Some features of feeding,respiration and energy conversion of Dapnia Magna

Some results of studies with Daphnia magna are presented. These results can be used as background information for toxicologists, but the techniques referred to might well be used for toxicity tests. Daphnia magna is a filter-feeder. With the Coulter Counter it was shown that the feeding mechanism is aselective for size of the food particles. It was also shown that algal cells can pass the gut of Daphnia magna several times before being completely digested. The uptake of food is proportional to the food concentration up to a critical concentration. Above this concentration the food uptake is constant. Respiration is also dependent on the food concentration, and has a maximum value at food concentrations near the critical concentration of the feeding process. Growth efficiency is independent of the food concentration. The effect of temperature on the feeding process is different for low and high food concentrations. Growth efficiency is maximal at 10°C and above 22°C growth efficiency was negative, which means that the population cannot survive under the experimental conditions used, at temperatures above 22°C.     

Low filtering rates of <Emphasis Type="Italic">Daphnia magna</Emphasis> in a hypertrophic lake: laboratory and in situ experiments using synthetic microspheres

Zooplankton grazing was measured in Albufera of València (Spain), a shallow turbid hypertrophic lagoon dominated by filamentous bluegreens, during the period of Daphnia magna growth, to evaluate the role of this cladoceran in maintaining a clear water phase which takes place after flushing for rice cultivation practices. We found extremely low ingestion and clearance rates (CR) on latex beads in situ, using a Haney trap suggesting potentially strong inhibition of grazing by the filamentous cyanobacteria, still flourishing in the lagoon. To test the effect of filaments, we undertook laboratory feeding experiments using six different dilutions of the lagoon water to vary filament concentrations, and four different bead concentrations. A highly significant power function between CR and water dilution associated with filament concentrations was found, indicating that the ability of Daphnia to exploit smaller edible algae and thereby control phytoplankton growth would depend on filament concentration levels in the lagoon. From our results only the two more diluted treatments, 9 × 10³ and 18 × 10³ filament ml⁻¹ showed CRs not far from the range of what would be normally expected on the basis of the general relationship of feeding rate as a function of total food concentration. This means that at lower food concentrations, filaments reduce D. magna CR by increasing total food concentration beyond the incipient limiting level, as well as by moderate mechanical interference with the animal’s feeding. However, at lagoon water concentrations above 25% (corresponding to filament concentrations of 75 × 10⁵ filaments ml⁻¹), extremely low CR’s, which did not respond to any food addition, were obtained for D. magna. From these results we can infer that at concentrations beyond the above-mentioned critical level, feeding inhibition by filamentous cyanobacteria is most probably due entirely to mechanical interference. Guest editor: Piet Spaak Cladocera: Proceedings of the 7th International Symposium on Cladocera     

Response of the respiratory rate of Daphnia magna to changing food conditions

Summary The respiratory rate of the cladoceran Daphnia magna was measured at varying concentrations of a green alga in a flow-through respirometer. Daphnids were either preconditioned to the respective food concentration or the food concentration was suddenly changed during the experiment and the response of the respiratory rate monitored. Previously starved animals were provided with food or prefed animals were deprived of food. The respiratory rate increased considerably with increasing concentrations of algae until a maximum rate was reached at a critical algal concentration corresponding to the incipient limiting level for feeding. The response of the respiratory rate to changing food conditions was fast, lagging only a few minutes behind the change in food. The respiratory rate of starved daphnids increased quickly when they received food, even at low concentrations. Prefed daphnids responded to the replacement of the food suspension by filtered water with reductions in their respiratory rates. A linear relationship between the assimilation rate and the respiration rate was found, indicating that the more than twofold range of the respiratory rate was due to specific dynamic action. The SDA coefficients of 0.15–0.24 found for Daphnia are consistent with values for marine zooplankton.     

Does excess dietary carbon affect respiration of <Emphasis Type="Italic">Daphnia</Emphasis>?

