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

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

Chaoborus crystallinus predation on Daphnia pulex: can induced morphological changes balance effects of body size on vulnerability?

Juvenile Daphnia pulex form neckteeth in reponse to chemicals released by predatory Chaoborus crystallinus larvae. Formation of neckteeth is strongest in the second instar followed by the third instar, whereas only small neckteeth are found in the first and fourth instar of experimental clones. Predation experiments showed that body-size-dependent vulnerability of animals without neckteeth to fourth instar C. crystallinus larvae matched the pattern of neckteeth formation over the four juvenile instars. Predation experiments on D. pulex of the same clone with neckteeth showed that vulnerability to C. crystallinus predation is reduced, and that the induced protection is correlated with the degree of neckteeth formation. The pattern of neckteeth formation in successive instars is probably adaptive, and it can be concluded that neckteeth are formed to different degrees in successive instars as an evolutionary compromise to balance prediation risk and protective costs.     

Genetic differentiation in life history between Daphnia galeata populations: an adaptation to local predation regimes?

Species of the water flea Daphnia exhibit constitutive as wellas phenotypically inducible anti-predator defence strategies,involving life history, morphological and behavioural traits.We explored the hypothesis of genetic differentiation in anti-predatordefence strategies using Daphnia clones originating from twodifferent water bodies: Tjeukemeer (the Netherlands) and FishPond (Belgium). Both water bodies are inhabited by zooplanktivorousfish. In contrast to Tjeukemeer, Fish Pond is also inhabitedby larvae of the phantom midge Chaoborus. The life history responsesof the two sets of clones to kairomones from fish (Perca), tokairomones from Chaoborus, and to a mixture of both were compared.Clones from Tjeukemeer and Fish Pond showed strong responsesto the presence of fish kairomone, with reductions in adultand neonate body length, in age at first reproduction and inthe total number of neonates produced during the first threeadult instars. Responses to Chaoborus kairomone were much lesspronounced, although there was a tendency towards an increasein the number of neonates in the first brood. Significant inter-populationgenetic differences were found for all the investigated traits.However, there was no indication for genetic adaptation of theFish Pond clones to negative size-selective predation by Chaoborus.Compared to Tjeukemeer clones, Fish Pond clones had a lowersize at first reproduction, produced smaller neonates and produceda higher number of neonates in their first brood. This suggestsadaptation to positive rather than to negative size selectivepredation. Genetic differences between populations were observedmainly for constitutive life history traits, rather than forphenotypic shifts in response to the presence of predator kairomones.     

Swimming behaviour of Daphnia clones: differentiation through predator infochemicals

We studied variation in small-scale swimming behavior (SSB)in four clones of Daphnia galeata (water flea) in response topredator infochemicals. The aim of this study was 3-fold. First,we tested for differences in SSB in Daphnia; second, we examinedthe potential of differences in SSB to explain survival probabilityunder predation; and third we tested the effect of differencesin SSB on survival under predation. Four treatments were applied:one kairomone-free control, one Chaoborus- (phantom midge) andone Perca (perch)-conditioned treatment, and a mixed treatmentcontaining both infochemicals. All of the three tested behavioralparameters (swimming speed, trajectory length and vertical distribution)were affected by the presence of the Chaoborus infochemical,and swimming speed and vertical distribution were also affectedby the presence of Perca infochemical. The effect of the treatmentwas interfered with by a clone effect: genetic differences werepronounced in all traits. These results illustrate that clonescan be responsive in only a subset of traits. The general theorythat clones are either responsive or non-responsive is not validfor SSB. The outcome of the predation trial confirms that adecrease in activity is a main factor in lowering Daphnia vulnerabilityto Chaoborus predation.     

