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     

Sexual reproduction in Daphnia: interspecific differences in a hybrid species complex

To investigate whether reproductive variation in the Daphnia galeata-cucullata-hyalina hybrid species complex can explain the occurrence of hybrids and backcrosses, a total of 43 clones from three north German lakes were tested for allocation to sexual reproduction under equal stress conditions. Six replicates per clone were followed until the seventh adult instar. I used the following cues to promote sexual reproduction: short photoperiod, water from a crowded Daphnia culture, fish-conditioned water and low food concentration. For each animal, clutch size and clutch sex were recorded. Ephippia, which were empty since the animals were cultured individually, were only produced by D. cucullata and D. cucullata x hyalina (26% and 6% of the broods, respectively), whereas almost all taxa produced males (the range was 2–15%). Intraspecific variation for male and ephippial production was also found. The fact that the taxa tested show different responses to the stimuli of-fered may indicate that there is reproductive isolation between them. This is supported by field data from the Tjeukemeer (The Netherlands) which shows that D. galeata mostly produces ephippia in spring and D. cucullata in autumn. The presence of hybrids with ephippia in both seasons, however, shows that backcrossing is also possible.     

Sexual reproduction in Daphnia pulex (Crustacea: Cladocera): observations on male mating behaviour and avoidance of inbreeding

1.  Mating behaviour in Daphnia appears to rely on random contact between males and sexual females rather than diffusible pheromones. Males may be able to discriminate sexually receptive females from females in other developmental stages and increase their mating efficiency. Males may also use chemical signals to avoid mating with females from the same clone and avoid the severe inbreeding depression that has been documented for intraclonal mating. The present study used experiments to test for the avoidance of intraclonal mating and assess male mating efficiency in D. pulex .
2.  Three clones were examined for the avoidance of intraclonal mating by providing males with an opportunity to mate with females of the same or two different clones. The proportion of intraclonal matings did not differ from the proportion of interclonal matings, suggesting that D. pulex males do not use kin discrimination to avoid mating with females from the same clone.
3.  The proportion of mated females decreased with increasing numbers and density of sexual females when exposed to a single male. This observation suggests that a male spends more time pursuing and copulating with sexually receptive females than non-receptive females and there is insufficient time to mate with all sexual females. The decrease in proportion of females mated could also be the result of sperm depletion in the male. Sperm depletion is unlikely to occur in nature because sexually receptive females are much rarer than in the experimental conditions.     

Effects of winter conditions on Daphnia dynamics and genetic diversity in a dimictic temperate reservoir

1. Winter conditions shape plankton dynamics and community composition in temperate regions, but their effect on dynamics and genetic composition of cyclical parthenogens like Daphnia is largely unclear. 2. For 5 years, we studied the dynamics, hatching from resting eggs and genetic structure of a D. galeata × longispina hybrid complex in a dimictic, temperate reservoir. Our main hypothesis was that higher spring densities and an earlier population peak will be observed after warmer winters, with a lower genetic diversity because of a lower contribution of resting eggs to population growth. 3. The study period could clearly be categorised into cold‐winter years (n = 3) and warm‐winter years (n = 2). Daphnia densities at the end of spring overturn were ～10‐fold lower after cold winters than after warm ones, but no pattern emerged concerning the timing and the height of the population peak in early summer. 4. Hatching intensity from resting eggs was higher and contributed up to 8.5% to Daphnia abundance in a cold‐winter year compared to a negligible contribution in a warm‐winter year. Consistent with this finding, new multilocus genotypes (MLGs) adding to the overwintering stock after the end of spring overturn and presumably originating from resting eggs increased genetic diversity and attained high frequencies within the population only after a cold winter. New MLGs were recorded also after warm winters, but they never gained dominance and no shift in genetic diversity was observed. However, genetic diversity was not generally reduced after warm winters. 5. Our results confirm earlier findings that winter conditions have only a limited effect on the main growth phase and the peak of Daphnia during late spring and early summer. However, winter conditions determine the contribution of resting eggs to the population development, which may profoundly alter the genetic composition of the population compared to the previous season.     

