A global assessment of freshwater mollusk shell oxygen isotope signatures and their relation to precipitation and stream water.

Records of δ18O in stream flow are critical for understanding and modeling hydrological, ecological, biogeochemical and atmospheric processes. However, the number of such records are extremely limited globally and the length of such time series are usually less than a decade. This situation severely handicaps their use in model testing and evaluation. Here we present a global assessment of freshwater mollusk (bivalves & gastropods) isotope data from 25 river basins that have stream water isotope values, water temperature data and shell material isotope signatures. Our data span a latitude range of 37.50°S to 52.06°N. We show that δ18O signatures in freshwater mollusks are able to explain 95% of the variance of stream water δ18O. We use shell δ18O values and water temperature data to reconstruct stream water δ18O signatures. With freshwater mussel life expectancy ranging from a few years up to 200 years, this translation of mollusk metabolic properties into long term stream water isotope records is a promising approach for substantially extending global stream water isotope records in time and space.

Impact of the cestode Triaenophorus nodulosus on the exotic Lepomis gibbosus and the autochthonous Perca fluviatilis.

The effects of plerocercoids of the cestode Triaenophorus nodulosus infecting the livers of native Eurasian perch Perca fluviatilis and non-native pumpkinseed Lepomis gibbosus was investigated in 17 sites along the Moselle watershed. With a single exception, infected individuals were not observed in the main channel whether or not northern pike Esox lucius, a final host, was present. In ponds where the pike was present, the prevalence of T. nodulosus averaged 86% in Eurasian perch and 15% in pumpkinseed. The parasite was not present at all in ponds when pike were absent. Parasite load, hepatosomatic index (HSI), gonadosomatic index (GSI) and body condition index (CI) were compared between hosts in one site where parasite prevalence and fish abundance was highest. HSI in infected perch was significantly higher than in uninfected perch, whereas no differences in HSI were detected between infected and uninfected pumpkinseed. While perch were more frequently infected and had a greater average parasite load than pumpkinseed, there were no significant differences in either indicator between the two species. Furthermore, no significant differences in GSI or CI were observed between infected and uninfected fish in either species, by either gender or maturity stage. We hypothesize that pumpkinseed is more resistant to the parasite or less likely to feed upon infected copepods than perch.

Do male and female gammarids defend themselves differently during chemical stress?

To investigate xenobiotic impacts on organism physiology, several studies involve biomarker assessment. However, most studies do not take into account the toxic effect on both males and females. Here, we have investigated the influence of gender on the detoxification response (reduced glutathione, metallothionein, γ-glutamylcystein ligase and carotenoid), energy reserves (protein, lipids and glycogen) and biomarker of toxic effects (malondialdehyde) in Gammarus roeseli exposed to cadmium. A principal component analysis revealed that G. roeseli males and females were differently impacted by cadmium. We observed lower malondialdehyde levels in females than in males, whatever the condition tested (i.e. control, 2 and 8 µg CdL(-1)), although the pattern of responses of control and exposures to 2 or 8 µgL(-1) was the same for both genders. Results could be linked to apparently more effective detoxification displayed by females than by males. Protein concentrations were unchanged in both genders, lipids contents were always significantly decreased and glycogen contents decreased only in females. This study supports the importance of taking into account the gender in ecotoxicological studies to have an overview of xenobiotics effects on a population.

Acanthocephalan parasites: help or burden in gammarid amphipods exposed to cadmium?

We investigated the influence of the acanthocephalan parasite Polymorphus minutus on the mortality of its intermediate host, Gammarus roeseli, exposed to cadmium, by the measure of LC(50-96h) values as well as the bioaccumulation of cadmium both in the host and in its parasite. LC(50) results revealed that infected G. roeseli males died less under cadmium stress than uninfected ones; while the converse has been observed in females. Cadmium resistance of infected males could be explained by a weaker bioconcentration factor (BCF) than in females. The lower BCF in infected individuals was closely related with an uptake of cadmium by P. minutus in its host. Nevertheless, although infected females had both weaker BCF and cadmium concentration in their body, the presence of P. minutus did not induce lower mortality than uninfected females. On the contrary, their sensitivity to cadmium was increased by the presence of P. minutus. We discuss the hypothesis that differences of mortality between uninfected and infected gammarids could be explained by a difference of cadmium bioconcentration in host, and by the cadmium bioaccumulation in the parasite. Indeed, results suggested that P. minutus could help G. roeseli to face with stress, what contributed to keep the host alive and favour the parasite transmission.

Does the acanthocephalan parasite Polymorphus minutus modify the energy reserves and antitoxic defences of its intermediate host Gammarus roeseli?

