The round goby, Neogobius melanostomus, is a Ponto-Caspian fish considered as an invasive species in a wide range of aquatic ecosystems. To understand the role that parasites may play in its successful invasion across Western Europe, we investigated the parasitic diversity of the round goby along its invasion corridor, from the Danube to the Upper Rhine rivers, using data from literature and a molecular barcoding approach, respectively. Among 1666 parasites extracted from 179 gobies of the Upper Rhine, all of the 248 parasites barcoded on the c oxidase subunit I gene were identified as Pomphorhynchus laevis. This lack of macroparasite diversity was interpreted as a loss of parasites along its invasion corridor without spillback compensation. The genetic diversity of P. laevis was represented by 33 haplotypes corresponding to a haplotype diversity of 0·65 ± 0·032, but a weak nucleotide diversity of 0·0018 ± 0·00015. Eight of these haplotypes were found in 88·4% of the 248 parasites. These haplotypes belong to a single lineage so far restricted to the Danube, Vistula and Volga rivers (Eastern Europe). This result underlines the exotic status of this Ponto-Caspian lineage in the Upper Rhine, putatively disseminated by the round goby along its invasion corridor.

Predicting how elevated temperatures from climate change alter host–parasite interactions requires understandings of how warming affects host susceptibility and parasite virulence. Here, the effect of elevated water temperature and parasite exposure level was tested on parasite prevalence, abundance and burden, and on fish growth, using Pomphorhynchus laevis and its fish host Squalius cephalus. At 60 days post-exposure, prevalence was higher at the elevated temperature (22 °C) than ambient temperature (18 °C), with infections achieved at considerably lower levels of exposure. Whilst parasite number was significantly higher in infected fish at 22 °C, both mean parasite weight and parasite burden was significantly higher at 18 °C. There were, however, no significant relationships between fish growth rate and temperature, parasite exposure, and the infection parameters. Thus, whilst elevated temperature significantly influenced parasite infection rates, it also impacted parasite development rates, suggesting warming could have complex implications for parasite dynamics and host resistance.

Concentrations of the elements As, Cd, Co, Cu, Fe, Mn, Mo, Ni, Pb, Sn, V, Zn were analysed by inductively coupled plasma mass spectrometry (ICP-MS) in the acanthocephalan Pomphorhynchus laevis and its fish host Barbus barbus. A total of 27 barbels were collected from the Danube River in autumn 2006 close to the town Kozloduy (685 river kilometer) on the Bulgarian river bank. Fish were divided into 3 groups. According to their P. laevis infrapopulation size hosts were considered as heavily infected (>100 worms per fish) and lightly infected (<20 worms per fish). The third group was used to compare heavy metal concentrations between male and female P. laevis. The 5 elements As, Cd, Cu, Pb and Zn were detected in significantly higher concentrations in parasites compared to host tissues (muscle, intestine, liver). According to the calculated mean bioconcentration factors, 3 more elements (Co, Mn, V) showed usually higher concentrations in P. laevis. Comparisons between heavily and lightly infected fish revealed significant differences only for V with higher concentrations for the heavily infected group. Concerning sex-specific metal accumulation V and Zn showed significant differences (V, at P<0·05; Zn, at P=0·05), with higher levels of both metals in females of P. laevis. Our results suggest that – for the metals analysed – the size of the parasite infrapopulation plays no role in the degree of metal accumulation. Similarly, parasite sex seems not to be a crucial factor for metal accumulation in the parasites. Thus, for metal monitoring purposes there is no need to take these aspects into account, which makes the use of parasites as bioindicators more applicable.

Biological invasions expose parasites to new invasive hosts in addition to their local hosts. However, local parasites are often less successful in infecting and exploiting their new hosts. This may have major consequences for the competitive ability of hosts, and finally on the fate of the parasite-host community. In Burgundy (Eastern France), the acanthocephalan parasite, Pomphorhynchus laevis, infects 2 amphipod species living in sympatry: the native Gammarus pulex and the invasive Gammarus roeseli. While P. laevis affects the behaviour and the immunity of G. pulex, G. roeseli seems unaffected by the infection. In this study, we examined in detail the ability of the parasite to affect the immune system and resource storage of both gammarid species. We found that the infection was associated with a general decrease of the prophenoloxidase activity, haemocyte density, resistance to an artificial bacterial infection and level of sugar reserves in G. pulex, but not in G. roeseli. These results demonstrate a differential ability of P. laevis to exploit its local and its invasive gammarid hosts. Potential mechanisms of these differential physiological alterations and their potential consequences on the coexistence of both gammarid species in sympatry are discussed.

