Scope for growth in Gammarus pulex,a freshwater benthic detritivore

Although toxic substances affect the physiological processes of individual organisms, their ecological impacts occur at the population and community levels. However, physiological processes can often be assessed more easily and precisely than population and community ones. Here we argue that scope for growth, the difference between the energy input to an organism from its food and the output from respiratory metabolism, can give a good physiological measure of stress that, at least in principle, is straightforwardly related to population and community processes. We describe, in detail, how scope for growth can be measured in Gammarus pulex (Crustacea, Amphipoda). The results indicate that both zinc and low pH can significantly reduce the scope for growth of individuals and that the most sensitive component of the energy budget is food absorption.     

Population responses of the freshwater amphipod crustacean Gammarus pulex (L.) to copper

1. Populations of Gammarus pulex (density 4000m^?2) were exposed, over a 100-day period in a continuous-flow dosing system, to concentrations of copper below the 240 h median lethal concentration (LC₅₀) for juveniles. 2. There was a significant effect of copper on the resulting population density. In the control and lowest treatment (11.0μg1-^?1) it was double the initial density, but with increasing copper concentration there was a decrease in density until at the highest treatment (23.1 μg1-^?1), it was less than that of the initial population. The lowest observed effect concentration (LOEC) for population density was 14.6 μg 1-^?1. 3. Copper significantly affected the age composition of the final populations. In the control and lowest treatment concentration, the population was composed mainly of juvenile animals. With increasing copper concentration, however, there was a decrease in the number of juveniles, the LOEC being 14.6 μg1-^?1. The number of adults in the final population was also significantly reduced by copper, with an LOEC of 18.2 μg1-^?1. 4. The relationship between the body length and number of antennal segments of animals in the populations at each treatment concentration showed that growth was impaired by increased copper concentrations.     

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.     

Substrate selection and assortative mating in Gammarus pulex L.

We reject the published proposition that assortative mating for size in Gammarus is brought about purely as a result of spatial heterogeneity in substrate particle size. We confirm that, in general, large Gammarus prefer substrates with larger particle size than small Gammarus. However, in artificial streams in the laboratory, in choice experiments Gammarus males and females showed positive assortative mating for size in each of three substrates differing in mean particle size. We propose an explanation for assortative mating based on sexual selection.     

Cadmium and calcium interactions in the freshwater amphipod Gammarus pulex

SUMMARY. The accumulation of cadmium from an experimental medium by the freshwater amphipod Gammarus pulex is described.
Much of the uptake is internal as opposed to adsorption on the body surface, and after an apparent saturation of the exoskeleton the hepatopancreas becomes an increasingly important site of cadmium storage. The haemolymph cadmium concentration reaches a high level compared with marine crustaceans, achieving a concentration factor (ratio of internal Cd/ external Cd) of 100 after about 60 h uptake.
The cadmium uptake process is severely inhibited after exposure of experimental animals to 0.5 mM 2:4 Dinitrophenol, indicating the mediation of an active process. This fact together with the negative relationship between cadmium uptake rate and the calcium concentration of the animal suggests that cadmium accumulation by this species may be at least partially accounted for by a process of 'accidental' active cadmium uptake, with cadmium substituting for calcium on a calcium regulatory mechanism.
As yet it has not been possible to establish a true stoichiometric relationship between the two metals. Although calcium influx and cadmium uptake (influx) rates are similar over a wide range of external concentrations, calcium influx is clearly inhibited by a low external ratio of cadmium to calcium. This indicates that the relationship between the two metals is far from an equimolar one and the possibility of non-competitive inhibition of calcium influx by cadmium cannot be eliminated.     

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.     

The effect of copper on osmoregulation in the freshwater amphipod Gammarus pulex

The influence of copper on osmoregulation in the freshwater amphipod Gammarus pulex was determined from the analysis of water permeability, haemolymph sodium concentration, sodium influx and gill Na(+)/K(+) ATPase and Mg(2+) ATPase activity. Exposure to nominal copper concentrations of 100 microg l(-1) or greater caused a significant reduction in both haemolymph sodium concentration and sodium influx within 4 h. Measurements of water permeability, expressed as the half-time of exchange of body water (t(1/2)), excluded structural gill damage as the cause of this fall in haemolymph sodium. Copper at 10 microg l(-1) or above in the assay solution significantly reduced gill Na(+)/K(+) ATPase activity. In contrast gill Mg(2+) ATPase activity was markedly less affected by copper. These differences in enzyme sensitivity were considered with respect to the potential mechanisms of copper toxicity.     

Production of Gammarus pulex L. (Amphipoda) in a small Danish stream

Quantitative samples of Gammarus pulex L. taken from a small Danish stream during 1975 showed mean annual population densities varying from 500 m^–2 in early May to 5 500 m^–2 in late September. The mean annual biomass was 1.5 g dry weight m^–2. No discrete cohorts could be distinguished from the size frequency distributions. Annual production, estimated by the size-frequency method, was 3.9 g dry weight M^-2 and P/B ratio was 2.6. The contribution to trout energetics may have been as much as 17%.     

