Seasonal variation of egg size and number in a Daphnia pulex population

Seasonal variation of egg size and number was examined in a Daphnia pulex population inhabiting a vernal pond. In this population, size at maturity declines at midseason, probably as an adaptive response to size-selective predation by larvae of the salamander Ambystoma. The larger early season individuals produce more and larger eggs than the smaller late season individuals. Age at maturity does not vary between seasons. Laboratory experiments indicate that temperature may affect egg size, egg number and size at maturity. However, field data suggest that temperature accounts for only a small fraction of the total variation in egg size and number. Indirect measures of nutrition indicate that food limitation does not cause the seasonal decline in egg size and number. The seasonal change in reproductive traits is well correlated with changes in invertebrate and vertebrate predation. Examination of predator feeding preferences and their impact on Daphnia mortality indicate that variation of reproductive traits is most likely a complex adaptation to changing predation regimes.     

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.     

Demographic costs of Chaoborus-induced defences in Daphnia pulex: a sensitivity analysis

Summary We examined the demographic costs of Chaoborus-induced defensive spine structures in Daphnia pulex. Our aim was to assess the role of resource limitation and the interaction effects of limiting food level and antipredator structures on fitness of D. pulex and to pinpoint those life stages that are most sensitive to changes in the defence regime. Chaoborus-induced and typical morphotypes of D. pulex were reared at high and low food concentrations. Instar-based matrix population models were used to quantify the effects of predator-induction, food and their interaction on fitness of D. pulex. Predator-induction caused a statistically significant reduction in fitness at low food levels, but not at high food levels. Sensitivity analyses revealed that the fitness effects were primarily due to changes in the growth rate in instars 1–5, and secondarily to small reductions in the fertility of instars 5–10. The interaction between Chaoborus exposure and low food concentration was negative, and mediated through growth and fertility components. Both these components were reduced more in the Chaoborus-exposed, low food treatment than would be expected in the absence of interaction.     

A population of sexual Daphnia pulex resists invasion by asexual clones

Asexual reproduction avoids the costs associated with sex, predicting that invading asexual clones can quickly replace sexual populations. Daphnia pulex populations in the Great Lakes area are predominately asexual, but the elimination of sexual populations by invading clones is poorly understood. Asexual clones were detected at low frequency in one rare sexual population in 1995, with some increase in frequency during 2003 and 2004. However, these clones remained at low frequency during further yearly sampling (2005–2013) with no evidence that the resident sexual population was in danger of elimination. There was evidence for hybridization between rare males produced by asexual clones and sexual females with the potential to produce new asexual genotypes and spread the genetic factors for asexuality. In a short-term laboratory competition experiment, the two most common asexual clones did not increase in frequency relative to a genetically diverse sexual population due in part to a greater investment in diapausing eggs that trades-off current population growth for increased contribution to the egg bank. Our results suggest that a successful invasion can be prolonged, requiring a combination of clonal genotypes with high fitness, persistence of clones in the egg bank and negative factors affecting the sexual population such as inbreeding depression resulting from population bottlenecks.     

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.     

Reproductive costs of Chaoborus-induced polymorphism in Daphnia pulex

Although the Chaoborus-induced spined morph of Daphnia pulex survives attacks by Chaoborus over twice as frequently as the typical morph, the spined morph is never found in the absence of Chaoborus. This implies that a disadvantage is associated with the spined morph in the absence of Chaoborus predation. The present study tested the hypothesis that the typical morph has a higher intrinsic rate of increase than the spined morph, by measuring several life history characteristics in controlled laboratory experiments at constant temperature and unlimited food.The results suggest that the spined morph of D. pulex takes longer to reach maturity, is smaller at maturity, but has similar egg number and egg size as the typical morph. These results are consistent with the hypothesis that the Chaoborus-induced spined morph is reproductively inferior to the typical morph.     

