Effects of prey-size and predator-instar on the predation of Daphnia by Notonecta

Abstract. 1. Attack rates and handling times are measured by a series of separate functional response experiments for each instar of Notonecta glauca attacking four size classes of Daphnia magna as prey. The resulting attack rate and handling time surfaces are complex, with maximum attack rates for small predators attacking small prey, and large predators attacking large prey. Adult Notonecta have lower attack rates than the two previous juvenile instars (4 and 5).
2. The literature on attack rates and handling times in other predator—prey interactions that involve a series of different predator and prey size or age classes is reviewed in the context of the Notonecta-Daphnia results. The data suggest that small predator instars will usually compete with large instars for food, unless there is spatial or temporal separation between them.
3. Complex attack rate and handling time surfaces are to be expected wherever a wide range of prey and predator sizes is involved.
4. Size related changes in attack rates and handling times can introduce very complex dynamics into predator-prey interactions.     

Interspecific differences in movement and hunting success in damselfly larvae (Zygoptera: Insecta): responses to prey availability and predation threat

SUMMARY. 1. Freshwater invertebrate predators can alter their hunting behaviour in response to prey availability and threat from their own enemies. Movement patterns and hunting success of the larvae of two species of damselfly (Zygoptera: Insecta), Enallagma cyathigerum (L.) and L. sponsa (Hansemann), were monitored in four combinations of prey availability and enemy threat. Aeshna juncea (L.) (Anisoptera: Insecta) larvae acted as predator and Daphnia magna Straus (Cladocera: Crustacea) as prey in the combinations: no predator/no prey; one Aeshna present/no prey; no predator/thirty Daphnia present; one Aeshna present/ thirty Daphnia present.
2. E. cyathigerum showed significantly reduced movement in the +Aeshna/no prey treatment but hunting success was not significantly affected. No other treatment effects were noted. L. sponsa movement patterns differed significantly across all four treatments and hunting success was significantly reduced in the presence of Aeshna .
Interspecifically, movement patterns of the two species differed markedly in all four treatments. L. sponsa larvae were much more active, and caught many more prey. Despite their activity L. sponsa larvae did not appear markedly more vulnerable than the immobile E. cyathigerum .
3. The interspecific differences between the two damselflies reflect predictions based on larval life-history. Activity patterns and ability to capture adequate prey under varying levels of predation may be important in the ecology of damselfly species.     

Estimating predation rates in experimental systems: scale-dependent effects of aggregative behaviour

We studied the effect of arena size on estimates of prey mortality rate in predation experiments. In a laboratory experiment involving two of the dominating benthic species in the northern Baltic Sea, the predacious isopod Saduria entomon , and its main prey, the amphipod Monoporeia affinis , the prey mortality rate increased with container radius. The densities of both predator and prey close to the wall increased with container size. We hypothesised that this scale-dependent coaggregation of predator and prey caused the mortality rates to increase with arena size. We tested the hypothesis with the help of a simple model, by calculating the expected number of prey eaten in containers of different size from experimental data on the distributions of predator and prey within the arenas. A significant relationship between expected and observed numbers eaten supported our hypothesis. As the aggregative response was most pronounced in large arenas, this leads to the counterintuitive conclusion that large containers produced more biased estimates of morality rates than small containers.
To further study the effects of coaggregation we explored a general simulation model where both predator and prey preferred the habitat close to the arena wall. The model predicted a humpshaped relationship between encounter rate and arena size. This suggests that when predators and prey show a scale-dependent tendency to aggregate along arena walls, the most accurate estimates of predation rates may be obtained with very small or very large arenas.     

Changes in food quality of the green alga Scenedesmus induced by Daphnia infochemicals: biochemical composition and morphology

1. The effects of Daphnia infochemicals on the morphology and biochemical composition of Scenedesmus were studied and subsequently their influence on Daphnia life history. Three species of Scenedesmus ( S. acutus, S. obliquus and S. subspicatus ) were tested for Daphnia -induced colony formation. Life history experiments were performed with Daphnia cucullata and D. magna feeding on unicellular or colonial S. acutus.
2. Colony formation was promoted when S. acutus and S. obliquus were exposed to filtered water from a D. magna culture. S. subspicatus did not form colonies when exposed to culture water, but showed some colony formation when exposed to live D. magna.
3. No clear differences were found in total lipid, protein or carbohydrate content between the unicellular and colonial ecomorphs of the three Scenedesmus species. However, fatty acid (FA) concentration and composition were changed. Total FA as a percentage of frozen dry weight (DW) were increased in colonies. The ratio of monounsaturated (MUFA) and polyunsaturated fatty acids (PUFA) and the ratio of ω3 : ω6 fatty acids decreased in colonies.
4. A short-term grazing experiment with three size classes of Scenedesmus revealed lower clearance rates for small Daphnia when feeding on large colonial Scenedesmus.
5. Life history parameters of both Daphnia species were influenced by food type. The intrinsic rate of population increase ( r) was significantly lower for daphnids feeding on colonies.
6. The negative influence of colonial Scenedesmus on population growth of Daphnia seems to be determined by algal morphology rather than biochemical composition.     

