Good food versus bad food: the role of sterols and polyunsaturated fatty acids in determining growth and reproduction of <Emphasis Type="Italic">Daphnia magna</Emphasis>

The absence of dietary sterols and polyunsaturated fatty acids (PUFAs) has been shown to affect the performance of the freshwater herbivore Daphnia. Here, we compared somatic growth rates and clutch sizes of Daphnia magna reared on a diet of low food quality (Synechococcus elongatus) and of high food quality (Cryptomonas sp.) and investigated if and to what extent the absence of sterols or PUFAs in the cyanobacterium S. elongatus accounts for the observed differences in food quality. The supplementation of S. elongatus with cell-free lipid extracts (fatty acids, sterols, total lipids) obtained from the flagellate Cryptomonas sp. suggested that the superior food quality of Cryptomonas sp. is predominantly, but not completely, a combined effect of its sterol and PUFA composition. Our laboratory study suggests that somatic growth of D. magna feeding on S. elongatus is primarily constrained by the absence of sterols, whereas egg production is primarily limited by the absence of long chain PUFAs.     

Application of individual growth and population models of Daphnia pulex to Daphnia magna, Daphnia galeata and Bosmina longirostris

Daphnia models for individual growth and population dynamics have been developed in the manner of models developed by Gurney, McCauley, Andersen and others. All or most of the earlier models were parameterized for Daphnia pulex; we have used the D. pulex model as a baseline model for other species of Daphnia such as magna, galeata and also Bosmina longirostris. Because of the lack of ample data for D. magna, D. galeata and B. longirostris, some of the physiological data had to be relied on the other species whose data were available and in some case calibrated. We were able to produce reasonable results for individual growth as well as population dynamics under the controlled laboratory conditions. Most of the results were compared with the available laboratory data for population as well as growth. All the simulations have been done under high and low food concentrations. The animals are assumed to be feeding on green algae (Chlamydomonas reinhardtti) under the laboratory conditions of 18–20°C. The continuous growth until the end of the life was observed in smaller B. longirostris, whereas rapid growth in the beginning and slower after the start of the reproduction was observed in Daphnia species. The smaller species matured earlier than larger species. B. longirostris population sustained better than Daphnia species in medium food concentrations.     

Effects of crowding and different food levels on growth and reproductive investment of Daphnia

The effects of daphniid crowding on juvenile growth rate, length at first reproduction, clutch size and egg size of four species of Daphnia were compared with the effects of food level. Juvenile Daphnia were grown to primipary in a flow-through system in water conditioned by different densities of the same, or another, species. At high ambient food levels, water from Daphnia that had been crowded at densities 150 l^–1 depressed growth rate and lowered body size and clutch size of D. hyalina and D. galeata; effects on the same traits of D. magna and D. pulicaria were variable (stimulation, depression, or no effect). D. hyalina and D. galeata responded to gradients of increasing daphniid density (0–300 l^–1) by altering egg mass, somatic mass and clutch size to maintain a relatively constant reproductive investment; egg mass increased with crowding and then decreased in a pattern consistent with Glazier's (1992) hypothetical model of changes in offspring size in relation to food quantity and maternal demand. Effects of crowding by conspecifics were indistinguishable from those of other species. This study, which uncouples the effect of crowding per se from ambient resource depletion, shows that chemical substances released by high densities of Daphnia can cause changes in life-history traits comparable to those that occur in response to low food levels.     

Coupling of growth rate and body stoichiometry in Daphnia: a role for maintenance processes?

