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To test the hypotheses that multigenerational predation risk favors clones with longer postero-lateral spines and a larger body, and that fitness consequences of selection depend on both tested fitness variables and selection intensity, we expose replicate Brachionus dorcas populations in low (LK) and high (HK) concentrations of Asplanchna kairomones. Over 60 days (23 asexual generations) of selection, LK-selected populations showed the highest equilibrium population densities, and median mictic ratios and postero-lateral spine lengths, and HK-selected populations showed the lowest equilibrium population densities, the highest mictic ratios, and the longest postero-lateral spines, among all the control and treatment populations. In the absence of Asplanchna kairomones, and compared with populations with the control selection history, populations with an LK selection history showed increased growth rates/intrinsic rates of population increase and net reproductive rates, but decreased mictic ratios, indicating three fitness gains and a fitness cost; and those with an HK selection history showed increased life expectancy at hatching and mictic ratios, but decreased growth rates, indicating two fitness gains and a fitness cost. Asplanchna kairomones might increase the heritability of higher equilibrium densities or mictic ratios, and longer postero-lateral spines, and thus enable the prey to survive predation. [ABSTRACT FROM AUTHOR]

To test the hypothesis that non-consumptive predation pressures favor genotypes that have higher equilibrium densities or mictic ratios, we expose replicate populations of the rotifer Brachionus angularis to low (LK) and high (HK) concentrations of predator Asplanchna kairomones. Over 75 days (25 asexual generations) of selection, LK-treated populations have higher equilibrium densities and lower mictic ratios than the control populations. The opposite occurs for HK-treated populations. These results suggest that compared with the control, LK and HK select for genotypes with higher equilibrium densities and mictic ratios, respectively. Common garden bioassays reveal that in the absence of Asplanchna kairomones, populations with an LK selection history have a higher average intrinsic rate of population increase, and those with an HK selection history have a higher average mictic ratio than those with the control selection history, which suggest that adaptations to LK and HK increase average intrinsic rate of population increase and mictic ratio of prey populations, respectively. Increased intrinsic rate of population increase and mictic ratio can serve as strategies of offsetting mortality from predation and escaping the predator, and thus enable prey to survive low and high predation pressures, respectively. [ABSTRACT FROM AUTHOR]

The aim of this study was to explore whether the widespread zooplankter Daphnia magna is able to differentiate between the kairomones of planktivorous common bleak (Alburnus alburnus) and omnivorous black bullhead (Ameiurus melas). We expected different responses in some plastic defensive traits because the presence of a planktivorous and an omnivorous fish pose a different degree of threat to daphnids. Our mesocosm study showed that the size distribution of D. magna in the bleak treatment tended to differ from both black bullhead and control treatments, while there was no apparent difference between the latter two treatments. The eye diameter/body length ratio and tail spine/body length ratio of D. magna were highest in the bleak treatment, differing from both black bullhead and control treatments, and without any significant difference between the latter two treatments. We found that only bleak triggered detectable responses in D. magna, suggesting either that kairomones of the studied fish species could differ in their chemical composition, or that black bullhead may exude less kairomone than bleak. [ABSTRACT FROM AUTHOR]

Non-native species introductions are becoming increasingly common, but long-term consequences of the introduction of non-native predators on native prey species remains poorly understood. Bythotrephes cederströmii is an invasive zooplanktivore with potential negative impacts on Daphnia across North America. However, Daphnia may be responding to this invader by developing anti-predator defenses. We investigated whether escape ability of three Daphnia species (D. ambigua, D. mendotae, and D. pulicaria) differed in the absence and presence of Bythotrephes chemical cues, whether the escape ability of pre- and post- invasion clones of each Daphnia species differed, and whether the three Daphnia species differed in their escape ability. We conducted a series of transfer experiments in the presence and absence of Bythotrephes chemical cues to evaluate the escape ability of the three Daphnia species from both pre- and post-Bythotrephes invasion time periods. Comparison of Daphnia from pre- and post-invasion time periods revealed a constitutive increase in the escape ability of D. pulicaria which may help them avoid attack by Bythotrephes. These data contribute to a growing body of research that demonstrates that ecological invasions can prompt adaptive responses, altering the nature of interactions between invasive predators and native prey species. [ABSTRACT FROM AUTHOR]

