Parasite altered micro-distribution of Gammarus pulex (Crustacea: Amphipoda)

In a river survey, Gammarus pulex amphipods both unparasitised and parasitised with the acanthocephalan Echinorhynchus truttae were distributed similarly with respect to flow regimen, tending to be more abundant in faster, shallower, riffle patches. However, there was a higher prevalence of parasitism in faster, shallower areas than in slower, deeper areas and abundance correlated with macrophyte coverage for unparasitised but not parasitised amphipods, indicating subtle differences in habitat usage. A laboratory 'patch' simulation indicated that parasitism influenced micro-distribution. There were higher proportions of unparasitised amphipods in/under stone substrates and within weed. In contrast, there were higher proportions of parasitised amphipods in the water column and at the water surface. As the experiment progressed, unparasitised but not parasitised amphipod habitat usage shifted from those micro-habitats above the substrate and in the water column to those in/under the substrates. Experiments also demonstrated that parasitised amphipods were more active and had a greater preference for illumination. Previous studies of the effects of acanthocephalan parasitism of amphipod hosts have focussed on how drift behaviour is altered, now we show that subtle differences in micro-habitat usage could translate to greatly increased vulnerability to fish predation. We discuss how aggregation of parasitised individuals within specific habitats could promote parasite transmission.     

Gut passage times in Gammarus pulex (Crustacea,Amphipoda) and aspects of summer feeding in a stony stream

Gut passage times in Gammarus pulex (Crustacea, Amphipoda) were calculated by counting and measuring voided faecal pellets. Following feeding on elm leaves conditioned in a stream, or summer fine detritus from the same source, calculated gut passage times were usually short, about 2 h or less at 15 °C. Using harder oak and beech leaves, not previously conditioned in the stream, apparent gut passage times were much longer, 8–48 h, but since intermittent feeding occurred these figures are not comparable to the others.In connection with summer feeding of the animal in the stream, the microbiological status of the fine detritus there was studied. It contained fungal hyphae, which were mostly empty, viable fungal spores of terrestrial derivation, and bacteria, all largely carried on fragments of vascular plant tissues. In experiments, such fungal spores (of Aureobasidium pullulans and of Mucor sp.), incorporated into fine detritus, were not digested by the animal. The bacterium Bacillus cereus was present in the fine detritus and when the latter was consumed it survived passage through the gut of the animal. Using B. cereus as a proportion marker it was concluded that other unicellular bacteria in the fine detritus were not digested either. In other experiments, not immediately related to the summer season, nutrient extraction by the animal from hyphae of the fungus Nectria lugdunensis was examined; it probably occurs through sub-microscopic pores connecting the cells. This mode of extraction may apply when the animal consumes vascular plant tissues.     

Presence of sculpins (Cottus gobio) reduces drift and activity of Gammarus pulex (Amphipoda)

Introduction of sculpins into a stream previously devoid of these predators significantly reduced drift rate of Gammarus pulex. The drift of insect larvae was not affected. High amounts of exudates after implantation of sculpins were probably responsible for the low number of drifting G. pulex specimens. Laboratory experiments confirmed reduced locomotory activity of G. pulex when exposed to caged sculpins, an observation that excludes reduced drift activity as a result only of predation.The average size of drifting G. pulex specimens was larger during the night than during the day. This result is in accordance with the hypothesis that large individuals should, in relation to small ones, turn nocturnal because of greater predation risk during daytime. Presence of sculpins did not alter the size composition of drifting G. pulex.     

Effects of two acanthocephalan species on the reproduction of Hyalella patagonica (Amphipoda, Hyalellidae) in an Andean Patagonian Lake (Argentina)

The aim of the present study was to evaluate alterations in the reproduction induced by acanthellae and cystacanths of the acanthocephalans Acanthocephalus tumescens and Corynosoma sp. in the amphipod Hyalella patagonica from Lake Mascardi. Specimens of H. patagonica were separated in two categories: paired amphipods (joined specimens during precopulatory mate guarding period until fertilization) and unpaired amphipods (alone specimens). Different analyses were performed: first with paired (n = 406) and unpaired (n = 375) amphipods, and second only with female amphipods (n = 1949), that were classified into three categories (without internal oocytes and eggs, only with internal oocytes, and with eggs). Also, carotenoid extraction was performed of amphipods uninfected (n = 75) and infected (n = 105) by cystacanths of Corynosoma sp. Unpaired amphipods had significantly higher prevalence of cystacanths of both acanthocephalan species than paired ones; but such differences were not found in prevalence of acanthellae. Female amphipods without internal oocytes and eggs showed significantly higher prevalence of cystacanths of both acanthocephalan species than the two other female categories; while females with eggs had significantly higher prevalence of A. tumescens acanthellae. Amphipods infected by Corynosoma sp. showed lower carotenoid concentration than uninfected ones. In Lake Mascardi, there is indirect evidence of both reduced mating success and female fecundity of H. patagonica provoked by both cystacanths species (A. tumescens and Corynosoma sp.). However, infections by acanthellae seem to have no effects.     

