The effect of the acanthocephalan parasite (Pomphorhynchus laevis) on the drift of its intermediate host (Gammarus pulex)

SUMMARY 1. The drift of Gammarus pulex in a population in which approximately 20% of adults are infected with the acanthocephalan parasite Pomphorhynchus laevis was monitored at margin and mid-river sites in the River Teme, England over a 24-h period in mid-summer.
2. Drift densities showed a diurnal pattern with a large increase at night, independent of parasite burden.
3. There was no significant effect of site (margin or mid-river) on the proportion of parasitized and unparasitized G. pulex found in the drift or the benthos.
4. The drift of parasitized G. pulex was significantly greater than unparasitized animals. Gammarids harbouring only one parasite were found in significantly higher proportions in the drift than those with two or more parasites.
5. At both sites (margin and mid-river) the proportion of unparasitized adult G. pulex in the drift was significantly lower than that in the benthos. However, there was a significantly higher proportion of parasitized animals in the drift than in the benthos.     

Introduction of the non-indigenous amphipod Gammarus pulex alters population dynamics and diet of juvenile trout Salmo trutta

Summary 1. The amphipod Gammarus pulex , introduced to Irish rivers with the aim of enhancing trout feeding, is displacing the native Gammarus duebeni celticus . These two species are generally associated with different environmental conditions and macroinvertebrate communities, confounding assessment of effects of the invader as compared with the native on fish populations. Here, we uncouple effects of the two Gammarus species from environmental gradients.
2. A weir dissects a lowland stretch of the River Lissan, slowing the upstream invasion by G. pulex and resulting in contiguous G. pulex , mixed species and G. d. celticus reaches. Total invertebrate abundance and biomass in the benthos were significantly higher in the G. pulex reach, driven by high invader abundance, with low abundance of other taxa. Gammarus pulex was particularly prominent in night-time drift.
3. Correspondingly, densities and biomass of 0+ trout were significantly higher in the G. pulex reach, while instantaneous loss rates were lower. Fish growth rates were similar among the three reaches.
4. In the G. pulex reach, this invader dominated the diet of 0+ trout, leading to ingestion of significantly higher invertebrate biomass than fish in the other reaches. Fish generally preyed on Gammarus in proportion to its abundance, but exhibited some positive selection for G. pulex in the invaded reach.
5. The negative effects of the invader on native invertebrates are contrasted with positive effects on juvenile trout. This indicates changes in energy flux after invasion, with differential resource use or assimilation by G. pulex probable underlying mechanisms. As the frequency of amphipod invasions increases globally, investigations of their role as strong interactors at multiple levels of ecological organisation is required if the consequences of deliberate and unintentional introductions are to be predicted, and ultimately, prevented.     

Effects of low pH and humus on the survivorship, growth and feeding of Gammarus pulex (L.) (Amphipoda)

SUMMARY. 1. The toxicity to the freshwater amphipod Gammarus pulex (L.) of soft water (pH6.0) with three concentrations of XAD-extracted aquatic humus was treated in the laboratory.
2. Exposure to pH6.0 water without humus added resulted in 92% mortality after 3 weeks. Humus, added at concentrations of 7 and 20 g Cm ³, decreased the mortality to 80% and 64% in the same period.
3. Surviving animals kept at pH6.0 had a lower growth rate, lower food conversion efficiency and higher body water content than animals kept at pH 7.3.
4. Humus had no significant effects on growth, food conversion or body composition of G. pulex kept at pH 6.0. However, there was a tendency for growth and food conversion to increase, and for body water content to decrease with increased humus concentration. The effects of humus on growth and food conversion of G. pulex observed in this study do not support the contention that humus acts as a free coupler to lower metabolic efficiency. On the contrary, humus tended to benefit food conversion.
5. It is suggested that low concentrations of humus can be directly beneficial to organisms in acidified water in ways other than by complexing toxic metals.     