氯化高汞对大型溞的慢性毒性

在换水静置条件下(25℃),氯化高汞(以Hg++计)对大型溞的48小时LC50值及其可信限为13.5(±2.1)微克/升。试验个体饲以斜生栅藻单个培养,在汞含量为1—21微克/升浓度下,产仔总数经方差分析表明组间差异显著(FF0.05)。各浓度组(X)与产仔总数(Y)的关系为:Y=-51.823X+4320.960(r=-0.904,p0.01)。净增殖率(R0)随浓度升高而逐渐下降。存活和生长用方差分析及D值检验表明对照组与各浓度组之间(差异数分别小于各自的D值20.89和0.1749),没有显著差异。各浓度组对内禀增长能力(rm)和世代平均周期(T)影响不大。根据大型溞的生物学基本参数,其体长的生长模型为: lt=4.57-3.5177e-0.00838t.       

Coping with predator stress: interclonal differences in induction of heat-shock proteins in the water flea Daphnia magna

Although predation is a strong selection pressure, little is known about the molecular mechanisms to cope with predator stress. This is crucial to understanding of the mechanisms and constraints involved in the evolution of antipredator traits. We quantified the expression of heat-shock protein 60 (Hsp60), a potential marker for predator stress, in four clones of the water flea Daphnia magna, when exposed to fish kairomones. Expression of Hsp60 induction increased after 6 h and returned to base levels after 24 h of predator stress. This suggests that it is a costly transient mechanism to temporarily cope with novel predator stress, before other defences are induced. We found genetic variation in the fixed levels and in the fish-induced levels of Hsp60, which seemed to be linked to each clone's history of fish predation. Our data suggest that Hsp60 can be considered part of a multiple-trait antipredator defence strategy of Daphnia clones to cope with predator stress.     

Predation risk perception and food scarcity induce alterations of life-cycle traits of the mosquito Culex pipiens

Abstract.  1. Predators may affect prey populations by direct consumption, and by inducing defensive reactions of prey to the predation risk. Food scarcity frequently has effects on the inducible defences of prey, but no consistent pattern of food–predation risk interaction is known.
2. In this study the combined effect of food shortage and predation-risk perception in larvae of the mosquito Culex pipiens was investigated. Water exposed to the aquatic predator bug Notonecta glauca was used as a source of predation intimidation. Mosquito larvae were reared in three different media containing either no predator cues or the cues of N. glauca that had been fed on either C. pipiens larvae or on Daphnia magna . Food was provided in favourable or limited amount for these set-ups.
3. The results showed that chemical cues from the predators fed with prey's conspecifics caused a decreased survival, delayed pre-imaginal development, and reduction in body size of emerged mosquitoes, whereas chemical cues from predators fed with D. magna caused only delayed development. Food scarcity significantly exacerbates the negative effect of the predator cues on pre-imaginal development of C. pipiens . Effects of the cues on larval development and body size of imagoes are significantly stronger for females than for males.
4. The present study suggests that when food is limited, predators can affect population dynamics of prey not only by direct predation, but also by inducing lethal and sublethal effects due to perception of risk imposed by chemical cues. To understand the effects of predators on mosquito population dynamics, environmental parameters such as food deficiency should be considered.     

Predation and priority effects in experimental zooplankton communities

During colonization of new habitats, the sequence of arrival among species is in many cases determined by chance. Priority effects imply that differences in arrival time may lead to long-lasting differences in species dominance. To evaluate the importance of priority effects, we performed a community assembly experiment, manipulating the inoculation order of three large cladoceran zooplankton species. The inoculation treatments were crossed with a predation treatment to test whether the presence of a predator (larvae of the phantom midge Chaoborus ) results in changes in the strength of species sorting and priority effects. Our results clearly demonstrate priority effects, but also that the presence of a predator impacts both community assembly and the strength of priority effects. In the predation-free treatments, communities were dominated by either Daphnia magna or Daphnia obtusa , depending on the species arrival sequence. Whenever D. obtusa was inoculated after D. magna , the species displayed negative growth. In the presence of Chaoborus predation, the communities were generally dominated by the third species, Simocephalus vetulus . Here, the growth of D. magna was negative when the species was inoculated as second or third. Overall, our results underscore the importance of both priority effects and species sorting during community assembly.     