Assortative pairing in Gammarus insensibilis (Amphipoda) infected by a trematode parasite

We have investigated the influence of Microphallus papillorobustus (Trematoda) on the reproductive biology and mating patterns of its intermediate host Gammarus insensibilis (Amphipoda). Infected Gammarus species show altered behaviour which renders them more susceptible to predation by Charadriiform birds, the parasite's definitive hosts. In a natural population of G. insensibilis, mean parasite intensity was higher for unpaired individuals than for paired individuals. Fecundity was reduced in infected amphipods. Size-assortative pairing was significant, although infected males were found with smaller females compared to uninfected males of the same size. There was also a positive assortative pairing by parasitic prevalence. Vertical segregation between infected and uninfected individuals, male-male competition for access to uninfected females, and female choice may explain assortative mating for prevalence. This study provides the first empirical evidence that parasites can have a direct effect on patterns of mating in gammarids.     

Effect of zinc on cadmium toxicity to the amphipod Gammarus pulex

The freshwater amphipod Gammarus pulex was sampled from several sites in Northumberland, England and analysed for total body zinc and cadmium concentrations.The toxicity of cadmium to animals from the different sites was determined and a positive relationship was found between the initial total body zinc concentration and the ability to survive in an artificial medium containing 0.5 mg 1^–1 cadmium as cadmium sulphate.Exposure in the laboratory to a low concentration of zinc (0.01 mg 1^–1) as zinc sulphate for two weeks resulted in a decreased susceptibility to cadmium poisoning.     

Population characteristics of Gammarus pulex (L.) from five English streams

Gammarus pulex were sampled from five English streams during April 1992. The population density, number of precopula pairs and incidence of parasitic infection were recorded, and the biomass was estimated from subsamples by relating body area to dry weight. Physical and chemical measurements were taken from each stream. The abundance and standing crop biomass differed significantly between streams, probably due to the influence of pollutants or the physical structure of the stream bed. The size of individual G. pulex also differed significantly between streams, although there was no obvious causal explanation for this. Few individuals were visibly parasitised in any of the populations. Males were significantly larger than females, both in precopula pairs and in the general populations. The sex ratio differed between populations and may explain inter-stream differences in the relationship between precopula male and female size.     

Isolation and characterization of the first eight microsatellite loci in Gammarus fossarum (Crustacea, Amphipoda) and cross-amplification in Gammarus pulex and Gammarus orinos

We characterized the first microsatellite markers for Gammarus fossarum. Eight loci gave satisfactory amplification patterns in two stream populations (Southern France) with number of alleles ranging from 2 to 10 and expected heterozygosity from 0.076 to 0.857. We performed cross-amplification in two closely related gammarid species, Gammarus pulex and Gammarus orinos. Among the eight tested microsatellite loci, four correctly amplified in G. pulex and three in G. orinos.     

High food quality increases infection of Gammarus pulex (Crustacea: Amphipoda) by the acanthocephalan parasite Pomphorhynchus laevis

Parasitism is an important process in ecosystems, but has been largely neglected in ecosystem research. However, parasites are involved in most trophic links in food webs with, in turn, a major role in community structure and ecosystem processes. Several studies have shown that higher nutrient availability in ecosystems tends to increase the prevalence of parasites. Yet, most of these studies focused on resource availability, whereas studies investigating resource quality remain scarce. In this study, we tested the impact of the quality of host food resources on infection by parasites, as well as on the consequences for the host. Three resources were used to individually feed Gammarus pulex (Crustacea: Amphipoda) experimentally infected or not infected with the acanthocephalan species Pomphorhynchus laevis: microbially conditioned leaf litter without phosphorus input (standard resource); microbially conditioned leaf litter enriched in phosphorus; and microbially conditioned leaf litter without phosphorus input but complemented with additional inputs of benthic diatoms rich in both phosphorus and eicosapentaenoic acid. During the 110 day experiment, infection rate, parasite load, host survival, and parasite-mediated behavioral traits implicated in trophic transmission were measured (refuge use, geotaxis and locomotor activity). The resources of higher quality, regardless of the infection status, reduced gammarid mortality and increased gammarid growth. In addition, higher quality resources increased the proportion of infected gammarids, and led to more cases of multi-infections. While slightly modifying the geotaxis behavior of uninfected gammarids, resource quality did not modulate the impact of parasites on host behavior. Finally, for most parameters, consumption of algal resources had a greater impact than did phosphorus-enriched leaf litter. Therefore, manipulation of resource quality significantly affected host–parasite relationships, which stressed the need for future research to investigate in natura the relationships between resource availability, resource quality and parasite prevalence.     

