Depth selection by Daphnia pulex in response to Chaoborus kairomone

1. The presence of kairomone from the predaceous larval dipteran Chaoborus americanus can result in juvenile Daphnia pulex positioning themselves higher in the water column. Chaoborus normally lives at greater depth during the day, so this behavioural response by Daphnia will reduce its encounter rate with the predator and enhance survival. 2. Behavioural observations of seven clones from a single population were made under four treatments (with and without Chaoborus kairomone prior to and/or during experiments). 3. All clones moved higher when exposed to kairomones during behavioural observations. Six moved higher when kairomone was added prior to the experiment (i.e. pre‐conditioned), while the other clone went lower, providing evidence for the presence of genetic variation in induced behaviour. When this clone was removed from the analysis, the evidence for genetic variation in induced response disappeared. 4. Juvenile Daphnia that were pre‐conditioned had a significantly greater response than those that had no previous exposure. Of the total shift in depth (comparing treatment and control means), 38% was due to prior exposure to the kairomone (‘preconditioning’), while 62% was due to exposure during the 2‐h experiments when the depth selection was assessed. When the effect of the one clone with a qualitatively different response was removed, these two figures became 45% and 55%, respectively. 5. The enhanced effect of prior exposure to the kairomone during development suggests that such exposure causes greater sensitivity to the kairomone.     

Progress towards characterization of a predator/prey kairomone: Daphnia pulex and Chaoborus americanus

Predatory larvae of the midge Chaoborus americanus induce an antipredator defense (neckteeth) in prey individuals of the cladoceran Daphnia pulex. The signal for presence of predator is a water-soluble chemical. We provide evidence that this kairomone originates in the intestinal tract of the predator. The active compound is an organic molecule of intermediate polarity which is heat stable and partially destroyed by acid and base digestion. It is stable to digestion by the general peptidase Pronase. Hydroxyl groups, but not primary amines, carbonyls or thiols are essential to activity. Low-pressure liquid chromatography on a reverse-phase silica gel (Amicon Matrex C-18) column suggests there may be more than one active component.     

Complex effects of a kairomone of Chaoborus and an insecticide on Daphnia pulex

Daphnia pulex were reared in Chaoborus-conditioned water containingthe insecticide carbaryl, and their life history parametersand morphologies were investigated. The insecticide inhibitedthe animals' growth and reproduction and delayed their maturationtime more intensely in the chaoborus-conditioned water thanin the control Chaoborus-free water, indicating that a kairomoneof Chaoborus made the Daphnia more sensitive to the insecticide.The Chaoborus conditioned water induced neckteeth formationof D.pulex in instars 1–2 and elongated the intermoultingperiod of juveniles. The moulting to the spined morphs and elongationin duration of juvenile stages seemed to increase the risk ofdamage from the insecticide. The potential population growthrate of D.pulex in treatments was estimated as a possible fitnessindicator of the animals. It was reduced synergistically bythe kairomone of Chaoborus and the insecticide. Some individualskept neckteeth until the third or fourth instar stage when theywere exposed to sublethal concentrations of the insecticidein the Chaoborus-conditioned water. This was considered as aresult of synergistic effects of both the kairomone and theinsecticide. Insecticides may be a factor inducing further developmentof protuberant structures in cyclomorphic Daphnia in naturalwater bodies.     

Multiple predator defence strategies in Daphnia pulex and their relation to native habitat

Daphnia may respond with an array of anti-predator defences(behavioural, morphological and life history) to a chemicalcue (kairomone) exuded by its predators: fish and Chaoborus.Given the wide array of potential responses, it is an interestingquestion whether anti-predator defences are coupled or independentof each other. Since anti-predator responses are costly andeven possessing the genetic information to respond to a certainpredator might involve a cost, clones may only react to predatorsthey co-occur with in nature. In this study, we provide evidencefor an uncoupling of responses by Daphnia pulex in several anti-predatordefences against Chaoborus. We were unable to detect a correlationbetween behavioural (migration), morphological (neck-spine induction)and life history [growth rate, neonate size and size at firstreproduction (SFR)] responses. Furthermore, anti-predator responsesdid not always comply with what is commonly believed. We foundthat Daphnia clones can migrate up or down when exposed to fishor Chaoborus kairomone and that population growth rate, neonatesize and SFR can increase or decrease in response to Chaoboruskairomone. We also show patterns in anti-predator defences thatseem to relate to the habitat from which clones were derived.Daphnia clones that were collected in habitats with Chaoborusas the dominant predator tended to react strongly to Chaoboruskairomone by migrating upward and producing neck-spines. Themigration behaviour against fish kairomone in these clones wasoften an unexpected upward migration. The Daphnia clone thatco-existed with fish predators showed a downward migration inthe presence of fish as well as Chaoborus kairomone. Clonesthat had occurred with either both or no predators had mixedresponses. We sometimes found an upward migration in combinationwith smaller body size as a response to Chaoborus kairomone.This may be interpreted as a behavioural defence against Chaoborusand a life-history defence against fish. Daphnia seem not toexhibit defence behaviour against predators they do not co-occurwith. It might be costly for Daphnia to maintain genetic informationto respond to these predators and protect that information fromgenetic drift.     