Exposure of prey to predation risk usually leads to increased metabolism and decreased nutrient deposition as physiological stress responses (PSRs) of prey to their predators. Many studies using freshwater animals as models failed to find evidences as predicted from PSRs. One possible reason is that energetic costs during development of inducible defenses may lower the quantity of resources, which can be allocated to production of costly PSRs. Results obtained from the classic Brachionus-Asplanchna model indicated that inducible defenses did not alleviate PSRs in B. calyciflorus, which were manifested as increased respiration and enhanced body molar ratios of carbon (C) to nitrogen (N) and C to phosphorus (P). Moreover, activities of antioxidant systems also increased in defended B. calyciflorus. There were no alterations in somatic growth and swimming activity in B. calyciflorus when they were exposed to Asplanchna kairomones; however, we observed predator-induced –reduction of food intake in Brachionus. Our results suggest that enhanced metabolisms of defended prey in support of fight-or-flight responses are likely to be the physiological adaptation of prey to diminish hydrodynamic costs accompanied with morphological alterations (e.g., decreased swimming speed) and maintain their fitness under predation risk. [ABSTRACT FROM AUTHOR]

Calcium decline is an emerging environmental concern for thousands of soft-water lakes. Daphnia, an important freshwater herbivore, has reduced reproduction, a weaker, less-rigid carapace, and decreased survival under low aqueous calcium concentrations, but there has been little attention to study how calcium decline will influence predator-prey relationships. We conducted a mesocosm experiment to determine the joint effects of calcium concentration (0.3, 1.2, and 2.3 mg/l) and kairomones from Chaoborus americanus, a ubiquitous invertebrate predator, on Daphnia minnehaha populations. In the presence of Chaoborus kairomones, chemical signals of Chaoborus presence, Daphnia populations had delayed peak abundance and grew antipredator neck spine defenses. However, aqueous calcium concentration influenced the strength of the response. Daphnia in both 0.3 and 2.3 mg Ca/l had stronger neck spine responses and larger reproductive delays than Daphnia in 1.2 mg Ca/l. Our results suggest that as lake calcium concentrations continue to fall, Daphnia populations will respond in complex, nonlinear ways. [ABSTRACT FROM AUTHOR]

Here we test for the possible coupling of two kairomone-induced, anti-fish defences in Daphnia, life-history changes (LHC) and diel vertical migration (DVM) mediated by the environmental factor light. A gradient of five different light intensities that represents naturally occurring intensities in the lake water column was used in life-history experiments, and we show that LHC of a single Daphnia clone are inversely coupled to the ambient light intensity. Furthermore, we could show that the light intensity has to exceed a threshold to induce the LHC. We also observed an effect of the light intensity on the fish kairomone-mediated expression of a candidate gene ( actin 3) in a way that the gene response differs between groups of low and high light intensities. We argue that the ambient light intensity an individual is exposed to and that is dependent on the position in the water column of a lake inversely determines the degree of LHC. These findings suggest a plastic coupling of these two anti-fish defences in Daphnia (LHC and DVM), which allows for an adjustment to fluctuating environments of standing freshwater ecosystems. [ABSTRACT FROM AUTHOR]

Predators have been widely demonstrated to influence developmental plasticity in aquatic prey. We investigated the body size of the amphipod Hyalella azteca raised with predatory sunfish by exposing solitary or groups of three amphipods to sunfish kairomones or a control for 30 days. Field studies indicate that small body size in H. azteca is likely an adaptive response to size-selective sunfish predation. Grouped amphipods chronically exposed to the kairomones had significantly smaller body lengths than grouped amphipods exposed to controls, while individual amphipods exhibited no significant differences. Behavioral trials revealed that grouped amphipods sought refuge less than solitary amphipods. To examine if a conspecific chemical cue was related to the reduction in body size of the grouped amphipods exposed to kairomones, individual amphipods were exposed to a combination of conspecific chemical cues and kairomones or to controls for 30 days. Amphipods exposed to kairomones in combination with conspecific cues had significantly smaller body length than amphipods exposed to conspecific cues alone. These results mirrored the group versus individual exposure experiment, suggesting that a conspecific chemical cue is required to induce an anti-predator response. The present study highlights the importance of conspecific cues for plastic responses to predators. [ABSTRACT FROM AUTHOR]

