Condition-dependent ecdysis and immunocompetence in the amphipod crustacean,Gammarus pulex

The exoskeleton of arthropods forms an efficient protection against pathogens, but this first line of defence is periodically weakened during ecdysis, increasing the opportunity for surrounding pathogens to invade the body cavity. Since the richness of pathogens in the environment can be spatially and temporally variable, arthropods may have a fitness advantage in moulting in a place and time of low infection risk. Consistent with this hypothesis, we found that the amphipod crustacean, Gammarus pulex, exhibits temporal adjustment of the moult cycle in response to elevated risks of infection. Interestingly, this phenomenon is variable between two populations and independent of levels of immune defences. These results suggest that plasticity of the moult cycle in response to elevated risks of infection is adaptive and may result from adaptation to local variations in the risk of infection.     

Effects of temperature, age and body size on moulting and growth in the freshwater amphipods Gammarus fossarum and G. roeseli

1. The total body length, cephalic length, wet weight and dry weight was measured in juveniles, males and females of Gammarus fossarum and G. roeseli kept in the laboratory. Numbers of flagellar segments on the first and second pairs of antennae of G. fossarum and G. roeseli were quantitatively related to body size and in stars. The addition of segments cannot be used to identify particular instars of individuals or to determine their ages in natural populations. 2. At experimental temperatures ranging from 3.8 to 20.2°C, the number of moults, duration of intermoults, maturation times and specific growth rates were studied from birth in isolated specimens. Sexual maturity was reached after 9 or 10 moults, at a mean wet weight of c. 5mg for females and c. 7mg for males of G. fossarum, and at c. 10 mg for females and c. 13 mg for males of G. roeseli. At 3.8°C neither species reached sexual maturity within 550 days. 3. The mean interval between moults was observed from birth to sexual maturity and was linearly related to moult number and exponentially related to age. The relationship between each intermoult interval, or the maturation time, and the experimental water temperature was described by a power function. Maturation times increased from 96 days at 20.2°C to 355 days at 7.9°C in G. fossarum, and 85 days to 403 days in G. roeseli. 4. Over the range 3.8–20.2°C there was a log—log relationship between temperature and specific growth rates. Growth was maximal at 20.2°C in newborn animals as well as in small sexually mature animals; interspecific differences were highly significant. 5. Increase in body wet weight of G. fossarum and G. roeseli fed ad libitum on a constant mixture of autumn-shed, naturally decaying, tree leaves and aquatic macrophytes was followed to senescence and death. The instantaneous or specific growth rate was maximal near birth, at c. 7.98% wet weight day^?1 in G. fossarum and 9.03% in G. roeseli. At ≥12°C, growth conformed to a logistic curve; maximum absolute increments in weight occurred about half-way through a life span of 280–300 days at 20°C, 380–420 days at 16°C and 550–600 days at 12°C. Some individuals lived longer than 850 days at ≤12°C. The wet weight at birth was 0.112mg for G. fossarum and 0.123mg for G. roeseli. Asymptotic mean body weights of males and females were, respectively, 61 and 41 mg for G. fossarum and 87 and 58 mg for G. roeseli. However, G. roeseli reached the inflection point of the logistic curve significantly faster than G. fossarum. In the latter species, growth and maturation were relatively faster at temperatures below 12°C, whereas they were faster in G. roeseli at 16–20°C. Thus G. fossarum is adapted to summer-cool streams and G. roeseli is adapted to summer-warm streams.     

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.     

Number of flagellar segments and moulting in the amphipod Gammarus pulex

SUMMARY. The number of flagellar segments on the antennules of Gammarus pulex was quantitatively related to body size in animals from natural populations and to moulting and instar number of isolated animals grown from birth in the laboratory. The number of segments was 5.0 at birth increasing to means of 19–20 in females and 21–22 in males at sexual maturity or c. 5 mg wet wt reached after nine or ten moults. Addition of segments to the paired flagellae of juveniles was not regular and the number of segments cannot be used to identify particular instars of individuals or to determine their age. The mean interval (days) between moults progressively increased in the later juvenile instars and was exponentially related to moult number, age and the number of flagellar segments; the moult interval is also exponentially related to these variables in adults.     

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.     

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.     

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.     