Differences in composition of macroinvertebrate communities with invasive and native Gammarus spp. (Crustacea: Amphipoda)

SUMMARY 1. Assessing the effects on communities of invasive species is often confounded by environmental factors. In Irish rivers, the introduced amphipod Gammarus pulex replaces the native G. duebeni celticus in lowland stretches. The two amphipods are associated with different macroinvertebrate communities, which may in part be the result of natural longitudinal physicochemical change. However, this hinders assessment of any direct community impacts of the invasive as compared with the native species. Here, we report on a fortuitous circumstance that allowed us to uncouple the community effects of Gammarus species from environmental differences.
2. The lowland stretch of the River Lissan is dissected by a weir, which has slowed the upstream invasion by G. pulex . We took physicochemical measurements and macroinvertebrate samples from three contiguous 150 m reaches of this stretch with G. pulex only, mixed Gammarus and G. d. celticus only communities.
3. We found no biologically significant differences in physicochemistry among the three reaches. Overall invertebrate densities did not differ among reaches. However, G. pulex numerically dominated its reach, whilst G. d. celticus abundance was relatively low in its reach. The G. pulex reach had significantly higher overall biomass because of high invader abundance. In addition, both diversity and species richness of macroinvertebrate communities were lower in the G. pulex than the G. d. celticus reach, with the mixed Gammarus reach intermediate.
4. Ordination indicated distinctly different associations of invertebrate community samples and taxa that were best explained by the distributions of the Gammarus species. Again, the mixed Gammarus species samples were intermediate.
5. This study indicates that the invasive G. pulex has a greater impact on macroinvertebrate community composition than the native G. d. celticus , probably through biotic interactions such as predation.     

Changes in Na, K, Ca, Mg and Al content of submersed leaf litter, related to ingestion by the amphipod Gammarus pulex (L.)

SUMMARY. 1. Changes in the amounts of Na, K, Ca, Mg and Al in fallen leaves of oak, alder and mat grass were measured during 13 weeks of submersion in seven streams differing in pH and ionic content.
2. In the first 2 weeks, Na, K and Mg leached from the leaves, but in the following weeks there was a net increase of Al, K and Mg and, in some instances, Na and Ca. Increases of K and Mg were relatively larger in circumneutral streams whereas the greatest increases of Al occurred in acid streams. Net gains are attributed to absorption of cations from water by saprophytic micro-organisms degrading the leaves, direct adsorption of Al on to leaf tissues - especially in acid streams - and capture of small particles of magnesium and aluminium silicates (clays).
3. Daily ingestion of elements was calculated as a percentage of whole body contents in Gammarus pulex (L.) and compared with daily loss rates. Leaf litter may be an important source of some major elements for ionic balance in juvenile G. pulex and a major source for uptake of potentially toxic aluminium in acidic, soft waters.     

Predation on mayfly nymph, Baetis rhodani, by native and introduced Gammarus: direct effects and the facilitation of predation by salmonids

1. In a series of laboratory experiments, we assessed the predatory nature of the native Irish amphipod, Gammarus duebeni celticus , and the introduced G. pulex , towards the mayfly nymph Baetis rhodani . We also investigated alterations in microhabitat use and drift behaviour of B . rhodani in the presence of Gammarus , and indirect predatory interactions with juvenile Atlantic salmon, Salmo salar .
2. In trials with single predators and prey, B. rhodani survival was significantly lower when Gammarus were free to interact with nymphs as than when Gammarus were isolated from them. The invader G. pulex reduced the survival of B. rhodani more rapidly than did the native G. d. celticus . Both Gammarus spp. were active predators.
3. In `patch' experiments, B. rhodani survival was significantly lower both when G. pulex and G. d. celticus were present, although the effect of the two Gammarus species did not differ. Again, active predation of nymphs by Gammarus was observed. Significantly more nymphs occurred on the top and sides of a tile, and per capita drifts were significantly higher, when Gammarus were present. Baetis rhodani per capita drift was also significantly higher in the presence of the introduced G. pulex than with the native G. d. celticus .
4.  Gammarus facilitated predation by salmon parr of B. rhodani by significantly increasing fish–nymph encounters on exposed gravel and in the drift. There were no differential effects of the two Gammarus spp. on fish – B . rhodani encounters or consumption.
5. We conclude that Gammarus as a predator can have lethal, nonlethal, direct and indirect effects in freshwaters. We stress the need for recognition of this predatory role when assigning Gammarus spp. to a `Functional Feeding Group'.     