Comparative ecology of Gammarus pulex (L.) and Asellus aquaticus (L.) I: population dynamics and microdistribution

Gammarus pulex and Asellus aquaticus generally occupy different zones in rivers; the former occurs in upper reaches but is replaced by the latter in lower reaches. Microdistribution and life-history patterns of G. pulex and A. aquaticus in sympatry and allopatry, were analyzed. Both species exhibited similar patterns of microhabitat selection, with larger individuals associated predominantly with large-sized substratum particles, and juveniles with weed. Coexisting populations of G. pulex and A. aquaticus had similar densities and population dynamics. Within each species, differences in population dynamics of allopatric and sympatric populations were observed. Although variation in population dynamics of G. pulex may be explained in terms of competition between the two species, the evidence is weak and equivocal. Differences in the dynamics of the two A. aquaticus populations were possibly a consequence of coal-mine and organic pollution, reducing the survival of offspring in the allopatric population.     

Mate guarding in relation to seasonal changes in the energy reserves of two freshwater amphipods (Gammarus fossarum and G. pulex)

1. We assessed sex‐specific seasonal changes in major energy storage compounds (triglycerides, glycogen) in Gammarus fossarum and Gammarus pulex collected from the field, with respect to their reproductive activity. 2. The dynamics of stored energy followed a seasonal pattern in both species and sexes. Moreover, over a 4‐year period, these changes were independent of the year in which they were investigated. Stored energy reached a peak in late winter, but was depleted in late summer and early autumn, coinciding with the reproductive periods. 3. Triglyceride (annual mean ± SD) accounted for 79.7 ± 11.9% of the total stored energy and was responsible for the seasonal pattern. In contrast, glycogen contributed a lesser percentage (20.3 ± 11.9%). Over the study period, the amount of stored energy ranged between 0.39 and 4.08 kJ g^?1 dry mass (triglyceride: 0.19–3.69 kJ g^?1 dry mass; glycogen: 0.14–0.80 kJ g^?1 dry mass). 4. In both species, the energy reserves of males were drastically depleted shortly before the cessation of precopulatory mate guarding in the field, thus offering a bioenergetic explanation for the reproductive period in these two widespread species.     

Anaerobic capacity of a crustacean sensitive to low environmental oxygen tensions,the freshwater amphipod Gammarus pulex (L.)

We investigated the accumulation of haemolymph L-lactate in response to anoxia, progressive hypoxia and activity, in a species that rarely encounters low PO₂ in its natural environment, the freshwater amphipod Gammarus pulex (L.). Individuals survived <1.5 h anoxia and accumulated L-lactate in the haemolymph at a rate of 10.26 mmol l^–1 h^–1. No lactate was excreted into the medium during this time. Despite the fact that most individuals of G. pulex were oxyconformers, exposure to acutely declining PO₂s did not result in a switch to anaerobic pathways until PO₂ < 4.4 kPa. Even then the concentrations accumulated remained very low until PO₂ < 0.46 kPa. There was no accumulation of L-lactate in the haemolymph as a result of exhaustive activity. Consequently, it was suggested that (a) it cannot be automatically assumed that anaerobic metabolism is switched on only at the point at which oxyregulation breaks down, (b) that the poor anaerobic capacity observed plays little, if any, role in the metabolic response to hypoxia by Gammarus pulex.     

Life-cycle, drift and production of Gammarus pulex L. (Amphipoda) in a Danish spring

SUMMARY. Gammarus pulex L. was studied at three sites in a Danish spring. Breeding took place during the summer and therefore cohorts could be followed. Sexual maturity was not reached until the next summer and maximum age appeared to be two years.
Changes in population size differed between stations and was tentatively related to differences in drift and numbers of predators. Generally the drift was non-selective between cohorts and within cohorts.
Yearly production estimated from production curves was 2.6 g dry weight m⁻² (the young cohort) and 1.2 g dry weight (the old cohort) at Station 1. The respective P / ratios were 3.35 and 1.22. Production of the youngest cohort at the three stations in the period September-May was 0.9–1.3 g m⁻² and P / ratios were 2.14–2.31. Total yearly production at the three stations was estimated at 4–7 g dry weight m⁻².     

