Clonal variation in the survival and reproduction of Daphniapulicaria under low-food stress

1. Survivorship and reproductive parameters of eight Daphnia pulicaria clones were evaluated in a life-table experiment under four dietary regimes ranging from food limitation to complete food deprivation.
2. D . pulicaria exhibited both diet-independent interclonal differences and genotype–environment interactions in life-span.
3. Reproduction ceased entirely or nearly so under the three treatments with lowest food availability. Under life-long food limitation, differences between clones were evident in clutch sizes and age at first reproduction. Some clones produced ephippia at a markedly higher frequency than others under food-limiting conditions.
4. For most of the life-history traits evaluated, within-lake variation among clones exceeded differences between the two lake populations they represented.
5. Intraspecific variations in response to periods of food deprivation and to extended quantitative or qualitative food-limitation could affect the clonal composition of Daphnia populations. They are therefore of potential importance in determining the effect of altered phytoplankton assemblages on zooplankton communities.     

Effects of algal food quality on fecundity and population growth rates of Daphnia

1. Food quality was at least as important as food quantity for both fecundity and population growth responses of the cladoceran Daphnia pulicaria fed the green alga Ankistrodesmus falcatus grown under N limitation, P limitation, or non-limited condition.
2. The fecundity of D. pulicaria was reduced under conditions of low food quality (low N or low P) compared with that for animals fed control non-limited algae regardless of ration size. The reduced fecundity of D. pulicaria fed P-limited food could be partially alleviated by increasing the ration (hence, compensation), but such was not the case for animals fed N-limited food.
3. Population growth rates of D. pulicaria ( r _max) were significantly reduced under conditions of low-quality food for both N-limited and P-limited algae. Population growth rates were unaffected by ration size, indicating no compensation.     

Genome size evolution and polyploidy in the Daphnia pulex complex (Cladocera: Daphniidae)

Genome size was estimated in 49 clones of the Daphnia pulex complex from temperate and subarctic locations using flow cytometry and microsatellite DNA analyses. Significant genome size differences were found in diploid species belonging to the two genetically distinct groups (the pulicaria and the tenebrosa groups), with clones from the tenebrosa group having genome sizes 22% larger than those in the pulicaria group. Combined flow cytometry and microsatellite DNA analyses revealed that nearly all polyploid clones in the D. pulex complex are triploid and not tetraploid, as was previously suggested. Sequencing analyses of the ND5 gene to position clones in their respective clades within the D. pulex complex have uncovered three triploid clones of Daphnia middendorffiana with a D. pulex maternal parent. This result was unexpected because Daphnia pulicaria has always been identified as the maternal parent of these hybrid polyploid clones. Triploid clones likely owe their origins to interactions between sexual and asexual populations. Further interactions in the tenebrosa group have generated tetraploid clones but these events have been rare.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 97 , 68–79.     

Seasonal analysis of Daphnia pulicaria swimming behavior

Our study documents individual swimming behavior of Daphnia pulicaria over a yearly cycle in a temperate lake. We collected D. pulicaria, a common freshwater zooplankton, from Lake Mendota on 10 dates between July 1994 and June 1995 from two depths, 2 m and 10 m. The Daphnia were rushed to the laboratory and video-taped as they swam in lake water under lake-ambient temperature and light conditions. Five-second swimming tracks of individual Daphnia were filmed and digitized using a motion analysis system. We measured average turning angle, swimming speed and sinking rate for each track. D. pulicaria swimming behavior varied over the annual cycle. We found significant differences in turning angle between depths and among months. Sinking rate and swimming speed were significantly different among months but not depths. Sinking rate and swimming speed were not significantly correlated with water temperature. Our results were contrary to Stokes' Law predictions, in that D. pulicaria had the slowest sinking speed in June, not in the winter when water temperatures were lowest and viscosity was highest. Body length was significantly correlated with all three swimming variables. We also studied swimming behavior in clonal populations of D. pulicaria in different concentrations of the alga, Chlamydomonas reinhardtii. D. pulicaria did not change swimming speed, turning angle or sinking rate over a range of food concentrations. Finally, swimming behavior in a D. pulicaria clone, tested at two temperatures in the laboratory, confirmed the results from our seasonal study; Daphnia did not sink as predicted by changes in viscosity.     

Perch or plankton: top‐down control of Daphnia by yellow perch (Perca flavescens) or Bythotrephes cederstroemi in an inland lake?