Distribution and microhabitats of native and non-native gammarids (Amphipoda, Crustacea) in Brittany, with particular reference to the endangered endemic sub-species Gammarus duebeni celticus

Aim  To assess temporal changes in gammarid distribution in Brittany and microhabitat-use overlap between the endangered endemic Gammarus duebeni celticus Stock & Pinkster, 1970 , the expanding natives G. pulex (Linnaeus, 1758) and Echinogammarus berilloni (Catta, 1878), and the introduced G. tigrinus Sexton, 1939.
Location  Brittany and adjacent regions in western France.
Methods  The spatial and temporal patterns in distribution of gammarids at the scale of Brittany were studied using 351 sites. Longitudinal distributions (from the source to the estuary of the river) and microhabitat-use (substratum type and water velocity) were also considered in selected rivers.
Results  At the regional scale, all species occurred together less often than expected statistically, with significant deviations from expected for G. pulex vs. both G. duebeni celticus and G. tigrinus , and for E. berilloni vs. both G. duebeni celticus and G. tigrinus . However, at the microhabitat scale, E. berilloni occurred significantly more often than expected with the endemic G. duebeni celticus , and this appears to be due to similar substratum and water velocity preferences, although at both the regional and microhabitat scales E. berilloni prefers wider streams than G. duebeni celticus . This study reveals a decline in the endangered G. duebeni celticus since 1970.
Main conclusions  The longitudinal and local distributions of G. duebeni celticus , and the higher-than-expected co-occurrence of the species with G. pulex , suggest that the decline of the endemic species may be due to changes in the environment and/or interference from native G. pulex , which is expanding its range in Brittany. The results are discussed as regards to the consequences for regional biodiversity.     

Cadmium toxicity to the freshwater amphipod Gammarus pulex (L.) during the moult cycle

SUMMARY. 1. The toxicity of cadmium to mature Gammarus pulex at different stages in the moult cycle is described.
2. Immediate post-moult animals are significantly more sensitive than intermoult specimens at cadmium concentrations between 1.0 and 0.1 mg 1⁻¹ but not at 0.03 or 0.01 mg Cd 1⁻¹.
3. At a calcium concentration of 40mgl⁻¹, post-moult animals undergo recalcification within 7 days and thereafter there is little variation in their response to cadmium.
4. External calcium concentrations of 40 and 115mg 1⁻¹ do not affect cadmium toxicity but at 180 mg Ca1⁻¹ the sensitivity of immediate post-moult specimens is significantly reduced.
5. The results are discussed with regard to the protection of G. pulex by present water quality standards.     

The role of fungi in the diet of the amphipod Gammarus pulex (L.): an enzymatic study

SUMMARY. 1. Sets of ten Gammarus pulex fed on controlled diets of sterile alder leaves, or fungal mycelium, or alder leaves incubated for 10 days with an aquatic hyphomycete, were assayed for cellulase, β-1,3-glucanase an d chiitinase activity and compared with (a) animals taken directly from the stream, (b) animals starved for 2 days, and (c) enzyme activity in fungal mycelium.
2. Gut enzyme activity was compared on natural substrates of sterile leaves, mycelium and inoculated leaves as well as on model substrates.
3. G. pulex secretes an endogenous coupled cellulase system capable of degrading native cellulose in plant cell walls. It also secretes β-1,3-glucanase and chitinase capable of degrading fungal cell walls thus affording access for gut enzymes to cell contents.
4. Secretion of enzymes active on native cellulose is enhanced on a diet of leaves already partially degraded by fungal enzymes. Gut enzymes extract more reducing sugar from this substrate than from sterile leaves. Specific enzyme secretion is enhanced by the presence in the diet of exposed, accessible substrates. Fungal enzymes do not appear to contribute to the digestive processes of G. pulex.     