Effects of Cyanobacteria on Survival and Reproduction of the Littoral Crustacean Gammarus zaddachi (Amphipoda)

Toxic cyanobacteria can have harmful or fatal impacts on aquatic organisms. In the archipelagos of the northern Baltic Sea, the open sea blooms often drift into littoral areas, where they decompose and release toxins and other chemical compounds in the water. However, the effects of cyanobacteria on the littoral organisms have not previously been investigated. We studied the effects of three cyanobacteria species (toxic Nodularia spumigena, non-toxic N. sphaerocarpa and non-toxic Aphanizomenon flos-aquae) and purified dissolved nodularin (produced by N. spumigena) on a common littoral amphipod Gammarus zaddachi. Nodularin was transferred to eggs, juveniles and adults of G. zaddachi, but no significant negative effects of dissolved nodularin were detected on adults, eggs or juveniles. However, survival of adults decreased by the exposure to toxic N. spumigena cells. The egg hatching rate and juvenile survival were not affected when exposed to the three cyanobacteria species. In contrast, a weak decrease in the egg production and an increased abortion of embryos from the brood pouch of females was observed, the later indicating a failure in parental care. Further, a decrease in grazing rate on the filamentous green alga Enteromorpha intestinalis was observed. The results suggest that toxic cyanobacteria blooms are not extremely fatal, but may have, in high concentrations, negative effects on the adult survival, fecundity, and feeding behaviour of gammarids inhabiting the littoral zone.     

Effects of pH,humic substances and animal interactions on survival and physiological status of Asellus aquaticus L. and Gammarus pulex (L.)

Summary The mortality and physiological status (body water content) of Asellus aquaticus (Isopoda) and Gammarus pulex (Amphipoda) were measured after 25 days exposure in 20 natural streams with a pH range of 4.3–7.5 and a colour range of 8–280 mg Pt L^–1. In addition, the effects of keeping the animals as single species or together were studied. The response of Gammarus to low pH was an increased mortality and lower physiological status of surviving individuals in streams with a pH lower than 6.0. In Asellus the physiological status was correlated with pH, while the mortality was not pH dependent. The effects of humus on the physiological status of Asellus was significant when fitted to a second order polynomial function. The influence of humus can, however, be regarded as small relative to pH. The interactions between the species could be described as asymmetric under optimal conditions of high pH and low humus concentrations, where the presence of Gammarus decreased the survival and physiological status of Asellus. Acid stress did not seem to reverse the direction of this asymmetry, but the presence of Gammarus improved the physiological status of Asellus at pH lower than 6.0. Since the presence of Asellus did not increase the mortality or decrease the physiological status of Gammarus, this could be explained by Asellus feeding on Gammarus that died from physiological stress solely. This mechanism suggests that food quality, and thus effects of diffuse competition, can be important for the ability to withstand acid stress. The results, though, give no support for the hypothesis that competition from Asellus is important for the disappearance of Gammarus during the acidification of streams.     

Seasonal variability in the grazing potential of the invasive amphipod Gammarus tigrinus and the native amphipod Gammarus salinus (Amphipoda: Crustacea) in the northern Baltic Sea

