Competition in natural populations of Daphnia

I investigated the competitive relationships between two species of Daphnia, D. galeata and D. cucullata, and their interspecific hybrid. The term hemispecific competition was introduced to describe competition between parental species and hybrids. In eutrophic Tjeukemeer both parental species were found to compete with the hybrid, whereas competition between D. galeata and D. cucullata seemed limited. Although the effect of competition on life history traits of daphnids may be profound, the influence of the competitors on the seasonal dynamics of the Daphnia species seems limited.     

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.     

Morphological differentiation within the Daphnia longispina group

Although morphological evolution is assumed to be slow within Daphnia species complexes, discontinuities in morphological space can be detected. Here, morphological data derived from females of genetically-defined clones (cf. Gießler et al., 1999) are presented, in order to estimate the genetic component of phenotypic variance under standardised laboratory conditions. Animals originated from clonal assemblages of pre-alpine lakes and ponds, and a remote lake in western Germany, covering a wide range of morphotypes known from the traditional species D. cucullata, D. galeata, D. hyalina, D. rosea, and a variety of interspecific hybrids. Phenotypic analyses were based on quantitative and qualitative morphological characters of females in the first and fifth instars. Morphological divergence between clones was analysed using discriminant analysis or multidimensional scaling and the significance of the morphological groupings was estimated using neighbour-joining trees and bootstrapping. All analyses confirmed that (a) phenotypic similarities among taxa change with instar, (b) in contrast to low genetic divergence, pronounced morphological divergence exists between animals separated on the lake/pond level favouring speciation by the habitat shift hypothesis.     

Life-history variation in a hybrid species complex ofDaphnia

Summary Life-history variation was examined among members of theDaphnia longispina group, which consists ofD. galeata, D. hyalina, D. cucullata, and hybrids. Factorial experiments were conducted at two temperatures (14° and 20° C) and two food concentrations (0.2 and 1.0 mg Cl⁻¹). Differences in life-history features (size at maturity, age at first reproduction, size of first clutch, offspring size in first clutch) under the different environmental conditions were assessed among eightDaphnia clones, which represented members of this species complex. Significant differences between parentals and hybrids for most life-history features were observed under various treatments; generally, hybrid clones showed intermediate life-history traits when compared with parentals. When comparisons were made among clones within a given species (i.e.D. galeata, D. galeata xcucullata, D. cucullata), clonal differences were also noted for certain life-history traits. The data are discussed with reference to the formation and maintenance of hybrid species complexes in nature. This paper is dedicated to the memory of Hans Georg Wolf, who died suddenly in May 1990 at the age of 39 years.     

Interspecific hybridization betweenDaphnia hyalina,D. galeata,andD. cucullata and seasonal abundances of these species and their hybrids

The three cladoceran speciesDaphnia hyalina, D. galeata, andD. cucullata frequently coexist in the lakes of northern Germany. Although there are some problems in distinguishing them morphologically, they are easily determined by gelelectrophoresis: each species carries a different allele at the glutamate oxaloacetate transaminase (GOT) locus. Animals morphologically intermediate between two species are heterozygous for the alleles carried by the species they resemble. This pattern is in agreement with the findings at other loci, where also diagnostic alleles exist. These findings are most easily explained by interspecific hybridization between the three species. No evidence is found for backcrosses involving hybrids ofD. cucullata, whereas some backcrosses betweenD. hyalina, D. galeata, and their hybrids are found in some lakes. In four lakes the seasonal abundances of the three species and their hybrids are determined.     

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.     

