Embryogenesis plasticity and the transmission of maternal effects in Daphnia pulex

Understanding how genetic, nongenetic, and environmental cues are integrated during development may be critical in understanding if, and how, organisms will respond to rapid environmental change. Normally, only post‐embryonic studies are possible. But in this study, we developed a real‐time, high‐throughput confocal microscope assay that allowed us to link Daphnia embryogenesis to offspring life history variation at the individual level. Our assay identified eight clear developmental phenotypes linked by seven developmental stages, the duration of which were correlated with the expression of specific offspring life history traits. Daphnia embryogenesis varied not only between clones reared in the same environment, but also within a single clone when mothers were of different ages or reared in different food environments. Our results support the hypothesis that Daphnia embryogenesis is plastic and can be altered by changes in maternal state or maternal environment. As well as furthering our understanding of the mechanisms underpinning parental effects, our assay may also have an industrial application if it can be used as a rapid ecotoxicological prescreen for testing the effect that pollutant doses have on offspring life histories traditionally assayed with a 21‐day Daphnia reproduction test.     

Daily and seasonal vertical migration rhythms in Daphnia

1. Clones of Daphnia longispina (O.F.M.), collected from ponds with different levels of predation, were monitored using video techniques in order to test the effects of temperature and visual cues on vertical migration behaviour.
2. Endogenous rhythms were found to have a part in the control of vertical migration in some clones but responses were highly variable.
3. Low temperature caused a marked increase in the amplitude of vertical migration in D. longispina , consistent with field studies.
4. Clones from ponds with fish predators had larger vertical migration amplitudes than clones from ponds with no fish, providing further evidence for the genetic basis of this mechanism.     

Daily and seasonal vertical migration rhythms in Daphnia

1. Clones of Daphnia longispina (O.F.M.), collected from ponds with different levels of predation, were monitored using video techniques in order to test the effects of temperature and visual cues on vertical migration behaviour.
2. Endogenous rhythms were found to have a part in the control of vertical migration in some clones but responses were highly variable.
3. Low temperature caused a marked increase in the amplitude of vertical migration in D. longispina , consistent with field studies.
4. Clones from ponds with fish predators had larger vertical migration amplitudes than clones from ponds with no fish, providing further evidence for the genetic basis of this mechanism.     

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.     

The effect of accelerations in light increase on the phototactic downward swimming of Daphnia and the relevance to diel vertical migration

The effect of relative increases in light intensity on photobehaviourwas studied in the hybrid Daphnia galeata x hyalina. We firstcarried out a series of experiments to study the influence offish kairomone on several response variables of light-inducedswimming. With fish kairomone present, an increase in the percentageof reacting daphnids to 100% was found at almost all ecologicallyoccurring relative light change rates that were above threshold.The relationship between the relative increase in light intensity(stimulus) and the time expiring between the onset of the stimulusand the start of the downward swimming response was not influencedby fish kairomone, nor did kairomone alter the functional relationshipbetween stimulus strength and downward displacement velocity,although velocity increased. During the previous experiments,various light change rates were applied, but per test run theserates were constant. The natural relative light increase inthe early morning consists of continuously increasing relativelight change rates, turning into decreasing rates after themaximum is reached     

Morphology of the Daphnia nervous system: A comparative study on Daphnia pulex,Daphnia lumholtzi,and Daphnia longicephala

The freshwater crustacean Daphnia is well known for its phenotypic plasticity, in which environmental cues are perceived by the nervous system and transformed into phenotypic adaptations beneficial under current conditions. Critical knowledge regarding the distribution and localization of neuronal antigens or neurotransmitters and differentially expressed proteins is sparse. Here, we applied immunohistochemical and histological‐staining methods in combination with confocal laser scanning microscopy on whole mount preparations in Daphnia pulex, Daphnia longicephala, and Daphnialumholtzi. We document the nervous system, elements of the sensory system, and cell clusters with secretory characteristics in the Daphnia head. This is the first report on the nervous system of the species D.longicephala and D.lumholtzi. The methods established in this investigation will help to foster research on specific neuronal mechanisms in this rapidly advancing model system of life science research. J. Morphol., 2012. © 2012 Wiley Periodicals, Inc.     