Salamander predation and vertical distributions of zooplankton

SUMMARY. 1. The impact of different predation regimes of zooplanktivorous larval tiger salamanders, Ambystoma tigrinum nebulosum , on the vertical temporal distributions of Daphnia pulex, Diaptomus nudus and larval Chaoborus flavicans was studied in limnetic enclosures in a pond in east-central Arizona. Zooplankton densities were monitored 1-2 h before sunset, at sunset, and 1-2 h after sunset at three depths (0-0.4, 1.0 and 2.0 m) in four salamander treatments (0, 20, 40 salamanders per enclosure, and unenclosed limnetic areas).
2. Densities and body size of Daphnia pulex were consistently greater in salamander-free enclosures than in other treatments. More Daphnia were generally nearer the surface (0-1.0 m) than along the bottom before and at sunset than after sunset in enclosures without salamanders. After sunset, Daphnia was by far most dense along the bottom. Densities were uniform among depths and sample times in treatments with salamanders. Salamanders moved throughout the water column. Frequencies of D. pulex spined morphs (with prominent dorsal crest) did not differ among salamander treatments.
3. Chaoborus densities were positively correlated with D. pulex distributions among depths and sample times in treatments. Spatio-temporal distributions of Chaoborus were influenced more by their daphnid prey than by risk of predation from larval salamanders.
4. Densities of Diaptomus nudus were highest in open water controls and lowest in enclosures without salamanders. Diaptomus was generally more dense between 0 and 1 m than at 2.0 m depths in all treatments, and vertical distributions were not dependent on salamander density.     

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.     

On Longevity of Adult Chaoborids (Diptera,Chaoboridae) under Sugar Feeding Conditions

Entomological Review - Longevity of the adult male and female Mochlonyx velutinus (Ruthe) and the adult male Chaoborus crystallinus (De Geer) that emerged under laboratory conditions was measured,...     

Environmental correlates of the intrinsic rate of natural increase in primates

Morphological and life history traits of two clones of the cladoceran Daphnia pulex were measured in the presence and absence of size-selective insect predators, the midge larva Chaoborus flavicans, which preys on small Daphnia, and the water bug Notonecta glauca, which preys on large Daphnia. The aim was to detect predator-induced phenotypic changes, particularly the effect of simultaneous exposure to both types of predators. Other work has shown that in the presence of Chaoborus americanus, Daphnia pulex produce a socalled neck spine which may carry several teeth. The morphological modifications are supposed to serve as an anti-predator device. Furthermore, females exposed to Chaoborus often delay their maturation; this has been interpreted as a cost that balances the benefits of the neck teeth. In this investigation, females of both clones produced fewer but larger offspring than control animals when reared in the presence of Chaoborus flavicans. The offspring showed the typical neck spine and delayed first reproduction. In the presence of Notonecta glauca, one of the clones produced more and smaller offspring, and maturation occurred at earlier instars. The other clone also produced more offspring than the control but there was no size difference. When both predators were present, in most cases the reactions of the daphnids were similar to those in the Notonecta experiment. The response to Chaoborus appeared to be suppressed. The observed modifications are interpreted as evolved strategies that reduce the impact of size-selective predation. They are consistent with predictions of life-history theory.     

Instar-specific mortalities of coexisting Daphnia species in relation to food and invertebrate predation

Instar-specific mortalities of Daphnia hyalina and D.cucullatawere studied from May 19 to September 29, 1988 in combinationwith invertebrate predator and phytoplankton abundance. Simultaneouslife-table experiments were conducted under semi-natural conditionsin the laboratory to estimate juvenile mortality in a predator-freeenvironment. Juvenile mortality by predation was calculatedas the difference between juvenile mortality in the field andin the experiments and was the most important factor for thedifferences in abundance of the two species. For D.hyalina juvenilemortality was higher in early summer and probably caused byselective predation by Chaoborus flavicans. Predation by Leptodorakindtii was probably more important during the rest of the summer.Estimated mortality by predation adequately explained juvenilemortality, except for a 3-week period in August. Decreasingflagellate densities in July were accompanied by increased juvenilemortalities of D.hyalina and D.cucullata in the life-table experimentsin August and coincided with a Daphnia population decrease.     