Chaoborus predation and delayed reproduction in Daphnia: a demographic modeling approach

I develop a demographic model that examines the impact of Chaoborus predation on the population dynamics and life history of Daphnia. Predation effects are determined through analysis of the various components of the predator-prey interaction (encounter, attack, strike efficiency), and are integrated into a stage-classified matrix population model. The model is parameterized with data from interactions between D. pulex and fourth-instar C. americanus. I test this model with two laboratory experiments that examine population growth of D. pulex under the influence of five different levels of Chaoborus predation. With the exception of a single predation treatment in each experiment, the model accurately predicted the observed reduction in Daphnia numbers with increasing Chaoborus predation. I then use this model to investigate the evolution of delayed reproduction in D. pulex that are exposed to Chaoborus. I ask whether delayed reproduction may evolve in Daphnia that are subjected to Chaoborus predation as a trade-off for the benefits of larger body size. The model predicts that the effectiveness of such a life history trade-off depends on the relative sizes of predator and prey. In some interactions between Chaoborus and Daphnia, increasing Daphnia body length by as little as 5% from base growth trajectories sufficiently increases fitness (by reducing vulnerability to Chaoborus predation) to compensate for the cost of delayed reproduction. In other interactions, however, increased body length provides no benefit to Daphnia (and may even reduce fitness), and selection would act against the evolution of delayed reproduction. This revised version was published online in July 2006 with corrections to the Cover Date.     

Predicting a change in the order of spring phenology in temperate forests

The rise in spring temperatures over the past half‐century has led to advances in the phenology of many nontropical plants and animals. As species and populations differ in their phenological responses to temperature, an increase in temperatures has the potential to alter timing‐dependent species interactions. One species‐interaction that may be affected is the competition for light in deciduous forests, where early vernal species have a narrow window of opportunity for growth before late spring species cast shade. Here we consider the Marsham phenology time series of first leafing dates of thirteen tree species and flowering dates of one ground flora species, which spans two centuries. The exceptional length of this time series permits a rare comparison of the statistical support for parameter‐rich regression and mechanistic thermal sensitivity phenology models. While mechanistic models perform best in the majority of cases, both they and the regression models provide remarkably consistent insights into the relative sensitivity of each species to forcing and chilling effects. All species are sensitive to spring forcing, but we also find that vernal and northern European species are responsive to cold temperatures in the previous autumn. Whether this sensitivity reflects a chilling requirement or a delaying of dormancy remains to be tested. We then apply the models to projected future temperature data under a fossil fuel intensive emissions scenario and predict that while some species will advance substantially others will advance by less and may even be delayed due to a rise in autumn and winter temperatures. Considering the projected responses of all fourteen species, we anticipate a change in the order of spring events, which may lead to changes in competitive advantage for light with potential implications for the composition of temperate forests.     

Chaos and stability of age-0 fish assemblages in a temperate deep reservoir: unpredictable success and stable habitat use

Large year-to-year variability in different fish species recruitment has been confirmed by previous studies while diurnal patterns of occupation in two basic reservoir habitats (pelagic and littoral) by different age-0 fish species in late summer are still unclear. Data collected over an 11-year period regarding late-summer age-0 fish assemblages in pelagic and littoral habitats of a reservoir were used to test the recruitment instability and to investigate diurnal habitat use. Trawling was conducted in the pelagic habitat at night while beach seining was conducted in the littoral habitat during day and night. Fluctuations in age-0 fish abundance and species composition were observed with both sampling methods; however, the following spatio-temporal patterns were relatively stable in most investigated years: (1) pelagic species (pikeperch; Sander lucioperca, small perch; Perca fluviatilis, bream; Abramis brama at night), (2) littoral species (large perch, asp; Leuciscus aspius, dace; Leuciscus leuciscus), (3) migratory species likely performing diel horizontal migrations (bleak; Alburnus alburnus), (4) species abundant in the littoral habitat both during day and night and also in pelagic habitat at night (roach; Rutilus rutilus) and (5) species detected in both habitats exclusively at night (ruffe; Gymnocephalus cernuus).     

Evolutionary genetics of aging in Daphnia

Evolutionary theory of aging stipulates that aging is inevitable consequence of low effectiveness of natural selection acting on traits expressed late in the life span of the organisms. Two main hypotheses exist: the neutralist mutation-accumulation theory and selectionist antagonistic pleiotropy theory. Both theories predict the increase of genetic variance with age; the antagonistic pleiotropy theory also predicts negative genetic correlation between fitness related traits in the beginning and in the end of the life span. In order to test these predictions we measured life expectancy and age specific mortality in cohorts of 26 c lones of Daphnia magna extracted from a single cyclic parthenogen population. Simultaneously, fecundity and age to maturity were measured in representatives of the same clones. Log mortality increased linearly with age, with little evidence for leveling off, although some replicate cohorts did show a significant leveling off of mortality. Genetic variance of log mortality was significantly higher in the last quarter of the life span than in earlier time intervals. There was a significant positive genetic correlation between early fecundity and early mortality, but not between early fecundity and late mortality. This indicates that, although there is a trade-off between fecundity and survival, this trade-off is not based on pleiotropy across ages and therefore the data does not support the prediction of antagonistic pleiotropy theory.     