A total of 112 flounder, Platichthys flesus, was surveyed for the presence of Pomphorhynchus laevis (Acanthocephala). No acanthocephalans were found in 35 fish, 40 fish were infected with 1–5 thorny-headed worms, and in 37 fish the intensity of infection exceeded 6. The fish surveyed were also studied for the composition of the resident microflora of the terminal portion of the intestine—the characteristic site for P. laevis. It turned out the number of worms was positively correlated with the bacterial count. The bacteria cultured from the material sampled from intestines of infected- and non-infected fish exhibited similar specific composition and the differences were only in the intensity of individual species. The most frequently isolated bacteria, represented the genera: Aeromonas, Pseudomonas and Shewanella. The least frequent were species of Chromobacterium, Escherichia, Providencia and Serratia. The presently reported study results, suggest that bacteria species, constituting physiological bacterial flora, may become, under certain conditions, potentially pathogenic, through substantial increase in their abundance. The factor responsible for disturbing the natural balance in the intestine may be the thorny-headed worm, Pomphorhynchus laevis.

Threshold density levels were investigated experimentally in rainbow trout, Oncorhynchus mykiss, for the acanthocephalan Pomphorhynchus laevis. Fifty small fish 120–150 mm long were infected with 50 cystacanths each every 4 weeks for 16 weeks, and samples of up to 10 fish examined at 1, 5, 9, 13 and 17 weeks after the original infection. The mean intestinal length was found to have increased significantly over this period, from a mean of 125 to 173 mm and the mean number of parasites recovered increased after each interval of 4 weeks, and significantly from 19·30 to 86·40 per fish over the 17 weeks. The distribution of the parasites in the alimentary tract of O. mykiss was unaltered, indicating that the favourable region for establishment had remained unchanged. The failure to detect any manifestations of competition is discussed, and the relevance of conventional approaches to experimental investigations of both intra- and interspecific competition in fish was queried, and contrasted to the situation in birds and mammals.

Chub and eels were experimentally infected via intermediate hosts harbouring cystacanths, with Pomphorhynchus laevis alone, or Acanthocephalus anguillae alone, or simultaneously with mixtures of both species in varying proportions, and sampled at 7, 56 or 112 days post-infection. Examination of chub revealed that both species showed low establishment and growth rates, differing markedly from British field data, where chub is apparently one of the most important hosts, and preventing further meaningful experiments. Both species showed higher establishment rates in eels, but A. anguillae both grew faster and matured to a greater extent than P. laevis, reflecting field data. No evidence of any interactions, competitive or otherwise, was found in the mixed infections in eels at low or high infection levels. The results were interpretated with regard to British field data and current theories of the role of interspecific competition in structuring fish helminth communities.

Rainbow trout were infected simultaneously with cystacanths of the two acanthocephalan species Pomphorhynchus laevis and Acanthocephalus anguillae in the laboratory. Infections consisted of varying proportions of the two species (from 0 to 100%) at low (20 cystacanths) and high (60 cystacanths) levels. Fish were sacrificed at 7, 56 and 112 days post-infection (p.i.) and the parasites recovered. At low-level infections there was a high degree of overlap in the range of intestine occupied by the two species (44·12 %) and no evidence for interspecific competition affecting either species. At high-level infections the establishment of both species was unaffected by the presence or number of individuals of the other species. The survivorship and the range of intestine occupied by A. anguillae were found to be reduced in the presence of P. laevis. The interaction was one-sided, as P. laevis remained unaffected by A. anguillae, and so indicated interference competition. The possibility that both intraspecific and interspecific competition are implicated in explaining the distribution of the two species in the British Isles is discussed.