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

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

Chaoborus and gasterosteus anti-predator responses in Daphnia pulex are mediated by independent cholinergic and gabaergic neuronal signals

Many prey species evolved inducible defense strategies that protect effectively against predation threats. Especially the crustacean Daphnia emerged as a model system for studying the ecology and evolution of inducible defenses. Daphnia pulex e.g. shows different phenotypic adaptations against vertebrate and invertebrate predators. In response to the invertebrate phantom midge larvae Chaoborus (Diptera) D. pulex develops defensive morphological defenses (neckteeth). Cues originating from predatory fish result in life history changes in which resources are allocated from somatic growth to reproduction. While there are hints that responses against Chaoborus cues are transmitted involving cholinergic neuronal pathways, nothing is known about the neurophysiology underlying the transmission of fish related cues. We investigated the neurophysiological basis underlying the activation of inducible defenses in D. pulex using induction assays with the invertebrate predator Chaoborus and the three-spined stickleback Gasterosteus aculeatus. Predator-specific cues were combined with neuro-effective substances that stimulated or inhibited the cholinergic and gabaergic nervous system. We show that cholinergic-dependent pathways are involved in the perception and transmission of Chaoborus cues, while GABA was not involved. Thus, the cholinergic nervous system independently mediates the development of morphological defenses in response to Chaoborus cues. In contrast, only the inhibitory effect of GABA significantly influence fish-induced life history changes, while the application of cholinergic stimulants had no effect in combination with fish related cues. Our results show that cholinergic stimulation mediates signal transmission of Chaoborus cues leading to morphological defenses. Fish cues, which are responsible for predator-specific life history adaptations involve gabaergic control. Our study shows that both pathways are independent and thus potentially allow for adjustment of responses to variable predation regimes.     

Components of additive variance in life-history traits of Daphnia hyalina: seasonal differences in the response to predator signals

We attempted to separate the additive variance of life-history parameters (size at first reproduction (SFR), proportion of total production allocated to reproduction) of Daphnia hyalina in a mesotrophic lake during different seasons into genetic components and phenotypic plasticity. Every month we randomly isolated juveniles of 20 clones from the lake and measured their life-history parameters immediately after isolation, i.e., under the influence of all modifying factors in the lake. The measurements were repeated after the clones had been kept in the laboratory for at least six generations without predator signals (controls) and with the addition of fish and Chaoborus kairomones. SFR increased monotonously from July to December in fresh field isolates. Laboratory controls showed always much larger SFR, but approached field value in December. Most of the size difference was due to maternal effects. Reproductive allocation showed a different pattern than SFR with a minimum in September both in field samples and laboratory controls. Fish kairomone reduced SFR in July and August when fish predation is high, but not later in the year. On the contrary, Chaoborus kairomone increased SFR in September and October when Chaoborus larvae are abundant in the lake, but not in summer. This indicates a seasonal shift in the clonal composition of D. hyalina populations towards clones adapted to the specific predation pressure. Reproductive allocation changed in response to fish kairomone in July and August, but not in response to Chaoborus kairomone. In this mesotrophic lake, fish have a stronger direct and indirect impact on Daphnia life history than Chaoborus larvae.     

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.     

Alternative antipredator responses of two coexisting Daphnia species to negative size selection by YOY perch

