Costs of predator‐induced morphological defences in Daphnia

1. Inducible defences are advantageous because they protect the prey while limiting associated fitness costs. The presence of these costs is an essential component of this conditional strategy, since their absence would favour constitutive (fixed) defences. In some cases, however, these costs have been difficult to measure because of complex interactions between the defences themselves, resultant life history changes and the organism’s environment. 2. The pond‐dwelling water flea, Daphnia pulex, forms defensive neck spines in response to kairomones released by predatory larvae of the phantom midge, Chaoborus. This predator–prey interaction and the formation of these inducible defences have been well studied, but costs associated with the development of neck spines remain unclear. In this study, I address this problem by analysing the effect of Chaoborus kairomones on the life history responses (and fitness costs associated with these responses) of two clones of D. pulex that are from the same pond population, but differ greatly in their degree of neck spine development. 3. Both D. pulex clones exhibited the same predator‐induced shifts in life history: larger size at birth, reduced juvenile growth rate (producing a smaller size at maturity), delayed reproduction and a reduction in the number of neonates produced after the first clutch. Relative fitness decreased significantly and to the same degree (c. 10% reduction in r) in each clone. This observed fitness cost was not directly related to the neck spines per se since the cost was the same in both clones, despite their considerable differences in neck spine development. Rather, it appears to be indirectly related to this antipredator morphology via a combination of delayed reproduction and a set of life history trade‐offs (decreased growth rate, decreased reproduction after the first clutch) for increased neonate body size, which is necessary for neck spines to be effective defences. This suite of induced responses is probably a result of local adaptation of these two D. pulex clones to their common pond environment. 4. Costs of inducible defences do not always entail direct allocation costs associated with forming and maintaining a defence, but may also involve indirect life history responses that are specific to particular environmental situations. This local adaptation would explain the highly variable life history responses observed among D. pulex clones from different pond environments.     

Food availability modulates temperature‐dependent effects on growth,reproduction, and survival in Daphnia magna

Reduced body size and accelerated life cycle due to warming are considered major ecological responses to climate change with fitness costs at the individual level. Surprisingly, we know little about how relevant ecological factors can alter these life history trade‐offs and their consequences for individual fitness. Here, we show that food modulates temperature‐dependent effects on body size in the water flea Daphnia magna and interacts with temperature to affect life history parameters. We exposed 412 individuals to a factorial manipulation of food abundance and temperature, tracked each reproductive event, and took daily measurements of body size from each individual. High temperature caused a reduction in maximum body size in both food treatments, but this effect was mediated by food abundance, such that low food conditions resulted in a reduction of 20% in maximum body size, compared with a reduction of 4% under high food conditions. High temperature resulted in an accelerated life cycle, with pronounced fitness cost at low levels of food where only a few individuals produced a clutch. These results suggest that the mechanisms affecting the trade‐off between fast growth and final body size are food‐dependent, and that the combination of low levels of food and high temperature could potentially threaten viability of ectotherms.     

有毒和无毒微囊藻对大型溞生长和繁殖的影响

近年来,随着工农业的快速发展和人们生活水平提高,一些城市生活污水和工农业废水流入江河、湖泊等淡水水体中,加剧了水体的富营养化和污染,导致蓝藻过度生长繁殖而形成水华,其中最常见的是微囊藻水华[1,2].     

Evolution of life histories and yields in experimental populations of Daphnia magna

An experiment on life-history evolution is described in which replicated populations of the Cladoceran Daphnia magna , made up of a standard mixture of clones, were subjected to two contrasting culling regimes, involving removal of small or large individuals. After approximately 150 days of culling, analysis of the life histories showed that genetic differences had emerged between culling regimes. Clones selected by culling small sized individuals grew rapidly through small size classes, whereas those selected by culling large sizes grew slowly through small size classes, with the result that the age at which they became vulnerable to harvesting was delayed. In addition, there was some redistribution of reproduction towards size classes that were not culled. This evolution is consistent with a major decline observed in the yield from populations in which large individuals were culled. We argue that changes of this general kind are likely to take place in exploited populations and that serious consideration should be given to evolutionary aspects of the management of such populations.     

