Taxon-specific reaction norms to predator cues in a hybrid Daphnia complex

1. Previous studies have shown that interspecific hybridisation is common among taxa from the Daphnia galeata/hyalina/cucullata species complex. We investigated the influence of predator kairomones on the morphology and life histories of nine clones belonging to three taxa (pure D. galeata, F₁ hybrids between D. galeata and D. hyalina, and backcrossed D. hyalina) of this species complex. Predators exerting positive (fish) and negative (Chaoborus larvae) size‐selective predation were tested. 2. The most responsive traits were size at maturity and size of neonates. Despite large between‐clone variation, discriminant analysis revealed that the three taxa were distinct from each other in key life‐history traits. F₁ hybrids did not react in an intermediate way compared to the other taxa: the multivariate distances between F₁ hybrids and either taxon were larger than between pure D. galeata and backcrossed D. hyalina. 3. The average plasticity (calculated across all traits) was similar for all three taxa. With regard to the size at maturity and neonate body size, the strength of the response was a function of the intrinsic values of these traits expressed in the control. For example, for size at maturity, smaller individuals showed a significantly stronger reaction to Chaoborus kairomones than larger ones. 4. Finally, we monitored seasonal changes in body size, egg number and population density of pure D. galeata and F₁ hybrids in Greifensee (Switzerland). The two taxa experienced similar seasonal changes in body size but, on some sampling dates, they differed in mean egg number. The observed seasonal changes in Daphnia body size were consistent with what would be expected if the predator assemblage shifted from fish to Chaoborus over the course of the summer. The fluctuations in the frequencies of Daphnia taxa, however, were not related to seasonal variation in Daphnia body size. 5. Experimental data suggest that temporally heterogeneous predation regimes might be an important condition stabilising the co‐occurrence of Daphnia hybrids with parental taxa. Predation regimes, however, cannot solely explain dynamic changes in taxon frequency in Greifensee.     

Amplified Fragment Length Polymorphism (AFLP) Reveals Species-Specific Markers in the Daphnia Galeata–HyalinaSpecies Complex

Daphnia often occur in species complexes that consist of two or more co-occurring species and their hybrids. Hybrid individuals are often capable of sexual reproduction and so backcrossing with introgression occurs. To better understand hybridization and backcrossing frequency, we sought to develop PCR-based, species-specific markers in the Daphnia galeata–hyalina species complex using amplified fragment length polymorphism (AFLP). This technique produces large numbers of reproducible markers for assessing diversity across the nuclear genome and provides several advantages over mtDNA and microsatellite approaches. We examined 28 clones of D. galeata, D. hyalina, and their hybrids isolated from Lake Constance on the Swiss-German border. Using a single AFLP primer combination we found five potential species-specific markers, defined as bands that occurred in >80% of one parental species and <20% of the other. Two bands appeared to be co-dominant and were present (homozygous) in D. galeata, absent in D. hyalina, and heterozygous in the hybrid. We conclude AFLP could provide enough PCR-based, species-specific markers to identify species, hybrids, and backcrosses from even small amounts of tissue (i.e. resting eggs).     

Plasticity in filtering screens of Daphnia cucullata×galeata hybrids and parental species at two food concentrations

The coexistence of Daphnia cucullata × galeata hybrids with the parental species D. galeata and D. cucullata was investigated by measuring areas and mesh sizes of filtering structures of these herbivorous zooplankton taxa cultivated at low and high food concentrations. The clearance rates and somatic growth rates were also determined. When reared at low food concentration, all taxa had larger filtering areas. Larger filtering areas also resulted in higher clearance rates. Differences between taxa in both filtering area and clearance rate were caused mainly by interspecific size differences. Hybrids had the largest absolute mesh sizes, and the parental species had smaller mesh sizes. Hybrids also showed heterosis in somatic growth rate at high food concentration. The observed taxon-specific differences in mesh size and somatic growth rate contribute to resource partitioning between the taxa and thus to their successful coexistence in lakes. Received: 1 March 1999 / Accepted: 25 May 1999     

