Fitness cost from fluctuating ultraviolet radiation in Daphnia magna

Solar ultraviolet radiation (UVR) is an important environmental threat for organisms in aquatic systems, but its temporally variable nature makes the understanding of its effects ambiguous. The aim of our study was to assess potential fitness costs associated with fluctuating UVR in the aquatic zooplankter Daphnia magna. We investigated individual survival, reproduction and behaviour when exposed to different UVR treatments. Individuals exposed to fluctuating UVR, resembling natural variations in cloud cover, had the lowest fitness (measured as the number of offspring produced during their lifespan). By contrast, individuals exposed to the same, but constant UVR dose had similar fitness to control individuals (not exposed to UVR), but they showed a significant reduction in daily movement. The re-occurring threat response to the fluctuating UVR treatment thus had strong fitness costs for D. magna, and we found no evidence for plastic behavioural responses when continually being exposed to UVR, despite the regular, predictable exposure schedule. In a broader context, our results imply that depending on how variable a stressor is in nature, populations may respond with alternative strategies, a framework that could promote rapid population differentiation and local adaptation.     

Daphnia community analysis in shallow Kenyan lakes and ponds using dormant eggs in surface sediments

1. Water fleas of the genus Daphnia are considered rare in tropical regions, and information on species distribution and community ecology is scarce and anecdotal. This study presents the results of a survey of Daphnia species distribution and community composition in 40 standing waterbodies in southern Kenya. The study sites cover a wide range of tropical standing aquatic habitats, from small ephemeral pools to large permanent lakes between approximately 700 and 2800 m a.s.l. Our analysis combines data on Daphnia distribution and abundance from zooplankton samples and dormant eggs in surface sediments. 2. Nearly 70% (27 of 40) of the sampled waterbodies were inhabited by Daphnia. Although their abundance in the active community was often very low, this high incidence shows that Daphnia can be equally widespread in tropical regions as in temperate regions. 3. Analysis of local species assemblages from dormant eggs in surface sediments was more productive than snapshot sampling of zooplankton communities. Surface‐sediment samples yielded eight Daphnia species in total, and allowed the detection of Daphnia in 25 waterbodies; zooplankton samples revealed the presence of only four Daphnia species in 16 waterbodies. 4. Daphnia barbata, D. laevis, and D. pulex were the most frequently recorded and most abundant Daphnia species. Canonical correspondence analysis of species–environment relationships indicates that variation in the Daphnia community composition of Kenyan waters was best explained by fish presence, temperature, macrophyte cover and altitude. Daphnia barbata and D. pulex tended to co‐occur with each other and with fish. Both species tended to occur in relatively large (>10 ha) and deeper (>2 m) alkaline waters (pH 8.5). Daphnia laevis mainly occurred in cool and clear, macrophyte‐dominated lakes at high altitudes.     

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.     

Competitive abilities and distribution ranges of four asexual Daphnia pulex lineages and Daphnia mitsukuri in Eurasian continental islands

Daphnia pulex and D. mitsukuri are morphologically similar and distributed in small lowland Japanese lakes. Daphnia pulex in Japan reproduces by obligate parthenogenesis and is composed of four different lineages (JPN1–JPN4) with North American origins. Although this species is found throughout Japan, JPN1 has the largest distribution range, followed by JPN2. Daphnia mitsukuri is a putatively endemic species that is now rarely found in Japan. Since Daphnia share the same algal food, the difference in distribution ranges among the four asexual D. pulex lineages and D. mitsukuri may be caused by exploitative competition for algal food. To examine this possibility, we measured the threshold food concentration (TFC) at zero-net growth rate and the food concentration required for 50% of the individuals to survive (FC50). In addition, we examined the effects of temperature on the somatic growth rate (SGR) and the mortality rates of four asexual D. pulex lineages and D. mitsukuri. The experiments showed that TFC was lowest in D. pulex JPN1 and highest in D. mitsukuri, supporting the idea that the largest distribution range of JPN1 is due to its superiority in inter-lineage competition, and that the distribution area of D. mitsukuri is reduced through exploitative competition with D. pulex. However, although JPN2 was found in relatively large areas, it had higher TFC than JPN3 and JPN4, which appear in very limited areas. These results clearly showed that the difference in distribution ranges among D. pulex lineages are not necessarily determined by their competitive superiority alone.     