Inter-genotype variation in reproductive response to crowding among Daphnia pulex

Crowding is known to have a major influence on reproduction in the freshwater microcrustacean Daphnia pulex. We analyzed reproductive output of six different D. pulex genotypes under two different density regimes in the laboratory. Four of these genotypes reproduce via obligate parthenogenesis, allowing thorough analysis of the life history strategies of some asexual lines. Among 30,109 neonate offspring and 1041 resting egg ephippia collected, several trends were evident. Crowding induced increased resting egg production and reduced neonate offspring production among all genotypes. Offspring sex ratios grew more male-biased with maternal age. The extent, but not direction, of each of these trends varied among genotypes. Offspring sex ratios, and the very direction in which they changed in response to crowding, differed significantly among genotypes with some genotypes producing more and others fewer males in response to crowding. Obligately parthenogenetic genotypes seemed to respond to the crowding stimulus in similar ways as the facultatively parthenogenetic genotypes, as expected from the sexual origins of their genomes. The inter-genotype variation in life-history traits observed in this and other investigations calls into question the common practice of extrapolating results from a single Daphnia genotype to an entire species. Our findings are considered in the context of other research in the field of environmental influences on Daphnia reproduction with a review of representative literature.     

Fitness and sexual response to population density in Daphnia pulex

1. The switch between asexual and sexual reproduction is an important fitness component in cyclically parthenogenetic populations as it is the key to persistence in unstable habitats and because it influences population genetic characteristics such as linkage disequilibrium and population genetic structure.
2. Genetic variation for sexual and asexual reproductive rate ( R _C) was examined, under varying population density, in Daphnia pulex sampled from a rockpool system and two other distinct European localities.
3. Density affected the switch to sexual behaviour, as found in other studies and reduced R _c. Production of males was not correlated with sexual reproduction, promoting outcrossing.
4. Genetic variation was found for the response to density, both within and between populations. There was substantial variation within the rockpools despite their demographic instability, indicating rapid recovery of life-history variation or maintenance during bottlenecks.     

Fish kairomone regulation of internal swarm structure in Daphnia pulex (Cladocera: Crustacea)

In this laboratory experiment, swarms of D. pulex were artificially created by placing thirty individuals in small chambers containing 130 ml of water. The swimming behaviour of the animals was measured using a video camera and a programme to digitize observations. The D. pulex swimming in water with fish kairomones had a more uniform swimming speed compared with animals in control water. If aggregated prey individuals benefit from a reduced predation risk, and this risk is further reduced by uniformity of swarm members, the uniformity of swimming speed can be interpreted as an behavioural adjustment to minimize the vulnerability to predation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Microsporidiosis in a Daphnia pulex population

The impact of parasitism by Thelohania sp., an intracellular microsporidian, on Daphnia pulex inhabiting a vernal pond was studied for three years. Three issues were considered: the distribution of parasites among hosts, the effect of parasites on individuals, and the impact of parasitism on population growth. Each year, Thelohania infected members of at least half of the generations of Daphnia produced in the pond. When the frequency of infection was low, parasites were found only in large adults. As infection frequency increased, parasitism spread downward through host size classes. However, parasites were rarely found in juveniles. Parasitism reduced clutch size drastically, increased mortality to a variable extent, had little impact on egg size or on per instar growth and none on molt frequency. Interaction with other stresses, such as food limitation, exacerbated some of these effects. Parasitism reduced instantaneous birth rate much more than it elevated instantaneous death rate. Population growth was reduced significantly but it is unlikely that Thelohania alone regulates the growth of this Daphnia population.     

Horizontal distribution of Daphnia pulex in response to toxic and non-toxic algal extracts

We studied the horizontal distribution of Daphnia pulex as aresponse to gradients of toxic extracts of two cyanobacteria,Microcystis aeruginosa and Oscillatoria agardhii and non-toxicextracts of the green alga Scenedesmus obtusiusculus. The experimentwas performed in the dark using a flow through system with fivechambers. Daphnia pulex aggregated at the inlet of Scenedesmusextract, but showed similar responses to extracts of toxic cyanobactenaand food-free medium. Chemical cues seem to affect food search,but do not effect Daphnia pulex's avoidance of toxic algae.     