In disturbed environments, infected organisms have to face both parasitic and chemical stresses. Although this situation is common, few studies have been devoted to the effects of infection on hosts' energy reserves and antitoxic defence capacities, while parasite survival depends on host survival. In this study, we tested the consequences of an infection by Polymorphus minutus on the energy reserves (protein, lipid and glycogen) and antioxidant defence capacities (reduced glutathione, γ-glutamylcysteine ligase activity) of Gammarus roeseli males and females, in the absence of chemical stress. Moreover, malondialdehyde concentration was used as a toxicity biomarker. The results revealed that in infected G. roeseli, whatever their gender and the sampling month, protein and lipid contents were lower, but glycogen contents were higher. This could be explained by the fact that the parasite diverts part of the host's energy for its own development. Moreover, glutathione concentrations and γ-glutamylcysteine ligase activity were both lower, which could lead to lower antitoxic defence in the host. These results suggest negative effects on individuals in the case of additional stress (e.g. pollutant exposure). In the absence of chemical stress, the lower malondialdehyde level in infected gammarids could imply a probable protective effect of the parasite.

Host switch and infestation by Ligula intestinalis L. in a silver bream (Blicca bjoerkna L.) population.

Sampling of the fish community was carried out for 20 years in the Mirgenbach reservoir, in North-Eastern France. The prevalence and the mean intensity of Ligula intestinalis (Cestoda) were analysed in roach (Rutilus rutilus) and silver bream (Blicca bjoerkna) populations, the main two infected species. The aim of this study was to investigate the host switch from roach to silver bream and the consequences of L. intestinalis infestation in silver bream, which is an unusual host for this parasite as Ligula parasitism in silver bream appears to be rare. We analysed in detail the relationships between parasitism index (PI), gonadosomatic index (GSI), perivisceral fat abundance (PFA) and condition index (CI) in the silver bream population. In 1998, prevalence of L. intestinalis highlighted a clear host switch from roach to silver bream. In the silver bream population, young fish were the most severely infected and the impact of plerocercoids appeared to be different depending on the host sex. In male silver bream, plerocercoids drew energy from fat reserves even if GSI was also slightly impacted. On the contrary, in females energy was diverted from gonad maturation rather than from perivisceral fat reserves. No significant difference was observed in terms of CI in either sex.

Parasite-induced changes in the diet of a freshwater amphipod: field and laboratory evidence.

Trophically transmitted parasites are likely to strongly influence food web-structure. The extent to which they change the trophic ecology of their host remains nevertheless poorly investigated and field evidence is lacking. This is particularly true for acanthocephalan parasites whose invertebrate hosts can prey on other invertebrates and contribute to leaf-litter breakdown. We used a multiple approach combining feeding experiments, neutral lipids and stable isotopes to investigate the trophic ecology of the freshwater amphipod Gammarus roeseli parasitized by the bird acanthocephalan Polymorphus minutus. Infected compared to uninfected amphipods consumed as many dead isopods, but fewer live isopods and less leaf material. Infection had no influence on the total concentration of neutral lipids. Contrary to what we expected based on laboratory findings, the nitrogen isotope signature, which allows for the estimation of consumer's trophic position, was not influenced by infection status. Conversely, the carbon isotope signature, which is used to identify food sources, changed with infection and suggested that the diet of infected G. roeseli includes less perilithon (i.e. fixed algae on rocks, stones) but more terrestrial inputs (e.g. leaf material) than that of uninfected conspecifics. This study shows evidence of changes in the trophic ecology of P. minutus-infected G. roeseli and we stress the need to complement feeding experiments with field data when investigating top-down effects of infection in an opportunistic feeder which adapts its diet to the available food sources.

An acanthocephalan parasite boosts the escape performance of its intermediate host facing non-host predators.

Among the potential effects of parasitism on host condition, the 'increased host abilities' hypothesis is a counterintuitive pattern which might be predicted in complex-life-cycle parasites. In the case of trophic transmission, a parasite increasing its intermediate host's performance facing non-host predators improves its probability of transmission to an adequate, definitive host. In the present study, we investigated the cost of infection with the acanthocephalan Polymorphus minutus on the locomotor/escape performance of its intermediate host, the crustacean Gammarus roeseli. This parasite alters the behaviour of its intermediate host making it more vulnerable to predation by avian definitive hosts. We assessed the swimming speeds of gammarids using a stressful treatment and their escape abilities under predation pressure. Despite the encystment of P. minutus in the abdomen of its intermediate host, infected amphipods had significantly higher swimming speeds than uninfected ones (increases of up to 35%). Furthermore, when interacting with the non-host crustacean predator Dikerogammarus villosus, the highest escape speeds and greatest distances covered by invertebrates were observed for parasitized animals. The altered behaviour observed among the manipulated invertebrates supported the 'increased host abilities' hypothesis, which has until now remained untested experimentally. The tactic of increasing the ability of infected intermediate hosts to evade potential predation attempts by non-host species is discussed.