Factors affecting the microdistribution of Gammarus pulex (Amphipoda): an experimental study

SUMMARY. 1. The tnierodistribution of Gammarus species is size assortative: larger animals are associated with larger substrate particles. Using an artificial stream with scrubbed substrate we investigate the hypothesis that the assortative mechanism is stimulated by current avoidance rather than food searching.
2. In uniformly mixed substrate, G. pulex tend to move upstream, but in patchy substrate they are found in a predictable substrate grade. There is no discrimination between patches by different size classes of G. pulex in static water. The size assortative field pattern can only be reproduced under flow conditions in the laboratory. It occurs in the absence of food and irrespective of the sequence or location of substrate patches.
3. As density increases, G. pulex are displaced from the optimal patches but move into the open stream rather than into the cover of alternative patch types.
4. We discuss the significance of these results and suggest that some earlier models explaining mechanisms for microdistribution patterns tnay have been unnecessarily complex. Contrasts between natural and simulated stream situations and their experimental advantages are noted.     

Parasitic infection manipulates sodium regulation in the freshwater amphipod Gammarus pulex (L.)

The acanthocephalan parasite Polymorphus minutus induces both physiological and behavioural effects in its intermediate host, Gammarus pulex. The net effect of parasite infection is to increase the likelihood of transmission to the definitive host. Osmoregulation is an energetically expensive mechanism that allows G. pulex to survive in dilute media. Any factor influencing osmoregulation is thus likely to affect the allocation of resources to other areas. This study investigated whether P. minutus infection alters sodium regulation in G. pulex. Haemolymph sodium concentration, water permeability and sodium fluxes were measured over the salinity acclimation range of G. pulex. Water permeability was unaltered by either acclimation salinity or parasite infection. Acclimation to 12‰ significantly raised the haemolymph sodium concentration, reduced the sodium influx, and increased the sodium efflux, to the same extent in both uninfected and infected G. pulex. However, parasite infection induced a significant increase in haemolymph sodium concentration in G. pulex acclimated to 6‰, which was not observed in uninfected G. pulex acclimated to the same salinity. Also, both sodium influx and sodium efflux were significantly lower in parasitized G. pulex acclimated to 6‰, when compared to uninfected G. pulex acclimated to the same salinity. It was concluded that the parasite induced disturbances to sodium regulation in G. pulex acclimated to 6‰ were a functional consequence of the manipulative strategy employed to alter behaviour, rather than a primary target.     

Short-term movements of chalk-stream invertebrates

SUMMARY 1. Trays of gravel were implanted for 24 h periods in several chalk streams. The trays were variously modified to monitor colonization from different sources, particularly drift (D), other downstream movements (DSM), upstream movements (USM), and all sources combined (ALL).
2. The abundance of invertebrates in the streams was high (maximum of 717 per 0.04 m²) and colonization of the trays was extensive, especially by Gammarus pulex , the predominant species in all but one of the streams (maximum number of individuals of G. pulex per tray: for D, 473; for DSM, 462; tor USM, 286; and tor ALL, 639). In some cases benthic densities were achieved in the trays in the 24 h exposure period, and in most cases any one source of colonization could provide benthic densities in a few days.
3. Patterns of movement differed between species, between size classes of G. pulex , and between sites for particular species. No one source of colonization could be regarded as generally predominant although downstream movements in total (D + DSM) usually far exceeded USM. DSM was usually an important source of colonization.
4. The extent and direction of movement appeared to be determined by local factors and were explained in terms of the high densities of invertebrates and the current and substratum characteristics of each site.
5. Estimates of the relative importance of different sources of colonization varied greatly according to the criteria used in determining proportions.     

Sexual dimorphism in Gammarus pulex: the effect of current flow on pre-copula pair formation

SUMMARY. 1. Male Gammarus pulex (L.) are typically twice as heavy as females in field caught pre-copula pairs and mating is positively size assortative. Earlier work had shown that relatively large males can swim in pre-copula at higher current speeds than males that are relatively small compared with their mates. We describe experiments on pair formation in the laboratory, in static water and in an artificial stream.
2. Pairs formed in the artificial stream had a significantly higher weight ratio than either those formed in static water or those caught in the field. In both experimental conditions, males in pairs were significantly larger than singles. In the stream, the weight ratio of pairs increased with formation sequence due to a significant decrease in the size of selected females.
3. It is concluded that these results are consistent with the hypothesis that the relatively large size of paired male G. pulex is a product of mechanical constraints rather than male–male competition.     