Like many invertebrate herbivores, Daphnia frequently face diets with excess carbon (C) relative to elements like phosphorus (P), and with limited ability to store C-rich compounds. To cope with this relative surplus of C they may either regulate the net uptake of C or dispose of excess assimilated C via increased release of dissolved organic carbon or CO₂. Here we investigate whether juvenile Daphnia magna use respiration as a means of stoichiometrically regulating excess C. Growth rate and respiration were measured under different algal food qualities (P-replete and P-depleted algae). Growth rate was strongly reduced by P-depleted food, implying a stoichiometric disposal of excess ingested C. Respiration rates of feeding animals were measured after short- (0.5 h), medium- (12 h) and long- (five days) term acclimation to P-limited food. The respiration rates of animals during active feeding were not affected by the acclimation period per se, whereas food quality had a significant effect; respiration rates of feeding animals increased slightly in individuals receiving low-P food under all acclimation regimes. Respiration was also measured on nonfeeding and fasting animals that had been acclimated for five days to P-limited food. Respiration rates of these animals were strongly affected by feeding conditions but not by food quality; feeding individuals had higher respiration rates than those deprived of food, which again had higher respiration than fasting animals. Although animals grown on low-P food had strongly reduced growth and thus were expected to have decreased respiration rates due to reduced growth-related costs, this seems to be canceled out by increased stoichiometric respiration under P-deficiency. These results indicate that D. magna partly releases excess C as CO₂, but other means of stoichiometric regulation most likely add to this.     

Optimal foraging and ontogeny; food selection by Haplochromis piceatus

Summary We examined the influence of satiation level, prey density and light intensity on food uptake rate through the ontogeny of Haplochromis piceatus. Prey handling in the buccal cavity was found to be the main factor limiting prey uptake rate under light circumstances and at a sufficiently high prey density. Food uptake rate per unit body weight of different sizes of H. piceatus was equal when feeding on Chaoborus but decreased with increasing fish size when feeding on Daphnia magna. In choice experiments with Chaoborus and D. magna, prey selection by H. piceatus of all sizes was according to the predictions based on Charnov's 1976 model.     

Temperature and the effects of elemental food quality on Daphnia

1. We examined the responses of two species of Daphnia to changes in food phosphorus (P) content, with animals reared at three different water temperatures. Specifically, we measured mass‐specific growth rate (MSGR), body P content and respiration rate of Daphnia magna and Daphnia pulex acclimatised to 10, 17.5 and 25 °C and fed food carbon : phosphorus (C : P) ratios of either 150 or 500. 2. The responses of these three physiological variables to temperature–food quality interactions were species‐specific. There was a significant interactive effect of temperature and food quality on D. magna, as the greatest proportional effect of food quality on growth was observed at 10 °C and reductions in body P because of low food P content were relatively greater at 25 °C. These effects may reflect the temperature dependence of mechanisms that reduce elemental constraints associated with food quality in D. magna. By contrast, there were no interactive effects between food quality and temperature on MSGR, body P or mass‐specific respiration of D. pulex. 3. It thus appears that temperature can alter food quality effects on Daphnia but the nature of these alterations depends upon the daphniid species and its thermal adaptability. Significant temperature–food quality interactions will complicate efforts to understand zooplankton nutrition in nature and warrant future consideration.     

Influence of food density on respiration rate of two crustacean plankters,Daphnia galeata and Bosmina longirostris

Summary The influence of food density on respiration rate was measured for two cladoceran plankton species, Daphnia galeata and Bosmina longirostris, over the range 0 to 2.5 mg C 1^-1, using the modified Winkler technique in order to examine how this affects the respiration rate and whether the functional response is the same in the two species. The respiration rate for animals of equivalent body size did not differ significantly between the two species in the absence of food, but was significantly lower in Bosmina longirostris than Daphnia galeata at high food density. Within a species, the response of respiration rate to changing food density did not differ among individuals of different body size. The respiration rate of D. galeata increased with increasing food density and reached a plateau at a high food density. A similar response curve was also found with the respiration rate of B. longirostris, although the response was less conspicuous. This response curve cannot be explained by the energy cost of known feeding behavior in cladocerans. Since the respiration rate related linearly with the assimilation rate, increase in food density seemed to increase the respiration rate by increasing the energetic cost required to process food biochemically, known as specific dynamic action.     