1. The growth rate hypothesis predicts positive relationships among growth rate (μ), body RNA (%RNA of dry mass) and body P (%P of dry mass) contents. 2. We tested this within‐ and across‐species by growing five species/clones of Daphnia (Daphnia magna, Daphnia pulex, Daphnia galeata and two isolates of Daphnia pulicaria) with different combinations of food quantity and stoichiometric food quality. 3. Within each species, positive correlations among μ, %RNA and %P were seen and across species there was a strong association between%RNA and %P, consistent with the growth rate hypothesis. However, coupling of growth to %RNA and to %P differed for different species. In particular, the %RNA–μ and %P–μ relationships had similar slopes but considerably different y‐intercepts (i.e.%P or %RNA at zero growth), with D. pulicaria and D. galeata having higher intercepts than D. magna and especially D. pulex. As a result of these displacements, the relative rankings of the species on the basis of %P and %RNA did not correspond to their rankings based on μ. 4. These findings suggest that within a narrow clade (e.g. the daphnids), interspecific differences in body P content may reflect not growth rate‐related RNA allocation but instead the amount of RNA required for support of maintenance processes.     

Effects of stoichiometric dietary mixing on<Emphasis Type="Italic"> Daphnia</Emphasis> growth and reproduction

Herbivores often encounter nutritional deficiencies in their diets because of low nutrient content of plant biomass. Consumption of various diet items with different nutrient contents can potentially alleviate these nutritional deficiencies. However, most laboratory studies and modeling of herbivorous animals have been done with diets in which all food has uniform nutrient content. It is not clear whether heterogeneous versus uniform food of equal overall nutrient content is of equivalent nutritional value. We tested the effects of dietary mixing on performance of a model organism, Daphnia. We fed two species of Daphnia ( D. galeata, D. pulicaria) with diets of equivalent bulk stoichiometric food quality (C:P) and studied whether they would produce equivalent performance when C:P was uniform among cells or when the diet involved a mixture of high C:P and low C:P cells. Daphnia were fed saturating and limiting concentrations of a uniform food of moderate C:P (UNI) or mixtures (MIX) of high C:P (LOP) and low C:P (HIP) algae prepared to match C:P in UNI. Daphnia were also fed HIP and LOP algae separately. Juvenile growth rate and adult fecundity were measured. D. galeata performance in UNI and MIX treatments did not differ, indicating that partitioning of C and P among particles did not affect dietary quality. Similarly, D. pulicarias performance was similar in the MIX and UNI treatments but only at low food abundance. In the high food treatment, both growth and reproduction were higher in the MIX treatment, indicating some benefit of a more heterogeneous diet. The mechanisms for this improvement are unclear. Also, food quality affected growth and reproduction even at low food levels for both D. pulicaria and D. galeata. Our results indicate that some species of zooplankton can benefit from stoichiometric heterogeneity on diet.     

Dietary lipid quality affects temperature-mediated reaction norms of a freshwater key herbivore

Temperature-mediated plasticity in life history traits strongly affects the capability of ectotherms to cope with changing environmental temperatures. We hypothesised that temperature-mediated reaction norms of ectotherms are constrained by the availability of essential dietary lipids, i.e. polyunsaturated fatty acids (PUFA) and sterols, as these lipids are involved in the homeoviscous adaptation of biological membranes to changing temperatures. A life history experiment was conducted in which the freshwater herbivore Daphnia magna was raised at four different temperatures (10, 15, 20, 25°C) with food sources differing in their PUFA and sterol composition. Somatic growth rates increased significantly with increasing temperature, but differences among food sources were obtained only at 10°C at which animals grew better on PUFA-rich diets than on PUFA-deficient diets. PUFA-rich food sources resulted in significantly higher population growth rates at 10°C than PUFA-deficient food, and the optimum temperature for offspring production was clearly shifted towards colder temperatures with an increased availability of dietary PUFA. Supplementation of PUFA-deficient food with single PUFA enabled the production of viable offspring and significantly increased population growth rates at 10°C, indicating that dietary PUFA are crucial for the acclimation to cold temperatures. In contrast, cumulative numbers of viable offspring increased significantly upon cholesterol supplementation at 25°C and the optimum temperature for offspring production was shifted towards warmer temperatures, implying that sterol requirements increase with temperature. In conclusion, essential dietary lipids significantly affect temperature-mediated reaction norms of ectotherms and thus temperature-mediated plasticity in life history traits is subject to strong food quality constraints.     