Costs, benefits and the evolution of inducible defences: a case study with Daphnia pulex

Phenotypic plasticity is one major source of variation in natural populations. Inducible defences, which can be considered threshold traits, are a form of plasticity that generates ecological and evolutionary consequences. A simple cost-benefit model underpins the maintenance and evolution of these threshold, inducible traits. In this model, a rank-order switch in expected fitness, defined by costs and benefits of induction between defended and undefended morphs, predicts the risk level at which individuals should induce defences. Here, taking predator-induced morphological defences in Daphnia pulex as a threshold trait, we provide the first comprehensive investigation into the costs and benefits of a threshold trait, and how they combine to reflect fitness and predict the switchpoint at which induction should occur. We develop reaction norms that show genetic variation in switchpoints. Further experiments show that induction can confer a survival benefit and a cost in terms of lifetime reproductive success. Together, these two traits combine to estimate expected fitness and can predict the switchpoint between an undefended and a defended strategy. The predictions match the reaction norm data for clones that experience these costs and benefits, and correspond well to independent field data on induction. However, predictions do not, and cannot, match for clones that do not gain a benefit from induction. This study confirms that a simple theory, based on life history costs and benefits, is a sufficient framework for understanding the ecology and evolution of inducible, threshold traits.     

The use of inorganic substances to stimulate gut evacuation in Daphnia magna

Observations were done on the effect of inorganic substances on the gut evacuation process in Daphnia magna. Procedures which accelerate this process are described.     

Evidence for strong host clone-parasite species interactions in the Daphnia microparasite system

Abstract Organisms are often confronted with multiple enemy species. Defenses against different parasite species may be traded off against each other. However, if resistance is based on (potentially costly) general defense mechanisms, it may be positively correlated among parasites. In an experimental study, we confronted 19 clones from one Daphnia magna population with two bacterial and three microsporidian parasite species. All parasites were isolated from the same pond as the hosts. Host clones were specific in their susceptibility towards different parasite species, and parasite species were host-clone specific in their infectivity, spore production, and virulence, resulting in highly significant host-parasite interactions. Since the Daphnia 's resistance to different parasite species showed no obvious correlation, neither general defense mechanisms nor trade-offs in resistance explain our findings. None of the Daphnia clones were resistant to all parasite species, and the average level of resistance was quite similar among clones. This may reflect a cost of defense, so that the cumulative cost of being resistant to all parasite species might be too high.     

The interaction of Chaoborus size and vertical distribution determines predation effects on Daphnia

1. Larvae of Chaoborus, the phantom midge, are important pelagic planktivores in many freshwater lakes and ponds. The effect of Chaoborus on its prey depends on its size, especially mouth gape diameter, and vertical migration pattern, which affects predator–prey spatial overlap. These two features vary considerably in different Chaoborus species and instars. In this study, the interacting effects of both Chaoborus size and vertical distribution on population growth of Daphnia pulex was analysed with a field enclosure experiment and a matrix population model. 2. In the field experiment, Daphnia were grown in four replicated treatments that included a control (no Chaoborus) and three combinations of instar III and IV Chaoborus of two species (C. trivittatus and C. americanus). Parameters of the matrix model were based on differences between Chaoborus species and instars in capture and ingestion of Daphnia of differing sizes (prey vulnerability) and in vertical overlap with Daphnia in each treatment (density risk). 3. In comparison with the control, the two treatments containing the smaller, migratory C. americanus showed a significant effect on Daphnia population growth rate, while the treatment containing only the larger, non‐migratory C. trivittatus did not. The model accurately simulated these effects. 4. A Daphnia predation risk model, which uses prey vulnerability and density risk parameters, illustrated the individual and combined effect of the different Chaoborus types on Daphnia. Daphnia have a high prey vulnerability to the large C. trivittatus, but overall predation risk was low because of very little overlap. On the contrary, the smaller C. americanus affects only a small range of Daphnia instars, each with a low vulnerability, yet those instars that were vulnerable had a very high density risk because of an increased overlap. 5. This analysis of Daphnia predation risk parameters with coexisting Chaoborus species strongly supports an integrated approach using both size and vertical distribution to determine the ultimate predation effect on Daphnia.     