Releases of ingested phytoplankton carbon by Daphnia magna

1. The carbon budgets and assimilation efficiencies (AEs) of adults and juveniles of Daphnia magna were quantified using ¹⁴C as a tracer. Animals were fed pure Chlamydomonas reinhardtii or Scenedesmus obliquus at different food concentrations. Carbon AEs (46–70%) were comparable at food concentrations of 0.03–0.30 mg C L^?1 for both algal species, but decreased to 34–49% when the food concentration further increased by 10‐fold. The carbon AEs were significantly and negatively correlated with the food level. 2. During the postdigestive period, partitioning of ingested carbon into different compartments including excretion, respiration and egestion was not influenced by the food species and life stage. There was a negative correlation between respiration (as % of total loss) and food concentration and a positive correlation between egestion (as % of total loss) and food concentration. Dissolved organic carbon (DOC) and CO₂ accounted for 55–72% and 9–37%, respectively, of the total carbon loss from juveniles fed both algal diets. For adults, DOC and CO₂ contributed to 44–64% and 20–47% of the total carbon loss, respectively. Particulate organic carbon (POC) was a minor pathway for the overall carbon loss. 3. The turnover and release budget of structural carbon (as moults and neonate reproduction) were further evaluated in long‐term experiments at different algal concentrations. Food concentration did not affect the carbon efflux or the carbon allocation into different physiological compartments except for respiration. Juveniles had twofold lower carbon turnover rate (0.12–0.16 day^?1) than those of the adults (0.32–0.35 day^?1). In adults, comparable carbon was allocated into DOC (35–42%) and reproduction (27–35%), which were the dominant routes for carbon loss. For the juveniles, DOC accounted for 42–64% of the total carbon loss. 4. About 21–38% of the total DOC released by adults and juveniles was associated with the high molecular weight organic carbon fraction (>5 kDa). Our results show that carbon was mainly lost by D. magna in the form of DOC during assimilation process as well as from the structural materials. Reproduction or maternal transfer was another major drain of body carbon for adult D. magna.     

How<Emphasis Type="Italic"> Daphnia</Emphasis> copes with excess carbon in its food

Animals that maintain near homeostatic elemental ratios may get rid of excess ingested elements from their food in different ways. C regulation was studied in juveniles of Daphnia magna feeding on two Selenastrum capricornutum cultures contrasting in P content (400 and 80 C:P atomic ratios). Both cultures were labelled with ¹⁴C in order to measure Daphnia ingestion and assimilation rates. No significant difference in ingestion rates was observed between P-low and P-rich food, whereas the net assimilation of ¹⁴C was higher in the treatment with P-rich algae. Some Daphnia were also homogeneously labelled over 5 days on radioactive algae to estimate respiration rates and excretion rates of dissolved organic C (DOC). The respiration rate for Daphnia fed with high C:P algae (38.7% of body C day^-1) was significantly higher than for those feeding on low C:P algae (25.3% of body C day^-1). The DOC excretion rate was also higher when animals were fed on P-low algae (13.4% of body C day^-1) than on P-rich algae (5.7% of body C day^-1) . When corrected for respiratory losses, total assimilation of C did not differ significantly between treatments (around 60% of body C day^-1). Judging from these experiments, D. magna can maintain its stoichiometric balance when feeding on unbalanced diets (high C:P) primarily by disposing of excess dietary C via respiration and excretion of DOC.     

Growth efficiency ofDaphinia magna. I. The effect of food concentration

Summary In 24 hour experiments in a flow-through set-up the growth efficiency ofDaphnia magna fed withChlorella vulgaris was determined at different food concentrations. All parameters were measured simultaneously. Food uptake was measured with the Coulter Counter and in each experiment the conversion factor for cell volumes to joules was determined. Growth was measured directly in joules by burningDaphnia at the end of the experiment in a calorimeter and a parallel group ofDaphnia at the start of the experiment. No effect of food concentration on growth efficiency was found.     

Life-History Characteristics and Fitness in Descendents of Parthenogenetic and Ex-Ephippio Females of Daphnia Magna

Daphnids of ex-ephippial and parthenogenetic origin differ substantially in life-history. Possible maternally transmitted ex-diapause effects of differing female origin on the fitness of their offspring were studied across multiple clones in Daphnia magna. Ex-ephippial daphnids responded in egg size to different constant food concentrations with the same pattern as parthenogenetic females. Significant differences in egg characteristics between females of different origin were only found for the first clutch produced under high food. The smaller size of eggs in ex-ephippial females, however, did not translate into size differences of first-clutch neonates. A trend for lower mass density per body volume was detected in offspring from ex-ephippial females in comparison with parthenogenetic daphnids. Hence, an ex-diapause effect transmitted by ex-ephippial mothers to their parthenogenetic offspring is likely. However, there was no difference in life-histories and fitness between offspring produced by females of ex-ephippial and parthenogenetic origin at both, high and low food concentrations across multiple clones. Thus, a significant ex-diapause effect on fitness in successive parthenogenetic generations may not be expected in D. magna at the population level.     

The use of the Coulter Counter for measuring the feeding rates of daphnia magna

The feeding rate of Daphnia magna was determined by measuring the decrease of the Chlorella vulgaris concentration as animals were fed in a certain volume of algal suspension. The algal concentration was measured with a Coulter Counter. The results are quite comparable with the results of other methods reported in the literature. The advantage of this method is the possibility of studying the feeding rate over longer periods. The effect of the temperature on the filtering and feeding rate was determined. The highest values of both rates were found above 22°C.     