Differences in genetic structure and ecological diversity between parental forms and hybrids in a Daphnia species complex

Hybridization is a common phenomenon in Daphnia species complexes. Hybrids often dominate in Daphnia populations; therefore it is worthwhile to look for principal differences between parental and hybrid populations with respect to their genetic structure and clonal differentiation. We studied natural populations of members of the Daphnia galeata/hyalina/cucullata complex in three lakes. In one of these lakes, one parental species (D. galeata) and one hybrid (D. galeata × cucullata) were investigated more intensively. The frequency of sexual reproduction was higher in parental populations, whereas clonal diversity was higher in hybrid populations. Ecological differentiation among clonal groups was more pronounced in the D. galeata × cucullata hybrid compared to D. galeata, whereas selection intensity was weaker. These results are discussed with respect to stability of clonal groups, multiple hybridizations and selective constraints.     

The possible origin ofDaphnia cucullata procurva Poppe 1887 in the lakes of the Pomeranian Lakeland by hybridization in the past

Besides the genusBosmina, the genusDaphnia offers considerable taxonomic difficulties due to the existence of a very great number of different morphotypes and their intergrades. Especially the morphotypes clustering aroundDaphnia galeata Sars, 1864,D. hyalina Leydig, 1860, andD. cucullata Sars, 1862 offer a bewildering picture. In the last years an attempt has been made to understand this phenomenon mainly as the result of hybridization and colonization of new biotopes by populations with hybridogenetic polymorphism and genotypic diversity (founder effect). Now it will be suggested on a morphological base thatD. cucullata procurva Poppe, 1887, a unique and aberrant formtype restricted to few lakes along the river Brda in Poland, is probably an old and stabilized hybrid originating from interspecific hybridization betweenD. galeata andD. cucullata.     

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.     

Daphnia Galeata × D. Longispina Hybrids in Western Norway

We describe the occurrence of D. galeata×longispina hybrids in two lakes of western Norway. Parental species and interspecific hybrids were characterised by both nuclear and mitochondrial molecular markers. In one of the populations, hybrids were shown to dominate the population over several years. A few individuals in both populations were probably not F1 hybrids, but possibly backcrosses or F2 hybrids. Most (possibly all) F1 hybrids were of D. galeata maternal origin. In addition, interspecific hybrids could be identified based on morphological characters, which were intermediate between the parental species. Interspecific hybridisation between these two species is remarkable, since they are distantly related.     

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.     

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.     

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.     

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.     

Life history patterns of parental and hybrid Daphnia differ between lakes

1. We investigated whether Daphnia galeata × hyalina hybrids of Lake Constance and Lake Greifensee show the same pattern of life history parameters as previously reported for D. galeata × cucullata hybrids and whether such a pattern is consistent between Daphnia populations from those two lakes. 2. Hybrids in Lake Constance were intermediate in size compared with the parental species. Hybrids in Lake Greifensee were smaller than D. galeata. The intrinsic growth rate (r) of hybrids from Lake Constance was not significantly different from the faster growing parental taxon D. galeata. However, r of hybrids from Lake Greifensee was significantly lower than that of D. galeata. 3. The observed juvenile body length differences between the taxa varied with the clutch number. The first clutch juvenile lengths of the three taxa did not differ for Lake Constance. First clutch juveniles of Lake Greifensee D. galeata were smaller than hybrid first clutch juveniles. The third clutch juvenile length did not differ between taxa from Lake Greifensee, but D. galeata juveniles from Lake Constance were bigger than those of D. hyalina. 4. The life history pattern found in Lake Constance corresponds to previous findings from other studies. The hybrids in this lake combine the faster population growth of one parental species with a relatively small size. In the case of Lake Greifensee hybrids, the relatively large size of first clutch juveniles and the small size of the adults could be interpreted as dual adaptations to invertebrate and fish predation. We speculate that the lower population growth rate of the hybrids is a trade‐off for this twofold protection.     

