Daphnia hybridization along ecological gradients in pelagic environments: the potential for the presence of hybrid zones in plankton

The relative homogeneity of pelagic environments has been regarded as the reason for the absence of hybrid zones for hybridizing planktonic Daphnia (Crustacea: Cladocera); occasional dominance of interspecific hybrids over parental species was explained by their temporal superiority in fluctuating environments. However, water bodies with spatially varying environmental conditions might facilitate the formation of hybrid zones in plankton. We studied the distribution of species and hybrids of the Daphnia longispina complex in 11 canyon-shaped reservoirs, localities characterized by horizontal environmental gradients (particularly of food supply and size-selective predation); we also analysed patterns of carapace size and fecundity among coexisting taxa. Spatial distribution of taxa agreed with their ecological characteristics; those showing different affinities along longitudinal reservoir profiles differed in size according to the presumed fish predation gradient. Only hybrids of Daphnia galeata with Daphnia cucullata and D. longispina (=hyalina) were recorded. The latter two species preferred opposite ends of gradients, such spatial segregation probably explaining the absence of their hybrids. Distributional patterns were relatively stable in two consecutive summers, apart from a substantial decline of D. galeata X cucullata in the second year. The observed pattern of a hybrid-dominated zone in intermediate conditions suggests that local Daphnia hybrid zones may indeed form within reservoirs.     

The edibility of Staurastrum chaetoceras and Cosmarium abbreviatum (desmidiaceae) for Daphnia galeata/hyalina and the role of desmids in the aquatic food web

A laboratory clone of Daphnia galeata/hyalina was fed with two different planktic desmid taxa: Staurastrum chaetoceras and Cosmarium abbreviatum var. planctonicum, being about equal in cell size. Whereas Staurastrum chaetoceras was readily ingested and assimilated to a high degree, Cosmarium was hardly incorporated. This could be partly due to the presence of an extracellular mucilaginous envelope in the latter species. When decapsulated by mild sonification, Cosmarium cells were significantly better ingested but digestion was still inferior to that of Staurastrum, presumably because of the more compact cell shape of Cosmarium.From literature it appears that small-sized planktic desmid species occasionally may constitute a main food source for zooplankton, especially in eutrophic lakes. Most likely however, desmids, particularly large-sized species, play a much more important role in the food chain in the benthic compartment of shallow, oligotrophic water bodies where they serve as a food source for various macro-invertebrate taxa.     

西藏湖泊长刺溞复合种的系统进化关系

本文通过对西藏湖泊长刺溞复合种(Daphnia longispina complex)中分布最广的3个物种, 即长刺溞(D. longispina)、盔形溞(D. galeata)和颈齿溞(D. dentifera)线粒体COI基因序列以及GenBank中欧洲的长刺溞、加拿大的颈齿溞和我国东部低海拔地区的盔型溞COI基因序列的比较分析, 研究了西藏湖泊长刺溞复合种的系统进化关系, 发现西藏地区的盔型溞、颈齿溞和长刺溞均已出现较大分化。颈齿溞种群内遗传差异度为0.33-2.32%, 盔型溞为0.33-2.74%, 长刺溞的遗传差异度最高, 为1.31-5.50%。基于COI基因序列构建的最大似然树和贝叶斯系统树均表明, 长刺溞复合种由3个进化分支组成, 分别对应长刺溞、盔型溞和颈齿溞, 三者之间的遗传差异度为9.40-16.98%(Kimura 2-parameter双参数模型)。基于COI基因单倍型(haplotype)所构建的网络关系也支持上述3个分支的存在。早期记录虽然显示长刺溞在我国分布较广, 但本次调查只在班公错有发现, 相比之下, 盔形溞和颈齿溞则分布更广。我们的研究表明, 由于形态学鉴定上的局限性, 早期的长刺溞记录很可能混杂了容易引起混淆的盔型溞或颈齿溞。     

Genetic assessment of species boundaries in the North American Daphnia longispina complex (Crustacea: Daphniidae)