Crowding‐induced changes in growth,reproduction and morphology of Daphnia

• 1 Daphnia may reach high population densities seasonally, or in patches, in lakes. To test the effects of chemicals released by high daphniid densities on their life‐history traits, nine species of Daphnia, D. magna, D. pulicaria, D. pulex, D. hyalina, D. galeata, D. laevis, D. lumholtzi, D. ambigua and D. cucullata, were grown in water from crowded Daphnia cultures in a flow‐through system in the presence of abundant food.
• 2 Water from Daphnia at 85 L^‐1 depressed growth rate, and lowered body size and clutch at first reproduction of six species of small‐bodied Daphnia (adult body length < 1.8 mm), but had no significant effects on larger species. Two clones of D. pulex differed in their growth rate in response to crowding, indicating that response patterns may vary within species.
• 3 Chemicals released by crowded D. magna reduced tail spine length in D. lumholtzi and D. cucullata by 37% and 11%, respectively, and induced changes in carapace morphology in D. lumholtzi and D. ambigua.
• 4 Chemicals released by crowded conspecifics may provide an additional, density‐dependent mechanism of population regulation; when large species of Daphnia coexist at a high population density with small species, these chemicals may reinforce the competitive advantage of large species.

     

Antibiotics affect the growth responses of <Emphasis Type="Italic">Daphnia magna</Emphasis> to poor food quality

We tested the hypothesis that exposure to antibiotics alters the growth and reproductive responses of Daphnia magna to changing stoichiometric food quality. To do so, we measured growth and reproduction of differentially P-nourished Daphnia in the presence and absence of sublethal concentrations of antibiotics. We found that exposure to an antibiotic cocktail significantly reduced an index of the microbial load of Daphnia and altered its growth responses to changing dietary P-content. Growth rates of Daphnia consuming the most P-rich and P-poor food increased with antibiotic exposure but were negatively or not affected in animals eating mildly to moderately P-limiting food. Similar effects were found in a subsequent experiment where daphnid neonates were exposed to natural bacterial communities prior to receiving antibiotics and being fed different food C:P ratios. In contrast, antibiotic effects on Daphnia reproduction were either not detected (number and size of broods) or were relatively minor (day of first reproduction). We also found no evidence that gut flora provides defense against pathogenic bacterial infection; instead, infection rates in Daphnia by a bacterial microparasite, Pasteuria ramosa, decreased in animals that had experienced prior antibiotic exposure. Our results demonstrate that antibiotic exposure reduced the microbial load and altered growth rates of an important zooplankton herbivore. Given the mediating role of animal’s food C:P ratio, our results show that interactions between Daphnia and its microbial symbionts vary in strength and nature partly with the host’s nutritional state.     

Allocation strategies in crustacean stoichiometry: the potential role of phosphorus in the limitation of reproduction

1. Elemental composition (carbon : nitrogen : phosphorus, C : N : P) was analysed in eggs and juveniles of two crustaceans, Daphnia magna (Cladocera) and the crayfish Astacus astacus (Decapoda). Stoichiometry was also analysed for the carapace, muscle tissue, hepatopanchreas and gills of Astacus. 2. For both species the C : P ratio was significantly higher in eggs than juveniles, but there was a constant, homeostatic elemental ratio in eggs during embryogenesis (Astacus) and with different C : N : P in maternal food (Daphnia). 3. Differences in the stoichiometry of major tissue categories in Astacus suggest that there are distinct allocation strategies of elements to various somatic tissues as well as to reproduction versus somatic tissues overall. 4. There are strong ontogenetic shifts in the allocation of energy and elements in both species, as for crustaceans in general. During maturity there may be a trade‐off with regard to the allocation of C, N or P to somatic or reproductive tissue, and poor food quality (high C : P in food) could pose other constraints on reproductive capacity than does food shortage (low C). 5. Egg production may be at least as sensitive to low P as is somatic growth and could result in a marked decrease in overall population growth rate more severe than would be expected from individual growth rate alone.     