The micro-crustacean Daphnia pulex is a model species for studying predator-induced defenses. When exposed to chemical cues released by its predator, the phantom midge larvae Chaoborus (Diptera), it develops protective neckteeth that reduce the predator's success of predation in the juvenile instars. Defensive traits need to be expressed as soon as possible, which requires an early sensitivity to predator cues. We investigated the exact kairomone-sensitive period in three D. pulex strains and the timeline of neckteeth expression in early juvenile instars. We divided embryonic development into five major stages based on successive morphological landmarks. We exposed animals in these developmental stages to kairomones in order to determine the sensitive periods for neckteeth expression in the 1st and 2nd juvenile instar. Our results indicate that kairomone sensitivity starts during embryogenesis when compound eye spots begin to fuse and egg membranes are shed. Neckteeth develop with a stage-dependent time lag, being shorter when exposed in the first kairomone-sensitive stage and longer when exposed in the following developmental stages. Evolution of early kairomone sensitivity and fast defense development is a crucial step in D. pulex's defenses against Chaoborus as it allows for protection of the most vulnerable juvenile stages. [ABSTRACT FROM AUTHOR]

Water hyacinth ( Eichhornia crassipes) is one of the most problematic invasive macrophyte in the world. Although it has become a key element of the systems where it was introduced several decades ago, there is insufficient information to determine its role on biological interactions. To elucidate water hyacinth-littoral zooplankton-planktivorous fish interactions, we conducted habitat choice experiments in the absence and presence of fish chemical cues (kairomones) for two tropical littoral cladocerans, Chydorus brevilabris and Simocephalus vetulus. We classified habitat selection as a habitat (preference for the plant without predation risk) and/or as a refuge (preference for the plant with predation risk). Our results showed that E. crassipes is used as a habitat by C. brevilabris and as a refuge by S. vetulus against fish predation. Although C. brevilabris did not use the water hyacinth as a refuge in the presence of kairomones, the results suggested that both C. brevilabris and S. vetulus exhibited behavioral responses that further reduced their predation risk. To our knowledge, this study is the first experimental evidence for littoral cladocerans using free-floating macrophytes as a refuge against fish predation in tropical systems. Our results support the assertion that habitat association is not necessarily an evidence of habitat preference. [ABSTRACT FROM AUTHOR]

Maternal effects on the induction of defensive morphology are regarded as an adaptive strategy. For instance, a female can transmit environmental conditions related to predation risk to offspring of the same generation or succeeding generations. However, studies have presented limited information regarding such an adaptive maternal effect on monogonont rotifers. In the present study, the maternal effects on the spine development of rotifers were evaluated using Brachionus calyciflorus- Asplanchna models. Asplanchna can release soluble kairomones into the environments, thereby inducing B. calyciflorus to form defensive morphs (e.g., elongated posterolateral spines). Our empirical data supported the hypothesis that an amictic Brachionus-experienced Asplanchna kairomones could produce offspring with longer posterolateral spine in the current or succeeding generation than those of a non-experienced amictic female. On the basis of our data, we speculated that the maternal effect of B. calyciflorus may involve the transfer and dilution of inducers (e.g., hormone) within and between generations. On one hand, our results reinforced the within-generation maternal effects on induction of defensive morphology of rotifers; on the other hand, our results enhanced transgenerational inducible defense to other planktonic taxa that have been only observed in cladocera. [ABSTRACT FROM AUTHOR]

Since cladocerans from the genus Daphnia are known to have evolved several inducible defenses (morphological and life history shifts) against the notostracan predator Triops, we investigated whether hatching was also altered in response to Triops. We tested whether dormant eggs of Daphnia magna are able to detect Triops cancriformis kairomones in the water as a signal of predation pressure and alter their hatching response accordingly to avoid predation. We predicted that, in the presence of Triops kairomones, hatching fractions might be reduced (postponing hatching to a next growing season) and/or that hatching might peak earlier (increasing chances to reproduce before Triops becomes predatory). We also tested whether this response depended on the origin of the population. Ephippia from three D. magna populations, originating from one permanent lake and two temporary pond systems, were exposed to Triops kairomone and control treatments. We observed significant population differences in hatching patterns, both in terms of the fraction of eggs that hatch as well as the timing of hatching, with evidence for within-season bet-hedging through delayed hatching in the populations inhabiting temporary habitats. However, no indication was found that the populations also adjust their hatching pattern to the presence of Triops kairomones. [ABSTRACT FROM AUTHOR]