Our study showed for the first time in nature that two coexistingDaphnia adopted alternative life history and behavioural strategiesto cope with negative size-selection predation by gape-limitedyoung-of-the-year (YOY) perch. We evaluated the phenotypic plasticityin life history and behavioural traits of two coexisting Daphniaspecies, D. pulicaria (2 mm) and D. galeata mendotae (1.4 mm),in response to seasonal changes in predation by YOY yellow perch(Perca flavescens) in a mesotrophic lake. We expected that thelarge-sized D. pulicaria, the most likely subjected to size-selectivepredation by YOY perch, will show stronger antipredator responsesthan the small-sized D. galeata mendotae. To test this hypothesis,we examined changes in life history and behavioural traits injuveniles and adults of both species during four YOY fish predationperiods that were selected based on the presence of YOY perchin the pelagic zone and the relative abundance of Daphnia preyin their gut contents. Our study supports the scenario of negativesize-selective predation by gape-limited YOY perch on both Daphniaspecies. The electivity index indicated that no daphnids witha body length > 1.75 mm were predated by YOY yellow perch.Coexisting Daphnia exhibited phenotypic plasticity in theirantipredator defenses based on their vulnerability to seasonalchanges in size-selective predation of YOY perch. Juvenile Daphniawere the targeted prey and they responded by a decreased bodylength. Behavioural defenses were the dominant strategy usedby both adult Daphnia populations to withstand high predation.A decreased size at maturity was not employed by Daphnia, exceptat the very end of the predation period. Behavioural defensesare short-term strategy adopted to avoid predation. Both antipredatordefenses became unnecessary expenses and were no longer sustainedafter the predation period.     

Where to stay by night and day: Size-specific and seasonal differences in horizontal and vertical distribution of Chaoborus flavicans larvae

1. Data on the distributions of pelagic and benthic Chaoborus flavicans larvae were gathered in 1994 and tested for their agreement with the predator avoidance hypotheses. The development of all Chaoborus life stages, as well as the horizontal and vertical distribution in the four larval instars, was followed from May until October. We expected the largest larvae to dwell deeper by day, thus avoiding predation by visually foraging fish.
2. In agreement with this prediction body size increased with daytime depth, and this was true both between and within instars. The migration amplitude consequently increased with larval instar.
3. There was also evidence for horizontal migration, mainly in the third but also in the fourth instar.
4. Along a horizontal transect with increasing depth, locations with many benthic larvae had fewer pelagic larvae. Oxygen concentration was a good predictor of maximum benthic larval depth for most of the season but failed to predict their distribution in autumn.     

Size distribution of Daphnia longispina in the vertical profile

D. longispina of the meromictic lake El Tobar is a round-headed form. It never has a helmet, but in summer a small proportion of immature individuals (0.9–1.2 mm females and males) have one or two neck teeth. The size structure of this Daphnia population, as well as the vertical distribution and migration of different size-classes, were studied in September and November of 1991 and April of 1992. The large variation in mean size and size at first reproduction, as well as the occurrence of different patterns of vertical migration are interpreted as responses to different predator situations. At the end of April, when Daphnia mortality by visually hunting predators is dominating, a typical nocturnal migration is adopted and size distribution is biased to smaller size classes. In November, when mortality is mainly attributed to the nocturnally migrating Chaoborus, Daphnia shows a reversed migration pattern. In September, when the population of Daphnia is responding to both visual (fish) and non-visual predators (Chaoborus), it adopts a pattern of twilight migration. The presence of neck teeth in vulnerable size classes in September might be an additional adaptative response to Chaoborus predation. In September, the size structure of the Daphnia population is shifted to larger classes and the vertical distribution of size classes shows a pronounced segregation between juveniles and adults. Juveniles are found closer to the surface, while adults dwell predominantly in the rich, deep waters near the oxicline. This suggests that an additional advantage of the ascent of the adult Daphnia exploiting those deep resources is the release of young in more oxygenated and warmer waters. The Daphnia population of lake El Tobar is known to be clonally diverse, and the changing frequency of genotypes could play an important part in the observed seasonal differences in behaviour and size.     