1. Seasonal termination of the vernal clear-water phase in Long Lake, Grand Traverse Co., Michigan coincided with severe size-selective predation on juvenile Daphnia pulicaria from 0.8 to 1.8 mm in length. This could be caused by predation by age-0 yellow perch ( Perca flavescens ) or by the exotic predatory zooplankter Bythotrephes cederstroemi .
2. During the initial decline of Daphnia , Ivlev's electivity coefficient for yellow perch from 15.0 to 20.0 mm in length was 0.50 for copepods and −0.75 for D. pulicaria .
3. Bioenergetics modelling of both yellow perch and Bythotrephes demonstrates that, during the initial Daphnia decline, Bythotrephes consumed 1.5–5 times greater total mass than yellow perch. Furthermore, models in which Bythotrephes consumed juvenile Daphnia were more consistent with the timing of the Daphnia decline than those in which yellow perch consumed juvenile Daphnia .
4. The invasion of Bythotrephes into Long Lake seems to be a significant perturbation, introducing effects that propagate throughout the food chain. Bythotrephes created a possible bottleneck for age-0 yellow perch in late June by suppressing Daphnia .     

Environmental gradients structure Daphnia pulex × pulicaria clonal distribution

The rarity of eukaryotic asexual reproduction is frequently attributed to the disadvantage of reduced genetic variation relative to sexual reproduction. However, parthenogenetic lineages that evolved repeatedly from sexual ancestors can generate regional pools of phenotypically diverse clones. Various theories to explain the maintenance of this genetic diversity as a result of environmental and spatial heterogeneity [frozen niche variation (FNV), general-purpose genotype] are conceptually similar to community ecological explanations for the maintenance of regional species diversity. We employed multivariate statistics common in community ecological research to study population genetic structure in the freshwater crustacean, Daphnia pulex × pulicaria. This parthenogenetic hybrid arose repeatedly from sexual ancestors. Daphnia pulex × pulicaria populations harboured substantial genetic variation among populations and the clonal composition at each pond corresponded to nutrient levels and invertebrate predator densities. The interclonal selection process described by the FNV hypothesis likely structured our D. pulex × pulicaria populations.     

Temperature-dependent energy budget of an Arctic Cladoceran, Daphnia middendorffiana

1. Global change models predict the greatest impact in climate to occur in the northern polar region. Change in temperature will alter individual metabolism and has the potential to change community structure to an unknown degree.
2. The temperature-dependent energy budget of Arctic Daphnia middendorffiana was investigated by measuring respiration rates, ingestion rates and assimilation rates. The scope for growth and reproduction was determined and compared with data from the literature for a clone of Daphnia pulicaria collected in the temperate zone.
3. A difference was observed between the Arctic species and the temperate zone clone in both temperature tolerance, and the energy available for growth and reproduction at various temperatures. A low availability of energy for growth and reproduction indicated that life history patterns as well as physiological mechanisms are important in allowing D. middendorffiana to exist successfully in Arctic environments.
4. The lower available energy for growth compared to Daphnia clones from temperate zones may be detrimental to D. middendorffiana , which might have to compete with species expanding their range under the predicted temperature increase for Arctic regions.     

Responses of Daphnia pulex populations to toxic cyanobacteria

1. We studied the effects of toxic cyanobacteria, Microcystis aeruginosa , on Daphnia pulex populations. The experiment was performed at room temperature in laboratory microcosms to which we added toxic Microcystis in two pulses. Green alga, Scenedesmus obtusiusculus, was provided in two concentrations.
2. Microcystis exposure resulted in a decreased population density of Daphnia. The proportion of adolescents was higher in the Microcystis treatment than in the control, while the proportion of newborns did not differ significantly from the control. This indicates delayed maturation of Daphnia exposed to Microcystis . We found no significant impacts of cyanobacteria, food level or clonal origin on population variability after correcting for differences in population size.
3. Juveniles of the two clones studied showed different sensitivities to toxic Microcystis in the acute test. However, in the population experiment we did not find clonal differences between Microcystis and control treatments in the relative proportion of juveniles.
4. The number of ephippia produced per adult was highest in the treatments with Microcystis. This indicates that cyanobacterial toxins may be one of several factors inducing ephippia formation, acting directly or through inhibition of feeding.     

How well do laboratory experiments explain field patterns of zooplankton emergence?