Influence of food density on the effects of a Chaoborus-released chemical on Daphnia ambigua

SUMMARY 1. Daphnia ambigua was reared individually at four different food levels in water conditioned by a predator, Chaoborus flavicans. Daphnia ambigua developed spike-like helmets during the first four instars in each treatment with the Chaoborus-conditioned water, but only during the first instar in untreated water. The helmet development at instars 2–4 was probably induced by a chemical released from the Chaoborus .
2. The helmets decreased in size with decreasing food levels, but never ceased to form, even at a very low food concentration.
3. Daphnids cultured in the Chaoboras -conditioned water exhibited reduced growth, reproduction and survival rates at low food levels. These results might be induced by the energy loss associated with producing the helmets.
4. It is suggested that the predator releases a chemical, which reduces the tolerance of the cyclomorphic Daphnia to food deficiency, thus accentuating the summer decline in the Daphnia population caused by food shortage.     

Effect of vertebrate and invertebrate kairomones on the life history of Daphnia magna Straus (Crustacea: Branchiopoda)

The history of selection of Daphnia magna populations living in North African temporary ponds may differ from populations inhabiting permanent ponds. Laboratory experiments were conducted to examine the effect of fish Gambusia holbrooki and invertebrate Notonecta glauca kairomones on the life history traits of the freshwater Cladocera Daphnia magna Straus. With fish kairomones, Daphnia reproduced early and had a significantly smaller size at first reproduction (SFR) and a smaller size of neonates compared to control. In contrast, daphnids reared in water treated with Notonecta glauca had no effect on the age at first reproduction but females were also smaller and produced smaller neonates.     

Food quality of heterotrophic bacteria for Daphnia magna: evidence for a limitation by sterols

The quality of heterotrophic bacteria as food for metazoan grazers has been investigated poorly. We conducted growth experiments with juvenile Daphnia magna feeding on different strains of heterotrophic bacteria that represent typical pelagic bacteria of five phylogenetically distinct groups. The bacterial food suspensions were supplemented with cholesterol and/or the polyunsaturated fatty acid eicosapentaenoic acid (EPA), two essential nutrients that are either absent or scarcely represented in bacteria. Our data imply that the selected heterotrophic bacteria are of poor food quality for D. magna, which was indicated either by very low somatic growth rates or by high mortality. However, with four out of six bacterial strains tested, the somatic growth rates increased significantly upon supplementation with cholesterol, which shows that the lack of sterols in bacteria is a major food quality constraint. We did not find clear evidence for a limitation by EPA on bacterial diets within our growth experiments. High mortality was observed when D. magna was fed with Hydrogenophaga sp. or Pseudomonas sp., which indicates that these two bacterial strains are toxic to D. magna. Our findings highlight the limitations of bacteria as a carbon source for Daphnia and point to a so far underestimated diversity of interactions between grazers and its bacterial food.     

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.     

Importance of consumptive and non-consumptive prey mortality in a coupled predator–prey system

1. Laboratory experiments were completed to identify the mechanisms by which the predatory flatworm, Dugesia tigrina , imposes mortality on its Aedes aegypti and Daphnia magna prey. Feeding trials were completed in glass microcosms which contained one of three – nine densities of small or large individuals of each prey species.
2. Mortality by Dugesia on small and large Aedes followed a type II functional response, whereas the mortality of Daphnia resembled a type III functional response. Prey mortality imposed by Dugesia consisted of consumptive and non-consumptive elements. Non-consumptive mortality occurred when prey individuals trapped in mucus trails subsequently died but were not ingested.
3. Additional experiments were conducted to quantify consumptive (capture followed by ingestion) and non-consumptive mortality (death not followed by ingestion).
4. Consumptive mortality followed a type II functional response for small and large individuals of both prey species, whereas non-consumptive mortality increased with prey density, although the relationships differed with prey species and size. The non-consumptive mortality of large Daphnia increased at an accelerating rate with prey density and exceeded consumptive mortality at intermediate prey abundances. In contrast, non-consumptive mortality of small Aedes and small Daphnia was lower than consumptive mortality and either increased with prey density at a decelerating (small Aedes ) or accelerating (small Daphnia ) rate.
5. These results suggest that the importance of consumptive and non-consumptive mortality to total prey mortality needs to be considered when modelling predator–prey dynamics.     