Daphnia size structure,vertical migration,and phosphorus redistribution

The timing and magnitude of diel migration in two daphnid assemblages were determined from a series of vertical profiles of daphnid size distribution. Animals were collected concurrently for gut fullness determination. Only large daphnids (> 1.4 mm) migrated, but these animals could account for substantial vertical and diel differences in phosphorus excretion rate. Gut fullness measurements and time courses of diel vertical migration suggested that large Daphnia can cause a net downward flux of phosphorus during summer in thermally stratified lakes.     

Karyologic observations on the maturation of the summer and winter Eggs of Daphnia pulex and Daphnia middendorffiana

The cladoceran Daphnia middendorffiana is an arctic species able to produce winter eggs which develop parthenogenetically. The cytological study of the maturation of summer and winter eggs of an italian population of this species has shown that both types of eggs undergo only one maturation division of equational type with the expulsion of only one diploid polar body, the same maturation process as had been found in the pseudosexual eggs of a race of Daphnia pulex (Schrader, 1925).—The chromosomes behaviour during the maturation of summer and winter eggs of D. middendorffiana has been compared with that of a population of D. pulex which shows a normal heterogonic cycle.This investigation was supported by the Consiglio Nazionale delle Ricerche (C.N.R.) of Italy.     

Predation by chaoborids on typical and spined Daphnia pulex

SUMMARY. 1. In laboratory predation experiments, predation rates were determined for fourth-instar larvae of the chaoborids (Insecta: Diptera) Mochlonyx sp. and Chaoborus americanus preying on four juvenile instars and adult Daphnia pulex (clone SBL). In the case of Mochlonyx , predation was most intense on second-instar juvenile Daphnia while Chaoborus preyed equally on second and third instars.
2. Mochlonyx and Chaoborus induced an antipredator defence (neck-teeth) which was most frequently expressed in second-instar juvenile Daphnia . Expression of this antipredator phenotype was correlated with a reduced rate of Mochlonyx predation.     

Life history consequences for Daphnia pulex feeding on nutrient-limited phytoplankton

1. The growth and feeding of Daphnia pulex De Geer on different algal species was examined. The green algae Chlamydomonas reinhardtii Dangeard and Scenedesmus acutus Meyen, the diatom Synedra tenuissima Kützing, the cryptophyte Cryptomonas pyrenoidifera Geitler and the cyanobacterium Microcystis aeruginosa Kützing were cultured in non-limiting and in N- or P-limiting medium and used as food for D. pulex.
2. Growth limitations were reflected in the elemental and biochemical composition and the morphological characteristics of the algal resources.
3. The clearance rates of D. pulex feeding on nutrient-limited algae were reduced. This was not observed when nutrient-limited mutant Chlamydomonas cells without cell walls were used as food, indicating that the cell wall may play an important part.
4. In comparison with animals grown on nutrient-sufficient cells, nutrient-limited algae resulted in smaller body length, reduced brood sizes, reduced size at maturity, increased age at first reproduction and, consequently, in reduced Daphnia population growth rates.
5. Daphnia population growth rates ( r ) were negatively correlated with the C : P ratio and the carbohydrate content of the food. Moreover, significant correlations between r and clearance rates were found.
6. The observed differences in the grazing and the life history parameters of Daphnia feeding on non-limited and nutrient-limited algae may be the result of both reduced nutritional value and reduced digestibility of nutrient-limited algae.     

