Heavy ionic pollution disrupts assemblages of algae, macroinvertebrates and riparian vegetation.

Urban streams display consistent ecological symptoms that commonly express degraded biological, physical, and chemical conditions: the urban stream syndrome (USS). Changes linked to the USS result in consistent declines in the abundance and richness of algae, invertebrates, and riparian vegetation. In this paper, we assessed the impacts of extreme ionic pollution from an industrial effluent in an urban stream. We studied the community composition of benthic algae and benthic invertebrates and the indicator traits of riparian vegetation. The dominant pool of benthic algae, benthic invertebrates and riparian species were considered as euryece. However, ionic pollution impacted these three biotic compartments' communities, disrupting these tolerant species assemblages. Indeed, after the effluent, we observed the higher occurrence of conductivity-tolerant benthic taxa, like Nitzschia palea or Potamopyrgus antipodarum and plant species reflecting nitrogen and salt contents in soils. Providing insights into organisms' responses and resistance to heavy ionic pollution, this study sheds light on how industrial environmental perturbations could alter the ecology of freshwater aquatic biodiversity and riparian vegetation.

Acanthocephalan parasites reflect ecological status of freshwater ecosystem.

Acanthocephalans' position in food webs, in close interaction with free-living species, could provide valuable information about freshwater ecosystem health through the viability of the parasites' host populations. We explored Pomphorhynchus laevis cystacanths' and adults' intensities of infection, and the prevalence of infected hosts respectively in their Gammarus pulex intermediate hosts and Squalius cephalus definitive hosts in a Mediterranean river. First, we analysed the relationship between P. laevis intensity of infection, its two hosts populations and the other acanthocephalan species found (Pomphorhynchus tereticollis and Polymorphus minutus). Second, we characterised the influence of bacteriological, physicochemical and biological water parameters on these acanthocephalans, and their intermediate and definitive hosts. This research highlights that P. laevis infection was closely related to their two preferential hosts population in the river. Moreover, P. laevis intensity of infection was positively correlated with organic pollution in the river but negatively correlated with biodiversity and with ecological indexes of quality. Pomphorhynchus laevis could thus benefit from moderate freshwater pollution, which promotes their tolerant intermediate and definitive hosts.

Carotenoid-based colour of acanthocephalan cystacanths plays no role in host manipulation.

Manipulation by parasites is a catchy concept that has been applied to a large range of phenotypic alterations brought about by parasites in their hosts. It has, for instance, been suggested that the carotenoid-based colour of acanthocephalan cystacanths is adaptive through increasing the conspicuousness of infected intermediate hosts and, hence, their vulnerability to appropriate final hosts such as fish predators. We revisited the evidence in favour of adaptive coloration of acanthocephalan parasites in relation to increased trophic transmission using the crustacean amphipod Gammarus pulex and two species of acanthocephalans, Pomphorhynchus laevis and Polymorphus minutus. Both species show carotenoid-based colorations, but rely, respectively, on freshwater fish and aquatic bird species as final hosts. In addition, the two parasites differ in the type of behavioural alteration brought to their common intermediate host. Pomphorhynchus laevis reverses negative phototaxis in G. pulex, whereas P. minutus reverses positive geotaxis. In aquaria, trout showed selective predation for P. laevis-infected gammarids, whereas P. minutus-infected ones did not differ from uninfected controls in their vulnerability to predation. We tested for an effect of parasite coloration on increased trophic transmission by painting a yellow-orange spot on the cuticle of uninfected gammarids and by masking the yellow-orange spot of infected individuals with inconspicuous brown paint. To enhance realism, match of colour between painted mimics and true parasite was carefully checked using a spectrometer. We found no evidence for a role of parasite coloration in the increased vulnerability of gammarids to predation by trout. Painted mimics did not differ from control uninfected gammarids in their vulnerability to predation by trout. In addition, covering the place through which the parasite was visible did not reduce the vulnerability of infected gammarids to predation by trout. We discuss alternative evolutionary explanations for the origin and maintenance of carotenoid-based colorations in acanthocephalan parasites.

Habitat segregation mediates predation by the benthic fish Cottus gobio on the exotic amphipod species Gammarus roeseli.