The prevalence of the diapause response of the freshwater crustacean Daphnia magna to chemical cues on fish predation was evaluated in 35 clones originating from 14 European water bodies-inhabited or not inhabited by fish. Clonal lineages of experimental animals were cultured for 4 weeks in the presence or absence of water to which an extract of faeces of crucian carps ( Carassius carassius), that were fed with Daphnia, was added. The proportion of females producing diapausing forms during the experimental period was used as a measure of the diapause response. A positive diapause response to fish predation cues was observed in 43% of investigated clones that originated from 71% of the water bodies tested. This indicated that the diapause response to fish scent is a common phenomenon in Daphnia magna from various locations in Europe. Surprisingly, no signs of a stronger diapause reaction to the tested cues were found in Daphnia originating from water bodies inhabited by fish compared with those from fish-free habitats. [ABSTRACT FROM AUTHOR]

Dormant propagule pools (egg banks) of zooplankton populations that accumulate in sediments provide biological archives of past conditions and enable the investigation of evolutionary changes in populations over relatively long periods of time (many decades). This study examined the egg bank of a Daphnia pulicaria population in a lake that has been stocked annually with rainbow trout (a zooplanktivore) since 1961. Resting eggs from sediments from the 1920s to 2001 (determined by Pb dating) were hatched and established as isofemale clonal lines. The phototactic behavior (a proxy for vertical migration behavior) of clones was assessed in the presence and absence of fish kairomones to evaluate the hypothesis that the heavy and consistent level of predation imposed by the stocked trout would have selected for clones that are more negatively phototactic. In addition, exposure to kairomones was expected to induce stronger negative phototaxis for all clones relative to control conditions. The behavior of clones derived from resting eggs from the trout era (after 1961) was significantly more negatively phototactic than it was for pre-trout era clones. Kairomone exposure induced a more negative phototactic response in clones from both eras, but the response was much greater for the pre-trout era clones. These results suggest that the consistently high level of predation by trout over several decades has selected for a conservative (less plastic) vertical migration strategy in which Daphnia maintain a deep daytime distribution with or without the presence of chemical cues from fish predators. [ABSTRACT FROM AUTHOR]

A Patagonian strain of Keratella tropica exhibits extensive non-genetic variation in spine development and body size. This study experimentally investigates the effect of temperature (10, 15, 20, and 25°C), both alone and with Daphnia and Asplanchna kairomones, on spine and lorica length. Temperature by itself had a major effect on right and left posterior spines and the lorica; these increased in length as temperature decreased, most notably from 20 to 15°C. At these two temperatures and a high food level (1 × 10 cells Cryptomonas erosa ml), lengths were, respectively, ca. 24 and 38 μm for the right posterior spine, 19 and 32 μm for the left posterior spine, and 116 and 126 μm for the lorica. A lower food concentration (2 × 10 cells C. erosa ml) at these two temperatures had little effect; the left posterior spine and lorica were slightly but significantly smaller. The growth-promoting effects of low temperature and kairomones generally were additive. Daphnia induced a moderate increase in the length of both posterior spines and lorica; these increases were significantly greater at 15 than 20°C. Asplanchna induced a great elongation of the right posterior spine, a reduction or loss of the left posterior spine, and moderate elongation of the outer two pairs of anterior spines. The right posterior spine and anterior spines were of similar length at 15 and 20°C, but the left spine and lorica were significantly longer at 15°C. Temperature and kairomones from Asplanchna and some crustaceans control the expression of a great variety of phenotypes and probably are largely responsible for seasonal changes in the spine development of this rotifer in natural communities. [ABSTRACT FROM AUTHOR]

We quantified the indirect effects of different densities (1, 4, 16 individuals per jar) of the predator Asplanchna brightwellii on the morphometry and demography of their prey Brachionus calyciflorus and Plationus macracanthus. Population growth and life table demography experiments were separately conducted for each of the two prey rotifer species while keeping A. brightwellii in indirect contact. The brachionids were fed the green alga Chlorella vulgaris at a density of 1 × 10 cells ml. As compared to those cultured in the absence of the predator, in the presence of A. brightwellii the postero-lateral spines of P. macracanthus increased by about 15 μm, while in B. calyciflorus the increase was more (>80 μm). For both the brachionid species, the peak population density significantly decreased in the presence of A. brightwellii. The reproductive variables viz., net reproductive rate, generation time, and the rate of population increase of B. calyciflorus and P. macracanthus were negatively affected in the presence of kairomones from A. brightwellii. [ABSTRACT FROM AUTHOR]

