Daphnia Species Research Articles

Toxicity of lead and mancozeb differs in two monophyletic Daphnia species

Lead and mancozeb are two important chemicals used for different human purposes and activities worldwide. Hazard assessment in different areas of the world is carried out with different but phylogenetically similar species, adapted to different climatic conditions, in order to increase relevance. This study evaluated the sensitivity of two monophyletic species, the tropical species Daphnia similis and the temperate species Daphnia magna, to the two chemicals lead and mancozeb. Standard acute and chronic ecotoxicological tests (reproduction and growth), as well as other sublethal measurements such as the intrinsic rate of population increase (r), feeding rate (FR) and O2 consumption, were recorded along with the analysis of the AChE activity to determine the neurotoxicity of both contaminants. Albeit their similar evolutionary status, D. magna generally presented a lower sensitivity to Pb in comparison to D. similis. Despite the differences in sensitivity, both species presented similar patterns of response under Pb exposure, with diminished reproductive outputs, feeding impairment, reduced O2 consumption and no effect on AChE activity. Mancozeb decreased the reproduction, rate of population increase and feeding rate, increased the AChE activity in both species and increased O2 consumption only in D. magna. While D. magna increased O2 consumption under mancozeb exposure, no effects were observed for D. similis. Thus, species may present different responses and sensitivities to different pollutants, regardless of their phylogeny. Therefore, the use of ecotoxicological assays with native species is crucial for a better ecological risk assessment in contaminated areas.

Identifying invasive Daphnia species by morphological analysis of postabdominal claws in Sierra Nevada alpine lakes

Sedimentary postabdominal claws, among other remains, have been used successfully in paleolimnological studies to reconstruct past environmental conditions and the distribution pattern of certain Daphnia species. However, morphological analysis of postabdominal claws has not proven adequate for the clear taxonomic differentiation among species within a complex, such as the Daphnia pulex complex. The presence of the invasive North American (NA) D. pulex lineage was recently detected in an alpine lake in the Sierra Nevada mountain range (Southeast Spain). This lineage has spread throughout Africa and the Mediterranean basin, suggesting a trend towards its increased presence in Europe. The aim of this study was to examine whether this invasive lineage could be differentiated from the native European (Eu) Daphnia pulicaria lineage based on morphological differences in postabdominal claws recovered from three alpine lakes in the Sierra Nevada. The most useful differential variables were the postabdominal claw length (PCL), ratio of distal comb length to PCL (Cldist/PCL), and number of stout spines. Thus, NA D. pulex may be identifiable by a short PCL, low Cldist/PCL value and the presence of < 5 stout spines. Because of a wide variability in PCL within the Eu D. pulicaria species and an overlap in stout spine number between the species, morphological analysis results cannot unequivocally differentiate these lineages. However, they make a useful contribution to recognition of the possible presence of this invasive lineage. The present findings assist identification of the invasive NA D. pulex lineage in potentially affected regions, facilitating reconstruction of its historic dispersion and colonization.

Eutrophication mediates rapid clonal evolution in Daphnia pulicaria

Abstract Laboratory studies have revealed that Daphnia species can evolve to tolerate toxic cyanobacteria in the diet. Specifically, Daphnia from eutrophic lakes where cyanobacteria are common tend to have higher growth rates and survival when fed toxic cyanobacteria than populations from oligotrophic environments with low abundance of cyanobacteria. We conducted an in‐lake mesocosm (i.e. limnocorral) experiment during the autumn of 2009 to assess the effects of nutrient enrichment on clonal evolution in Daphnia pulicaria. As nutrient enrichment often favours grazing‐resistant cyanobacteria, we hypothesised that fertilisation would influence the genotypic composition of D. pulicaria that vary in tolerance to cyanobacteria. Mesocosms were fertilised to manipulate phytoplankton and cyanobacterial abundance and concentrations of a cyanobacterial toxin (microcystin). Thus, half of the mesocosms were high‐nutrient and half were low‐nutrient. We then stocked half of the mesocosms with a mixture of six genetically‐distinct D. pulicaria genotypes (three genotypes from oligotrophic lakes and three from eutrophic lakes) leaving half of the mesocosms Daphnia‐free to assess grazing effects, using a fully factorial design. When compared to the low nutrient treatment, high nutrient mesocosms had nearly five‐fold higher chlorophyll a concentrations, eight‐fold higher cyanobacterial dry biomass, and three‐fold higher microcystin levels at the start of the experiment. In contrast, low nutrient mesocosms had phytoplankton concentrations typical of mesotrophic lakes. Fertilisation strongly affected Daphnia genetic diversity in the mesocosms. Final Daphnia genotype diversity in the mesocosms with low‐cyanobacteria (richness = 5.83, Shannon–Weiner index = 1.55, evenness = 0.88) was similar to the initial stocked diversity (richness = 5.50, Shannon–Weiner index = 1.48, evenness = 0.87). In contrast, final diversity in fertilised mesocosms with high cyanobacteria was greatly reduced (richness = 2, Shannon–Weiner index = 0.17), with one Daphnia genotype that originated from the most‐eutrophic lake being highly dominant (evenness = 0.25). Thus, eutrophication mediated strong clonal selection of a cyanobacteria‐tolerant Daphnia genotype over just 10 weeks. By the end of the experiment, Daphnia significantly reduced phytoplankton biomass in the high‐nutrient, but not in the low‐nutrient treatment. This difference in effect size was largely driven by the five‐fold higher initial phytoplankton biomass in the high‐nutrient treatment. Thus, the ability of Daphnia to reduce phytoplankton biomass in eutrophic lakes may be driven more so by the abundance of planktivorous fishes, as opposed to the prevalence of cyanobacteria and their associated toxins.