Cyclomorphosis as a factor explaining success of a Daphnia hybrid in Tjeukemeer

Recent studies have shown that Daphnia hybrids are intermediate to their parental species for several life history traits, and that they combine advantageous traits of the parental species, thereby decreasing their risk of being preyed upon by planktivorous fish. In this study D. galeata, D. cucullata and their interspecific hybrid are compared with respect to cyclomorphosis and juvenile growth as a potential mechanism to avoid invertebrate predation. Helmets and tail spines are known to reduce predation risk, whereas fast juvenile growth reduces the period that Daphnia are vulnerable to predation. In laboratory cultures, I tested the hypothesis that, with regard to cyclomorphosis, juvenile hybrids are more comparable to the juveniles of D. cucullata, and with respect to growth rate, they are more comparable to D. galeata. The hybrid did not differ significantly from D. galeata in the time needed to reach 0.9 mm body length. In contrast, the relative spine and helmet length of juvenile hybrids were more comparable with those of D. cucullata, whereas in adult instars these characters were more similar to D. galeata. Thus during the first juvenile instars, the hybrids combine the relative long helmet and spine of D. cucullata, with the faster growth of D. galeata. This combination of characteristics might reduce hybrid vulnerability to invertebrate predation in field situations.     

Effects of crowding and different food levels on growth and reproductive investment of Daphnia

The effects of daphniid crowding on juvenile growth rate, length at first reproduction, clutch size and egg size of four species of Daphnia were compared with the effects of food level. Juvenile Daphnia were grown to primipary in a flow-through system in water conditioned by different densities of the same, or another, species. At high ambient food levels, water from Daphnia that had been crowded at densities 150 l^–1 depressed growth rate and lowered body size and clutch size of D. hyalina and D. galeata; effects on the same traits of D. magna and D. pulicaria were variable (stimulation, depression, or no effect). D. hyalina and D. galeata responded to gradients of increasing daphniid density (0–300 l^–1) by altering egg mass, somatic mass and clutch size to maintain a relatively constant reproductive investment; egg mass increased with crowding and then decreased in a pattern consistent with Glazier's (1992) hypothetical model of changes in offspring size in relation to food quantity and maternal demand. Effects of crowding by conspecifics were indistinguishable from those of other species. This study, which uncouples the effect of crowding per se from ambient resource depletion, shows that chemical substances released by high densities of Daphnia can cause changes in life-history traits comparable to those that occur in response to low food levels.     

No Evidence for Adaptive Micro-Evolution to a Decrease in Phosphorus-Loading of a Daphnia Population Inhabiting A Pre-Alpine Lake

Many polluted lakes in Europe are being restored and phosphorus concentrations have dropped dramatically in these lakes. We studied the genetic structure of Daphnia galeata over the past 30 years in Lake Greifensee, Switzerland, a period during which the phosphorus concentration in the lake reduced dramatically. Distinct genotypes of D. galeata were hatched from diapausing eggs extracted from six different time horizons in dated sediments. We compared juvenile growth, size and egg production of D. galeata reared on high-P and low-P algae to investigate whether Daphnia have evolved to grow better on phosphorus-limited algae. Our results indicate that life histories of D. galeata differed significantly between both food types. We also found significant clone effects for size and egg number. But we found no significant interaction between the depth from which the clones were selected and food quality. This means that we found no evidence for adaptive micro-evolution in response to P reduction in the lake. We discuss our results in relation to other studies that found evidence for adaptive micro-evolution in comparable time frames.     

Filamentous cyanobacteria alter the relative fitness in a Daphnia hybrid species complex

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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.     

Experimental interspecific hybridization in Daphnia

Hybridization is a common phenomenon in Daphnia (Cladocera; Anomopoda); interspecific hybrids have been found between several species and hybrids are found in many European lakes. Although much information on the morphology, ecology and genetics of hybrids is available, little is known about the level of reproductive isolation among species or about the relative fitness of hybrids and parental species. In order to facilitate studies on differentiation and speciation processes and comparative experimental studies on hybrids and recombinant genotypes, we present the first successful laboratory crossing experiments of two different Daphnia species, D. galeata and D. cucullata. Males and sexual females from two D. galeata and two D. cucullata clones were reciprocally crossed, juveniles hatched from resting eggs and reared until maturity. Hatching and juvenile survival rates of hybrids were relatively low (12.1% and 24%, respectively). D. galeata and D. cucullata clones vary in their level of successful interspecific matings and in the number of subsequent offspring. In general, hybrid crosses between D. cucullata females and D. galeata males were more successful than reciprocal crosses.     