Intraspecific variability in response to stimuli for male and ephippia formation in Daphnia pulex

Natural clones and clones from laboratory crossings of Daphnia pulex have been tested in their response to stimuli for male and ephippia formation. The clones are from ponds in Illinois, USA, an area with both obligate and cyclical clones of D. pulex. The progeny from two types of crosses were studied: 1) crosses between an obligate clone producing males and a cyclical clone producing only females, and 2) self-fertilization within a cyclical clone producing both sexes. Both the natural and the artificial clones showed great variation in response to environmental stimuli for males. Ephippia response seems more equal among the clones, but two of the artificial clones could hardly be stimulated to produce ephippia in the induction experiments. In crowded cultures, however, they produced a few. Crossings between cyclicals and obligates yielded mostly cyclical progeny. Progeny from the selfed clone had low survival and fecundity.     

Physiological responses of Daphnia pulex to acid stress

Background  

Acidity exerts a determining influence on the composition and diversity of freshwater faunas. While the physiological implications of freshwater acidification have been intensively studied in teleost fish and crayfish, much less is known about the acid-stress physiology of ecologically important groups such as cladoceran zooplankton. This study analyzed the extracellular acid-base state and CO₂ partial pressure (P _CO2), circulation and ventilation, as well as the respiration rate of Daphnia pulex acclimated to acidic (pH 5.5 and 6.0) and circumneutral (pH 7.8) conditions.     

Responses of Daphnia pulex populations to toxic cyanobacteria

1. We studied the effects of toxic cyanobacteria, Microcystis aeruginosa , on Daphnia pulex populations. The experiment was performed at room temperature in laboratory microcosms to which we added toxic Microcystis in two pulses. Green alga, Scenedesmus obtusiusculus, was provided in two concentrations.
2. Microcystis exposure resulted in a decreased population density of Daphnia. The proportion of adolescents was higher in the Microcystis treatment than in the control, while the proportion of newborns did not differ significantly from the control. This indicates delayed maturation of Daphnia exposed to Microcystis . We found no significant impacts of cyanobacteria, food level or clonal origin on population variability after correcting for differences in population size.
3. Juveniles of the two clones studied showed different sensitivities to toxic Microcystis in the acute test. However, in the population experiment we did not find clonal differences between Microcystis and control treatments in the relative proportion of juveniles.
4. The number of ephippia produced per adult was highest in the treatments with Microcystis. This indicates that cyanobacterial toxins may be one of several factors inducing ephippia formation, acting directly or through inhibition of feeding.     

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.     

Sexual reproduction of Daphnia pulex in a temporary habitat

Species of Daphnia (Crustacea: Cladocera) typically reproduce by cyclical parthenogenesis, in which a period of all-female parthenogenetic reproduction is followed by sexual reproduction. Sex in Daphnia is determined by the environment, with factors such as temperature, photoperiod and crowding stimulating the production of males and sexual females. Previous studies on Daphnia pulex from temporary pond habitats demonstrated the coexistence of male-producing and non-male-producing (NMP) females, as determined under crowding in the laboratory. A strong genetic component to this sex allocation variation suggested that sex expression in D. pulex is better described as a result of genotype-environment interaction. The present study examined the switch from parthenogenetic to sexual reproduction in two temporary-pond populations of D. pulex. Both populations showed a very early investment in sexual reproduction, independent of population density, by producing males very soon after the populations were reestablished from resting eggs in the early spring. Approximately 40% of the initial broods were male. Additional evidence for gender specialization was obtained by observing the sex of two or three successive broods for 85 individual females. Fifty-eight females produced successive broods of females, 13 females produced successive broods of males and 14 females produced successive broods which included both male and female broods. Females that produced successive female broods under natural conditions included a higher frequency of NMP females compared to a random sample of females, confirming the existence of NMP females. Sexual females were observed in both populations after the first appearence of males, suggesting that the presence of males may stimulate the production of sexual females. For D. pulex populations in a temporary environment, there appears to be an increased emphasis on sexual reproduction and a decreased influence of the environment on sex determination, compared to Daphnia populations in more permanent habitats. Received: 19 February 1996 / Accepted: 20 January 1997     

Plasticity of Daphnia magna life history traits in response to temperature and information about a predator

1. Exudates from predators often elicit early maturation in Daphnia, which may protect them from predation. Diel vertical migration (DVM) is also a predator-avoidance device and affects life history traits because of the variable temperature experienced during migration. This study asks, therefore, how do these two effects interact and what are the net costs and benefits of the two defences combined? 2. Key life history features were studied in a two factorial life table experiment in a monoclonal cohort of Daphnia magna to quantify the costs of predator-induced defences. 3. The costs of DVM, associated with low temperature, yielded a 30% decrease in the intrinsic rate of population increase. This was caused by later maturation and longer egg development time, despite a higher fecundity. 4. Chemical information that predators were present resulted in smaller, more numerous offspring, but had no significant effect on size and age at first reproduction. The costs of induction were therefore associated with smaller, and thus presumably lower quality, offspring. 5. Changes in life history induced by fish exudates were independent of the shifts caused by low temperature. 6. The measure of fitness chosen to assess the costs of induced defences is important because the costs often depend on the environment.     