We studied the attachment strength and aggregation behaviour of Dreissena polymorpha in the presence of large roach Rutilus rutilus (>180 mm total length) (efficient molluscivore), small roach (<110 mm) (unable to feed on zebra mussels) and perch Perca fluviatilis (not feeding on mussels). The intention was to check whether small (<10 mm) and large (>10 mm) mussels would respond specifically to fish capable of consuming them (i.e. large roach). After 1 day of exposure, we found no significant differences in mussel attachment strength. After 6 days in the presence of large roach, mussels were attached more strongly than in the other treatments. After a 1-day exposure to all kinds of fish, mussels were more aggregated than in the control treatment. After 6 days, the largest percentage of aggregated mussels was found in the presence of large roach, while the aggregation levels in the other treatments were lower and did not differ from one another. Perhaps, an initial response was a non-specific reaction to the presence of any fish, while a specific response to large roach appeared later. Thus, zebra mussels were able to recognize their potential predators. The observed behaviour of mussels may enhance their resistance to molluscivores in the field by limiting the access of predators to their potential prey (due to the increased aggregation of prey) and by increasing predator handling costs (due to the stronger attachment of prey). [ABSTRACT FROM AUTHOR]

Previous studies confirmed the presence of melatonin in Daphnia magna and demonstrated diurnal fluctuations in its concentration. It is also known that in several invertebrate species, melatonin affects locomotor activity. We tested the hypothesis that this hormone is involved in the regulation of Daphnia diel vertical migration (DVM) behaviour that is well recognized as the adaptive response to predation threat. Using ‘plankton organs’, we studied the effect of three concentrations of exogenous melatonin (10−5, 10−7, 10−9 M) on DVM of both female and male D. magna in the presence or absence of chemical cue (kairomone) of planktivorous fish. Depth distribution was measured six times a day, using infrared-sensitive closed circuit television cameras. Our results showed a significant effect of melatonin on the mean depth of experimental populations, both males and females, but only when melatonin was combined with fish kairomone. Females stayed, on average, closer to the surface than males, both responding to the presence of kairomone by descending to deeper strata. In the presence of exogenous melatonin and with the threat of predation, Daphnia stayed closer to the surface and their distribution was more variable than that of individuals, which were exposed to the kairomone alone. Approaching the surface in the presence of predation threat seems to be maladaptive. We postulate the role of melatonin as a stress signal inhibitor in molecular pathways of response to predation threat in Cladocera. [ABSTRACT FROM AUTHOR]

Life-history theory predicts adaptive shifts in response to size-selective predation, namely earlier reproduction, smaller age/size at maturity, and higher relative investment into reproduction. Such shifts should bring about reduced lifespan of potential prey. We tested this prediction in life-table experiments with clones of Daphnia hyalina and Diaphanosoma brachyurum, two species of contrasting anti-predatory strategies. The clones were derived from seven lakes of different trophy and held in water with and without fish kairomone, under standard laboratory conditions. Exposure to the kairomone caused a decrease in age of first reproduction and an increase in early-life reproductive effort but also an about 20% decrease of longevity in both species. Although shortened lifespan did not result in significant decrease in fitness of the tested species (in terms of lifetime reproductive output) it should be taken into account in considerations of costs and benefits of inducible defenses in cladocerans. [ABSTRACT FROM AUTHOR]

The life history of Daphnia exposed to fish kairomone at different developmental stages was examined in a laboratory experiment. The strongest life history response to the applied predation threat was observed in females exposed during the 4th instar. Compared to Daphnia experiencing the presence of fish at earlier or later instars, these individuals reached maturity at a smaller size and released fewer neonates. Moreover, their offspring also demonstrated the strongest reaction to predation threat, exhibiting the broadest phenotypic plasticity in the life history response to predation. The breadth of their reaction norm was, on average, two times larger comparing with individuals from other treatments. Broader phenotypic plasticity may offer clear selective advantages under the unpredictable predation regime. This finding highlights the adaptive role of maternal effect in shaping life history of cladocerans. [ABSTRACT FROM AUTHOR]