Colonization dynamics and life history traits of seven Daphnia pulex genotypes

Summary A series of experiments revealed significant differences in the potential ability of seven Daphnia pulex genotypes to colonize two lakes in which this species does not naturally reside. Life table experiments, in which individuals from each genotype were raised separately on water and natural phytoplankton from the two lakes, revealed several significant differences among genotypes in life history traits, including age and size at first reproduction, clutch size and offspring body size. Significant differences among genotypes were also found in mean genotype fitness and rate of population increase, although all genotypes were able to increase in absolute numbers. Significant genotypexlake interaction was found for several life history traits and mean fitness, indicating that the relative success of invading genotypes may depend on habitat characteristics. Enclosure experiments, in which all seven genotypes were introduced together into enclosures in both lakes, revealed that some genotypes increased greatly while others declined in relative abundance, and that the most successful genotypes differed between lakes. In addition, the most successful genotypes in the enclosures were not necessarily the genotypes that displayed the highest fitness in the life table experiments, possibly because individuals in the enclosures competed for food resources, leading to exclusion of certain genotypes.     

Esterase genes in Daphnia pulex: linked inheritance and genotypic distribution in natural populations

Summary Esterase patterns were examined in three populations of Daphnia pulex. The total number of bands showing esterase activity was 17. Three major genes Est-1, Est-2, Est-3 controlling esterase synthesis were identified and genetically studied. These genes were found to be located in the same linkage group. It was shown that two or three homologous chromosomes differing in sets of esterase alleles predominantly occur in the populations considered.     

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.     

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.     

Effects of Microcystis aeruginosa on interference competition between Daphnia pulex and Keratella cochlearis

Laboratory experiments tested the hypothesis that a toxic strain of Microcystis aeruginosa decreases the ability of Daphnia pulex to interfere with Keratella cochlearis. To test a variety of conditions, juvenile and adult Daphnia were exposed to the cyanobacterium for different time periods prior to, and during the experiments. Adult Daphnia not only suppressed rotifers over successive two-day intervals, but also had a significant impact within a 24-hour period. However, the presence of Microcystis (5 × 105 cells ml^–1) decreased the Daphnia effect in both experiments. Although juvenile Daphnia also significantly suppressed Keratella population growth, the presence of Microcystis (10⁵ and 5 × 10⁵ cells ml^–1) caused a significant reduction in daphniid body size and decreased the ability of both nonacclimated and acclimated daphniids to suppress rotifers. Keratella inhalation and mortality are positively correlated with filtering rates and body size of Daphnia. Therefore, the feeding rates and size structure of a Daphnia population will determine its potential to interfere with vulnerable rotifers. In all experiments the presence of Microcystis significantly decreased the ability of Daphnia to interfere with this rotifer despite the fact that Keratella was also inhibited. In the field this effect might be augmented if Microcystis colonies are more easily ingested by cladocerans than by the rotifers.     

Specificity of predator-induced neck spine and alteration in life history traits in Daphnia pulex

It has been proposed that the predator-induced defensive neck spine in Daphnia pulex has a demographic cost. Our results show that this cost is not merely an allocation cost related to the formation and maintenance of the neck spine. In a life table experiment, we tested whether spine induction and life history traits in D. pulex are affected by different invertebrate predators: first and third instar Chaoborus, fourth instar Mochlonyx and two size classes of Notonecta and Dytiscus larvae. D. pulex showed sensitivity to the different predators. Predator-exposure affected one or more of the following life history traits of D. pulex: the timing of first reproduction, clutch size, and growth. In some cases, exposure to predators altered life history traits when neck spine induction did not occur. These shifts in life history traits occurring in the absence of spine induction may be caused by behavioral or physiological changes triggered by the predators.     