Comparisons were made between the protein concentrations and protein profiles of uninfected Gammarus pulex and those infected with cystacanths of Pomphorhynchus laevis. Comparisons of the extractable haemolymph volumes from uninfected and infected G. pulex, when related to host tissue weight, showed that there was no difference in the host tissue/haemolymph volume ratio between infected and uninfected individuals. SDS-PAGE of haemolymph samples resolved 36 bands of protein, the major components being band 6(19 kDa) and 21(33 kDa). Densitometric analysis of haemolymph samples showed that infected gammarids had total protein concentrations elevated by 84%. The (88 kDa) subunit was identified as the monomer of the respiratory protein haemocyanin. This band was significantly elevated in infected individuals (one-way ANOVA P < 0.001); increasing from 30 to 45% of the total haemolymph proteins.

In and around the British Isles, three biologically specialized strains of the fish acanthocephalan Pomphorhynchus laevis have been recognized, each differing in geographical distribution and both final and intermediate host utilization. Six restriction endonuclease digests of the genomic DNA of the English freshwater and marine strains were compared by genomic cross-hybridization using the randomly sheared and radio-isotope labelled genomic DNA of freshwater P. laevis, and also by using a heterologous isotope labelled ribosomal DNA probe. Autoradiographs of hybridized Southern blots provided evidence for considerable homogeneity in the total repetitive DNA of the two strains as well as some preliminary evidence for possible restriction fragment length polymorphisms using EcoR I and Pvu II.

Phenotypic alterations induced by parasites in their intermediate hosts often result in enhanced trophic transmission to appropriate final hosts. However, such alterations may also increase the vulnerability of intermediate hosts to predation by non-host species. We studied the influence of both infection with 3 different acanthocephalan parasites (Pomphorhynchus laevis, P. tereticollis, and Polymorphus minutus) and the availability of refuges on the susceptibility of the amphipod Gammarus pulex to predation by 2 non-host predators in microcosms. Only infection with P. laevis increased the vulnerability of amphipods to predation by crayfish, Orconectes limosus. In contrast, in the absence of refuges, the selectivity of water scorpions, Nepa cinerea, for infected prey was significant and did not differ according to parasite species. When a refuge was available for infected prey, however, water scorpion selectivity for infected prey differed between parasite species. Both P. tereticollis- and P. laevis-infected gammarids were more vulnerable than uninfected ones, whereas the reverse was true of P. minutus-infected gammarids. These results suggest that the true consequences of phenotypic changes associated with parasitic infection in terms of increased trophic transmission of parasites deserve further assessment.

A hypothesis has been erected stating that in the British Isles the acanthocephalan, Pomphorhynchus laevis can be separated into 3 strains, an English, Irish and marine strain. Ecological and morphological evidence exists in support of this hypothesis. An investigation at the molecular level was conducted in order to test the validity of the existing evidence. A mitochondrial gene, subunit I of cytochrome c oxidase was partially sequenced from 3 Irish populations of P. laevis, 1 Scottish population and 3 English populations. P. laevis sequences from brown trout from Ireland, England and Scotland were very similar, showing a mean sequence divergence of 0·7%. Sequences from two populations of P. laevis from English chub and bullhead were also similar to each other (0·35% divergence). These two groups of sequences, the brown trout group and the chub/bullhead group were 2·2% different. These data confirm the existence of at least 2 strains in Ireland and Britain, although there is evidence to suggest that these strains are defined by their host species rather than by their geographical distributions.

This paper describes two approaches to evaluate the use of fish macroparasites as bioindicators of heavy metal pollution at selected river stretches in Austria. Firstly changes in the diversity and richness of endoparasites of the cyprinid barbel, Barbus barbus (L.), were tested in relation to heavy metal contents in the aquatic system. Secondly, the bioaccumulation potential of cadmium, lead and zinc was assessed in the acanthocephalan, Pomphorhynchus laevis (Müller, 1776), and compared with that in the muscle, liver and intestine of its barbel host. The present results indicated that in order to validate the role of parasite community patterns related to heavy metal pollution, more investigations on food web dynamics, interelationships between parasites and the presence/absence of intermediate hosts will be essential. Heavy metal concentrations differed significantly between the organs of barbel and P. laevis (P=0·001) with levels up to 2860 fold in the parasite. The high level of heavy metal accumulation in P. laevis compared with that in its barbel host, suggests that despite variability in the parasite infrapopulation, host mobility and feeding behaviour, P. laevis is a most sensitive indicator of heavy metals in aquatic ecosystems.