Predation is often considered as one of the most important biotic factor determining the success of exotic species. The freshwater amphipod Gammarus roeseli has widely colonized Western Europe, where it is frequently found in sympatry with the native species (Gammarus pulex). Previous laboratory experiments revealed that G. roeseli may have an advantage over G. pulex through differential predation by native fish (brown trout). Morphological anti-predator defences (spines) were found responsible for lower rates of predation on the invasive G. roeseli. Here, using both field surveys and laboratory experiments, we tested if a differential of predation exists with other fish predators naturally encountered by gammarids. The main predators present in our field site were nocturnal benthic feeders (mainly bullheads, Cottus gobio). Fish diet analysis showed that, compared to its global availability in the river, G. roeseli was less consumed than G. pulex. In the field, however, G. roeseli was found mainly in the aquatic vegetation whereas G. pulex was found in all habitat types. Laboratory experiments in microcosms revealed that G. roeseli was less prone to predation by C. gobio only when vegetation was present. Depending on the type of predator, the differential of predation could therefore be mediated by antipredator behaviour, and a better usage of refuges, rather than by morphological defences.

Modification of hosts' behavior by a parasite: field evidence for adaptive manipulation.

Parasites relying on trophic transmission to complete their life cycles often induce modifications of their host's behavior in ways that may increase their susceptibility to predation by final hosts. These modifications have often been interpreted as parasite adaptations, but very few studies have demonstrated that host manipulation has fitness benefits for the parasite. The aim of the present study was to address the adaptive significance of parasite manipulation by coupling observations of behavioral manipulation to estimates of trophic transmission to the definitive host in the natural environment. We show that the acanthocephalan parasite Pomphorhynchus laevis manipulates the drifting behavior of one of its intermediate hosts, the amphipod Gammarus pulex, but not of a sympatric host, the introduced amphipod Gammarus roeseli. We found a 26.3-28.3 times higher proportion of infected G. pulex in the stomach content of one of the definitive hosts of P. laevis, the bullhead Cottus gobio, than in the benthos. No such trend was observed for G. roeseli. The bell-shaped curve of mean parasite abundance (MPA) relative to host size observed in G. pulex also supported an increased predation mortality of P. laevis-infected individuals compared to uninfected amphipods. Again, no such pattern was observed in G. roeseli. Furthermore, our results indicate that the modifications induced by P. laevis are specific to the definitive host and do not increase the risk of predation by inappropriate hosts, here the adult edible frog Rana esculenta. Overall, our study is original in that it establishes, under field conditions, a direct link between parasitic manipulation and increased transmission to the definitive host, and more importantly, identifies the specificity of the manipulation both in the intermediate host species and toward the definitive host.

Increased susceptibility to predation and altered anti-predator behaviour in an acanthocephalan-infected amphipod.

According to the 'parasitic manipulation hypothesis', phenotypic changes induced by parasites in their intermediate hosts are effective means of increasing trophic transmission to final hosts. One obvious prediction, although seldom tested, is that increased vulnerability of infected prey to an appropriate predator should be achieved by the parasite altering the anti-predator behaviour of its intermediate host. In this study, we tested this prediction using the fish acanthocephalan Pomphorhynchus tereticollis and the freshwater amphipod Gammarus pulex. Firstly, we estimated the relative vulnerability of infected and uninfected gammarids to predation by the bullhead Cottus gobio in the field. Second, we investigated under experimental conditions how two common anti-predator behaviours of aquatic invertebrates, refuge use and short-distance reaction to predator chemical cues, were affected by infection status. We found that the prevalence of infection in the field was 10 times higher among gammarids collected from the stomach contents of bullheads compared with free-ranging individuals collected in the same river. In a microcosm uninfected gammarids, but not infected ones, increased the use of refuge in the presence of a bullhead. Finally, a behavioural experiment using an Y-maze olfactometer showed opposite reactions to predator odour. Whereas uninfected gammarids were significantly repulsed by the chemical cues originating from bullheads, infected ones were significantly attracted to the odour of the predator. Taken together, our results suggest that the alteration of anti-predator behaviour in infected G. pulex might enhance predation by bullheads in the field. Reversing anti-predator behaviour might thus be an efficient device by which parasites with complex life-cycles increase their trophic transmission to final hosts. Further studies should pay more attention to both the increased vulnerability of infected prey to an appropriate predator in the field and the influence of parasitic infection on the anti-predator behaviour of intermediate hosts.