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.     

Seasonal dynamics of sestonic protease inhibition: impact on Daphnia populations

Daphnia populations often show rapid microevolutionary adaptation to environmental changes. Here, we investigated the possibility that microevolution of Daphnia populations could be driven by natural sestonic Protease Inhibition (PI). We hypothesized that PI changes seasonally, which might lead to concomitant changes in tolerance to PI in a co-occurring Daphnia magna population. In order to test this, seston from a eutrophic pond was sampled regularly over two successive years. Extracts of these freeze-dried samples were used to determine their Inhibitory Potential (IP) on D. magna gut proteases. In the summer seston the IP against chymotrypsins exceeded that of spring seston 200-fold. In order to test for possible impacts on the co-existing D. magna population, we isolated clones before (spring) and after (fall) the peak of the IP. Microsatellite analyses revealed that the two subpopulations were genetically distinct. Individual exposure of three clones from each population to varying concentrations of a cyanobacterium that contains chymotrypsin inhibitors revealed a decrease in population and somatic growth rate for each clone, but no seasonal effects on Daphnia’s tolerance. In order to include maternal effects, we conducted a multi-clonal competition experiment on various cyanobacterial concentrations. However, no evidence for seasonally increased tolerance of D. magna to dietary protease inhibitors could be found.     

The importance of highly unsaturated fatty acids in zooplankton nutrition: evidence from experiments with Daphnia, a cyanobacterium and lipid emulsions

1. We used laboratory growth and feeding experiments to evaluate the role of ω3 highly unsaturated fatty acids (HUFA) in zooplankton nutrition. Polyunsaturated fatty acids (PUFA) comprised 40% of total fatty acids (FA) in the green alga Scenedesmus acutus but only 6% in the cyanobacterium Synechococcus elongatus.Scenedesmus contained modest amounts of the ω3 HUFA eicosapentaenoic acid (EPA, 20 : 5ω3) and docosahexaenoic acid (DHA, 22 : 6ω3), whereas Synechococcus contained only a trace of EPA and no DHA. 2. One-day-old Daphnia maintained high growth rates for 2–3 days on a diet of Synechococcus but exhibited reduced growth or even lost weight over the later part of the 6- or 7-day experiments. Daphnia magna grew better than D. galeata or D. pulicaria on a diet of pure Synechococcus. Daphnids fed the same concentration (0.5 mgC l^–1) of green algae or a mixture of green algae and Synechococcus maintained high growth rates. 3. Supplementing Synechococcus with fish oil emulsions rich in ω3 HUFA markedly improved the growth and reproduction of all three Daphnia species. Supplementation with an emulsion of monounsaturated oleic acid (18 : 1ω9), however, did not affect the growth of D. galeata and caused a decrease in the growth of D. magna. 4. Short-term (7 min) and long-term (3 h) feeding trials with ¹⁴C-labelled Synechococcus were used to evaluate the effects of acclimation and mixed diets on carbon incorporation. D. galeata that had fed on unlabelled Synechococcus for 3 days exhibited no decline in clearance rate but a marked decline in carbon incorporation in comparison with animals acclimated with Scenedesmus or fed a mixture of Synechococcus and Scenedesmus. 5. Our results support the hypothesis that the poor nutritional quality of a cyanobacterium is at least partially due to a HUFA deficiency. Growth and feeding experiments both suggest that the utilization of Synechococcus is enhanced by HUFA from lipid reserves or a mixed diet.     

Competition in natural populations of Daphnia

I investigated the competitive relationships between two species of Daphnia, D. galeata and D. cucullata, and their interspecific hybrid. The term hemispecific competition was introduced to describe competition between parental species and hybrids. In eutrophic Tjeukemeer both parental species were found to compete with the hybrid, whereas competition between D. galeata and D. cucullata seemed limited. Although the effect of competition on life history traits of daphnids may be profound, the influence of the competitors on the seasonal dynamics of the Daphnia species seems limited.     