Changes in vulnerability of Daphnia to an insecticide application depending on the population phase

1. Laboratory populations of cloned Daphnia pulex at three population phases (growing phase, density peak, declining phase) were exposed to the insecticide carbaryl at 15 μg L^?1, which killed juveniles but not adults. 2. The population decline was largest and recovery was slowest when the chemical was applied at the density peak, indicating that the population was most sensitive to the chemical at that stage. Populations in the declining phase were least affected by the treatment. 3. We propose that food condition, which varies with population density, was a major factor affecting vulnerability to carbaryl. In that case, Daphnia populations in lakes would be most vulnerable to toxic chemicals during food‐limited condition such as the spring clear‐water phase.     

Ideal free distribution in Daphnia? Are daphnids able to consider both the food patch quality and the position of competitors?

The distribution of the planktonic crustacean Daphnia pulex was tested in a ring-formedflow-through chamber divided into eleven sections. Thedistribution of the animals under both homogeneousfood conditions and in a food gradient were studied.The distribution of repeated registrations of singleanimals was randomly distributed in a homogeneous,low food environment. Single daphnids exposed to afood gradient tended to reside during 88% of the timeat the highest or next highest food concentration,suggesting that daphnids can detect food gradients.When a group of approximately 100 co-occurring animalswere given low homogeneous food conditions, they alsotended to be randomly distributed. In one out of threecases, however, they were slightly less aggregatedthan expected from a random distribution. Exposed toa food gradient (0.5–0.0015 mg C l^-1), thedistribution of the daphnids approximated the idealfree distribution. A very high maximum density in thefood gradient (2 mg C l^-1) resulted in less strongaggregations than expected from the ideal freedistribution.     

Mechanisms of energy allocation to reproduction in the cladoceran Daphnia magna Straus

Understanding rules of resource allocation within individuals is helpful in explaining population dynamics. This is particularly the case under the conditions of resource limitation that are commonly experienced by zooplankton. Here, we evaluate assumptions underlying models of resource allocation in Daphnia and test the predictions of two models of response to starvation.
We also test the predictions of two simple models concerning the mechanisms of egg provisioning in Daphnia magna. Our results: (1) demonstrate that provisioning is discontinuous, occurring over a discrete period of the instar; (2) support the hypothesis that egg production occurs in a serial fashion; (3) show that Daphnia magna responds to starvation by ceasing egg production but that there is an increase in mean size of eggs provisioned during the instar, prior to starvation; and (4) broadly support predictions that the response of Daphnia to starvation is an instantaneous cessation of reproduction but that there is a time lag before death.     

Climate change affects colonization dynamics in a metacommunity of three Daphnia species

Climate change is expected to alter the range and abundance of many species by influencing habitat qualities. For species living in fragmented populations, not only the quality of the present patches but also access to new habitat patches may be affected. Here, we show that colonization in a metacommunity can be directly influenced by weather changes, and that these observed weather changes are consistent with global climate change models. Using a long‐term dataset from a rock pool metacommunity of the three species Daphnia magna, Daphnia longispina and Daphnia pulex with 507 monitored habitat patches, we correlated a four‐fold increase in colonization rate with warmer, drier weather for the period from 1982 to 2006. The higher colonization rate after warm and dry summers led to an increase in metacommunity dynamics over time. A mechanistic explanation for the increased colonization rate is that the resting stages have a higher exposure to animal and wind dispersal in desiccated rock pools. Although colonization rates reacted in the same direction in all three species, there were significant species‐specific effects that resulted in an overall change in the metacommunity composition. Increased local instability and colonization dynamics may even lead to higher global stability of the metacommunity. Thus, whereas climate change has been reported to cause a unidirectional change in species range for many other species, it changes the dynamics and composition of an entire community in this metacommunity, with winners and losers difficult to predict.     