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.     

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.     

Temperature and kairomone induced life history plasticity in coexisting Daphnia

We investigated the life history alterations of coexisting Daphnia species responding to environmental temperature and predator cues. In a laboratory experiment, we measured Daphnia life history plasticity under different predation risk and temperature treatments that simulate changing environmental conditions. Daphnia pulicaria abundance and size at first reproduction (SFR) declined, while ephippia (resting egg) formation increased at high temperatures. Daphnia mendotae abundance and clutch size increased with predation risk at high temperatures, but produced few ephippia. Thus, each species exhibited phenotypic plasticity, but responded in sharply different ways to the same environmental cues. In Glen Elder reservoir, Kansas USA, D. pulicaria dominance shifted to D. mendotae dominance as temperature and predation risk increased from March to June in both 1999 and 2000. Field estimates of life history shifts mirrored the laboratory experiment results, suggesting that similar phenotypic responses to seasonal cues contribute to seasonal Daphnia population trends. These results illustrate species-specific differences in life history plasticity among coexisting zooplankton taxa.     

Temperature adaptation in a geographically widespread zooplankter,Daphnia magna

Evidence for temperature adaptation in Daphnia magna was inferred from variation in the shape of temperature reaction norms for somatic growth rate, a fitness‐related trait. Ex‐ephippial clones from eight populations across Europe were grown under standardized conditions after preacclimation at five temperatures (17–29 °C). Significant variation for grand mean growth rates occurred both within populations (among clones) and between populations. Genetic variation for reaction norm shape was found within populations, with temperature‐dependent trade‐offs in clone relative fitness. However, the population average responses to temperature were similar, following approximately parallel reaction norms. The among‐population variation is not evidence for temperature adaptation. Lack of temperature adaptation at the population level may be a feature of intermittent populations where environmentally terminated diapause can entrain the planktonic stage of the life‐history within a similar range of temperatures.     

Variation in the neonate size of Daphnia pulex: the effects of predator exposure and clonal origin

Daphnia pulex clones originating from twelve small pond habitats were exposed to chemical cues from a size-selective predator, larvae of the phantom midge Chaoborus. Exposure delayed the onset of reproduction and increased the size at first reproduction. On the other hand, the neonates produced by these larger mothers were slightly smaller than the neonates produced by the smaller control mothers. In cladocerans, neonate size is usually positively correlated to the size of the mother. Thus exposure to Chaoborus kairomone apparently had direct effects on neonate size counterbalancing the maternal effects. Daphnia clones collected from Chaoborus-free and Chaoborus-rich environments exhibited different responses. In first adult instar, the clones from Chaoborus inhabited environments increased their offspring size under Chaoborus exposure whereas clones from Chaoborus-free environments did not. This may reflect clonal adaptation to the predation prehistory of their original habitat since larger neonates more quickly reach a size protected from the predator.     

富营养化水体中自然食物对透明溞(Daphnia hyalina)生长与繁殖影响的初步实验

以广东省流溪河水库中分离驯化的透明溞为研究对象,通过喂养来自富营养化湖泊(暨南大学明湖)的自然食物,探讨富营养化水体中的食物对透明溞生长与繁殖的影响。来自明湖的4种自然食物分别由2个网孔为38μm和110μm的浮游生物网过滤所得的,并将这2种过滤水稀释3倍后得到另外2种食物。在4种明湖自然食物培养条件下,透明溞最大体长、体长生长率和寿命均随食物浓度的稀释而下降,最大体长为2.63mm,最高体长生长率为0.0314mm·d^-1,最长寿命为65d。稀释后食物培养的透明溞,首次怀卵个体体长为1.46mm,大于非稀释食物条件下首次怀卵个体体长1.28mm;在110μm过滤的非稀释食物条件下,内禀增长率达到最大值为0.42。本实验结果表明,冬季富营养水体中食物对透明溞生长与繁殖没有产生抑制作用,食物浓度对透明溞的生长与繁殖的影响高于食物大小的影响。     