Transcontinental Phylogeography of the Daphnia pulex Species Complex

Daphnia pulex is quickly becoming an attractive model species in the field of ecological genomics due to the recent release of its complete genome sequence, a wide variety of new genomic resources, and a rich history of ecological data. Sequences of the mitochondrial NADH dehydrogenase subunit 5 and cytochrome c oxidase subunit 1 genes were used to assess the global phylogeography of this species, and to further elucidate its phylogenetic relationship to other members of the Daphnia pulex species complex. Using both newly acquired and previously published data, we analyzed 398 individuals from collections spanning five continents. Eleven strongly supported lineages were found within the D. pulex complex, and one lineage in particular, panarctic D. pulex, has very little phylogeographical structure and a near worldwide distribution. Mismatch distribution, haplotype network, and population genetic analyses are compatible with a North American origin for this lineage and subsequent spatial expansion in the Late Pleistocene. In addition, our analyses suggest that dispersal between North and South America of this and other species in the D. pulex complex has occurred multiple times, and is predominantly from north to south. Our results provide additional support for the evolutionary relationships of the eleven main mitochondrial lineages of the D. pulex complex. We found that the well-studied panarctic D. pulex is present on every continent except Australia and Antarctica. Despite being geographically very widespread, there is a lack of strong regionalism in the mitochondrial genomes of panarctic D. pulex – a pattern that differs from that of most studied cladocerans. Moreover, our analyses suggest recent expansion of the panarctic D. pulex lineage, with some continents sharing haplotypes. The hypothesis that hybrid asexuality has contributed to the recent and unusual geographic success of the panarctic D. pulex lineage warrants further study.     

Activation of the Ephippial Egg of Daphnia pulex

The ephippial eggs of Daphnia pulex require light for the initiation of development. The ephippial capsule prevents the completion of development but is not a barrier to an adequate light stimulus. Working with decapsulated eggs, the response to light increased to 100 % within 9 days of storage in the dark and remained at 100% for up to 60 days of storage in the dark. The response was not dependent on drying the ephippia. Ephippia stored in the light did not reach 100% response to illumination when decapsulated, indicating that activation was dependent on prior dark reactions. About 4500 ft-c-min of fluorescent light energy was required for 100% activation. The effective wavelengths were between 350 and 475 mµ with 2 x 10⁶ ergs/cm² sufficient to initiate nearly 100% development at 410 mµ, the most effective wavelength. Low temperature interfered with photoactivation but not with subsequent development. Chilling the ephippia resulted in an increased light requirement. Kinetic studies with chilled ephippia stored for various times in the dark indicated a diphasic process of photoactivation which has tentatively been interpreted as a light-dependent release of inhibition.     

Some Effects of Light on the Growth,Molting, Reproduction and Survival of the Cladoceran,Daphnia pulex

Daphnia pulex were raised under nine light intensities, 0, 1.7, 3.5, 7, 14, 28, 55 and 110 ft-c, polarized light, 6.6 ft-c, and four wavelength ranges, violet, blue, green and red.Of the nine light intensities studied, only 28 ft-c significantly depressed growth. Light generally may retard growth between molts. There was no intensity effect on molting rate although there was a significant affect on the number of preadult molts. Light may affect the amount of time prior to the release of the first brood. Light intensity did not affect the average number of broods per adult but significantly affected the number of young per brood and young per adult and the effects varied. High light intensity significantly affected the number of undeveloped young and nonviable eggs and abortion rate. Reproduction curves under darkness, 3.5, 7, 14, 28 and 55 ft-c do not drop rapidly after an initial peak and apparent plateaus exist with a decline in reproduction during later instars. Light did not significantly affect survival.All effects of polarized light (6.6 ft-c) were indistinguishable from the effects of light intensity.Red wavelengths significantly retard total growth and green wavelengths significantly stimulate molting and retard growth between molts. There was no effect of wavelength on the number of preadult molts but red populations took significantly longer to release the first brood. Wavelengths did not affect the number of broods per adult but blue wavelength significantly increased the number of young per brood and per adult. Wavelength did not affect the number of undeveloped young, nonviable eggs or abortion rate. Reproduction curves under vioîlet and green wavelength do not decline after an initial peak. Animals raised under only a portion of the visible spectrum, and specifically green wavelenghts, had an increased mortality. The effects of wavelengths are probably independent of light intensity.This paper is part of a Ph.D. Thesis submitted to the Faculty of the Graduate School of The University of Kansas.Supported in part by a NSF Summer Traineeship and Grant # 1175 from the University of Kansas Computation Center.Supported in part by a NSF Summer Traineeship and Grant # 1175 from the University of Kansas Computation Center.     