It has been hypothesized that the production of diapausing eggs in Daphnia can be induced by fish kairomones. A population of Daphnia could survive severe predation using this predator avoidance strategy. However, in changing environments, diapausing eggs experience various temperature conditions, and hatchlings at emergence may be exposed to the same predation risks as their mothers. Therefore, staying in diapause or an immediate response upon hatching to available environmental information could be important for hatchling survival. For this study, we investigated the impact of water temperature (10, 15, 20, and 25°C) in the presence and absence of fish kairomones ( Lepomis macrochirus) on the hatching success of resting eggs ( D. galeata). Results show that no diapausing eggs hatched at the lowest temperature (10°C), and the highest hatch percentage occurred at 15°C. Although higher water temperatures reduced hatching success, diapausing eggs hatched more quickly. The number of hatchlings was significantly higher after exposure to fish kairomones, and this was more noticeable at higher temperatures (20 and 25°C). The present results suggest that the diapausing eggs were produced as a predator avoidance strategy in Daphnia; however, the presence of fish works as a positive signal to increase hatchlings when the diapausing stage is terminated. [ABSTRACT FROM AUTHOR]

In the present study, the effect of chemical cues from two fish species (mosquitofish and pumpkinseed), at different concentrations, was tested in life history experiments with Daphnia longispina. The two fish species used represent the most abundant planktivores of many Mediterranean shallow lakes (SW Europe), where the indigenous fish communities have been replaced by such exotic assemblages. Results have shown a similar response of D. longispina to both fish species: kairomones stimulated daphnids to produce more offspring, which resulted in higher fitness ( r), relatively to a fishless control. Fish presence also induced an earlier first reproduction, a smaller size at maturity of daphnids, and the production of smaller-sized neonates. Significant correlations with fish concentration (indirect measure of fish kairomone concentration) were found for size at maturity and neonate size, for both fish species. These results are in accordance to the “positive response” observed by other authors, which represents a defence mechanism to face losses caused by fish predators. The chemically mediated size reduction of mature females and neonates is an adaptive response to the size-selective predation exerted by fish. Pumpkinseed introduction is very recent in the lake of origin of the daphnids used in the experiments and its kairomone produced similar effects to mosquitofish in the life history of D. longispina. These results are contrary to the existence of a species-specific kairomone and support the hypothesis of a general fish kairomone. [ABSTRACT FROM AUTHOR]

In shallow temperate lakes, zooplankton populations may exhibit diel horizontal migration (DHM) and move towards macrophytes during the day to avoid fish. Using a natural Daphnia magna population, we undertook an experimental investigation aimed to describe the genetic variation for DHM and to study whether an adaptive micro-evolutionary response occurred to changes in macrophyte coverage and fish predation pressure through time. Twenty-seven D. magna clones were hatched from ephippia in the sediment of shallow Lake Ring, Denmark. This lake was eutrophied during the 20th century and was subject to restoration measures in the 1970s. The DHM behaviour of the clones was observed both in the presence and absence of fish kairomone. Significant interclonal variation in DHM behaviour occurred in both treatments. To study the micro-evolutionary response of the Lake Ring D. magna population, two approaches were used. First, we compared the DHM behaviour of clones derived from ephippia collected at different depths. A comparison was conducted between clones resurrected from the period of eutrophication (1960–1980) and from the period of recovery (1986–2000). A significant treatment (presence and absence of fish kairomone) × period interaction effect was identified, suggesting a significant micro-evolutionary response for DHM behaviour. The D. magna clones exhibited a significantly stronger horizontal migration response during the period of eutrophication than in the recovery phase. Second, clonal means, representing the influence of the genotype on the trait, were correlated with environmental conditions (macrophyte cover, fish predation pressure and Secchi depth). The results of this analysis also suggest that a micro-evolutionary response by Daphnia has occurred in reaction to changes in fish predation pressure. In periods with high fish predation pressure, Daphnia migrated more strongly towards the plants. [ABSTRACT FROM AUTHOR]

The combined effect of temperature, food level and the presence of an invertebrate predator on the body size of the rotifer Brachionus havanaensis were tested in this study. B. havanaensis was cultured at 15, 20, and 25°C under three different Chlorella vulgaris levels (0.5 × 106, 1.0 × 106 and 2.0 × 106 cells ml−1) in the presence and in the absence of Asplanchna girodi. For each treatment we maintained three replicates and constant (0.4 ind ml−1) population density of B. havanaensis. In treatments containing A. girodi, the predator was separated from the prey by a mesh (pore size 50 μm). On the last day of the experiment, a portion of the B. havanaensis population was sampled for several morphometric measurements (adult lorica length, width, posterior spine length, body volume, and the egg volume). Size measurements were done by drawing the specimens using a calibrated camera lucida. Statistically significant impact of temperature as well as the predator’s presence was observed on the lorica length, posterior spine, and egg volume of B. havanaensis. The interactions of food × temperature, or predator´s presence × food × temperature were non-significant ( P > 0.05) for lorica length, spine length, body volume, and egg volume. Regardless of the type of treatment, there was a direct positive correlation between lorica length and width. Egg volume was linearly related to the adult size. Notably long posterior spines were observed in treatments containing the presence of A. girodi. [ABSTRACT FROM AUTHOR]