Contrasting migration behaviour of Daphnia pulicaria and D. galeata x hyalina, in avoidance of predation by 0+perch (Perca fluviatilis)

In spring and early summer, a small population of the large-bodied Daphnia pulicaria coexists with a much larger population of the medium-sized hybrid Daphnia galeata × hyalina in the epilimnion of Lake Maarsseveen (The Netherlands). When large shoals of juvenile perch (Perca fluviatilis) appear in the open water, both species start to migrate vertically. Since D. pulicaria has a larger body-size than D. galeata × hyalina, and is therefore competitive dominant over the hybrid, it is unlikely that both species interact via their common food resource, but they react both to fish predation. However, since they differ in size, and therefore in vulnerability for fish predation, both species adopt different strategies. The smaller bodied, and less vulnerable D. galeata × hyalina exhibits diel vertical migration ascending to the surface at dusk, and staying there during the night. In this way, benefiting from the higher temperatures of the surface layers. In contrast, the large-bodied, and more vulnerable D. pulicaria selects the deep cold hypolimnion water layers as refuge against fish predation. In this way it benefits from a safe habitat, free from fish predators, but on the other hand suffers from low water temperatures, which decrease its fitness. It is likely that the relatively higher temperature in the upper water layers serves as a proximate factor for the downward migration of D. pulicaria.     

Elemental composition of two coexisting Daphnia species during the seasonal course of population development in Lake Constance

Summary In 1986 and 1987 the seasonal course of population density, life history traits and biomass composition (CHN-elemental and ash) of two coexisting Daphnia species from L. Constance (Überlinger See) were studied. The two daphnids are morphologically similar, but demonstrate different seasonal variation of behaviour (i.e. diurnal vertical migration). With the exception of nitrogen content, the mean biomass composition of adults and that of eggs were significantly different: the dry biomass of eggs (adults) contained on average 55.6 (46.1) % C, 7.9 (6.5) % H, 9.7 (9.7) % N, and 10.0 (23.8) % ash. These values were similar to those of other daphnids; it may be concluded that the average daphnid composition is not species specific, but is determined mainly by the position in the life cycle and by the nutritional state of the animal. A distinct seasonal variation in the elemental composition of the biomass of adult daphnids was observed. It related significantly to the concentration of food, as indicated by the epilimnetic chlorophyll-a concentrations and by Secchi depth. From the amplitude of seasonal variation of elemental composition and from the shape of relationships between the elemental composition of daphnids and their food, it can be concluded that in summer the non-migrating D. galeata lives under stronger food limitation than the migrating H. hyalina. In spring, there were no differences in the biomass composition of either species. In summer, the adult fcmales of D. galeata contained significantly less carbon, and hydrogen than those of D. hyalina. In most parameters of biomass (i.e. biomass composition of adults) and of life history (i.e. clutch size, mean adult weight), D. galeata displayed higher coefficients of variation. A tendency in D. galeata to have peaks of population development carlier in the season and in D. hyalina to have them towards autumn was observed. These contrasting responses of the populations development, the seasonal variability of the biomass composition seem to support the view of Geller (1986) who called D. galeata and D. hyalina exploitative and conservative strategists respectively.This research was supported by the Deutsche Forschungsge-meinschaft within the Sonderforschungsbereich Cycling of Matter in Lake Constance (SFB 248), and by scholarship to R.B. from the Deutscher Akademischer Austauschdienst (DAAD)     

The contribution of differential hatching success to the fitness of species and interspecific hybrids