Predator-induced phenotypic plasticity in Daphnia pulex: uncoupling morphological defenses and life history shifts

Chemical cues from a predator Chaoborus sp. induce morphological defense (neck spine) and life history shifts (later reproduction, decreased fecundity but larger juvenile size) in the waterflea Daphnia pulex. These shifts have been interpreted either as costs of defense or as separate adaptation. In order to investigate if the life history shifts can be separated from the morphological defense, Daphnia pulex individuals were exposed to chemical cues from Chaoborus at different stages of life for variable periods. The daphnids that were exposed to Chaoborus started their reproduction later than the controls, although the differences were not statistically significant. Neck spine was induced only if daphnids were exposed to Chaoborus in an early stage of their life. Numbers of eggs produced were not affected by the different treatments, but egg mortality was higher in mothers exposed to Chaoborus. With these treatments it was possible to see neck spine induction without measurable life history changes or costs. On the other hand, irrespective of neck spine presence, the Chaoborus chemical(s) had an effect on Daphnia pulex mothers.Publication no 2159. Netherlands Institute of Ecology, Centre for LimnologyPublication no 2159. Netherlands Institute of Ecology, Centre for Limnology     

DNA transposon dynamics in populations of Daphnia pulex with and without sex

We investigate the role of recombination in transposable element (TE) proliferation in the cyclical parthenogen, Daphnia pulex. Recombination provides a mechanism by which the rate of both TE gain and loss can be accelerated, a duality that has long intrigued many biologists interested in the influence of sex on mutation accumulation. We compared TE loads among populations of D. pulex where sex occurs regularly (cyclical parthenogens or ‘sexuals’) with those in which the ability to reproduce sexually has been completely lost (obligate ‘asexuals’) for six different families of DNA transposons. Transposon display assays showed that sexuals have more TEs than asexuals, contrary to the expectations under Muller''s ratchet but consistent with the idea that sex facilitates TE spread. Sexuals also exhibit higher insertion site polymorphism among lineages, as predicted because recombination accelerates rates of loss and gain. Asexuals, however, have proportionally more singletons (loci occupied in a single isolate), which differs from previous studies where selfing and outcrossing were used as a proxy for high and low recombination. Our multi-element survey reveals that the impact of sex on TE proliferation is consistent among different Class II TE families and we discuss the genomic consequences of different reproductive strategies over long time periods.     

Diel horizontal migration and swarm formation in Daphnia in response to Chaoborus

Dense swarms of Daphnia longispina (up to 4000 animals l^–1) were recorded along the littoral zone in a lake where Chaoborus flavicans is considered the main predator. D. longispina coexisted with D. pulex, but there were no D. pulex in the littoral swarms. Swarms were less dense at night (about 1/10 the density), and D. longispina exhibited diel horizontal migrations by aggregating in the littoral during the day and spreading out at night. Laboratory experiments showed that Chaoborus capture efficiency on juvenile daphnids was higher in the light compared to darkness, and that Daphnia exhibited a behavioural response to water that had previously contained Chaoborus. We conclude that predation from Chaoborus can be an important factor affecting the distribution patterns of Daphnia observed in this lake. The behavioural experiments indicated that this influence might be partly mediated by chemical agents.     

Ammonia-nitrogen excretion in Daphnia pulex

Ammonia-nitrogen excretion rates were measured in natural summer and cultured populations of Daphnia pulex from Silver Lake, Clay County, Minnesota, USA during 1973. The mean rate of ammonia-nitrogen excretion for the summer populations was 0.20 µg N animal^–1 day^–1 or 5.11 µg N mg^–1 dry body weight day^–1 (N = 80) measured at 15°, 20°, and 25°C. These rates appear to be temperature and weight dependent, but they are probably affected by factors other than temperature and dry body weight. Ammonia-nitrogen excretion rates of Daphnia pulex cultured on Chlamydomonas reinhardi yielded the following relationship with temperature: Log₁₀E = (0.061) T 1.773, where E is µg N animal^–1 day^–1 and T is temperature °C. The ammonia-nitrogen excretion on a mg^–1 dry body weight day^–1 basis was related to temperature according to the following similar expression Log₁₀E = (0.043) T + 0.153, where E is µg N mg^–1 dry body weight day^–1, and T is temperature °C. The length-weight relationship of Daphnia pulex for the summer populations (N = 1583) was log₁₀W = (0.526) Log₁₀L + 1.357, where W is weight in µg and L is length in mm.     

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.