Sexual abstinence and the cost of reproduction in adult male water snakes, Nerodia sipedon

Low frequency of reproduction among iteroparous organisms is most often observed among female ectothermic vertebrates and is thought to be a strategy used to defer reproductive costs. We assessed reproductive costs of male water snakes ( Nerodia sipedon ) to determine why half of adult males abstain from reproduction each year. There was no evidence of a short-term energetic cost of reproduction. Change in mass did not differ between reproductive and non-reproductive males during the one-month mating season or during the entire four-month activity season. Changes in mass of reproductive males were similar at two sites in which the spatial distribution of females differed. However, there were size-specific differences in growth and survival between reproductive and non-reproductive males. Among reproductive males growth rate decreased with body size at a lower rate than among non-reproductive males. Survival increased with body size for reproductive males, but decreased with body size among non-reproductive males. Most of the differential survival between reproductive and non-reproductive males did not occur during the mating season but rather during hibernation. Size-related differences between reproductive and non-reproductive males may reflect selection having eliminated low quality males from the larger size classes. Overall our results appear most consistent with there being high variance in male quality, such that the best males can bear the cost of reproducing and still grow and survive as well or better than low quality males that abstain from reproduction.     

Spring leaf phenology and the diurnal temperature range in a temperate maple forest

Spring leaf phenology in temperate climates is intricately related to numerous aspects of the lower atmosphere [e.g., surface energy balance, carbon flux, humidity, the diurnal temperature range (DTR)]. To further develop and improve the accuracy of ecosystem and climate models, additional investigations of the specific nature of the relationships between spring leaf phenology and various ecosystem and climate processes are required in different environments. This study used visual observations of maple leaf phenology, below-canopy light intensities, and micrometeorological data collected during the spring seasons of 2008, 2009, and 2010 to examine the potential influence of leaf phenology on a seasonal transition in the trend of the DTR. The timing of a reversal in the DTR trend occurred near the time when the leaves were unfolding and expanding. The results suggest that the spring decline in the DTR can be attributed primarily to the effect of canopy closure on daily maximum temperature. These findings improve our understanding of the relationship between leaf phenology and the diurnal temperature range in temperate maple forests during the spring. They also demonstrate the necessity of incorporating accurate phenological data into ecosystem and climate models and warrant a careful examination of the extent to which canopy phenology is currently incorporated into existing models.     

The phenology of growth and reproduction in plants

The study of phenological aspects of plants involves the observation, recording and interpretation of the timing of their life history events. This review considers the phenology of leafing, flowering and fruit production in a range of species and communities. The selective forces (both abiotic and biotic) that influence the timing of these events are discussed. Within the limits imposed by phylogenetic constraints, the phenological patterns (timing, frequency, duration, degree of synchrony, etc.) of each phase are probably the result of a compromise between a variety of selective pressures, such as seasonal climatic changes, resource availability, and the presence of pollinators, predators and seed dispersers. Many studies on flowering times stress the role of interactions between plant species which share pollinators or predators. The timing of fruiting plays a key role in controlling the abundance and variety of obligate frugivores in many tropical communities. The importance of long-term recording is stressed, particularly in species which fruit irregularly. An understanding of the phenology of plants is crucial to the understanding of community function and diversity.     

The importance of hatching from resting eggs for population dynamics and genetic composition of Daphnia in a deep reservoir

1. Cyclical parthenogens, organisms switching between sexual and asexual mode of reproduction, are common in aquatic habitats. In permanent waterbodies with year‐round persistence of populations, the relevance of sexually produced resting eggs for population dynamics and structure is still largely unexplored. 2. The aim of this study was to investigate the importance of recruitment from resting eggs to Daphnia spp. in a deep dimictic reservoir with overwintering populations (Saidenbach Reservoir, Germany). We linked field studies on Daphnia population dynamics, hatching rates and diapausing egg abundances in the sediment with allozyme analysis of the genetic variability of both the pelagic population and hatchlings from resting eggs. 3. We found higher hatching rates at intermediate depths (10–18 m), although highest diapausing egg abundances were found in the deeper parts of the reservoir. From the extensive field data, we calculated a population budget for the main basin of the reservoir, which showed that hatched daphnids accounted for <1% of the total abundance and explained only a tiny fraction of population growth. 4. The genetic composition (MultiLocus Genotypes, MLG) of the pelagic population was relatively stable throughout the stratified period with up to five among 39 detected MLGs attaining frequencies >5% per date. From 11 MLGs differentiated among hatched daphnids, three were recovered in the pelagic population in perceptible, albeit low frequencies (1–4%). 5. We conclude that the contribution of hatched animals to the spring population dynamics is negligible in situations with an overwintering population present and will only become relevant if densities are extremely low. In this type of system, the significance of the resting egg bank is presumably mainly related to the maintenance of genetic diversity.     