Antipredator reaction norms for life history traits in Daphnia pulex: dependence on temperature and food

Synergistic effects between temperature and food level on the vulnerability of Daphnia life histories to predation have previously received little attention, despite their potential significance for summer population dynamics. In this investigation, most traits in the early life history of Daphnia pulex altered in response to increasing concentrations of fish kairomone. Although there was some variation attributable to experimental temperature and food treatments, traits exhibited reaction norms which suggested at least some degree of kairomone dose-dependence. Temperature, food level and their interaction affected the mean values of every trait examined and in some cases also influenced the antipredator response via a three-way interaction with fish kairomone. The insertion of an extra juvenile instar resulted in later maturation at a larger size for most females raised under conditions of low temperature and low food, but this response was largely suppressed in the presence of fish kairomone. Earlier maturation due to deletion of an instar was observed most frequently under high food and temperature conditions with fish kairomone present, suggesting an effect of kairomone on the size threshold for reproductive development. Principal components analysis was used to produce an index reflecting the net potential vulnerability to fish predation of the suite of life history traits. Vulnerability generally declined with increasing kairomone level as a result of the apparently adaptive alterations in most life history traits. Raised temperature and food level also generally reduced potential vulnerability, but a highly significant interaction between these factors was also found. Potentially important implications of these results for optimal vertical migration and summer population dynamics are discussed.     

Effects of competitors and Chaoborus predation on the cladocerans of a eutrophic lake: an enclosure study

The role of large laboratory grown food competitors ofthe genus Daphnia as well as the predationimpact of Chaoborus on the cladoceran communityof an eutrophic lake was assessed in five insitu enclosure experiments. The hypothesis tested wasthat the outcome of competition and gape-limitedpredation on cladocerans is size dependent. Accordingto the generally accepted assumptions on competitionand invertebrate predation, large-bodied cladocerantaxa were expected to be less affected by competingcongeners and by Chaoborus than weresmall-bodied taxa. Effects of the predator upon anassemblage of differently sized cladoceran taxa weremuch more pronounced than effects of competition.There was a tendency of predation and competitionimpact to decrease with cladoceran size, but predationpressure was also low for some small cladocerans andhigh for some large cladocerans. The general trendswere further obscured by factors not or indirectlylinked to body size.     

Effect of food and larval density on survival and growth of early instar greyback canegrub, Dermolepida albohirtum (Waterhouse) (Coleoptera: Scarabaeidae)

Abstract First- and early second-instar larvae of Dermolepida albohirtum (Coleoptera: Scarabaeidae) survived best and gained most weight on living roots of lawn seedlings, sugarcane or Guinea grass ( Panicum maximum var. maximum ) when compared with decaying plant material or soil alone. Survival of first-instar D. albohirtum was not density-dependent, but survival of older larvae (late first-instar and early second-instar) was reduced at high larval density. There was no evidence for larval combat between first instars. First instars kept at high density gained less weight and were slower to develop into second instars compared with first instars kept at low density. When food was limited, survival at high larval densities was reduced. Survival of early instars was high in sugarcane fields, probably reflecting the availability of suitable food. We conclude that food type and supply are critical factors affecting the survival and development of early instar D. albohirtum .     

How do predator-induced changes affect prey vulnerability? Larvae of Chaoborus flavicans (Diptera: Chaoboridae) feeding on Daphnia pulex (Crustacea: Cladocera)

1. The relative food consumption and prey preferences of fourth-instar larvae of the phantom midge Chaoborus flavicans were investigated in laboratory experiments. Neonates and second instars of two clones of the cladoceran Daphnia pulex - an Arctic and an European clone - were offered to the predatory larvae. Each clone was divided into two groups: one group was exposed to the midge larvae during embryogenesis, whereas the other was not exposed to the predator. 2. While both exposed and unexposed neonates of the European clone had neck teeth, animals of the Arctic clone did not develop these spines. 3. A consistent rank order was observed in the vulnerability of the four prey types. Newborn daphnids from the Arctic clone were more vulnerable to predation than those from the European clone. Moreover, differences existed within each clone, regardless of neck teeth induction: Daphnia previously exposed to Chaoborus were less susceptible to the predators than unexposed prey. 4. The results show differences in the vulnerability of exposed and unexposed neonates that are not caused by the presence or absence of neck teeth. Perhaps the reduced vulnerability of exposed neonates was due to predator-avoidance behaviours.