The response of European Daphnia magna Straus and Australian Daphnia carinata King to changes in geomagnetic field

This study investigates the effects of lifelong exposure to reversed geomagnetic and zero geomagnetic fields (the latter means absence of geomagnetic field) on the life history of Daphnia carinata King from Australia and Daphnia magna Straus from Europe. Considerable deviation in the geomagnetic field from the usual strength, leads to a decrease in daphnia size and life span. Reduced brood sizes and increased body length of neonates are observed in D. magna exposed to unusual magnetic background. The most apparent effects are induced by zero geomagnetic field in both species of Daphnia. A delay in the first reproduction in zero geomagnetic field is observed only in D. magna. No adaptive maternal effects to reversed geomagnetic field are found in a line of D. magna maintained in these magnetic conditions for eight generations. Integrally, the responses of D. magna to unusual geomagnetic conditions are more extensive than that in D. carinata. We suggest that the mechanism of the effects of geomagnetic field reversal on Daphnia may be related to differences in the pattern of distribution of the particles that have a magnetic moment, or to moving charged organic molecules owing to a change in combined outcome and orientation of the geomagnetic field and Earth's gravitational field. The possibility of modulation of self-oscillating processes with changes in geomagnetic field is also discussed.     

Larval fish-induced phenotypic plasticity of coexisting Daphnia: an enclosure experiment

1. The indirect effects of predators on lower trophic levels have been studied without much attention to phenotypically plastic traits of key food web components. Phenotypic plasticity among species creates phenotypic diversity over a changing environmental landscape. 2. We measured the indirect effects of planktivorous larval walleye (Stizostedion vitreum) on phytoplankton biomass through their effects on the dominant herbivore species, Daphnia pulicaria and D. mendotae. 3. Fish had no effect on phytoplankton biomass or overall Daphnia density. We observed a compensatory response to predation by functionally comparable species within a trophic level in the form of shifting dominance and coexistence of Daphnia species. We hypothesized that this phenotypically plastic response to predation decoupled a potential trophic cascade in this freshwater pelagic system. Daphnia pulicaria density decreased over time with fish predation, but D. mendotae density increased over time with fish predation. 4. Phenotypically plastic life history trait shifts and reproductive rates differed between species in fishless and fish enclosures, accounting for population trends. Daphnia pulicaria were also proportionally higher in walleye larvae stomachs than in the enclosures, indicating that walleye preferred to feed on D. pulcaria over D. mendotae. The resultant shift in dominance may partially explain the overall benign effect of fish on grazers and supports the hypothesis that trophic level diversity can decouple a trophic cascade.     

Measuring Daphnia life history in the wild: The efficacy of individual field cages

Life‐history studies are often conducted in a laboratory environment where it is easy to assay individual animals. However, factors such as temperature, photoperiod, and nutrition vary greatly between laboratory and field environments, making it difficult to compare results. Consequently, there is a need to study individual life histories in the field, but this is currently difficult in systems such as Daphnia where it is not possible to mark and track individual animals. Here, we present a proof of principle study showing that field cages are a reliable method for collecting individual‐level life‐history data in Daphnia magna. As a first step, we compared the life history of paired animals reared outside and inside cages to test the hypothesis that cages allow free flow of algal food resources. We then used a seminatural mesocosm setting to compare the performance of individual field cages versus glass jars refilled with mesocosm water each day. We found that cages did not inhibit food flow and that differences in life histories between three clones detected in the jar assays were also detectable using the much less labor‐intensive field cages. We conclude that field cages are a feasible approach for collecting individual‐level life‐history data in systems such as Daphnia where individual animals cannot be marked and tracked.     

