benefits to native species. Oikos 116: 295–301.
  80. 80. Telfer S, Bown KJ, Sekules R, Begon M, Hayden T, et al. (2005) Disruption of a host-parasite system following the introduction of an exotic host species. Parasitology 130: 661–668.
  81. 81. Brodniewicz I (1968) On glochidia of the genera Unio and Anodonta from the quaternary fresh-water sediments of Poland. Acta Palaeontol Pol XIII: 619–631.
  82. 82. Zieritz A, Gum B, Kuehn R, Geist J (2012) Identifying freshwater mussels (Unionoida) and parasitic glochidia larvae from host fish gills: a molecular key to the North and Central European species. Ecol Evol 2: 740–750.
  83. 83. Taeubert JE, Gum B, Geist J (2012) Host-specificity of the endangered thick-shelled river mussel (Unio crassus, Philipsson 1788) and implications for conservation. Aquat Conserv 22: 36–46.
  84. 84. Poulin R (1996) Helminth growth in vertebrate hosts: Does host sex matter? Int J Parasitol 2: 1311–1315.
  85. 85. Robinson SA, Forbes MR, Hebert CE, McLauglin JD (2010) Male biased parasitism in cormorants and relationships with foraging ecology on Lake Erie, Canada. Waterbirds 33: 307–313.
  86. 86. Koehler A, Hildenbrand P, Schleucher E, Riesch R, Arias-Rodriguez L, et al. (2011) Effects of male sexual harassment on female time budgets, feeding behavior, and metabolic rates in a tropical livebearing fish (Poecilia mexicana). Behav Ecol Sociobiol 65: 1513–1523.
  87. 87. Scharnweber K, Plath M, Tobler M (2011) Trophic niche segregation between the sexes in two species of livebearing fishes (Poeciliidae). Bull Fish Biol 13: 11–20.
  88. 88. Charlebois PM, Marsden JE, Goettel RG, Wolfe RK, Jude DJ, et al.. (1997) The round goby, Neogobius melanostomus (Pallas): a review of European and North American literature. Illinois-Indiana Sea Grant Program and Illinois Natural History Survey. INHS Special Publication No.20.