Theoretical studies have predicted that inducible defenses affect food chain dynamics and persistence. Here we review and evaluate laboratory experiments that tested hypotheses developed from these theoretical studies. This review specifically focuses on the effects of inducible defenses in phytoplankton-rotifer food chain dynamics. First, we describe the occurrence of colony formation within different strains of green algae (Scenedesmaceae) in response to infochemicals released during grazing by the herbivorous rotifer Brachionus calyciflorus. Then we examined the effects of inducible defenses on the population dynamics of this planktonic system in which algal strains that differed in their defense strategies were used. Simple food chains were composed of green algae (Scenedesmaceae), herbivorous rotifers ( Brachionus calyciflorus) and carnivorous rotifers ( Asplanchna brightwellii). In this system B. calyciflorus exhibits an inducible defense against predation by developing long postero-lateral spines. Experimental studies showed that inducible defenses, as opposed to their absence, could prevent high-amplitude population fluctuations. We discuss the dual effects of induced defenses on extinction probabilities and consider the fit of a theoretical model to experimental data to understand the mechanisms that underlie the observed dynamics. [ABSTRACT FROM AUTHOR]

Exposing Daphnia to the presence of a predator at different stages of ontogenesis leads to different reactions. Predator-induced changes of life history parameters related to the first reproduction schedule, as age and size at first reproduction, take place in the stage preceding that one in which vitellogenesis occurs. Determination of the amount of energy allocated to this first reproduction, including offspring number, occurs in the first 24-h of life. In case of parameters related to reproduction schedule, time since induction until the release of the first clutch of offspring is tens of hours; in case of allocation patterns – more than 100. This suggests that Daphnia do not “remember” the information of a potential threat but, instead, the decisions on future development are undertaken once they have estimated the reigning predation regime. Shortening of the period during which eggs are carried in the brood chamber, thus shortening the time when Daphnia are more easily visible, may be another adaptation to reduce danger from fish predation and so increasing the probability of survival until the first successful reproduction. [ABSTRACT FROM AUTHOR]

Large-sized Daphnia hyalina × galeata hybrid and small-sized Ceriodaphnia reticulata were raised together for 3 weeks in thermally stratified flow-through tubes (‘plankton organ’), in the presence and the absence of fish kairomone, and their relative performance was monitored, as indicated by the populations' growth rates (r). In the kairomone-free medium, D. hyalina grew faster than C. reticulata; the presence of kairomone reversed this ratio, producing relatively higher r in C. reticulata. The simulated fish presence reduced growth in D. hyalina population also when it was separated from the competitor, while growth in C. reticulata was altered by the kairomone only in the presence of Daphnia. It is suggested that costs of surface avoidance in Daphnia can reduce its competitive superiority to small-sized cladocerans, permanently dwelling at the surface. [ABSTRACT FROM AUTHOR]