1. We conducted a laboratory experiment to explore potential mechanisms driving variation in zooplankton emergence from diapausing eggs observed in Oneida Lake, NY, U.S.A. We hypothesized that variation in timing of ice-out (date of thawing of ice) between 1994 and 1995, which resulted in variation in photoperiod–temperature cues, contributed to the differences in the observed field patterns. Environmental chambers were used to establish weekly photoperiod–temperature combinations that reproduced natural conditions in Oneida Lake in 1994 and 1995. In addition, a third treatment ('dark') exposed eggs only to the changes in temperature. We recorded zooplankton emergence for 2.5 simulated ice-free seasons.
2. Nine cladoceran taxa were found to hatch, but only Daphnia pulicaria in large numbers. Hatching of D. pulicaria was recorded throughout the season in the two light treatments and sporadically in the dark treatment. The early ice-out treatment had the highest emergence, followed by the late ice-out and dark treatments. Among taxa, there was temporal segregation with five hatching in the early weeks of sampling and two taxa hatching during the middle weeks. Alona hatched late in the first year, but earlier in the second year.
3. We compared our laboratory results of D. pulicaria hatching with the field data obtained by Cáceres (1998) . Hatching was continuous in the laboratory, whereas a synchronous spring emergence was found in the field. However, in both the laboratory and the field, more D. pulicaria hatched under conditions reflecting ice-out occurring in March as opposed to April. Because differences in rates and timing of emergence can affect the population and community dynamics of pelagic systems, we suggest caution when applying laboratory results to field populations.     

Effects of fatty acid and phosphorus content of food on the growth, survival and reproduction ofDaphnia

1. Mixtures of microencapsulated lipids and Scenedesmus quadricauda grown at different degrees of P limitation were used as food for Daphnia galeata in two growth experiments. Thereby, food quality in terms of ω3-fatty acid (ω3-FA) or phosphorus (P) content could be assessed without interference from other factors.
2. ω3-highly unsaturated fatty acids (ω3-HUFA), given to Daphnia as fish oil or eicosapentaenoic acid (EPA) together with non P-limited algae, decreased the time to first reproduction. When fed fish oil, somatic growth and survival were also enhanced. Linolenic acid also decreased the time to first reproduction but to a lesser extent than EPA.
3. Food quality depended to a large extent on the degree of P limitation of Scenedesmus , which is consistent with P limitation of Daphnia. The overall impact of P was always larger than the effect of ω3-FA. Growth, survival and reproduction were elevated when Daphnia were fed non P-limited Scenedesmus compared to treatments with P-limited algae.
4. The relative importance of ω3-HUFA and P content in the food changed over a C : P gradient, with stronger effects of ω3-HUFA at low C : P ratios.     

Influence of food quality on depth selection of Daphnia pulicaria

We studied the habitat choice of juvenile and adult Daphniapulicaria in thermally stratified water columns (plankton towers)with a deep water algal maximum (DCM). The DCM consisted ofeither filamentous cyanobacteria (Planktothrix agardhii), non-filamentousChlorophyceae (Scenedesmus obliquus) or a mixture of both. AdultD. pulicaria spent more time at colder temperatures in the presenceof P. agardhii than in the presence of S. obliquus, either asthe sole food source or when mixed with P. agardhii. JuvenileD. pulicaria did not show a different habitat choice in thethree food treatments. In a fourth treatment, we also determinedDaphnia distribution in the absence of food. Comparing the habitatchoice of juveniles and adults in each of the four treatments,the latter spent more time at colder temperatures when foodwas absent or when in the sole presence of P. agardhii. Additionalgrazing and stable isotopic marker experiments showed that D.pulicaria ingested and assimilated Planktothrix filaments. Theresults suggest that the differences in habitat choice betweenadult D. pulicaria in the presence of different food types wereinfluenced by food quality effects: adult Daphnia which moveto colder waters in the presence of low quality P. agardhiidecrease their metabolic rate and might thus be able to investmore resources into reproduction when environmental conditionsimprove.     