Temperature effects on ballistic prey capture by a dragonfly larva

Understanding the effects of temperature on prey–predator interactions is a key issue to predict the response of natural communities to climate change. Higher temperatures are expected to induce an increase in predation rates. However, little is known on how temperature influences close‐range encounter of prey–predator interactions, such as predator's attack velocities. Based on the speed–accuracy trade‐off concept, we hypothesized that the increase in predator attack velocity by increasing temperature reduces the accuracy of the attack, leading to a lower probability of capture. We tested this hypothesis on the dragonfly larvae Anax imperator and the zooplankton prey Daphnia magna. The prey–predator encounters were video‐recorded at high speed, and at three different temperatures. Overall, we found that (1) temperature had a strong effect on predator's attack velocities, (2) prey did not have the opportunity to move and/or escape due to the high velocity of the predator during the attack, and (3) neither velocity nor temperature had significant effects on the capture success. By contrast, the capture success mainly depended on the accuracy of the predator in capturing the prey. We found that (4) some 40% of mistakes were undershooting and some 60% aimed below or above the target. No lateral mistake was observed. These results did not support the speed–accuracy trade‐off hypothesis. Further studies on dragonfly larvae with different morphological labial masks and speeds of attacks, as well as on prey with different escape strategies, would provide new insights into the response to environmental changes in prey–predator interactions.     

1,8-二羟基蒽醌对大型溞的毒性效应

The study on the acute,sublethal and chronic toxicity of 1,8-dihydroxyanthraquinone (1,8-dihAQ) to Daphnia magna showed that the 48 h LC₅₀ was 0.37 mg稬^-1,and the feeding behavior of Daphnia magna was severely affected by the compound.When exposed to 0.2 mg稬^-1 of 1,8-dihAQ for 5 h,the filtration and ingestion rate of Daphnia magna was inhibited by 97%.Chronic toxicity test results indicated that the reproduction ability decreased dramatically after exposing to sublethal concentration of 1,8-dihAQ.It could be inferred that reproduction parameters and intrinsic rate of natural increase were the sensitive parameters in characterizing sublethal toxicity.The NOEC and LOEC values for reproduction parameters were also given.     

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.     

Grazer-induced changes in the desmid Staurastrum

In aquatic environments, predator kairomones have been shown to affect morphology of prey species. Past work on the interaction between zooplankton and phytoplankton was based mainly on the Daphnia–Scenedesmus model. Algae of the genus Staurastrum can produce mucilage, causing cell clumping and settling out of the water column. These clumps are too large to be eaten by daphniids. Thus, we hypothesised that this may be a grazer defence. We investigated whether Daphnia magna induces the formation of mucus globules in Staurastrum, how this occurs, and if the formation of clumps inhibits growth in juvenile Daphnia. Eight strains of Staurastrum were used to check whether mucus extrusion is induced by the presence of Daphniaor possibly by a chemical excreted by Daphnia magna. None of the strains reacted to the presence of Daphnia water alone, animals had to be present to induce clumping. Mechanical action (gentle stirring) caused the same strains to clump. The ecological relevance of clumping was then investigated. The different Staurastrum strains were used as food in a growth experiment with ecologically relevant densities of neonates of Daphnia hyalina. These small daphniids did not cause the same clumping observed for Daphnia magna when present in experiments at high densities. We observed that juvenile daphniids grew less well on strains with larger cell size.     

Long-term dynamics in rock pool Daphnia metapopulations

Metapopulations of Daphnia longispina , D. magna and D. pulex on 16 islands containing 507 rock pools were studied over 17 yr (autumns 1982–1998). Two yearly samples were taken representing early and late summer conditions. A local extinction was assumed when a species was absent from a pool for more than one year. Average yearly extinction rates, calculated from probability of survival in age grouped data, were close to 20%, and number of extinctions was on average balanced by the number of colonizations. Extinction rates during the first year following colonization were 45–50% in all species, but later decreased to low values. In first-year data, single species populations of D. magna had lower extinction rates than in multi-species pools. In later years all species combinations had similar extinction rates. The average proportion of rock pools occupied by D. longispina in any given year was 0.178, by D. magna 0.169 and by D. pulex 0.056. In 22% of cases, coexistence was observed. 71% of the pools were colonized at least once. The distribution of shortest colonization distances was left-skewed, and median distances (7–8 m) were same in all species. The distribution of colonizations mostly corresponded to the expected frequencies calculated as the product of frequencies of source and target pools. Both D. longispina and D. magna colonized pools already occupied with a somewhat higher probability than empty pools. Probably owing to variation of habitat quality, Daphnia populations in some rock pools appear to be less prone to extinction, providing sources of dispersal to patches with higher extinction risk. The system appears to be at an approximate equilibrium, with several characteristics close to classical metapopulation system.     

Sequencing of hemoglobin gene 4 (dmhb4) and Southern blot analysis provide evidence of more than four members of the Daphnia magna globin family

A genomic fragment containing the hemoglobin gene dmhb4 of Daphnia magna was cloned and its nucleotide sequence determined. Concerning induction under hypoxic conditions, dmhb4 was found to be expressed constitutively with similar mRNA quantities in D. magna bred in either normoxic or hypoxic medium. Southern blot analysis revealed at least six hemoglobin-like sequences in the genome of Daphnia magna.