Evidence for local adaptation in neighbouring Daphnia populations: a laboratory transplant experiment

1. We tested whether two neighbouring Daphnia galeata populations (from Lake Blankaart and Fish Pond), separated only by 5 m of land and with the occasional exchange of water were genetically differentiated in allozyme markers and life history traits. Allozyme electrophoresis revealed that the populations differed in allelic as well as in genotypic composition. 2. In a laboratory transplant experiment, in which animals of four clones of each of the populations were raised in the water of both ponds, survival in Blankaart water was high for both the Blankaart and Fish Pond clones, whereas survival in Fish Pond water was high for the Fish Pond clones, but low for the Blankaart clones. 3. Fish Pond clones produced fewer neonates than Blankaart clones when cultured in Blankaart water. High egg mortality was observed for animals that were raised in Blankaart water, and this egg mortality was higher for Fish Pond clones than for Blankaart clones. 4. Our results provide evidence for genetic differentiation between Daphnia populations inhabiting neighbouring water bodies and suggest local adaptation to environmental conditions other than direct predation.     

Chaoborus predation and delayed reproduction in Daphnia: a demographic modeling approach

I develop a demographic model that examines the impact of Chaoborus predation on the population dynamics and life history of Daphnia. Predation effects are determined through analysis of the various components of the predator-prey interaction (encounter, attack, strike efficiency), and are integrated into a stage-classified matrix population model. The model is parameterized with data from interactions between D. pulex and fourth-instar C. americanus. I test this model with two laboratory experiments that examine population growth of D. pulex under the influence of five different levels of Chaoborus predation. With the exception of a single predation treatment in each experiment, the model accurately predicted the observed reduction in Daphnia numbers with increasing Chaoborus predation. I then use this model to investigate the evolution of delayed reproduction in D. pulex that are exposed to Chaoborus. I ask whether delayed reproduction may evolve in Daphnia that are subjected to Chaoborus predation as a trade-off for the benefits of larger body size. The model predicts that the effectiveness of such a life history trade-off depends on the relative sizes of predator and prey. In some interactions between Chaoborus and Daphnia, increasing Daphnia body length by as little as 5% from base growth trajectories sufficiently increases fitness (by reducing vulnerability to Chaoborus predation) to compensate for the cost of delayed reproduction. In other interactions, however, increased body length provides no benefit to Daphnia (and may even reduce fitness), and selection would act against the evolution of delayed reproduction. This revised version was published online in July 2006 with corrections to the Cover Date.     

Life history of Daphnia galeata in a hypertrophic reservoir and consequences of non-consumptive mortality for the initiation of a midsummer decline

SUMMARY 1. Field and laboratory investigations were combined in a 2‐year study on the initiation of a midsummer decline of Daphnia galeata Sars in a hypertrophic reservoir. Quantitative field samples were taken twice a week, and, adult and juvenile mortality rates were calculated. Patterns of reproduction and survival of daphnids born during spring and early summer under fluctuating food conditions were determined in life‐table experiments. 2. The abundance of Daphnia increased strongly in early May and declined in June 1998 (midsummer decline). In 1999, Daphnia density increased only slowly in spring and remained constantly high throughout the summer. 3. Food conditions (concentrations of POC_{<30 μm}) for daphnids deteriorated in both years in response to increasing Daphnia densities, resulting in a clear‐water phase of about 4 weeks. When Daphnia abundance declined in 1998, POC_{<30 μm} concentrations increased greatly, whereas in 1999 food conditions improved only slightly and Secchi depth remained high. 4. Survival of daphnids in life‐table experiments decreased greatly after food became rare and was strongly reduced in those animals born during the clear‐water phase compared with those born later. In addition, age at first reproduction was retarded during the clear‐water phase, resulting in very low population growth rates. Survivorship patterns in life‐table experiments suggest a strong impact of non‐consumptive mortality on Daphnia population dynamics. 5. Field data of mortality point to differences in mortality patterns between years, probably resulting from different predation impacts of juvenile fish. In both years, however, adult mortality contributed substantially to overall mortality at the end of the clear‐water phase. As bottom‐up effects on D. galeata were very similar in both years, the significance of non‐consumptive mortality on the initiation of midsummer declines appears to depend largely on recruitment patterns before the clear‐water phase. A high impact can be expected when Daphnia populations are dominated by a peak cohort of nearly identical age during the clear‐water phase.     