Species boundaries in the North American Daphnia longispina group have proved difficult to establish on the basis of morphology alone. This confusion may be due to hydridization, phenotypic plasticity or the existence of sibling species. We therefore used genetic analysis to delineate species boundaries by examining 27 North American populations belonging to the longispina complex for variation at 15–26 allozyme loci. The populations consisted of Daphnia thorata from two western sites and two eastern sites, Daphnia galeata mendotae from its type location and seven sites across its range, and Daphnia rosea from eight temperate and seven arctic sites. Two populations from the Eurasian longispina complex were also included for reference. Populations assigned to D. galeata mendotae formed a genetically cohesive group, whereas a genetic dichotomy was found between temperate and arctic D. rosea , suggesting that this taxon includes two species. Genetic analysis also confirmed the distinctness of western D. thorata from other members of the longispina group. Unexpectedly, eastern populations resembling D. thorata were genetically more similar to temperate D. rosea than to any helmeted species ( D. galeata, Daphnia hyalina or D. thorata ). Our results suggest that the helmet character is a poor indicator of phylogenetic relationships, as the genetic ability to produce this feature has been lost or acquired several times in the evolution of the longispina group.     

Effects of food type on the life history of Daphnia clones from lakes differing in trophic state. I. Daphnia galeata feeding on Scenedesmus and Oscillatoria*

1. Effects of the green alga Scenedesmus obliquus and the filamentous cyanobacterium Oscillatoria limnetica as food sources on the growth and reproduction of nine Daphnia galeata clones were studied. A high concentration of these two algae species was fed to Daphnia clones collected from four lakes with varying trophic conditions in a life history experiment.
2. Although Oscillatoria is regarded as a low quality food for Daphnia because of its filamentous morphology and suspected nutritional inadequacy, the cyanobacterium sustained both body growth and reproduction in D. galeata. Growth and reproduction were, however, lower and the onset of reproduction was delayed on a diet of Oscillatoria compared to a diet of Scenedesmus. The intrinsic growth rate, r , for each D. galeata clone was positive on a diet of Oscillatoria , ranging from 0.06 to 0.16 mm day^–1, whereas the intrinsic growth rates on a diet of Scenedesmus ranged from 0.27 to 0.34 mm day^–1.
3. The degree of growth depression on the Oscillatoria diet varied among clones. Clonal responses were correlated with body size; clones with larger body size decreased their growth and reproduction more on a diet of Oscillatoria. The responses of different clones from the same lake to the two diets resembled each other, irrespective of geography or trophic state of the lake.     

Interspecific hybridization in Daphnia: distinction and origin of hybrid matrilines

Three coexisting Daphnia species belonging to the D. longispina group (D. galeata, D. hyalina, and D. cucullata) form species-hybrid complexes by producing interspecific hybrids in several lakes in Germany and The Netherlands. To evaluate the genetic consequences of interspecific hybridization, I studied the patterns of mitochondrial DNA (mtDNA) sequence variation. The directionality of interspecific hybridization and divergence of hybrids from parental species was tested, using the DNA sequences of a segment of mtDNA. Via the polymerase chain reaction, it was possible to investigate single animals and even single resting eggs. A species-specific marker was established, using restriction patterns of amplified cytochrome b segments. mtDNA genotypes of hybrids revealed unidirectional mitochondrial gene flow for two hybrids, which were investigated by using multiple clones. No evidence for introgression of mtDNA was found. On the basis of a phylogenetic analysis, the species exhibit considerable distinctness, whereas differences between clones within species and between hybrids and maternal species tend to be very low. These results indicate a recent origin of hybrids and suggest that the radiation of the D. longispina group occurred > 5 Mya.      

Influence of Temperature on Exotic Daphnia lumholtzi and Implications for Invasion Success

We investigated the effects of water temperature on the exoticcladoceran Daphnia lumholtzi in a eutrophic Kansas reservoir(USA) and under laboratory conditions. Daphnia lumholtzi demonstrateda distinct late summer appearance in the reservoir at temperaturesbetween 26 and 31°C, which corresponded with steep declinesin the densities of native Daphnia spp. Laboratory life-tableexperiments confirmed that D. lumholtzi performs well at elevatedwater temperatures. The intrinsic rate of increase (r), netreproductive rate (R_o), age at first reproduction, survivorship(l_x) and molting rates all demonstrate that D. lumholtzi hasa high temperature optimum between 20 and 30°C. A comparisonof literature-reported r-values indicates that the reproductiverate of D. lumholtzi is comparable with other Daphnia spp. between20 and 25°C, but also implies that D. lumholtzi may out-performsome Daphnia spp. at temperatures >25°C. Collectively,these results suggest that D. lumholtzi may be taking advantageof a late summer thermal niche, and that this invader may continueto colonize lakes and reservoirs in the southern US. However,life table data also indicate that D. lumholtzi performs poorlyat temperatures < 10°C, which may inhibit the range expansionof this invader into northern waters.     