Feeding behavior as an index of copper stress in Daphnia magna and Brachionus calyciflorus

1. The effect of short-term exposure to the fungicide copper sulfate, on the feeding behavior of two freshwater zooplankters, the rotifer Brachionus calyciflorus and the cladocera Daphnia magna, was studied.2. Both species were exposed to sublethal levels (0.01, 0.02, 0.05 and 0.08 mg/l) of copper sulfate during 24 hr in order to determine the effect of this toxicant on filtration and ingestion rates.3. The experiments were performed using the unicellular algae Nannochloris oculata in a density of 5 × 10⁵ cells/ml, as food for both organisms.4. Rates of filtration and ingestion declined on B. calyciflorus and D. magna with increasing toxicant concentrations.5. The effective copper sulfate concentration at which feeding rate was reduced to 50% of that in controls (ec₅₀) was 0.043 and 0.090 mg/1 for B. calyciflorus and D. magna, respectively.     

Effect of food type and concentration on the survival,longevity, and reproduction of Daphnia magna

The freshwater microalgae Ankistrodesmus falcatus and Scenedesmus incrassatulus, at three concentrations (6, 12 and 18 mg l⁻¹ t, dry weight), were tested as diets for Daphnia magna cultured in reconstituted hard water, at 19 ± 1 °C. Effects on survival, and reproductive characteristics were evaluated during a complete life-cycle. Test animals fed with the highest food concentration had the lowest survival, longevity, number of clutches, and average inter-brood times, but the total offspring was similar to that obtained with the lowest food concentration, for the same species of alga. Average longevity ranged from 40 to 85 days, with a minimum of 24 and a maximum of 119 days. The average inter-brood time ranged from 4.2 to 5.8 days, the highest values being observed at the lowest food concentration. The average total offspring was maximum for the intermediate concentrations of both algae. The maximum number of clutches ranged from 9 to 23, with the lowest values being observed at the highest food concentration. Even though both microalgae had similar effects on survival and reproduction, and the greatest differences observed were related to food concentration, it seems that S. incrassalulus is a comparatively better food for D. magna. The most suitable food level should be determined prior to carrying out chronic bioassays, and it is also an important factor in cultures for obtaining neonates for toxicological tests.     

Foraging strategies and growth inhibition in five daphnids feeding on mixtures of a toxic cyanobacterium and a green alga

1. Laboratory experiments were used to study the feeding, growth and reproduction of five daphnids in mixtures of a toxic cyanobacterium, Microcystis aeruginosa, and a green alga, Scenedesmus acutus. The mixtures included 0%, 20%, 50%, 80% and 100% Microcystis with a total food concentration of 0.5 mg C L^??1 in each treatment. The feeding rate was measured after 1 and 24 h of acclimatization to the mixtures. 2. Toxic Microcystis inhibited feeding in all the species, but they exhibited an unexpected diversity and complexity in the pattern of feeding inhibition. Daphnia magna exhibited the strongest inhibition of feeding after 1 h of exposure to toxic food, but had substantially recovered after 24 h in the same mixtures. This pattern of inhibition and recovery may balance the benefits of reduced ingestion of toxin with the disadvantage of a reduced energy intake. 3. All five daphnids grew quickly in the Scenedesmus control, whereas growth and reproduction declined with an increasing proportion of the toxic alga in the diet. Daphnia pulicaria showed the least inhibition of growth and reproduction, D. pulex showed the strongest inhibition and the three remaining species exhibited intermediate sensitivity. 4. Estimates of gross growth efficiency (GGE; growth/ingestion) provided a means for discriminating between the effects of feeding inhibition and direct toxicity on zooplankton growth. Daphnia pulex exhibited a sharp decline in GGE, suggesting that growth inhibition was a result of both feeding inhibition and direct toxicity. In contrast, D. magna exhibited a nearly constant GGE, indicating that feeding inhibition accounted for its decline in growth. However, two Daphnia species (i.e. D. pulicaria and D. galeata) exhibited improbable increases in GGE with toxic cyanobacteria, suggesting that their feeding rates were underestimated. Growth assays with sensitive and resistant zooplankton species are proposed for testing the potential impacts toxic cyanobacteria in lakes.     