Indirect effect of planktivorous fish on the growth and reproduction of Daphnia galeata

Both the size of Daphnia galeata primiparae and the average egg volume were smaller in individuals reared from neonata in water inhabited for the preceding 24 hours by Y-O-Y roach when compared with control animals reared in the same water with the exclusion of fish. The smaller size of experimental primiparae resulted from smaller increments in some instars and from the earlier maturation of about two-thirds of those that matured in the 4th instar, while the rest of the experimental animals and all the controls matured in the 5th instar. There were larger clutches and smaller eggs in experimentals when compared with the similar sized controls, but the difference hasn't proved to be statistically significant.     

Trait sorting in Daphnia colonising man‐made lakes

1. We used a zooplankton metacommunity to ask how dispersal, genetic drift and selection act to determine the local and regional distributions of trait variation. Since the formation of the lakes 80 years ago, cladoceran species have sorted into local assemblages that cluster by lake depth. Given this species sorting, we ask whether intraspecific variation in an ecologically important phenotypic trait – body size – has sorted as well. 2. We quantified changes in body size through time by measuring ephippia from D. pulicaria, D. dentifera and D. ambigua recovered from sediment cores from two lakes. We then estimated mean body size of contemporary populations of two competing species, Daphnia pulicaria and D. dentifera, in a laboratory common garden experiment. Finally, we used microsatellite loci to characterise genetic diversity and gene flow among local sites in the metacommunity. 3. Body size was variable both within and among years for the three species of Daphnia examined using sediment cores. For two lakes where we examined body size distributions through time, we observed a significant shift in body size of the first species to arrive after colonisation by other Daphnia species, which suggests selection has occurred historically. 4. Despite heritable variation in body size in the laboratory, evidence for trait sorting was only found for D. pulicaria, which was larger in deeper lakes. Mean body size varied among lakes, but did not sort relative to depth for D. dentifera. 5. Microsatellite data indicated that neutral genetic diversity was low in the region; only 27% of the individuals assayed were unique multi‐locus genotypes. We also found significant patterns of isolation by distance for both species. However, population structure was stronger in D. dentifera than in D. pulicaria. Hence, we conclude that a limited number of colonists have successfully invaded this metacommunity, and those genotypes arriving in this new region have experienced significant dispersal limitation among local sites. 6. Overall, while dispersal and selection have clearly led to the development of predictable community assemblages related to depth in this metacommunity, the distribution of phenotypic traits within species can differ substantially even between two trophically similar species. Our results highlight the complex roles of colonisation history, dispersal, selection and stochasticity in determining inter‐ and intra‐specific patterns in metacommunities.     

Short‐ and long‐term costs of reproduction in a migratory songbird

Costs of reproduction represent a common life‐history trade‐off. Critical to understanding these costs in migratory species is the ability to track individuals across successive stages of the annual cycle. We assessed the effects of total number of offspring fledged and date of breeding completion on pre‐migratory body condition, the schedule of moult and annual survival in a migratory songbird, the Savannah Sparrow Passerculus sandwichensis. Between 2008 and 2010, moult was delayed for individuals that finished breeding later in the breeding period and resulted in reduced lean tissue mass during the pre‐migratory period, suggesting an indirect trade‐off between the timing of breeding completion and condition just prior to migration. Lean tissue mass decreased as the number of offspring fledged increased in 2009, a particularly cool and wet year, illustrating a direct trade‐off between reproductive effort and condition just prior to migration in years when weather is poor. However, using a 17‐year dataset from the same population, we found that parents that fledged young late in the breeding period had the highest survival and that number of offspring fledged did not affect survival, suggesting that individuals do not experience long‐term trade‐offs between reproduction and survival. Taken together, our results suggest that adult Savannah Sparrows pay short‐term costs of reproduction, but that longer‐term costs are mitigated by individual quality, perhaps through individual variation in resource acquisition.