Correlations between nutrient concentrations and zooplankton populations in a mesotrophic reservoir

1. A year-round study was conducted in a mesotrophic reservoir to determine the dynamics of zooplankton populations as a function of food availability (edible phytoplankton), nutrient concentration, temperature and hydraulic regime.
2. Rotifer biomass was correlated with soluble reactive phosphorus (SRP) concentration. The abundance of the rotifers Keratella cochlearis and Anuraeopsis fissa were not correlated with food availability (measured by chlorophyll and cell counts) but showed a strong dependence on P availability. Another rotifer, Synchaeta oblonga , and crustacean species were not related to nutrient availability but seemed to be dependent on food concentrations, especially of some phytoplankton taxa.
3. In this field study, rotifers seemed more susceptible than Daphnia or copepods to P-limitation. Among rotifer species, Keratella seemed to be more susceptible than Anuraeopsis to P limitation. Different susceptibilities of zooplankton species to nutrient limitation may be important in explaining the dynamics of these organisms in natural situations. Further analyses are warranted to clarify the interactions between nutrient limitation and energy limitation among zooplankton.     

Phenotypic plasticity of Daphnia life history traits: the roles of predation, food level and toxic cyanobacteria

1. We studied the life history responses of Daphnia pulex under different biotic conditions. In a factorially designed experiment, we tested the impacts of water conditioned by the invertebrate predator Chaoborus, low and high food level (10 000 and 100 000 Scenedesmus cells ml^–1), and exposure to toxic Microcystis (5000 cells ml^–1) on twelve D. pulex clones originating from different habitats. Our aim was to compare the phenotypic plasticity of different clones, and to study the interactions among biotic factors.
2. Individuals cultured in Chaoborus -conditioned water started to reproduce at a larger size than individuals cultured in water not conditioned by the predators. We found interactions between food level and Chaoborus -conditioned water on age at first reproduction and total offspring number. In addition, the impact of the Chaoborus treatment on the size at first reproduction was reduced by Microcystis exposure.
3. Clonal differences were found in the degree of phenotypic plasticity of different life history traits. However, there was no obvious connection between the original habitat of the clone and the degree of plasticity.     

Impact of zooplankton grazing on the excystation, viability, and infectivity of the protozoan pathogens Cryptosporidium parvum and Giardia lamblia

Very little is known about the ability of the zooplankton grazer Daphnia pulicaria to reduce populations of Giardia lamblia cysts and Cryptosporidium parvum oocysts in surface waters. The potential for D. pulicaria to act as a biological filter of C. parvum and G. lamblia was tested under three grazing pressures (one, two, or four D. pulicaria grazers per 66 ml). (Oo)cysts (1 x 10(4) per 66 ml) were added to each grazing bottle along with the algal food Selenastrum capricornutum (6.6 x 10(4) cells per 66 ml) to stimulate normal grazing. Bottles were rotated (2 rpm) to prevent settling of (oo)cysts and algae for 24 h (a light:dark cycle of 16 h:8 h) at 20 degrees C. The impact of D. pulicaria grazing on (oo)cysts was assessed by (i) (oo)cyst clearance rates, (ii) (oo)cyst viability, (iii) (oo)cyst excystation, and (iv) oocyst infectivity in cell culture. Two D. pulicaria grazers significantly decreased the total number of C. parvum oocysts by 52% and G. lamblia cysts by 44%. Furthermore, two D. pulicaria grazers significantly decreased C. parvum excystation and infectivity by 5% and 87%, respectively. Two D. pulicaria grazers significantly decreased the viability of G. lamblia cysts by 52%, but analysis of G. lamblia excystation was confounded by observed mechanical disruption of the cysts after grazing. No mechanical disruption of the C. parvum oocysts was observed, presumably due to their smaller size. The data provide strong evidence that zooplankton grazers have the potential to substantially decrease the population of infectious C. parvum and G. lamblia in freshwater ecosystems.     

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.     

Population differentiation in Daphnia alters community assembly in experimental ponds

Most studies of community assembly ignore how genetic differentiation within species affects their colonization and extinction. However, genetic differentiation in ecologically relevant traits may be substantial enough to alter the colonization and extinction processes that drive community assembly. We measured significant molecular genetic and quantitative trait differentiation among three Daphnia pulex × pulicaria populations in southwestern Michigan ponds and investigated whether this differentiation could alter the assembly of pond zooplankton communities in experimental mesocosms. In this study, we monitored the invasion success of different D. pulex × pulicaria populations after their introduction into an established zooplankton community. We also monitored the invasion success of a diverse array of zooplankton species into different D. pulex × pulicaria populations. Zooplankton community composition depended on the D. pulex × pulicaria source population. Daphnia pulex × pulicaria from one population failed to invade zooplankton communities, while those from other populations successfully invaded similar communities. If population differentiation in other species plays a role in community assembly similar to that demonstrated in our study, assembly may be more sensitive to evolutionary processes than has been previously generally considered.     