Feeding rate inhibition in crowded Daphnia pulex

Feeding rates of Daphnia pulex fed a range of levels of the alga Chlamydomonas reinhardi of 15 °C are strongly density-dependent. At lower densities, Daphnia (30 1^–1) fed at higher rates than crowded (270 1^–1) Daphnia which manifest a relatively depressed saturation feeding response. At 30 individuals/liter, Daphnia consumed 8.5 – 15.7 × 10⁴ cells d^–1h^–1 (on a volume basis, 12.1 – 22.2 × 10⁶ m³), at 270 L^–1 3.7 – 3.9 × 10⁴ (5.2 – 5.5 = 10⁶ m³ cells d^–1h^–1 when feeding on algae at 80 000 cells ml^–1 (11.3 × 10⁶ m³ ml^–1). The feeding rate data best fit an Ivlev feeding function. An autoallelopath might be causing the repression. Water preconditioned with crowded Daphnia completely repressed feeding in uncrowded Daphnia after six hours.     

The cost of males in Daphnia pulex

Sex is paradoxical, because asexuals should replace their sexual ancestors by avoiding the demographic cost of producing males (hereafter referred to as the cost‐of‐males). Despite the large body of theoretical and empirical work dealing with the paradox of sex, the cost‐of‐males assumption has been rarely tested. In the present study, we tested the cost‐of‐males assumption in the cladoceran Daphnia pulex. Populations of this species consist of both cyclically parthenogenetic (i.e. sexuals) and obligately parthenogenetic (i.e. asexuals) lineages. In addition, some of the asexual lineages produce only female offspring, whereas others produce functional males, which can mate with sexual females. We compared the reproductive investment of sexuals, male‐producing asexuals, and non‐male‐producing asexuals when raised separately under various environmental conditions. We also determined the outcome of competition between pair‐wise combinations of these reproductive modes. The cost of males was evident when sexual and asexual females were raised separately: sexuals produced fewer female offspring. However, there was no cost of males when reproductive modes were raised in pairs, as sexuals won the competition with asexuals. Our results directly relate to the field conditions experienced by D. pulex. Sexuals might suffer the cost of males at the beginning of the season, when resource competition is low; but when conditions deteriorate as the population approaches carrying capacity, sexuals seem to be better competitors in spite of male production.     

Embryology of Chaoborus-induced spines in Daphnia pulex

Daphnia pulex (Crustacea: Cladocera) embryos were found to be sensitive to a chemical cue (kairomone) in an extract of the predator Chaoborus americanus (Insecta:Diptera). Sensitivity of embryos to the kairomone remains throughout embryonic development. Apparently declining sensitivity as development proceeds may be due to the amount of time the embryos are exposed to the kairomone. Male embryos were also found to be sensitive to the kairomone. The smallest eggs within a brood produced small offspring, which showed the antipredator morphology to a significantly lower degree than largest eggs. The production of the neckteeth is described, at the developmental stage in the maturation of the Daphnia coinciding approximately with the escape of the embryos from the brood chamber.     

Impact of Daphnia pulex on a metalimnetic microbial community

A well-structured metalimnetic community, composed mainly ofphototrophic purple bacteria, Cryptomonas, and a few speciesof ciliates and rotifers, was regularly found between 1984 and1991 in Lake Cis. These populations appeared during spring,reached high concentrations and remained in the metalimnionuntil winter mixing. We had previously postulated that thesemetalimnetic populations could persist in such high numbersbecause they were free of efficient predation, since the oxygen-sulfideinterface provided a refuge from predation. In September 1992,an abundant Daphnia pulex population developed in the lake whichresulted in drastic changes in the formerly stable community.The presence of D.pulex induced an extreme clear-water phasein the epilimnion and the metalimnetic populations were severelyreduced. Feeding experiments with Cryptomonas and heterotrophicand phototrophic bacteria as food for D.pulex resulted in clearancerates around 0.7 ml individual^–1 h^–1. The impactwas very high on the microaerophilic populations, whereas theanaerobic populations were least affected, since sulfide limitedthe vertical distribution of ^D.pulex. We conclude from thisevidence that the metalimnetic populations in previous yeanwere mostly food limited (bottom-up control) and that theirhigh abundances were in great part due to a lack of efficientpredation.