Acid-induced acute toxicity of aluminium to three species of filter feeding caddis larvae (Trichoptera, Arctopsychidae and Hydropsychidae)

1. Arctopsyche ladogensis , Hydropsyche angustipennis and Hydropsyche siltalai larvae were exposed to nominal aluminium concentrations of 0, 625, 1250, 2500 and 5000 μg Al l^–1 at pH 5.0 for 96 h. Larvae reared at pH 6.4 and without any aluminium treatment were used as controls. Morphological abnormalities in the anal papillae of the larvae were used as the response variable in estimating the median effective concentrations (EC₅₀) of aluminium.
2. No morphological abnormalities were observed in the control larvae. Only a few individuals of A. ladogensis had darkened anal papillae at pH 5.0 without additional aluminium treatment, whereas increasing aluminium concentrations significantly increased the number of individuals in all species displaying darkening and reduction of the papillae.
3. A. ladogensis appeared to be the most sensitive species to increasing aluminium concentrations, as reflected by the significantly lower mean EC₅₀ value for this species compared with those of H. siltalai and H. angustipennis . H. angustipennis larvae were the most tolerant to aluminium.
4. The results indicate that interspecific differences in sensitivity to aluminium may be a key factor influencing the guild structure of filter feeding caddis flies in acidified streams. The results also imply that toxic effects of aluminium on filter feeding caddis larvae occur due to the impairment of normal osmoregulation processes via damage to the ion-regulatory organs.     

Acid-induced acute toxicity of aluminium to three species of filter feeding caddis larvae (Trichoptera, Arctopsychidae and Hydropsychidae)

1. Arctopsyche ladogensis , Hydropsyche angustipennis and Hydropsyche siltalai larvae were exposed to nominal aluminium concentrations of 0, 625, 1250, 2500 and 5000 μg Al l^–1 at pH 5.0 for 96 h. Larvae reared at pH 6.4 and without any aluminium treatment were used as controls. Morphological abnormalities in the anal papillae of the larvae were used as the response variable in estimating the median effective concentrations (EC₅₀) of aluminium.
2. No morphological abnormalities were observed in the control larvae. Only a few individuals of A. ladogensis had darkened anal papillae at pH 5.0 without additional aluminium treatment, whereas increasing aluminium concentrations significantly increased the number of individuals in all species displaying darkening and reduction of the papillae.
3. A. ladogensis appeared to be the most sensitive species to increasing aluminium concentrations, as reflected by the significantly lower mean EC₅₀ value for this species compared with those of H. siltalai and H. angustipennis . H. angustipennis larvae were the most tolerant to aluminium.
4. The results indicate that interspecific differences in sensitivity to aluminium may be a key factor influencing the guild structure of filter feeding caddis flies in acidified streams. The results also imply that toxic effects of aluminium on filter feeding caddis larvae occur due to the impairment of normal osmoregulation processes via damage to the ion-regulatory organs.     

Population dynamics and morphometries of Gammarus pulex L.: evidence of seasonal food limitation in a freshwater detritivore

SUMMARY. 1. Seasonal changes in population size structure of Gammarus pulex L. in a Cotswold stream appeared to indicate a growth check in late summer.
2. The relationships between dry weight and body length, and between the number of primary flagellar segments on the first antenna and either dry weight or body length provided further evidence of a reduction in growth in mid and late summer.
3. Body fat content was minimal (4% of dry weight) at the end of summer, when large particulate organic detritus was scarce or of poor quality, and maximal in late winter (17.9% dry weight in females; 9.4% in males), after a period of high food availability. In a field experiment, the fat content of animals in summer was raised to levels typical of winter by providing high quality food.
4. Field and experimental evidence together strongly infer that this population of G. pulex was subject to severe food limitation from early summer until leaf fall in autumn.     