Cladoceran birth and death rates estimates: experimental comparisons of egg-ratio methods

I. Birth and death rates of natural cladoceran populations cannot be measured directly. Estimates of these population parameters must be calculated using methods that make assumptions about the form of population growth. These methods generally assume that the population has a stable age distribution.
2. To assess the effect of variable age distributions, we tested six egg ratio methods for estimating birth and death rates with data from thirty-seven laboratory populations of Daphnia pulicaria. The populations were grown under constant conditions, but the initial age distributions and egg ratios of the populations varied. Actual death rates were virtually zero, so the difference between the estimated and actual death rates measured the error in both birth and death rate estimates.
3. The results demonstrate that unstable population structures may produce large errors in the birth and death rates estimated by any of these methods. Among the methods tested, Taylor and Slatkin's formula and Paloheimo's formula were most reliable for the experimental data.
4. Further analyses of three of the methods were made using computer simulations of growth of age-structured populations with initially unstable age distributions. These analyses show that the time interval between sampling strongly influences the reliability of birth and death rate estimates. At a sampling interval of 2.5 days (equal to the duration of the egg stage), Paloheimo's formula was most accurate. At longer intervals (7.5–10 days), Taylor and Slatkin's formula which includes information on population structure was most accurate.     

Spring bloom development of microbenthic algae and associated invertebrates in two reaches of a small lowland stream with contrasting sediment stability

SUMMARY. 1. Population growth rates and relative competitive abilities of the rotifer Keratella cochlearis f. tecta and the small-bodied cladoceran Daphnia ambigua were studied under different schedules of food addition but equal total food quantity (per 4-day interval). The initial population growth rate of Keratella was significantly affected by the feeding schedule and by the presence of competitors, while that of Daphnia was affected by neither factor. Population densities of both species tended to increase as the frequency of food addition increased.
2. Daphnia suppressed and excluded Keratella from mixed-species cultures when food was provided intermittently at a high concentration, but it failed to exclude the rotifer when food was provided in a near-continuous supply at low concentration. Keratella had only a minor suppressive effect on Daphnia in all mixed-species treatments.
3. Starvation experiments indicate that Daphnia is able to withstand food shortages for significantly longer periods of time than Keratella . These and other results indicate that the outcome of interspecific competition between these species may be influenced by me frequency and concentration at which food is supplied. Daphnia ambigua is competitively superior to K. cochlearis when food is concentrated or 'pulsed', but much less so when ambient food levels are chronically low. Patterns of food availability may have important effects in determining the relative abundance of rotifers and small cladocerans in natural zooplankton communities.     

Effects of predation by Chaoborus flavicans on crustacean zooplankton of Lake Lenore, Washington

1. After Chaoborus flavicans became established in 1974, the species composition of zooplankton in Lake Lenore changed. Diaptomus nevadensis disappeared and densities of Diaptomus sicilis were reduced.
2. A transplant experiment showed the D. nevadensis could survive in Lake Lenore water in the absence of Chaoborus . The disappearance of D. nevadensis was probably a consequence of Chaoborus predation.
3. Although densities of Daphnia pulicaria † were similar before and after the colonization of C. flavicans , juvenile D. pulicaria began producing neck-teeth during the first summer Chaoborus was present.
4. The seasonal pattern of neck-teeth production by juvenile D. pilicaria did not correlate with density of Chaoborus but rather with a combination of water temperature and Chaoborus density, suggesting that metabolic processes are a component of induction of neck-teeth in this species.     

Sensitivity of Daphnia to toxic cyanobacteria: effects of genotype and temperature

1. We studied the effects of both acute and chronic exposure of Daphnia pulex to toxic Microcystis aeruginosa . We focused on the effects of Daphnia genotype and temperature (19 and 24 °C).
2. The study revealed variation among ten Daphnia pulex clones in survivorship under acute Microcystis exposure, measured as EC₅₀. An increase in temperature caused a clear decrease in EC₅₀, although the ranking of clones according to sensitivity remained the same at both temperatures.
3. In the chronic exposure of two of the clones, toxic Microcystis reduced survival and reproduction. The two clones differed in their responses, indicating different means of coping with toxic cyanobacteria. Toxic cyanobacteria reduced slightly more at 24 °C than 19 °C.
4. The clonal difference in sensitivity to toxic cyanobacteria at acute exposure was reversed at chronic exposure. This indicates that the results from short-term toxicity cannot be used to predict life history responses under sublethal exposure.