The use of biomarkers in the antioxidant responses of Daphnia magna to the acute and chronic exposure to no. 20 diesel oil and 2,4-dichlorophenol

Abstract

The effects of no. 20 diesel oil exposure, 2,4-dichlorophenol (2,4-DCP) exposure and combined exposure on the antioxidant defences of Daphnia magna have been studied systematically for the first time. Daphnia magna was exposed for 1 day or 10 days to several concentrations of 0, 0.005, 0.01, 0.05, 0.1, 0.5 or 1.0 mg L^?1 solutions. Antioxidant defences consisting of the levels of reduced glutathione (GSH) and glutathione disulfide (GSSG) and activities of superoxide dismutase (SOD), catalase (CAT), glutathione reductase (GR), selenium dependent glutathione peroxidase (Se-GPx) and glutathione S-transferase (GST) of daphnids were determined to evaluate their protective roles and to analyse the occurrence of oxidative stress. The possible antioxidant defence mechanisms are discussed. Furthermore, GST can be a potential biomarker and an early-warning index for the pollutants in waters in that GST responded sensitively to 1 day and 10 days of exposure to diesel oil and 2,4-DCP and 10 days of combined exposure. Crossover comparisons showed an antagonistic action about the no-observed-effect concentration (NOEC) against Daphnia magna, which needs further studies.     

Morphological defenses induced in situ by the invertebrate predator Chaoborus : comparison of responses between Daphnia pulex and D. rosea

The presence of plankton predators may induce altered morphology in their potential prey. To date, the mechanism of induction and adaptive value of such defensive responses have been examined in the laboratory. This study investigated the morphological defense structures induced by the invertebrate predator Chaoborus in two coexisting Daphnia species, D. pulex and D. rosea, in the field. In Piscivore Lake (Gr?fenhain, Germany), continuous and intense biomanipulation had led to near elimination of planktivorous fish and greatly increased abundances of Chaoborus (up to >10 larvae l^–1). Here, the density of Chaoborus was manipulated within the lake by an enclosure/exclosure setup and resulting morphological responses of Daphnia spp. were investigated in situ. Three replicate enclosures (4.6 m³) contained no Chaoborus (predator exclusion bags), whereas Chaoborus entered three others at ambient densities (predator enclosures). In both species of Daphnia, formation of neckteeth and elongation of the tail spine were recorded in the predator enclosures, but not in the predator exclusion treatments. Additionally, D. rosea responded to predator inclusion with an increase of the size at first reproduction. Despite the induced defense structures, the presence of Chaoborus caused increased mortality of both Daphnia species. In addition, Chaoborus affected the coexistence of the two populations of Daphnia by causing higher relative mortality in D. rosea. Neckteeth formation was always more pronounced in D. pulex than in D. rosea of the same size. Neckteeth were induced specifically in vulnerably sized juvenile instars of D. pulex, but were not found in all vulnerable instars of D. rosea. In D. rosea, neckteeth were few or absent in the ephippial hatchlings, and neckteeth formation ceased before juveniles reached a body size outside the range that larger larval stages of Chaoborus could ingest. This study provides the first experimental demonstration in the field of the inducibility of morphological defense structures in Daphnia at ambient densities of Chaoborus larvae, and quantifies these in situ responses. This expands on earlier observations of a correlation between predator density in the field and the expression of neckteeth in Daphnia. The term ”maximum size for neckteeth formation” (MSNF) is defined as the limit in body size above which no production of neckteeth was evident. This limit was used to distinguish the size classes of Daphnia that show a sensitive response to Chaoborus kairomone. This new term may be used for further comparisons among species and among different types of predator-induced responses as well as for the evaluation of the adaptive value of defense structures. Received: 10 April 1999 / Accepted: 6 April 2000     