The acanthocephalan parasite Pomphorhynchus laevis is transmitted by a suitable intermediate host such as the amphipod Gammarus pulex to its definitive fish host. Parasite-induced alterations in both appearance and behaviour concur to render infected gammarids more vulnerable to predation, thus promoting parasite transmission. Experimental infection of laboratory bred full- and half-sib groups of three-spined sticklebacks (Gasterosteus aculeatus) provided evidence that the parasite imposes a survival cost proportional to the severity of infection on its final host. Variation among sibships in the susceptibility to infection was consistent. When given a choice, fish consumed significantly more infected than uninfected prey. Overall, more resistant fish did not prey upon infected gammarids more often than did relatively susceptible fish. Only fish with a relatively high physical condition properly adjusted prey selection to the extent of their parasite susceptibility, thus exploiting the enhanced profitability of infected prey.

Larval helminths often share individual intermediate hosts with other larval worms of the same or different species. In the case of immature acanthocephalans capable of altering the phenotype of their intermediate hosts, the benefits or costs of host sharing can be evaluated in terms of increased or decreased probability of transmission to a suitable definitive host. Competitive interactions among the immature stages of acanthocephalans within the intermediate host could create additional costs of host sharing, however. The effects of intraspecific and interspecific interactions were measured in 3 sympatric species of acanthocephalans exploiting a population of the amphipod Echinogammarus stammeri in the River Brenta, Italy. The strength of interactions was assessed from differences in the size achieved by infective cystacanths in the intermediate host. The size of Pomphorhynchus laevis cystacanths was not correlated with host size, whereas the size of Acanthocephalus clavula and Polymorphus minutus cystacanths increased with host size. Reductions in cystacanth size caused by intraspecific competition were only detected in P. laevis, but may also occur in both A. clavula and P. minutus. When co-occurring in the same amphipod with cystacanths of A. clavula, cystacanths of P. laevis attained a smaller size than when they occurred on their own. This effect was not reciprocal, with the size of A. clavula cystacanths not being affected. This supports earlier suggestions that it is adaptive for A. clavula to associate with P. laevis in amphipod intermediate hosts, with both species going to the same fish definitive hosts. In contrast, cystacanths of P. laevis achieved their largest size when they co-occurred in an amphipod with a cystacanth of P. minutus, which has a different definitive host (i.e. birds). These findings suggest that the net benefits of sharing an intermediate host can only be estimated by taking into account both the effects on transmission success and the consequences for cystacanth development.

When two parasite species are manipulators and have different definitive hosts, there is a potential for conflict between them. Selection may then exist for either avoiding hosts infected with conflicting parasites, or for hijacking, i.e. competitive processes to gain control of the intermediate host. The evidence for both phenomena depends largely on the study of the relative competitive abilities of parasites within their common intermediate host. We studied the effects of simultaneous infection by a fish acanthocephalan parasite, Pomphorhynchus laevis, and a bird acanthocephalan parasite, Polymorphus minutus, on the behaviour of their common intermediate host, the amphipod Gammarus pulex. We compared the reaction to light and vertical distribution of individuals infected with both parasites to those of individuals harbouring a single parasite species and uninfected ones under controlled conditions. Compared to uninfected gammarids that were photophobic and tended to remain at the bottom of the water column, P. laevis-infected gammarids were attracted to light, whereas P. minutus-infected individuals showed a modified vertical distribution and were swimming closer to the water surface. The effects of both P. laevis and P. minutus appeared to be dependent only on their presence, not on their intensity. Depending on the behavioural trait under study, however, the outcome of the antagonism between P. laevis and P. minutus differed. The vertical distribution of gammarids harbouring both parasites was half-way between those of P. laevis- and P. minutus-infected individuals, whereas P. laevis was able to induce altered reaction to light even in the presence of P. minutus. We discuss our results in relation to the occurrence of active avoidance or hijacking between conflicting manipulative parasites and provide some recommendations for future research.