Resting egg banks of microcrustaceans have been used to reconstruct the evolutionary and ecological history of species. However, recent studies provided evidence for a discrepancy between dormant propagules in the sediment and the planktonic population. This pattern raises two questions: First, what is the value of data on resting egg banks for population dynamics over time and second, which component of the reproductive cycle causes the observed inconsistency? In our study we focussed on the second question by comparing the taxon composition of a resting egg bank with the reproductive success of ex-ephippial hatchlings. Species and interspecific hybrid identification of dormant and hatched stages was achieved through the application of restriction fragment length polymorphism analysis of an internal transcribed spacer region. We found no significant deviation between the proportion of hatched Daphnia galeata, D. galeata × hyalina and D. hyalina individuals and the observed taxon composition of the resting egg bank. However, species and hybrids differed in their mode and relative success of reproduction. We conclude that the components of reproductive success in Daphnia contribute differentially to the fitness of species and interspecific hybrids. The discrepancy between resting egg banks and “active” planktonic populations results not from differential hatching of species but from the reproductive success of ex-ephippial females and the timing and frequency of sexual reproduction of the different taxa.     

Costs of Predator Avoidance Reduce Competitive Ability of Daphnia

Large-sized Daphnia hyalina×galeata hybrid and small-sized Ceriodaphnia reticulata were raised together for 3 weeks in thermally stratified flow-through tubes ('plankton organ'), in the presence and the absence of fish kairomone, and their relative performance was monitored, as indicated by the populations' growth rates (r). In the kairomone-free medium, D. hyalina grew faster than C. reticulata; the presence of kairomone reversed this ratio, producing relatively higher r in C. reticulata. The simulated fish presence reduced growth in D. hyalina population also when it was separated from the competitor, while growth in C. reticulata was altered by the kairomone only in the presence of Daphnia. It is suggested that costs of surface avoidance in Daphnia can reduce its competitive superiority to small-sized cladocerans, permanently dwelling at the surface.     

The influence of fish kairomones on the induction and vertical distribution of sexual individuals of the Daphnia galeata species complex

To assess the potential production of hybrids and backcrosses in a semi-natural environment, we studied the combined effect of fish kairomone, and food level on the production of males and ephippial females in different clones of five Daphnia taxa from the D. galeata species complex. We also studied the diel vertical migration (DVM) of these sexual daphnids under the same varying conditions. This was done to test the hypothesis that males and ephippial females have different migrating strategies, which would increase their mating probability. The study was carried out in two large-scale indoor mesocosms, the so-called `plankton towers' in the Max-Planck Institute in Plön, Germany.Although all of the Daphnia taxa produced ephippial females in the course of the experiment, only D. galeata produced a significant number of males. Fish kairomones had a significant negative influence on the production of ephippial females. We found no DVM in the D. galeata males. They stayed at a depth between 5 and 6 m both day and night, 1 or 2 m above the thermocline. The ephippial females of D. cucullata x hyalina migrated, whereas ephippial females of the other taxa showed no DVM but came significantly closer to the surface in the presence of fish kairomones. We conclude that males and sexual females co-occur in this species complex both in time and space. Therefore, a regular production of hybrids and backcrosses in this species complex seems likely. Fish kairomones do not seem to significantly influence this process.     

Impact of fish predation on coexisting Daphnia taxa: a partial test of the temporal hybrid superiority hypothesis

Fish predation was tested as a factor mediating the coexistence of Daphnia taxa in the shallow, hypertrophic Lake Blankaart. Naturally co-occurring populations of D. galeata and the hybrid D. galeata x cucullata were subjected to different levels of fish predation in in situ enclosures. In control enclosures without fish, the largest taxon D. galeata rapidly became dominant over the intermediate sized D. galeata x cucullata, mainly as a result of higher birth rates. In enclosures with fish, population densities of D. galeata dropped relative to D. galeata x cucullata, due to higher mortality rates. These results are in concordance with the `temporal hybrid superiority hypothesis', and can be explained by a higher vulnerability of the large and more conspicuous D. galeata to the size selective predation exerted by visually hunting planktivorous fishes. After approximately one month, however, population growth rates of D. galeata and D. galeata x cucullata in the enclosures with fish converged, due to a relative reduction in the mortality rate of D. galeata. This suggests that, in the presence of fish, D. galeata may co-exist with hybrids due to a decrease in its relative vulnerability to visual predation with time. Indeed, both D. galeata and the hybrid showed strong reductions in adult body size in the enclosures with fish, but this size reduction tended to be stronger in D. galeata than in D. galeata x cucullata. In addition, turbidity increased in the enclosures with fish and may additionally have reduced the relative advantage of D. galeata x cucullata with regard to mortality caused by visual predation.     