Effects of a rainbow trout stocking moratorium on the Daphnia species composition and water quality of Square Lake (Minnesota)

Hembre, LK. 2019. Effects of a rainbow trout stocking moratorium on the Daphnia species composition and water quality of Square Lake (Minnesota). Lake Reserv Manage. 35:127–139.Square Lake is among the clearest lakes in the Minneapolis–St. Paul (Minnesota) metropolitan area, but its water clarity has decreased over the past several decades despite levels of total phosphorus (TP) remaining relatively stable. Predation by zooplanktivorous rainbow trout annually stocked since the early 1980s was hypothesized to be the cause for this eutrophication trend. To evaluate this hypothesis, a 3-year moratorium on trout stocking was imposed by the Minnesota Department of Natural Resources (MNDNR) and water quality and zooplankton monitoring data from 2 years prior to the moratorium (2010 and 2012) were compared to data from the 3 moratorium years (2013–2015). Significant changes observed during the moratorium years that support the hypothesis include (1) an increase in biomass concentrations of the large-bodied Daphnia pulicaria, (2) a coincident decrease in biomass concentrations of the smaller bodied D. mendotae, and (3) more pronounced spring clear-water phases when D. pulicaria reached peak densities. In addition, the volume of hypoxic water (dissolved oxygen [DO] < 1 mg/L) that developed in deep water was less in moratorium years compared to the premoratorium years. Unexpectedly, surface water TP concentrations were significantly lower during the moratorium years than in premoratorium years. Greater sequestration of P in the biomass of Daphnia during the moratorium years is the likely cause for the decrease in surface water TP levels in those years. Natural resource managers from the MNDNR have extended the moratorium and are using the conclusions from this study to determine the future fisheries management plan for Square Lake.

Impacts of increasing nitrogen:phosphorus ratios on zooplankton community composition and whitefish (Coregonus macrophthalmus) growth in a pre‐alpine lake

Abstract The combination of increasing atmospheric depositions of reactive nitrogen (N) and the highly effective diminishing of external phosphorus (P) loadings can change key nutrient ratios in lake ecosystems. Consequently, ratios of dissolved inorganic N (DIN) to dissolved P (DP) in lakes are increasing. However, potential consequences for aquatic organisms are as yet far from understood. We formulated three hypotheses on the potential effects of rising DIN:DP ratios on a lake food web: (1) increasing DIN:DP ratios intensify the P limitation of phytoplankton communities and lower their food quality; (2) densities of P rich zooplankters (e.g. cladocerans) will be negatively affected by P‐limited food algae; (3) as result, planktivorous fish will experience a reduction of their main prey (especially Daphnia species) and respond with lowered growth. These hypotheses were tested in a mesocosm experiment conducted in a pre‐alpine lake in southern Germany, Lake Brunnensee. For 76 days, the natural phytoplankton and zooplankton communities were exposed to a wide gradient of DIN:DP ratios. At the end of the experiment, juvenile planktivorous whitefish (Coregonus macrophthalmus) were released into the mesocosms and allowed to feed on zooplankton communities for 72 hr. Along the gradient of DIN:DP ratios, we found evidence for a rising P limitation of autotroph growth, which was indicated by increasing ratios of N:P (15:1–157:1) and C:P (91:1–797:1) in seston biomass. The rising P limitation in algae reduced the nutritional food quality for the majority of herbivorous zooplankton. Both the total zooplankton biomass and the Daphnia biomass declined substantially with increasing DIN:DP ratios. In contrast, increasing DIN:DP ratios favoured rotifer species, showing strong positive correlation with rotifer biomass. Whitefish weights decreased with increasing rotifer biomass and increased with rising Daphnia biomass in zooplankton communities. In summary, our results provide an experimental demonstration that increasing DIN:DP ratios can cause stoichiometric shifts in the biomass of primary producers towards higher N:P and C:P ratios. Effects on zooplankton were changes in the taxonomical community composition towards lower cladoceran biomass (mainly Daphnia spp.). The reduction in Daphnia biomass in turn caused significantly reduced growth rates of whitefish in our experiment. Our experimental results therefore support the assumption that stoichiometric effects can travel up the food chain. General consequences of such multi‐trophic effects induced by altered nutrient ratios could be potentially visible during re‐oligotrophication of water bodies, often resulting in high N:P ratios. Further empirical studies could look for signatures of these effects on the yield of economically important species.