Low pH leads to sister-chromatid exchanges and chromosomal aberrations,and its clastogenicity is S-dependent

The effect of low pH on sister-chromatid exchanges (SCE), chromosomal aberrations (CA), and the cell cycle were investigated in Chinese hamster cells. The cells were treated in media over the pH range 7.2–5.4 during 24-h continuous or 3-h pulse treatments. In Chinese hamster ovary K1 cells, slight increases in SCE frequency were induced by 3-h pulse treatment with a 28-h recovery time. In Chinese hamster V79 379A cells, similar slight increases in SCE frequency were observed with both treatments. A severe delay in the cell cycle was noted in both cell types. DNA analysis with flow cytometry indicated that the cell cycle delay occured in S phase. CA were observed in the first metaphase. Multiple fixation times over a 27-h period were used to determine whether or not CA could be induced in cells exposed to low pH medium in more than one part of the cell cycle. Only a few chromatid gaps were induced when the cells were fixed at 0–9 h after the 3-h treatment, most probably representing cells that were treated in their G2 or late S phase. CA were induced in cells fixed between 12 and 27 h after the 3-h treatment. These cells were most probably treated in early S phase, in G1, or in the previous G2/M. These results suggest that low pH clastogenicity is S-dependent.     

Salamander predation and vertical distributions of zooplankton

SUMMARY. 1. The impact of different predation regimes of zooplanktivorous larval tiger salamanders, Ambystoma tigrinum nebulosum , on the vertical temporal distributions of Daphnia pulex, Diaptomus nudus and larval Chaoborus flavicans was studied in limnetic enclosures in a pond in east-central Arizona. Zooplankton densities were monitored 1-2 h before sunset, at sunset, and 1-2 h after sunset at three depths (0-0.4, 1.0 and 2.0 m) in four salamander treatments (0, 20, 40 salamanders per enclosure, and unenclosed limnetic areas).
2. Densities and body size of Daphnia pulex were consistently greater in salamander-free enclosures than in other treatments. More Daphnia were generally nearer the surface (0-1.0 m) than along the bottom before and at sunset than after sunset in enclosures without salamanders. After sunset, Daphnia was by far most dense along the bottom. Densities were uniform among depths and sample times in treatments with salamanders. Salamanders moved throughout the water column. Frequencies of D. pulex spined morphs (with prominent dorsal crest) did not differ among salamander treatments.
3. Chaoborus densities were positively correlated with D. pulex distributions among depths and sample times in treatments. Spatio-temporal distributions of Chaoborus were influenced more by their daphnid prey than by risk of predation from larval salamanders.
4. Densities of Diaptomus nudus were highest in open water controls and lowest in enclosures without salamanders. Diaptomus was generally more dense between 0 and 1 m than at 2.0 m depths in all treatments, and vertical distributions were not dependent on salamander density.     

Effects of pH and aluminium on embryonic and early larval stages of Swedish brown frogs Rana arvalis, R. temporaria and R. dalmatina

The effects of pH and aluminium on embryonic and early larval stages of Swedish brown frogs Rana arvalis, R. temporaria and R. dalmatina were tested in laboratory bioassays. In all three species egg mortality and time needed for embryonic development to hatching increased when pH declined, but no significant effects were found on embryonic development when aluminium level was elevated. In R. arvalis and R. temporaria larval mortality was affected by both pH and aluminium. In both species the frequency of occurrence of larval deformities increased in acid water, and there was a synergistic effect of pH and aluminium. In R. arvalis swimming behaviour was disturbed by high levels of aluminium at pH 5. In all three species the frequency of stressed larvae increased when pH was depressed and aluminium concentration elevated, and there was a synergistic effect when both were combined. The three species differed significantly in egg mortality, time needed for embryonic development, larval mortality, larval deformities and larval stress al low pH and high aluminium levels. R. arvalis showed the highest acid tolerance and R. dalmatina was the most sensitive to low pH.