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.     

Thermal variation and factors influencing vertical migration behavior in Daphnia populations

The antipredator behavior diel vertical migration (DVM), common in aquatic keystone species Daphnia, involves daily migration from warmer surface waters before dawn to cooler deeper waters after dusk. Plasticity in Daphnia DVM behavior optimizes fitness via trade-offs between growth, reproduction, and predator avoidance. Migration behavior is affected by co-varying biotic and abiotic factors, including light, predator cues, and anthropogenic stressors making it difficult to determine each factor's individual contribution to the variation in this behavior. This study aims to better understand this ecologically significant behavior in Daphnia by: (1) determining how Daphnia pulicaria thermal preferences vary within and among natural populations; (2) distinguishing the role of temperature verses depth in Daphnia vertical migration; and (3) defining how two anthropogenic stressors (copper and nickel) impact Daphnia migratory behavior.Simulated natural lake stratification were constructed in 8 L (0.5 m tall, 14.5 cm wide) water columns to monitor under controlled laboratory conditions the individual effects of temperature gradients, depth, and metal stressors on Daphnia vertical migration. Three major findings are reported. First, while no difference in thermal preference was found among the four populations studied, within lake populations variability among isolates was high. Second, decoupling temperature and depth revealed that depth was a better predictor of Daphnia migratory patterns over temperature. Third, exposure to environmentally relevant concentrations of copper or nickel inhibited classic DVM behavior. These findings revealed the high variability in thermal preference found within Daphnia populations, elucidated the individual roles that depth and temperature have on migratory behavior, and showed how copper and nickel can interfere with the natural response of Daphnia to fish predator cues. Thus contributing to the body of knowledge necessary to predict how natural populations of Daphnia will be affected by climate related changes in lake temperatures and increased presence of anthropogenic stressors.     

Fish predators, food availability and diel vertical migration in Daphnia

Diel vertical migration of zooplankton is a highly variableand complex behaviour which apparently cannot be explained byany single factor. We determined the relative importance offish predation, food availability and water depth in shapingthe migratory behaviour of Daphnia. A modified 2x2x2 factorialexperiment provided two levels of fish density (present/absent),food availability (ambient/high) and depth (4–10 m); shallowtreatments with fish were excluded. Triplicate 1.2 m diameterenclosures for each of the six treatments were held in an 18unit array in Peter Lake, Gogebic Co., MI, USA. Repeated measuresANOVA identified significant trends in daphnid density, migrationand fitness (determined by lipid-ovary-egg index, LOE) as wellas in chlorophyll a content of the water column for part ofa 4-week experiment in July 1988. In deep enclosures with fish,Daphnia performed significantly more intense migrations thanin fishless enclosures, save those in fishless ambient-foodenclosures. Daphnia in deep fishless enclosures without abundantfood at depth performed significantly reduced migrations. DaphnidLOE index was significantly influenced only by food content.Our results were consistent with the predator-avoidance hypothesisas well as with observations of greatest migrations where largevertical differences in food abundance exist. They support ahierarchical view of vertical migration, with presence of fishthe primary factor, and food availability the secondary factor.     

Filtering rate of the cladoceran,Daphnia pulex as a function of body size,light and acclimation