Spatial and temporal patterns of sexual reproduction in a hybrid Daphnia species complex

Evidence for extensive interspecific hybridization among speciesof the genus Daphnia has been accumulating on a global scale.Although there is evidence for limited gene flow between taxavia hybridization, many species still maintain discrete morphologicaland molecular characteristics. We studied temporal and spatialpatterns of sexual reproduction within the Daphnia galeata–hyalina–cucullatahybrid species complex in a lake (Plußsee), located innorthern Germany. Allozyme electrophoresis allowed us to trackseasonal changes in taxon composition as well as the quantificationof back-crosses. Sexually-reproducing animals (ephippial femalesand males) were mainly found in autumn. The simultaneous presenceof sexual morphs of D. galeata and D. galeata x hyalina withthe dominant D. hyalina taxa makes recent hybridization, aswell as back-crossing, plausible. Males and ephippial femalesof D. hyalina were not back-crossed as were the parthenogeneticfemales. The low number of sexual clones of the hybrid D. galeatax hyalina might reflect its reduced fertility, although thesefew clones were detected in high densities. Only hybrid-clonesthat had a back-cross genotype (towards D. hyalina) exhibitedephippial females and males. This indicates that male and ephippialfemale production within the Daphnia taxa is not random, whichmight increase the chance for the parental Daphnia species toremain distinct.     

The energy budget of two sympatric Daphnia species in Lake Constance: productivity and energy residence times

Summary Two Daphnia species, D. hyalina and D. galeata, are living in Lake Constance. Both populations show logistic growth, with the phase of exponential increase in May and the phase of steady state during the summer. In spring, food is not limited and both daphnids reveal an exploitative strategy: females mature early, primiparae are small, egg weights are small, but clutch sizes are large. From June on, when food limiting conditions prevail, D. hyalina starts to migrate vertically with a diurnal rhythm whereas D. galeata keeps on living in the epilimnion over the whole season. The differing thermal environments of the populations lead to short generation times (age of first reproduction) of 11 days for D. galeata in mid-summer, and to 47 days (July) for D. hyalina. The annual production (P) of D. hyalina and D. galeata were 18 and 24 g DW/m², respectively. The average standing stocks (B) were 1.6 and 0.9 g DW/m². This corresponds to turnover rates of 11 x and 27 x per season (200 days), and to turnover times of 18 days and 7.5 days, respectively. From 1980 to 1982 the cumulative annual primary production (PPR) varied between 260 and 330 g C/m². The common productivity (P) of the functional component Daphnia (=sum of both Daphnia species) followed the PPR, but the Daphnia stading stock was constant at 2.5 g DW/m². The increase of secondary production (P) was a consequence of a shift in abundance between the two Daphnia species: at low PPR, D. hyalina was 4 x more abundant, but at high PPR, the two species were equally abundant. Such internal regulatory mechanism within a system component is in accordance with the hypothesis of Cheslak and Lamarra (1981): increase of energy residence time (B/R) with declining nutrients and increasing strength of competitive interrelationships within a given functional component of an ecosystem.This research was supported by the Deutsche Forschungsgemeinschaft within Sonderforschungsbereich Stoffhaushalt des Bodensees (SFB 248)     

Food Quality Constraints in Daphnia: Interspecific Differences in the Response to the Absence of a Long Chain Polyunsaturated Fatty Acid in the Food Source