Food quantity and quality regulation of trophic transfer between primary producers and a keystone grazer (Daphnia) in pelagic freshwater food webs

The transfer of energy and nutrients from plants to animals is a key process in all ecosystems. In lakes, inefficient transfer of primary producer derived energy can result in low animal growth rates, accumulation of nuisance phytoplankton blooms and dissipation of energy from the ecosystem. Most research on carbon transfer efficiency in pelagic food webs has focused on either food quantity or food quality, with the latter considered separately as either elemental stoichiometry or biochemical composition. The natural occurrence and magnitude of these types of growth limitations and their combined effects on Daphnia , a keystone grazer in pelagic freshwater ecosystems, are largely unknown. Our empirical models predict that the strength and nature of food quantity and quality limitation varies greatly with lake trophic state (total phosphorus, TP) and that Daphnia growth rates and thus energy and nutrient transfer efficiency are highest in lakes with intermediate trophic status (TP 10–25 μg l⁻¹). We predict that food availability place the greatest constraint on Daphnia growth in nutrient poor lakes (TP≤4 μg l⁻¹). Phosphorus limitation of Daphnia growth increased with decreasing TP, but the overall effect was never predicted to be the dominant constraining factor. Eicosapentaenoic acid (EPA, 20:5ω3) limitation was predicted to occur in both nutrient poor and nutrient rich lakes and placed the primary constraint on food quality in the most productive lakes. Two contrasting EPA-models gave different results on the magnitude of EPA-limitation, implying that additional food quality factors decrease Daphnia growth at high TP. In conclusion, the model predicts that Daphnia growth should peak in mesotrophic lakes, food quantity will place the greatest constraint on growth in oligotrophic lakes and EPA will primarily limit growth in eutrophic lakes.     

Spatial distribution of the Daphnia longispina species complex and other planktonic crustaceans in the heterogeneous environment of canyon-shaped reservoirs

Canyon-shaped reservoirs are often characterised by longitudinalgradients of environmental factors (including trophic level,phytoplankton and zooplankton biomass and abundance of planktivorousfish) affecting the taxonomic composition of the pelagic community.We tested the hypothesis that the spatial distribution of differentspecies and interspecific hybrids of the Daphnia longispinaspecies complex is non-random under such conditions. Duringthe summer stratification, we sampled crustacean zooplanktonfrom 11 reservoirs, covering both longitudinal (upstream, middle,dam) and vertical (epi-, meta- and hypolimnion) environmentalgradients. Allozyme electrophoresis was used to discriminateamong different Daphnia taxa. All three frequently hybridizingEuropean species of the complex (galeata, cucullata, longispina= hyalina) and hybrids with Daphnia galeata were commonly recorded.Smaller-bodied Daphnia cucullata and its hybrids, when present,preferred mostly the nutrient- and food-rich upstream regions;D. longispina and its hybrids were more commonly found in thedownstream part, and often dominated in the meta- or hypolimnion.Redundancy analyses confirmed significant differences in theDaphnia taxon composition, as well as in spatial distributionof other crustacean species, along both gradients. For the firsttime, we demonstrate regular patterns in the horizontal distributionof Daphnia species and hybrids within a water body, thus acceptingour hypothesis. Such spatial distributional patterns may stronglyimpact local hybridization processes.     

Mechanisms regulating zooplankton populations in a high-mountain lake

SUMMARY 1. We studied the seasonal succession of phyto- and zooplankton and the potential impact of predation by salmonids on zooplankton population dynamics in a high-mountain Swiss lake.
2. A comparison of patterns in the abundance, body length, fecundity and age structure in the Daphnia galeata population strongly suggests that trout predation had little impact on the population and was not the cause for a decline in summer.
3. The dominance in the lake of adult trout that feed mainly on benthic prey may buffer the effect of predation on the larger zooplankton. Further, the relatively high amount of phytoplankton after spring thaw could be important for sustaining the Daphnia population under moderate fish predation.
4. Partial correlation analyses proved circumstantial evidence for both exploitative and interference competition between some zooplankton taxa. D. galeata depressed performance of other plankton species through exploitative competition.
5. Our study shows that the impact of fish on zooplankton in high-mountain lakes depends strongly on food web structure and trophic state of the lake. Where fish predation is weak, invertebrate predation combined with competition for food may be responsible for the dominance of large-bodied zooplankton species.     