Mesograzers are known to reduce the biomass of their host plant and modify the structure of the whole macrophyte community in many ecosystems. Thus, the introduction of an efficient mesograzer may destabilize macrophyte community and also affect the native grazers. We estimated how large proportion of macrophyte production are consumed by the alien gammarid G. tigrinus and the native gammarid G. salinus in the species poor ecosystem of the northern Baltic Sea. We analysed whether G. tigrinus consumes different diet as the native G. salinus and whether the effect of G. tigrinus on the survival of the native G. salinus is macrophyte species specific. Grazing experiments showed that there was a clear difference in the grazing rates of gammarids among the studied macrophyte species in summer and autumn but not in spring. The grazing rates were significantly higher in the prevailing macrophyte Pilayella littoralis as compared to other macrophytes. The grazing was inversely related to the diurnal net photosynthetic values of macrophytes. The gammarid amphipods potentially removed only a minor part of plant primary production except for summer and autumn when grazing of a few perennial species exceeded macrophyte production. Macrophyte species and presence of G. salinus had no effect on the survival of G. tigrinus. The presence of G. tigrinus, however, reduced the survival of the native gammarids within P. littoralis in summer. To conclude it is likely that both native and alien gammarid amphipods do not exert significant pressure on the macroalgal communities in the northern Baltic Sea. Competitive interactions between G. tigrinus and G. salinus within the prevailing macrophyte P. littoralis is the likely explanation of the decline of the native gammarid amphipods after the establishment of G. tigrinus in the northern Baltic Sea.     

Changes in Ventilation and Locomotion of Gammarus pulex (Crustacea,Amphipoda) in Response to Low Concentrations of Pharmaceuticals

Exposure to contaminants below lethal concentrations may affect the performance of organisms, resulting in measurable differences in behavior. We measured the response of the benthic invertebrate Gammarus pulex (Crustacea, Amphipoda) to sublethal concentrations of three pharmaceuticals, fluoxetine, ibuprofen and carbamazepine, and the cationic surfactant cetyltrimethylammonium bromide (CTAB). Responses in behavior during exposure were analyzed using the multivariate method of Principal Response Curves (PRC). The PRC results show that exposure to low pharmaceutical concentrations (range 1–100 ng/l) resulted in increased ventilation, whereas exposure to control or to high concentrations (1 μ g/l–1 mg/L) resulted in increased locomotion. Exposure to CTAB resulted in decreased locomotion and increased ventilation at increasing concentrations. The results of our experiments indicate that increased ventilation can be used as a general sign of stress, but not necessarily an early warning signal for mortality.     

State-dependent pairing behaviour in male Gammarus pulex (L.) (Crustacea, Amphipoda): effects of time left to moult and prior pairing status

Because mating can be costly in terms of time and energy, an individual's propensity to engage in courtship and mating activities might be modulated by its physiological state. However, so far, state-dependent mate choice has received little attention The present study examined the effect of both prior pairing status and time left to the moult on the ability of male Gammarus pulex (Crustacea, Amphipoda) to enter in precopula with receptive females. In the lab, males that were freshly collected in precopula pairs in the field had a higher probability of re-pairing and were quicker to enter in precopula with receptive females compared to males of similar size that were freshly collected unpaired. In addition, unpaired males found in the field were closer to their moult than paired males. Considered together, our results strongly suggest that time left to the moult and prior mating status directly influence male propensity to pair in G. pulex.     

The dynamics of predation on Gammarus spp. (Crustacea: Amphipoda)

Gammarus spp. (Crustacea: Amphipoda) are widespread throughout a diverse range of marine, freshwater and estuarine/brackish habitats, often dominating benthic macroinvertebrate communities in terms of both numbers and/or biomass. Gammarus spp. are the dominant macroinvertebrate prey items of many fish, whether as a seasonal food source or a year-round staple. Selective predation by fish on Gammarus spp. is often linked to parasitism and the body size of the prey. Gammarus spp. populations are under increasing threat from both pollution and replacement/displacement by introduced species. Loss of populations and species invasions/replacements could have significant impacts on native predator species if the predator(s) cannot successfully adapt their feeding patterns to cope with non-indigenous Gammarus prey species. Despite this, many fish predation studies do not identify Gammarus prey to species level. This lack of precision could be important, as Gammarus spp. exhibit wide variations in physiochemical tolerances, habitat requirements, abilities to invade and susceptibility to replacement. Although rarely acknowledged, the impacts of nonpiscean predators (particularly macroinvertebrates) on Gammarus prey species may frequently be stronger than those exerted by fish. A major aim of this review is to ascertain the current importance of Gammarus as a prey species, such that the implications of changes in Gammarus spp. populations can be more accurately assessed by interested groups such as ecologists and fisheries managers. We also review the dynamics of Gammarus spp. as prey to a diverse array of mammals, birds, amphibians, insects, flatworms, other crustaceans such as crabs and crayfish and, perhaps most importantly, other Gammarus spp.