Life-history and production of the amphipod Gammarus pulex in a Dorset chalk stream

SUMMARY. The seasonal variation in population density of Gammarus pulex was studied in a Dorset chalk stream. The numbers increased markedly in June and July and reached a maximum of c. 10000m⁻² in September whilst the most rapid decline in density occurred in October-November and reached a minimum of 820 m⁻² in February. The animals occurred in greater densities in habitats containing Ranunculus or Callitriche than in those devoid of vegetation. The population structure was determined monthly and was split into juveniles (length <4mm), immature males, immature females, mature males and ovigerous females. The percentage of juveniles (39–76) was always the highest of any of the categories. Ovigerous females were found at all times of the year. The sex ratio varied with the time of year both for immatures and matures, although there was approximately a 1:1 ratio for the mature individuals. Seasonal variation in biomass showed a maximum of 7.l g dry wt m⁻² in September and a minimum of 1.4 g dry wt m⁻² in March. Production was calculated by two methods giving values of 12.9 g dry wt m⁻² year⁻¹ and 12.8 g dry wt m⁻² year⁻¹.     

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.     

Experiments on feeding and growth of the amphipod Gammarus pulex (L.) related to its distribution in the River Duddon

Individuals of Gammarus pulex were kept at 15°C for periods of 23–70 days on diets comprised of elm or oak leaves, Tricladium and Clavariopsis (Hyphomycete fungi), Molinia (grass), Zygogonium (alga), Nardia (liverwort). Both the mean interval between moults (m_i), and daily increments in body wet weight, were related to diet. The lowest value for m_i was 14·6 days on a diet of naturally decaying elm and oak leaves. The largest weight gains also occurred on this diet; the average daily gain in weight (D_w) = 130·8 μg/day, the mean specific gain in weight (G_w) = 1·69 μg %/day. On green Molinia and fungi diets D_w ranged from nil to 60·7 μg/day, and m_i= 17·4–18·4 days was significantly (P<0·01) longer than m_i on leaf diets. Survival and growth were poor on brown Molinia. Nardia did not support growth or survival. A mixed diet of Tricladium, Zygogonium, Nardia and decaying grasses was not sufficient to promote an increase in body weight, and m_i= 21·8 days. This diet represents the commonly available food materials in Mosedale Beck, an acid stream in the upper Duddon catchment. Survival and growth of G. pulex on fungi and leaf diets in media containing low concentrations of potassium ions are also described and discussed briefly in relation to the distribution of G. pulex in the upper catchment of the R. Duddon.     

Invasion by the amphipod Gammarus pulex alters community composition of native freshwater macroinvertebrates

Assessing the effects of invading species on native community structure is often confounded by environmental factors and weakened by lack of replicated, long-term pre- and post-invasion monitoring. Here, we uncouple the community effects of a freshwater amphipod invader from environmental differences. In Irish rivers, the introduced Gammarus pulex replaces the native Gammarus duebeni celticus . However, the River Lissan in Northern Ireland is dissected by a weir that has slowed the upstream invasion by G. pulex . This allowed us in 2000 to sample three contiguous 150-m reaches that were (1) G. pulex dominated; (2) mixed Gammarus spp.; and (3) G. duebeni celticus only. In 2003, we resampled these reaches and one additional of mixed Gammarus species and one with only G. duebeni celticus further upstream. In temperature, conductivity, and pH, there were statistically significant but no biologically relevant differences among the five reaches of 2003, and between the three reaches surveyed in both years. Although there was evidence of recovery in macroinvertebrate diversity and richness in invaded reaches between years, continued upstream invasion was associated with sustained reductions in these community metrics as compared to un-invaded sites. Community ordination indicated (1) different associations of community composition attributed to the distribution, abundance, and biomass of the invader; and (2) increasing similarity of invaded communities over time. The impact mechanisms of G. pulex on macroinvertebrate community composition may include predation and competition. The consequences of the observed community changes for ecosystem functioning require further investigation.     

Effects of oil on the amphipod Gammarus oceanicus

The effects of a number of oils (one crude and two refined products) were tested under various conditions on larvae and adults of the amphipod Gammarus oceanicus. Acute toxicity tests were performed, as well as long-term bioassays. Larvae were found to be several hundred times more sensitive to the oils than the adults during acute exposure. A number of sublethal effects appeared during long-term bioassays. The adults showed impaired swimming performance, decreased tendency to precopulate, impaired light reaction and decreased production of larvae. Decreased growth was found among larvae during chronic exposure to crude oil. Delayed mortality occurred among adults after a short-term exposure to crude oil with a long recovery period.     

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.     