The components of life-history traits variation in Daphnia magna population

The variation of six growth and reproduction traits in the parthenogenetic clones of Daphnia magna and its relationships with the genotypes at 5 isozyme loci were studied under controlled conditions at three food concentrations. A significant genetic component of age at maturity, growth rate and fecundity variation was detected. Nested ANOVA revealed the influences of the genotypes of the isozyme loci, that are more pronounced at the high food concentration. In some cases the genotype-environment interactions were found.The variation of body length at 6 days and total number of offspring includes a significant heterozygosity — environment interaction. The most heterozygous clones show the least differences in life history traits between food conditions.Life-history traits heritability, genotype-environment interactions and different reproductive strategies of the clones are discussed.     

Components of Daphnia feeding behaviour

Measurements of filter limb beat rate, mandible movement rate,and bolus swallowing rate from individual Daphnia magna arecollated with the grazing rates of groups of animals free-swimmingunder similar conditions. Significant effects of temperature,food concentration and angular light distribution are recorded,and the data are interpreted in the light of possible mechanismscontrolling the cladoceran feeding process.     

Biotic relations affecting species structure in zooplankton communities

Mathematical modelling and laboratory experiments were used to study how exploitative competition and predation influence the species structure in cladoceran community. For five species of Cladocera (Sida crystallina, Daphnia magna, Simocephalus vetulus, Daphnia longispina, and Diaphanosoma brachyurum), representing a gradient of body size, population characteristics were described as functions of food concentration. Abundance dynamics were simulated in mixed species cultures and invasion experiments under different levels of food supply corresponding to oligo-, meso-, and eutrophic conditions. Separate simulations were also run including and excluding (fish) predation. The competitive ability of each species was estimated as the values of the population equilibrium food concentration. Simulation results showed that for the no-predator scenario, increases in the level of food supply promoted species coexistence while under lower food concentrations only one species remained at the end of the simulation runs. When predation was allowed, the number of species that coexisted at the end of the simulations increased up to four species, indicating that predator pressure facilitated species coexistence because it shortened periods of food depletion. Simulation results were verified in laboratory experiments which suggested that population equilibrium food concentration can be used as an estimate of competitive ability. Finally, species structure and relative abundance in Lake Naroch (Belarus) during the summer of 2004 was found to be consistent with our results from computer simulation and laboratory experiments with regard to competition and predation impacts on zooplankton community. Handling editor: S. I. Dodson     

Energy allocation rules inDaphnia magna: clonal and age differences in the effects of food limitation

Summary The allocation of energy to carapace formation, respiration, growth, and reproduction were examined in two parthenogenetic clones ofDaphnia magna (Cladocera) cultured at two levels of food (Chlorella) concentration. Clonal differences in energy allocation were more apparent at high ration (1.5 g C mL^-1) than at low ration (0.3 g C mL^-1). These differences included respiratory and molting costs, and the timing of energy allocation to growth and reproduction. A comparison of active vs. anesthetized animals revealed that the interclonal difference in respiration rate was the result of a difference in activity level. In both clones mass-specific rates of respiration, growth, and brood production all decreased at low vs. high ration levels, whereas mass-specific molt-loss rate increased. Lowered food concentration decreased the relative allocation of energy to growth and reproduction, but increased allocation to maintenance (respiration and carapace formation). These allocation responses to food limitation indicated that for both clones the highest energy priority was carapace formation. However, the relative priority of respiration, growth and reproduction varied with age and clone. In juveniles (instars 1–4) the priority ranking of growth was essentially equal to that of respiration, whereas respiration always had higher priority in adults (instars 5–9). All three possibilities for the relative ranking of growth and reproduction (i.e., growth>reproduction, growth=reproduction, and reproduction>growth), as specified by different models in the literature, were observed depending on age and clone. The energy allocation rules were also shown to vary between other daphniid species. Furthermore, metabolic responses to chronic food limitation may be different from responses to acute food deprivation. In this study, one clone showed a greater decrease in respiration rate as a result of lifetime food limitation than did the other, but the opposite was true when these clones were exposed to 48 h of starvation. These differences in allocation rules and in acute vs. chronic responses may have to be considered when using physiological data to modelDaphnia populations.     

Reproducibility of short-term and reproduction toxicity experiments with Daphnia magna and comparison of the sensitivity of Daphnia magna with Daphnia pulex and Daphnia cucullata in short-term experiments

Short-term toxicity tests with Daphnia magna are reasonably reproducible. The sensitivity of Daphnia pulex, Daphnia cucullata and Daphnia magna proved to be about the same in short-term tests with 15 different chemical compounds.Reproduction toxicity experiments with Daphnia magna can easily be carried out within three weeks. At least duplicate experiments are necessary to arrive at an approximate no-effect level.