Predator-released compounds, ambient temperature and competitive exclusion among differently sized Daphnia species

1. We studied the effects of fish water and temperature on mechanisms of competitive exclusion among two Daphnia species in flow‐through microcosms. The large‐bodied D. pulicaria outcompeted the medium sized D. galeata × hyalina in fish water, but not in the control treatment. Daphnia galeata × hyalina was competitively displaced 36 days earlier at 18 °C than at 12 °C. 2. It is likely that the high phosphorus content of fish water increased the nutritional value of detrital seston particles by stimulating bacterial growth. Daphnia pulicaria was presumably better able to use these as food and hence showed a more rapid somatic growth than its competitor. This led to very high density of D. pulicaria in fish water, but not in the controls. The elevated D. pulicaria density coincided with high mortality and reduced fecundity in D. galeata × hyalina, resulting in competitive displacement of the hybrid. 3. It is clear that the daphnids competed for a limiting resource, as grazing caused a strong decrease in their seston food concentration. However, interference may also have played a role, as earlier studies have shown larger Daphnia species to be dominant in this respect. The high density of large‐bodied D. pulicaria in fish water may have had an allelopathic effect on the hybrid. Our data are inconclusive with respect to whether the reached seston concentration was below the threshold resource level (R*) of the hybrid, where population growth rate and mortality exactly balance, as it would be set in the absence of interference, or whether interference actually raised the hybrid's R* to a value above this equilibrium particle concentration. 4. Our results do clearly show that fish‐released compounds mediated competitive exclusion among zooplankton species and that such displacement occurred at a greatly enhanced rate at an elevated temperature. Fish may thus not only structure zooplankton communities directly through size‐selective predation, but also indirectly through the compounds they release.     

Maternal dopamine exposure provides offspring starvation resistance in Daphnia

The neurotransmitter dopamine has been shown to play an important role in modulating behavioral, morphological, and life history responses to food abundance. However, costs of expressing high dopamine levels remain poorly studied and are essential for understanding the evolution of the dopamine system. Negative maternal effects on offspring size from enhanced maternal dopamine levels have previously been documented in Daphnia. Here, we tested whether this translates into fitness costs in terms of lower starvation resistance in offspring. We exposed Daphnia magna mothers to aqueous dopamine (2.3 or 0 mg/L for the control) at two food levels (ad libitum vs. 30% ad libitum) and recorded a range of maternal life history traits. The longevity of their offspring was then quantified in the absence of food. In both control and dopamine treatments, mothers that experienced restricted food ration had lower somatic growth rates and higher age at maturation. Maternal food restriction also resulted in production of larger offspring that had a superior starvation resistance compared to ad libitum groups. However, although dopamine exposed mothers produced smaller offspring than controls at restricted food ration, these smaller offspring survived longer under starvation. Hence, maternal dopamine exposure provided an improved offspring starvation resistance. We discuss the relative importance of proximate and ultimate causes for why D. magna may not evolve toward higher endogenous dopamine levels despite the fitness benefits this appears to have.     

Supplementation with sterols improves food quality of a ciliate for Daphnia magna

Experimental results provide evidence that trophic interactions between ciliates and Daphnia are constrained by the comparatively low food quality of ciliates. The dietary sterol content is a crucial factor in determining food quality for Daphnia. Ciliates, however, presumably do not synthesize sterols de novo. We hypothesized that ciliates are nutritionally inadequate because of their lack of sterols and tested this hypothesis in growth experiments with Daphnia magna and the ciliate Colpidium campylum. The lipid content of the ciliate was altered by allowing them to feed on fluorescently labeled albumin beads supplemented with different sterols. Ciliates that preyed upon a sterol-free diet (bacteria) did not contain any sterols, and growth of D. magna on these ciliates was poor. Supplementation of the ciliates' food source with different sterols led to the incorporation of the supplemented sterols into the ciliates' cells and to enhanced somatic growth of D. magna. Sterol limitation was thereby identified as the major constraint of ciliate food quality for Daphnia. Furthermore, by supplementation of sterols unsuitable for supporting Daphnia growth, we provide evidence that ciliates as intermediary grazers biochemically upgrade unsuitable dietary sterols to sterols appropriate to meet the physiological demands of Daphnia.     