Daphnia pulex were raised under nine light intensities (0, 1.7, 3.5, 7, 14, 28, 55, and 110 ft-c), polarized light (6.6 ft-c), and four wavelength ranges.Light intensity significantly affected the relationship between filtering rate and body size of unacclimated animals. The b values were lowest at 7 and 14 ft-c and they increased above 7 ft-c as light intensity increased. There were significant differences among b values and adjusted means for unacclimated animals. Acclimation to their respective conditions resulted in some significant differences between b values and adjusted means. For acclimated animals there were no significant differences among b values but there were some differences among adjusted means. The filtering rates of unacclimated and acclimated animals were lower at 14 ft-c while 1.7 and 3.5 ft-c were generally stimulatory and the effect was more pronounced in larger animals. Light intensities above 28 ft-c tended to suppress the filtering rate of small unacclimated animals and stimulate filtering rate in larger unacclimated animals. After acclimation, intensities above 7 ft-c did not affect the filtering rate of either small or large animals.The effect of polarized light on filtering rate was inseparable from the effect of light intensity, however, acclimation to polarized light resulted in a significantly higher b value.There were no significant effects of wavelength among b values for unacclimated animals and the adjusted mean for blue wavelengths was significantly higher than for violet wavelengths. There were no significant differences among b values or adjusted means for acclimated animals. Acclimation to their respective wavelengths did result in some significant differences between b values and adjusted means. Except for the effects of blue wavelengths on unacclimated animals and red wavelengths on acclimated animals, the effects of wavelengths are inseparable from the effects of light intensity.This paper is part of a Ph.D. Thesis submitted to the Faculty of Graduate School of The University of Kansas.Supported in part by a NSF Summer Traineeship and a grant from the Research Corporation to St. Olaf College, Northfield, Minnesota 55057.     

A comparison of the sensitivities of Daphnia magna and Daphnia pulex to six different cyanobacteria

Cyanobacteria have become an important environmental concern due to their ability to produce a wide range of natural toxins. At present, very few studies describe concentration response curves for cyanobacteria other than Microcystis. However, field evidence highlights that both cyanobacterial concentration as well as cyanobacterial species composition vary considerably with season and year. Therefore, the aim of this study was to investigate the effects of different cyanobacteria at various concentrations of these cyanobacteria in the diet on the reproduction of Daphnia pulex and Daphnia magna. Those two species were chosen to assess whether the cyanobacteria-daphnid dynamics could be generalized for the Daphnia genus. Results demonstrated that both slope and EC50 of the concentration response curves depend upon the Daphnia species, the cyanobacteria species and the potential interaction between the two. This has two major consequences. First, the differences in sensitivity to cyanobacteria between D. magna and D. pulex depend upon concentration of the specific cyanobacteria. Second, we noted different mechanisms of toxicity for the two zooplankton species, a more general mechanism of toxicity for D. pulex and a more specific one for D. magna. Our data therefore suggest that results of studies investigating effects of cyanobacteria at different concentrations cannot be generalized across species. Furthermore, mechanisms of toxicity are not only cyanobacteria specific, but also dependent on the exposed species, even for rather closely related species such as in the Daphnia genus. Whenever possible, we therefore propose to combine a multi-species approach together with a full concentration response analysis to reach more general conclusions concerning the effects of cyanobacteria on zooplankton.     

Effects of a fluctuating temperature regime experienced by Daphnia during diel vertical migration on Daphnia life history parameters

Many freshwater zooplankton species perform a diel vertical migration (DVM) and spend the day within the lower, colder hypolimnion of stratified lakes. Trade-offs that arise from this migration have already attracted much attention and the cold temperature in the hypolimnion is thought to be the main cost of this behaviour. In this study we additionally looked at the extra costs daphnids have from being exposed to a fluctuating temperature regime (cold during the day and warm during the night) which is less well studied until today. In our experiment Daphnia hyalina Leydig and Daphnia magna Straus either spent 24 h in constant warm water (19 °C), 24 h in constant cold water (12 °C), or spent 12 h in warm and 12 h in cold water in an alternating way (fluctuating temperature regime). We expected the values of the life history parameters of Daphnia in the fluctuating temperature regime to be exactly halfway between the values of the life history parameters in the warm and cold treatments because the daphnids spent exactly half of the time in warm water, and half of the time in cold water. Concordant with earlier studies our results showed that age at first reproduction and egg development time were reduced at higher temperatures. In the fluctuating temperature regime the values of both parameters were exactly halfway between the values at permanently warm and cold temperature regimes. In contrast, somatic growth was higher at higher temperatures but was lower in the fluctuating temperature regime than expected from the mean somatic growth rate. This suggests that a fluctuating temperature regime experienced by migrating daphnids in stratified lakes involves additional costs for the daphnids.