Phytoplankton structure in different lake types in central Finland

Phyloplankton structure and its relation to physical and chemical properties of the water was studied in 58 central Finnish lakes. The biomass ranged from 0.2 to 14.2 g m⁻³ and the number of taxa per sample ranged from 33 to 152. The lakes were grouped into 5 types according to their trophic state: eutrophic, dyseutrophic, mesotrophic, oligotrophic, and acid oligotrophic lakes. The average biomass in eutrophic lakes was 5.57 g m⁻³, in dyseutrophic 3.54 g m⁻³, 1.23 g m⁻³ in mesotrophic, 0.52 g m⁻³ in oligotrophic and 0.39 g ⁻³ in acid oligotrophic lakes. The average number of taxa per sample in the corresponding lake types were 109. 1, 79.3, 97.9, 90.9 and 43.8, respectively. The phytoplankton communities in eutrophic lakes were characterized by blue-green algae (21.2% of total biomass) and green algae (18.7% of total biomass). In dyseutrophic lakes the proportion of green algae was much smaller (7.2% of total biomass) than in eutrophic lakes, whereas the proportion of diatoms and cryptophytes was higher (28.2 and 20.4% of total biomass, respectively). Chrysophytes dominated in the oligotrophic and mesotrophic lakes (27.3–39.9% of total biomass). The contribution of dinoflagellates to the total biomass was highest in the most oligotrophic acidified lakes and in those lakes the relative proportions of blue-green and green algae were much higher than in the typical oligotrophic lakes. The lakes were also grouped into 8 community types according to the dominating algal group. Cyanophyceae- and Chlorophyceae-types characterized the eutrophic lakes, whereas Chrysophyceae-Dinopheceae-type was typical for most oligotrophic lakes. The other 5 types occurred in mesotrophic and oligotrophic lakes but the physical and chemical properties of these lakes did not differ much.     

Seasonal variations in the condition of two Daphnia species and their hybrid in a eutrophic lake: evidence for food limitation

As seston and chlorophyll concentrations in eutrophic lakesare usually high all year round, it was hypothesized in thepast that food limitation is of major importance for the seasonaldynamics of herbivorous zooplankton in such lakes. Since directmeasurements of food are hampered by the lack of knowledge onthe exact nature of the food in eutrophic conditions, indirectmeasurements are necessary to estimate the degree of food limitationin these circumstances. Hence, we used laboratory- and field-derivedrelationships between the body length and body carbon contentof different species of Daphnia, which were collected from thehighly eutrophic Tjeukemeer, the Netherlands. From a seasonalsurvey of the carbon content of the daphnids. we concluded thatin Tjeukemeer D.galeata, D.galeata x cucullata and D.cucullataare food limited during the largest part of the year. Sincethe condition of the hybrids was relatively high as comparedwith the parental species when the food concentration was high,D.galeata x cucutlaia is expected to be the more successfultaxon during periods of high food availability.     

Relationships between heterotrophic nanoflagellates and the demographic response of Daphnia longispina in a eutrophic lake with poor food quality conditions

1. Based on both field data and laboratory studies, the summer population of Daphnia longispina living in a stratified eutrophic lake was examined in relation to the abundance of algae, nanoflagellates and picocyanobacteria.
2. In early July, the Daphnia population replaced Bosmina and remained the dominant zooplankter during summer 1994. Its development in July was concomitant with an increase of edible algae but, despite the apparent abundance in available food, the Daphnia population decreased throughout August suggesting poor food conditions.
3. From mid-August to the beginning of September, the biomass of inedible phytoplankton was greater than that of the smaller, edible fraction. Consequently the average rate of increase, birth rates and fecundity of Daphnia remained low. Although the biomass of heterotrophic nanoflagellates was consistently low, the demographic parameters of Daphnia were correlated throughout this period with these protozoans.
4. Life table experiments run in the laboratory showed that epilimnetic food supported both the growth in length of individual Daphnia and an increase in fecundity, but metalimnetic food supported only individual growth. D. longispina probably failed to reproduce because of the abundance of detritus mixed with the heterotrophic nanoflagellates in the metalimnetic water at that period of the year. The vertical migration of Daphnia into these deeper layers could be caused by a predator avoidance mechanism.     