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.     

Effect of Body Size on Toxicity of Zinc in Neonates of Four Differently Sized <Emphasis Type="Italic">Daphnia</Emphasis> Species

The sensitivity of neonates of four Daphnia species to zinc was tested in relation to their mean body size. These mean sizes of these four Daphnia spp were: D. magna, 0.813 ± 0.055 mm, D.␣pulicaria, 0.745 ± 0.063 mm, D. pulex, 0.645 ± 0.044 mm and D. galeata, 0.611 ± 0.058 mm. A positive relationship between EC₅₀ (24, 48) values and neonates size was found. The smaller the size of the daphnid the higher was the sensitivity to heavy metal toxicity. For all tested species did the EC₅₀ values decrease with time; the decrease was most marked for D. magna and the least for D. galeata. The EC₅₀ values of D. magna were higher than would be expected on basis of its body size.     

Maternal food quantity affects offspring feeding rate in Daphnia magna

Maternal effects have wide-ranging effects on life-history traits. Here, using the crustacean Daphnia magna, we document a new effect: maternal food quantity affects offspring feeding rate, with low quantities of food triggering mothers to produce slow-feeding offspring. Such a change in the rate of resource acquisition has broad implications for population growth or dynamics and for interactions with, for instance, predators and parasites. This maternal effect can also explain the previously puzzling situation that the offspring of well-fed mothers, despite being smaller, grow and reproduce better than the offspring of food-starved mothers. As an additional source of variation in resource acquisition, this maternal effect may also influence relationships between life-history traits, i.e. trade-offs, and thus constraints on adaptation. Maternal nutrition has long-lasting effects on health and particularly diet-related traits in humans; finding an effect of maternal nutrition on offspring feeding rate in Daphnia highlights the utility of this organism as a powerful experimental model for exploring the relationship between maternal diet and offspring fitness.     

Food quality triggers the reproductive mode in the cyclical parthenogen <Emphasis Type="Italic">Daphnia</Emphasis> (Cladocera)

Cyclic parthenogenesis (heterogony) is a widespread reproductive mode found in diverse taxa such as digenean trematodes, gall wasps, gall midges, aphids, cladocerans and rotifers. It is of particular interest as it combines the advantages of asexual reproduction (rapid population growth) and sexual reproduction (recombination). Usually sexual reproduction is initiated when, or slightly before, environmental conditions deteriorate, and often results in the production of resting stages. The optimal timing of diapause induction must thus be under strong natural selection. Using the cladoceran Daphnia as a model system, we show here for the first time that the switch from parthenogenetic to sexual reproduction in a cyclical parthenogenetic organism can be influenced by the chemical composition of food. Under crowding conditions Daphnia reproduced parthenogenetically with subitaneous eggs when fed the algal species Cryptomonas sp., but started the production of resting eggs when fed with the green algal species Scenedesmus obliquus. Supplementation experiments with lipids and especially proteins showed that the induction of resting egg production in two clones of different Daphnia species was due to a dietary deficiency in the green alga. Hence, the low food quality induced a switch in the reproductive mode that may contribute to optimal timing of the sexual reproduction of Daphnia in nature. Furthermore, our results have two other major implications: first, they suggest that protein compounds should be added to the list of diet constituents potentially limiting or influencing Daphnia reproduction. Second, we show that the role of food quality goes far beyond the up to now documented effects of food quality on somatic growth and trophic transfer efficiency of herbivores: due to its effects on sexual reproduction and the production of resting eggs, food quality might influence genetic diversity and long-term persistence of Daphnia in lakes. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.