Population dynamics of Pomphorhynchus bulbocolli in Gammarus pseudolimnaeus

The population dynamics of larval Pomphorhynchus bulbocolli were studied in the amphipod intermediate host, Gammarus pseudolimnaeus. Amphipods were collected monthly from April 1982 to March 1983 from a study site at the West Fork of Drake's Creek, Simpson Co., Kentucky. The number of infected amphipods per m2 was lowest (0.6) in May and highest (27.3) in November. A total of 2,502 amphipods was examined; 1,575 (62.9%) were infected with P. bulbocolli. Infection intensity ranged from 1 to 12 with a mean of 1.83 +/- 0.03. Single and multiple worm infections were found in 800 (50.8%) and 775 (49.2%) amphipods, respectively. Female amphipods had higher prevalence and intensity of infections than males but differences were not significant. Prevalence in the monthly samples was lowest in May (5.1%) and highest in October (88.1%). The monthly mean intensity of infection was lowest in July (1.10 +/- 0.04) and highest in November (2.02 +/- 0.08). Amphipods in length classes 3 through 15 were infected with P. bulbocolli. In late summer and fall, amphipods in all length classes were infected, while in late winter and spring only large amphipods were infected. Recruitment of P. bulbocolli into amphipods was seasonal and the total infrapopulation within an amphipod was usually acquired at one time. There was little correlation between size and number of larvae present.     

Environmental mediation of intraguild predation between the freshwater invader Gammarus pulex and the native G. duebeni celticus

Invasive species and environmental change often occur simultaneously across a habitat and therefore our understanding of their relative roles in the decline of native species is often poor. Here, the environmental mediation of a critical interspecific interaction, intraguild predation (IGP), was examined between invasive (Gammarus pulex) and native (G. d. celticus) freshwater amphipods. In the laboratory, IGP asymmetries (males preying on congeneric females) were examined in river water sourced from zones where: (1) the invader has completely displaced the native; (2) the two species currently co-exist, and (3) the native currently persists uninvaded. The invader was always a more effective IG predator, but this asymmetry was significantly weaker moving from ‘invader-only water’ through ‘co-existence water’ to ‘native-only water’. The constituent of the water that drives this mediation of IGP was not identified. However, balancing the rigour of laboratory experiments with field derived ‘environment’ has advanced understanding of known patterns in a native species decline, and its co-existence and persistence in the face of an invader.     

Comparative ecology of Gammarus pulex (L.) and Asellus aquaticus (L.) II: fungal preferences

This paper continues to explore niche differentiation in Gammarus pulex and Asellus aquaticus by analysis of their food preferences. Individuals from both species discriminated between leaf discs colonized by different fungal species and exhibited strong preferences for Anguillospora longissima and Heliscus lugdunensis. Fungal preferences were not correlated with the relative abundance of fungi in the field and there was considerable intra-population variability in food preferences — both between individuals and for the same individual through time. Niche overlap between animals from all four study populations was high and there was no evidence of differences in the potential trophic niches of animals from sympatric and allopatric populations.     

Effect of zinc on four populations and two generations of Gammarus pulex (L.)

1. The amphipod Gammarus pulex (L.) is increasingly used in toxicity assessments and is usually obtained from wild populations. Interpopulation variability in response to toxicants may be due to genetic or phenotypic differences and could be large in wild-caught organisms exposed to different environmental conditions. This paper describes an investigation designed to assess the extent of lethal and sublethal variability between four different G. pulex populations when exposed to zinc, and to determine whether differences observed in the parental generation were also apparent in offspring bred in the laboratory. 2. The mortality of G. pulex from one of the four field-caught populations (Crags Stream) was significantly lower than for animals from the other three populations for the first 2 days of exposure. After 6 days’ exposure the LC₅₀ values for all four populations did not differ significantly, and were approximately 1.0 mg Zn 1-^?1. Sublethal effects on feeding rate after 6 days’ exposure were also similar for all four populations (feeding rate EC₅₀ approximately 0.5 mg Zn 1-^?1). 3. Lethal effects on the F₁ generation were similar among the four populations, in contrast to the short-term differences observed in the parental generation. These results suggest that interpopulation differences in zinc tolerance in G. pulex are short-lived, and phenotypically rather than genetically based.     

Carotenoids of two freshwater amphipod species (Gammarus pulex and G. roeseli) and their common acanthocephalan parasite Polymorphus minutus

Carotenoid compositions of two freshwater Gammarus species (Crustacea: Amphipoda) and of their common acanthocephalan parasite Polymorphus minutus were characterized. The effect of carotenoid uptake by the parasite was addressed by comparing the carotenoid content of uninfected and infected female hosts. Using high-pressure liquid chromatography (HPLC), co-chromatography of reference pigments and electron ionization mass spectrometry of collected HPLC fractions (EI-MS), several xanthophylls and non-polar compounds were identified. Seven kinds of carotenoids, mainly xanthophylls, were identified in gammarids. Astaxanthin was predominant, amounting to 40 wt.% of total carotenoid in both uninfected G. pulex and G. roeseli. By contrast, we found only non-polar compounds with a predominance of esterified forms of astaxanthin in P. minutus larvae. No significant effect of infection on carotenoid content was evidenced in G. pulex and G. roeseli females. Our study highlights the use of a Matrix Solid Phase Dispersion as an efficient extraction method of both xanthophylls and non-polar pigments in small samples, including lipid-rich ones as P. minutus parasite. We discuss on the presumptive pathway leading to the formation of free astaxanthin in gammarids via hydroxy compounds, and on the accumulation of esters of astaxanthin in parasites.