We tested the reciprocal effects of water conditioned by exudates of Aeolosoma sp. (Aphanoneura, Oligochaeta) and fish (Skiffia lermae: Goodeidae) and by the presence of Aeolosoma sp. on the population dynamics of four species of anomopod cladocerans (C. cornuta, C. dubia, S. serrulatus and S. vetulus) at 25 °C for 26 days using Chlorella vulgaris (1 × 106 cells ml-1) as a basic food for all. We found that, regardless of treatment, C. cornuta and C. dubia had a long initial phase of slow growth, followed by a rapid increase after the second week and that, regardless of treatment, C. dubia had a higher rate of population increase per day than C. cornuta. S. vetulus had a longer lag phase than S. serrulatus. Fish-conditioned water resulted in growth inhibition in S. serrulatus, but had no effect on S. vetulus, while Aeolosoma-conditioned water inhibited growth of S. vetulus but not that of S. serrulatus. Our results suggest a scale of responses of cladocerans to exposure to fish- and worm-conditioned water and to the presence of worms, ranging from a mild stimulation to no effects to inhibited growth. Kairomone-effects on body size of C. cornuta were not significant, but beak length was strongly influenced, and more by fish than by worm kairomones. In S. vetulus, differences in size were not significant. However, individuals grown in the presence of worms had higher biomass while those grown in the presence of fish and worm kairomones showed a decreased weight. Effects of worm exudates and of their live biomass on cladocerans were thus opposite in Simocephalus. In Ceriodaphnia dubia, in contrast, they were additive. Aeolosoma was depressed by all four cladocerans, although worms tried to penetrate the valves of the cladocerans to feed on the tissue inside. In doing so, they suffered significant casualties, especially from blows of the powerful post-abdomen of Simocephalus. The nature of the cladoceran-worm interaction is therefore far from simple: in addition to the ‘chemical communication’ that is present, it has a predatory component (worms trying to feed on cladocerans) but the reverse might also be true (cladocerans filtering out tissues of disrupted Aeolosoma). Since both worms and cladocerans feed on algae, exploitative competition also seems to be involved. In this, the worms appear to be the inferior partner, although none of the experiments lasted long enough to drive any of the competing partners to extinction. [ABSTRACT FROM AUTHOR]

We quantified genetic variation for ecological relevant traits in the presence and the absence of fish chemicals of eleven Daphnia ambigua clones that were isolated from six interconnected ponds that differed in water transparency. In a cohort life table experiment, we tested whether genetic variation for a set of key life history traits was present among these clones. In addition the phototactic behaviour of these clones cultured in the presence and the absence of fish kairomones was quantified using a biotest. We detected a significant effect of fish kairomones on the phototactic behaviour and a highly significant genetic variation among clones for this trait in D. ambigua clones isolated from ponds in De Maten. Differences in size at maturity among D. ambigua clones in De Maten were highly significant, whereas differences in spina length among D. ambigua clones in De Maten were not significant. The presence of fish chemicals did not affect the studied life-history traits. We observed a significant positive relationship between average phototactic behaviour for each population and size at maturity both in the presence as in the absence of fish kairomones. Most of the genetic differences could be attributed to a clone isolated from one clearwater pond that is not directly connected to the remainder of the pond complex. [ABSTRACT FROM AUTHOR]

Life history traits of Daphnia galeata clones were affected differently by kairomones exuded by two species of fish. The exposure to Gasterosteus kairomone resulted in an increase in population intrinsic rate of increase and body growth rate relative to the exposure to Perca kairomone. These differences between fish treatments have not been reported earlier. The results suggest differences between fish kairomones that are not due to concentration effects alone. This raises the possibility that kairomones are cocktails of chemicals rather than a single `fish substance'. It also supports the view the effects of size-selective predation vary depending on the local predator–prey assemblage examined. This finding leads to an additional, previously underestimated but potentially complex aspect in zooplankton ecology. [ABSTRACT FROM AUTHOR]

Semiochemicals play an important role in interactions between living organisms in aquatic environments. Although the presence of chemical cues is confirmed in more and more systems, the chemical structures remain predominantly elusive. To create more accurate prey–predator interaction models and to advance the research on chemical communication, it is essential to identify these compounds. A literature overview of cues involving Daphnia (either as producer or receiver) is given and the progress towards their isolation and structure elucidation is described. Most of the research so far has concentrated on the elucidation of kairomones produced by predators of Daphnia (especially Chaoborus and several species of fish). Although some progress has been made, these cues have not been isolated and identified yet. Additionally new results on the isolation and identification of the kairomone responsible for the colony formation in Scenedesmus using differential diagnosis and bioassay-directed fractionation of Daphnia exudates are presented. The importance of suitable and well performing bioassays herein cannot be underestimated. Some preliminary results with solid-phase extraction with C18 proved to be reproducible for extracting the active compound from Daphnia water, although it was not possible to get the biological activity into a single fraction. The cue was not extractable with an anion exchanger (SAX). Subjecting the extract to HPLC led to one active fraction. [ABSTRACT FROM AUTHOR]

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 Daphnia or 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. [ABSTRACT FROM AUTHOR]

Examines the effects of water containing putative fish kairomones, chemicals released by fish predators, on the ingestion and filtration rates of Ceriodaphnia cf. dubia. Concentration level of fish kairomones and fish water culture; Methods use to measure the average filtration and ingestion rates; Reason behind the variations in the sensitivity of Ceriodaphnia cf. dubia to chemicals.