Effects of body size and temperature on population growth

For at least 200 years, since the time of Malthus, population growth has been recognized as providing a critical link between the performance of individual organisms and the ecology and evolution of species. We present a theory that shows how the intrinsic rate of exponential population growth, rmax, and the carrying capacity, K, depend on individual metabolic rate and resource supply rate. To do this, we construct equations for the metabolic rates of entire populations by summing over individuals, and then we combine these population-level equations with Malthusian growth. Thus, the theory makes explicit the relationship between rates of resource supply in the environment and rates of production of new biomass and individuals. These individual-level and population-level processes are inextricably linked because metabolism sets both the demand for environmental resources and the resource allocation to survival, growth, and reproduction. We use the theory to make explicit how and why rmax exhibits its characteristic dependence on body size and temperature. Data for aerobic eukaryotes, including algae, protists, insects, zooplankton, fishes, and mammals, support these predicted scalings for rmax. The metabolic flux of energy and materials also dictates that the carrying capacity or equilibrium density of populations should decrease with increasing body size and increasing temperature. Finally, we argue that body mass and body temperature, through their effects on metabolic rate, can explain most of the variation in fecundity and mortality rates. Data for marine fishes in the field support these predictions for instantaneous rates of mortality. This theory links the rates of metabolism and resource use of individuals to life-history attributes and population dynamics for a broad assortment of organisms, from unicellular organisms to mammals.     

Importance of fungi in the diet of Gammarus pulex and Asellus aquaticus

An important component of the interaction between macroinvertebrates and leaf litter in streams in the extent to which consumers can differentiate between undecomposed and decomposing leaves. The detritivores Gammarus pulex and Asellus aquaticus fed preferentially on conditioned rather on unconditioned leaf material. Growth in A. aquaticus was significantly reduced when unconditioned leaves were provided, but in G. pulex no significant effect of conditioning on growth was observed. The capacity of G. pulex to tolerate reductions in food quality seems to be a consequence of a compensatory system in which respiration rates change to compensate for reductions in food quality. In this way a constant growth rate is maintained. Increases in ingestion rates to compensate for low quality food were not observed.     

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.     

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.     

The digestion of cellulose and other dietary components, and pH of the gut in the amphipod Gammarus pulex (L.)

SUMMARY. Gut extracts from Gammarus pulex hydrolysed native and other cellulose substrates in vitro. Digestive fluid cellulase is probably endogenous as cell-free fluid mediated cellulose hydrolysis, but no bacteria were isolated from the fluid which produced a detectable extra-cellular cellulase. There was no apparent digestion of plant cell walls during their passage along the digestive tract, which took about 5–7 h at 10°C. The pH sensitivities of the digestive enzymes and the pH of the various regions of the gut suggest that carbohydrate digestion occurs in the proventriculus, midgut glands and anterior midgut, but protein digestion may be largely limited to the posterior midgut. The pH of the digestive fluid was altered slightly, but significantly, by the consumption of different natural and artificial test diets and by starvation. The most probable reason for the non-digestion of plant cell-walls is the lack of necessary enzymes other than cellulase. The role of cellulase may be confined to digesting the many small, non-cellular particles which are present in the gut.     

温度对水螅种群增长和个体大小的影响

应用群体累积培养法,以枝角类为食物,在10℃、20℃和30℃三种温度下,研究了温度对水螅(Hydra sp．)种群密度、种群增长率和个体大小的影响。结果表明温度对水螅种群密度和个体体积由极显著影响。在培养初期,30℃下的水螅种群密度最大;而在培养后期,20℃下的水螅种群密度则显著大于30℃下的值;10℃下的值则始终最小。在同一温度下,种群增长率均与时间呈曲线相关,10℃、20℃、30℃的回归方程分别为：Y=0．000433X^2-0．00262X 0．00332、Y=-0．003367X^2 0．068335X-0．066489、Y=-0．018469X^2 0．188952X-0．030933。在研究范围内,30℃的水螅个体最小。     

Effects of body size and water temperature on grazing rates of four intertidal gastropods

Abstract Rates of radular activity were measured in four species of Australian gastropods. The species were Austrocochlea constricta, Bembicium nanum, Cellana tramoserica, and Nerita atramentosa. Work was done in Botany Bay at Cape Banks, NSW, Australia from September 1989 to June 1990. A hydrophone was used to record radular raspings of snails in situ. Averages based on all observations showed Cellana had the fastest rate with 80 rasps/min, and was followed by Austrocochlea (71 rasps/min), Bembicium (57), and Nerita (39). Faster rates were associated with warmer water temperature and smaller body sizes in all species except Bembicium. The potential for these differences in radular activity affecting competitive interactions is discussed.