A population of sexual Daphnia pulex resists invasion by asexual clones

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

Zooplankton biomass dynamics in oligotrophic versus eutrophic conditions: a test of the PEG model

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Predictors of invasion success by Daphnia species: influence of food, temperature and species identity

Non-indigenous North American Daphnia ‘pulex’ has recently invaded lakes and reservoirs in South Island, New Zealand, that formerly contained only native Daphnia carinata. New Zealand is characterised by a wide range of freshwater ecosystems and low species diversity of planktonic crustaceans, particularly Cladocera. The potential success of a species to invade and establish in a new community is likely to be predicted more accurately when reproductive response norms and fitness of key resident species, as well as the non-indigenous species, have been established under a range of relevant environmental conditions. Based on the results of experiments to test aspects of reproduction and fitness of the invader, D. ‘pulex’, and D. carinata when grown together at a range of relevant temperatures (8–23 °C) and related photoperiods, I predict the species-specific potential of D. ‘pulex’ to be dispersed and colonise New Zealand lentic habitats, and the potential of D. carinata to persist with the invader in these habitats. Larger population densities of D. ‘pulex’ compared to D. carinata at higher temperatures and food level, and larger densities of D. carinata at low temperatures, imply a potential for both species to coexist in New Zealand lakes, facilitated by seasonal succession; increased water temperature and nutrient input associated with climate and land use changes appear likely to promote the wider establishment of D. ‘pulex’, with both negative and positive implications for the conservation and management of New Zealand’s freshwater ecosystems.     

Effects of food type on the life history of Daphnia clones from lakes differing in trophic state. II. Daphnia cucullata feeding on mixed diets*

1. The effects of feeding on suboptimal foods were investigated in Daphnia cucullata a zooplankton common in many types of lakes. Eleven clones of D. cucullata were collected from four lakes of varying trophic levels and fed a high (1 mg C l^–1) concentration of one of two diets: (i) a 1 : 9 mixture of the cryptophyte Cryptomonas pyrenoidifera with the green alga Scenedesmus obliquus ; and (ii) a 1 : 9 mixture of the cryptophyte C. pyrenoidifera with the filamentous cyanobacterium Oscillatoria limnetica.
2. As expected, the diet dominated by Oscillatoria was found to be significantly poorer compared with the diet dominated by Scenedesmus. All D. cucullata clones were, however, able to achieve positive population growth rates on both diets, supporting the view that daphnids can grow and reproduce on a diet dominated by filamentous cyanobacteria.
3. Clones originating from different types of lakes differed in their responses to the two diets. On the Scenedesmus -dominated diet, clones originating from moderately eutrophic lakes had lower population growth rates than clones from hypertrophic lakes. In contrast, on the Oscillatoria -dominated diet, the clones from moderately eutrophic lakes had higher population growth rates than clones from hypertrophic lakes. Contrary to expectation, clones originating from lakes dominated by filamentous cyanobacteria had more difficulty utilizing filamentous cyanobacteria as food than clones from less eutrophic lakes which contain less filamentous cyanobacteria.
4. As the reactions of clones originating from same type of lake resembled each other, it is hypothesized that the life histories of D. cucullata populations are locally adapted to environmental factors other than abundance of filamentous cyanobacteria, which are correlated with trophic levels.     

Colonization dynamics and life history traits of seven Daphnia pulex genotypes

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

Trade-offs in the vertical distribution of zooplankton: ideal free distribution with costs?

Zooplankton vertical migratory patterns are a classic example of optimal habitat choice. We hypothesize that zooplankton distribute themselves vertically in the water column according to an ideal free distribution (IFD) with costs such as to optimize their fitness. In lakes with a deep-water chlorophyll maximum, zooplankton are faced with a trade-off, either experiencing high food (high reproductive potential) but low temperature (slow development) in the hypolimnion or high temperature and low food in the epilimnion. Thus, in the absence of fish predation (e.g. at night) they should allocate the time spent in the different habitats according to fitness gain dependent on the temperature gradient and distribution of food. We tested this hypothesis with a Daphnia hyalina x galeata clone in large indoor columns (Plön Plankton Towers) and with a dynamic energy budget model. In the tower experiments, we simulated a deep-water algal maximum below the thermocline with epilimnetic/hypolimnetic temperature differences of 2, 5 and 10 degrees C. Experimental data supported the model. We found a significantly larger proportion of daphniids in the hypolimnion when the temperature difference was smaller. Our results are consistent with the concept of IFD with costs originally developed for stream fishes. This concept can be applied to predict the vertical distribution of zooplankton in habitats where fish predation is of minor importance.