Phylogeography of Daphnia magna in Europe

The phylogeographical structure of the water flea Daphnia magna in Europe was analysed using a 609-bp fragment of the cytochrome oxidase subunit I gene. Only moderate overall genetic divergence was detected within Europe. We detected four genetically and, to a large extent, geographically distinct phylogroups within Europe. Our results suggest that these groups recolonized large parts of Europe around 100,000 BP from different refugia. Overall, the pattern suggests a high degree of provincialism with a patchy occurrence of specific lineages, thus confirming the highly subdivided genetic structure usually observed in freshwater zooplankton populations. Although the region around the Mediterranean Sea was only sampled patchily, we obtained strong indications for the occurrence of more divergent genetic lineages in this region. Comparing our European samples to samples from North America and Japan revealed a higher level of differentiation, reflecting limited intercontinental dispersal.     

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].     

不同种类农药表面活性剂对大型溞的急性毒性

运用评价化学品对水生生物毒性的标准试验方法,比较了39种非离子型、6种阴离子型和3种阳离子型的常用代表性表面活性剂对大型溞的急性毒性.结果表明:3种阳离子型表面活性剂1427、1227及C8-10的急性毒性均为剧毒,其中1427毒性最高,EC50值为0.97×10-2 mg·L-1;非离子型表面活性剂中蓖麻油聚氧乙烯醚、吐温和斯潘系列乳化剂均为低毒,而烷基酚聚氧乙烯醚系列和脂肪醇聚氧乙烯醚系列表面活性剂的毒性稍偏高,而AEO-7和AEO-5的毒性达到高毒水平,EC50值分别为0.82和0.97 mg·L-1,且此类表面活性剂脂溶性越大,对大型溞的毒性越大;大部分阴离子表面活性剂的毒性为中毒,但NNO表现为高毒,EC50值为0.17 mg·L-1.     

Behavioural and morphological changes in polymorphic Daphnia related to different predation regimes

Morphology and behaviour of Daphnia galeata in Lake Lille Lungegårdsvann, Bergen, Norway, were studied in two consecutive years. A portion of the population showed helmet cyclomorphosis in 1992, but no helmets developed in 1993. The non-helmeted individuals in 1992 were found in the littoral habitat during summer, whereas the helmeted morph stayed mainly in the pelagial. These differences in morphology and behaviour within and between years are likely to be caused by changes in predation regimes. Larvae of the predacious phantom midge, Chaoborus flavicans, were observed only in 1992, whereas the three-spined stickleback, Gasterosteus aculeatus, was present both years.     

Differing effects of suspended sediments on the performance of native and exotic Daphnia

1. Although large cladocerans are usually uncommon in large rivers, Daphnia lumholtzi (an exotic species in North America) is widespread and occasionally abundant. We collected zooplankton on the Illinois River (Illinois, U.S.A.) in 1995 and 1996 and found that the peak density of D. lumholtzi (25 L⁻¹) typically exceeded that of all native species combined. Maximum density occurred during warm periods (up to 27 °C) when concentrations of inorganic suspended sediments were high (>50 mg L⁻¹).
2. Using a life table experiment, we examined the effects of variation in suspended sediment (0 and 80 mg L⁻¹) and food (10⁴ and 10⁵ Ankistrodesmus cells mL⁻¹) on fitness of D. lumholtzi and the native Daphnia parvula. Daphnia lumholtzi had greater survivorship than D. parvula in most treatments and higher life-time fertility than D. parvula in all treatments. Both species achieved their fastest intrinsic rates of growth in treatments with high food, but their responses to suspended solids differed. The growth rate of D. lumholtzi in high food was slightly increased by higher turbidity, whereas that of D. parvula was depressed.
3. Results suggest that the ability of D. lumholtzi to tolerate suspended solids is an important factor contributing to its success in invading North American rivers.