Sexual swarms in Daphnia magna, a cyclic parthenogen

SUMMARY. Dense swarms were observed in a population of Daphnia magna (Crustacea: Cladocera). The swarms appeared during a sexual period in which females were abundant but males were rare. Each swarm contained several males and large numbers of 'imminently sexual' females, a class of females that was rare except in the swarms. This implies that both sexes respond to attractant stimuli, and that the female response occurs only during a brief critical phase in the ovarian cycle.     

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.     

Interspecific variation in the phenology of advertisement calling in a temperate Australian frog community

Spatial and temporal partitioning of resources underlies the coexistence of species with similar niches. In communities of frogs and toads, the phenology of advertisement calling provides insights into temporal partitioning of reproductive effort and its implications for community dynamics. This study assessed the phenology of advertisement calling in an anuran community from Melbourne, in southern Australia. We collated data from 1432 surveys of 253 sites and used logistic regression to quantify seasonality in the nightly probability of calling and the influence of meteorological variables on this probability for six species of frogs. We found limited overlap in the predicted seasonal peaks of calling among these species. Those shown to have overlapping calling peaks are unlikely to be in direct competition, due to differences in larval ecology (Crinia signifera and Litoria ewingii) or differences in calling behavior and acoustics (Limnodynastes dumerilii and Litoria raniformis). In contrast, closely related and ecologically similar species (Crinia signfera and Crinia parinsignifera; Litoria ewingii and Litoria verreauxii) appear to have staggered seasonal peaks of calling. In combination with interspecific variation in the meteorological correlates of calling, these results may be indicative of temporal partitioning of reproductive activity to facilitate coexistence, as has been reported for tropical and temperate anurans from other parts of the globe.     

Population dynamics and production of Daphnia hyalina in a eutrophic reservoir

The population dynamics and production of Daphnia hyaiina^ the dominant cladoceran i n Eglwys Nynydd, a shallow eutrophic reservoir in South Wales, were studied for 2 years against a background of limnological measurements. The appearance and development of successive generations from egg to adult could be followed from changing numbers in arbitrarily defined size classes. Seasonal variations in mean length, mean brood-size and proportion of gravid adults were recorded and mean brood-size was related to changing food and temperature conditions. Egg-development times for D. hyaiina were determined in culture and the population parameters finite birth (S), instantaneous birth (b′), instantaneous population change (r′), instantaneous death (d′) and finite death rates (D) were estimated from field data. Turnover and production estimates were calculated from finite death rates and biomass. The calculated potential rate of increase (b′) was nearly always greater than the observed rate of increase (r′): seasonal changes in death rate (d′) generally parallel changes in birth rate (b′) but remain somewhat out of phase. Population oscillations are probably due t o a delay in the expression of the effects of population density upon birth and death rates. The mean biomass of Daphnia in 1970 was 0-57 mg dry wt/l (0-88 g C/m²) and in 1971 0-32 mg dry wt/l (0.49 g C/m²). Annual production for Daphnia was 11-8 mg dry wt/l (18-2 g C/m²) in 1970 and 8-30 mg dry wt/l (12 8 g C/m²) in 1971. Information available on primary production in the reservoir suggests that the production of Daphnia accounts for less than 2% of gross primary production. However, the pattern of population growth of Daphnia in Eglwys Nynydd almost certainly reflects a food limited system. In summer, blue-green algae may be abundant but serve as a poor food source: throughout the blue-green bloom egg production remains low, at times remaining below 0-5 eggs/adult.     

Effects of recent climate change on phytoplankton phenology in a temperate lake

1. A number of long-term studies have shown that spring biological events have advanced in recent decades and that this is a response to climate change. In lentic systems, changes in phytoplankton phenology have been attributed to various directly climate-related processes including changes in the onset and duration of thermal stratification, earlier ice-break up and increased water temperature. Both indirect climatic drivers and non-climate drivers such as elevated grazing pressure and nutrient enrichment can also affect phenology.
2. This study investigated whether phenological trends in phytoplankton could be detected in a relatively short time series in a shallow, ice-free, polymictic lake with a high annual discharge and whether any such trends could be causally explained.
3. It was found that the centre of gravity of the spring chlorophyll a bloom advanced significantly by 1.6 days per year over a 15-year period. This was accompanied by a significant increase in water temperature of 0.12 °C per year which is high compared to published rates of change over longer time series. No direct effects of ice cover, stratification or water discharge rates could be linked to the advancement of the spring bloom. Instead, the shift in timing was attributed to an advance in the timing of the dominant spring diatom, Aulacoseira spp., instigated by a temperature-driven increase in replication rate leading to an earlier onset of silica (SiO₂) limitation.