Optimising survival under predation: chemical cues modify curvature in Daphnia galeata

Morphological responses to the presence of predator info-chemicals havebeen described for many Daphnia (Cladocera) species, butD. galeata is generally considered to exhibit almost nomorphological changes that could increase its fitness under predation.Therefore, the aim of our study was to examine the nature and magnitude ofmorphological responses of D. galeata to their predatorsindetail and assess their potential role in decreasing the predation threat. Twoclones of Daphnia were exposed to predator info-chemicals(kairomones) from perch, a fish (Perca), and a phantommidge larvae (Chaoborus) an invertebrate, and a kairomone mixture fromboththese organisms. Laboratory life-table experiments were carried out and fiveparameters characterising the body shape of the daphnids were measured: helmetlength, head- and carapace width, eye diameter and body size. The last-namedthree parameters did not differ significantly between the clones or thetreatments. The differences found between the clones were significant for headwidth and helmet length, but only in combination with the treatment effects.Ourresults on genotype-dependent phenotypic plasticity indicated that, althoughphenotypic plasticity is present, the clonal composition of aDaphnia population can be altered by selection on themorphotype. This potential for a change in clonal frequencies is given by thedifferences measured between the two clones in head width and helmet length,altering the curvature of the Daphina body in response tokairomone presence.     

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     

Inter-specific hybridization underlies phenotypic variability in Daphnia populations

Summary In the glacial lakes of the Palaearctic three species of Cladocera commonly coexist: Daphnia hyalina, D. galeata, and D. cucullata. Frequently these populations contain not only animals which are morphologically typical for the species but also individuals of an intermediate phenotype. Electrophoretic investigations of allozyme-patterns in morphologically typical individuals reveal that each species is fixed for a different allele at the GOT locus. Morphologically intermediate animals are heterozygous for the alleles of the two species which they resemble. The allelic pattern at other loci is also consistent with the assumption that morphological intermediates are formed via interspecific hybridization. Very few backcrosses between galeata-hyalina hybrids and their parent species are found, and there is no indication of gene flow between D. cucullata and the other species.     

How selfing and intra- and interspecific crossing influence seed set, morphology and ploidy level in Euphrasia: An experimental study of species occurring in the Alps of Switzerland

Annual alpine species rely on selfing rather than on cross-pollination for successful reproduction. However, insect visits may occasionally cause cross-pollination not only within but also between closely related species. The aim of this study was to investigate four species of Euphrasia for their efficiency in spontaneous selfing and their success in intra- and interspecific crossing. We used the seed sets that followed spontaneous selfing and artificial cross- and selfpollination to measure the breeding success. We compared the morphological characters of species and hybrids and determined their ploidy level using flow cytometry. We verified the hybridogenous origin of plants resulting from interspecific crosses using RAPD banding patterns. While spontaneous seed set was high in the two small-flowered species, seed set in the large-flowered species was small and affected by external circumstances. We obtained F1 and F2 hybrids from interspecific crosses of two diploid species and detected polyploid individuals in both generations.     

Intraspecific spatial genetic differentiation of a Daphnia species within a long narrow reservoir

Environmental gradients influence the distribution and taxonomic composition of planktonic taxa, including Daphnia. In canyon-shaped reservoirs with pronounced horizontal gradients of food supply, predation pressure and other factors, not only species and interspecific hybrids but also clones within these taxa are non-randomly distributed. Using a long-term data set from a reservoir mostly dominated by a single Daphnia species, we evaluated whether intraspecific genetic differentiation can be frequently detected between upstream and downstream reservoir regions with different environmental conditions. We analysed variation at four allozyme loci (two species-specific and two polymorphic) to assess the taxonomic composition and intraspecific variation of Daphnia collected in different years (between 1995 and 2005) and periods of the growing season. D. galeata dominated in all samples, although other species and hybrids with D. galeata were also occasionally detected. Despite limited variation at the analysed loci, D. galeata from upstream and downstream regions were significantly genetically differentiated on seven out of twelve sampling dates. Although genetic drift in geographically distant subpopulations may contribute to differentiation, we presume that the observed patterns are primarily due to different selection regimes. We predict that a significant genetic differentiation within planktonic populations also occurs more frequently in natural water bodies.