The effect of the long chain polyunsaturated fatty acid (PUFA) eicosapentaenoic acid (EPA, 20:53) on food quality for Daphnia galeata, Daphnia hyalina and a D. galeata×hyalina hybrid was examined. Somatic growth rates of all three species were determined when growing on EPA-free Scenedesmus obliquus and on S. obliquus which had been supplemented with free EPA prior to the growth experiments. Growth rates on S. obliquus were substantially higher in D. galeata(0.45day^–1) than in D. hyalina(0.27day^–1) and D. galeata×hyalina(0.31day^–1). Supplementary EPA increased growth of D. galeata and D. hyalina, but not of the hybrid. Hence within the D. galeata/hyalina complex species differ in their ability to cope with an EPA-free diet. Analysis of fatty acids revealed that tissue concentrations of 18:33, 18:43, 20:33 and 20:43 were higher in D. galeata than in D. hyalina which indicated higher rates of assimilation and biosynthesis of PUFAs in D. galeata. This was contrasted by lower tissue concentrations of 20:53 D. galeata than in D. hyalina which suggests that 20:53 is metabolised with growth. In the D. galeata×hyalina hybrid high PUFA assimilation and biosynthesis were associated with low growth rates which explains the finding that tissue concentrations of 20:53 were highest and that growth was not constrained by the availability of 20:53.     

Seasonal variability in the relationship between body length and individual dry weight as related to food abundance and clutch size in two coexisting Daphnia species

Dry weights of individual Daphnia galeata and D. hyalina, whichcoexist in Lake Constance, were determined weekly during a twoyear investigation. From each sample and each species a length–weightrelationship (LWR) was calculated resulting in 54 (65) regressionequations based on N=2466 (3032) in D. galeata (D. hyalina).The constants (Ln(A)) and (B) of the LWRs showed a marked seasonalvariability, ranging from Ln(A)=1.0784–2.4740(1.0152–2.3860)and B=2.14–4.20 (2.11–4.15). The seasonal variabilityof the LWR was estimated with four different models. Condensingthe 54 (65) equations into one seasonal cycle resulted in amodel with an explained variance of r²=0.873 (0.869). Threemore models were established by calculating multiple regressionswith take stratification, average fecundity (E), and food concentrationas determining variates. Differences in LWRs of the two Daphniaspecies were significant (p <0.001). As the most useful approximationduring the period of stratification, LWRs are recommended forD. galeata: Ln(W)=(1.5674+0.0287^*(E))+Ln(L)^*(3.3611+0.0111^*(E)) as a generalized LWR for an epilimnetic daphnid, and for D.hyalina: Ln(W)=(1.5593+0.0613^*(E))+Ln(L)^*(3.2709+0.0017^*(E)) as LWR for a daphnid which migrates diurnally between epi andhypolimnion. The explained variance of this model is r²=0.843(0.826). The influence of food concentration in the epilimnionon LWRs was found to be significant on Ln(A) but not on (B). ^*This research project was supported by the Deutsche Forschungsgemeinschaft     

Seasonal dynamics of metabolic activity of the Daphnia community in Lake Bled (Slovenia)

Seasonal changes of the community composition, oxygen consumption (R) and respiratory electron transport system (ETS)-activity of the Daphnia community living in Lake Bled (Slovenia) were studied between January and December 1998. The ETS activity of ovigerous Daphnia females at in situ temperature ranged from 3.27 l O₂ mg dw^–1 h^–1 in February to 20.91 l O₂ mg dw^–1 h^–1 in July. Respiration rates at in situ temperature varied from 4.04 l O₂ mg dw^–1h^–1 in December to 18.68 l O₂ mg dw^–1 h^–1in July. The influence of four factors (temperature, body size, fecundity, genetic differences) on the metabolism were investigated. Both ETS activity and respiration rate significantly correlated with temperature. The proportion of hybrid D. cucullata× galeata in Daphnia community correlated significantly with respiration rate at in situtemperature and ETS activity at standard temperature also. ETS activity and respiration rate showed no significant correlation with body size and the fecundity of Daphnia, whereas ETS activity in D. hyalina × galeata and D. cucullata× galeataseparately correlated with body size. ETS activity of D. hyalina × galeata also was correlated with fecundity. Hybrid D. hyalina× galeata had up to one third lower ETS activity than D. cucullata× galeata. The mean ETS/R ratio in the Daphnia community was 1.16±0.28 (N= 12). The ETS/R ratio did not correlate significantly with temperature, body size, fecundity or the proportion of D. cucullata× galeatain the Daphnia community. Laboratory experiments showed that both hybrids had similar ETS/R ratios.