Effect of fractions of toxic cyanobacterium, Nodularia spumingena KAC 66 and its purified nodularin and nodulopeptin 901 on inhibition of colorimetric protein phosphatase 1 assay and lethality against two species of Daphnia

The strains of Nodularia are known to produce potent hepatotoxic nodularins (NOD) and other bioactive metabolites i.e. nodulopeptins and spumigins. Contemporary, three new nodulopeptins were also isolated from N. spumigena KAC 66 collected from the Baltic Sea. The wide distribution and toxicological effects of N. spumigena on food chain, has caused significant attention towards the effects of the species. In the present investigation the toxicity of NOD and nodulopeptin 901, produced by N. spumigena, was fractionated by Reversed Phase Flash Chromatography (RPFC) and their toxicity was determined by their lethality to water fleas, Daphnia pulex and D. magna along with inhibition of protein phosphatase 1 assay (PP1; eukaryotic threonine phosphatase/protein serine). All fractions showed lethality to Daphnids and inhibitory activity against PP1, the toxicity was due to additional compounds as NOD and nodulopeptin 901 were only detected in 7 fractions. The pure NOD was lethal to D. pulex and D. magna LC50=8.4 µg. ml-1 and 5.0 µg .ml-1, respectively. The newly characterized nodulopeptin 901 was also tested against D. magna (LC50=&gt;100 µg .ml-1). NOD and nodulopeptin 901 inhibited PP1 with weak IC500.038 μg .ml-1 and 25 µg .ml-1, respectively. The anabaenopeptin A (ANA) and anabaenopeptin B (ANAB) were used as reference peptides as they have similarity in structure with newly characterized nodulopeptin 901. The ANA, ANB and linear nodularin (LNOD) inhibited PP1 with IC5070μg .ml-1,100μg .ml-1 and20μg .ml-1, correspondingly. This is the first study that indicates the toxic and inhibitory activities of nodulopeptin 901 and fractions of N. spumigena KAC 66 against daphnids and PP1.

Ecotoxicological characterization of the antiepileptic drug carbamazepine using eight aquatic species: baseline study for future higher tier tests

Ecotoxicological effects of the antiepileptic drug carbamazepine (CBZ) were investigated in one primary producer (Desmodesmus subspicatus) and seven invertebrate species (Daphnia magna, Daphnia pulex, Ceriodaphnia dubia, Gammarus fossarum, Potamopyrgus antipodarum, Lumbriculus variegatus and Chironomus riparius) using OECD and US EPA guidelines for chronic toxicity testing. The present data set was used to conduct a hazard assessment for CBZ including confirmatory data. While most of our results were in accordance with previous studies, published effect data for C. dubia and D. pulex could not be confirmed, even though they have previously been considered to be the most sensitive invertebrate species to CBZ exposure. The non-biting midge, C. riparius, was the most sensitive test organism in the present study. From an EC10 of 406 µg/L and a no observed effect concentration (NOEC) of 400 µg/L, a predicted no effect concentration (PNEC) of 8 µg/L was calculated. With regard to realistic predicted and measured environmental concentrations, the environmental risk can be considered as low for CBZ when the assessment is based on laboratory-based effect data. To conduct a refined and more realistic assessment, this study provides foundational data for two future, higher tier studies: one multiple-stressor experiment and one mesocosm study.

Multi-generational exposure to Pb in two monophyletic Daphnia species: Individual, functional and population related endpoints

To better evaluate chemical damage in chronically contaminated habitats, a nine-generational exposure to Lead (Pb) was done with two monophyletic Daphnia species, from temperate (Daphnia magna) and tropical (Daphnia similis) environments. The multi-generational test consisted generally of a continuous Pb exposed set of organisms, plus an extra control set running simultaneously. To assess daphnids recovery after Pb exposure, some organisms from the sixth generation were transferred to clean media for three extra generations (recovery period; F6 to F9), while others were keep Pb exposed. All setups (control, Pb exposure and recovery period) were submitted to two different dietary regimes, the standard (3 × 105 cells/mL) and restricted food (1.5 × 105 cells/mL) regimes. To evaluate the effects of generational Pb exposure and food regimes, individual, functional and population related endpoints were assessed (number of offspring, body length and rate of population increase (r) and feeding rate (FR)). The tests were conducted on the first (F0) and last generations (F9). No differences were shown on number of offspring and feeding among F9 control and continuous Pb exposed D. magna, although a higher r was shown for F9 Pb exposed organisms. F9 Pb exposed D. similis also presented a higher r than F9 control, however, lethality was induced at high Pb exposure levels. At food restriction the patterns were opposite and D. magna died at high Pb exposure while Pb exposed D. similis was the only setup (compared to control and recovery period) to survive at high Pb exposure levels. Regarding the recovery period, D. magna (standard food) did not cope well with the Pb re-exposure and lethality was induced, while D. similis indicate a decreased Pb sensitivity (only setup that survived high Pb exposure levels). Under food restriction, both species presented a decreased Pb sensitivity and consequent failed recovery (possibly due to epigenetic changes). Both species presented similar patterns regarding generations. Organisms from F0 presented enhanced reproductive outputs in comparison to F9 and the contrary occurred to the FR (even in control organisms). Data show an acclimation under a generational Pb exposure, which could increase the population of adapted organisms in natural habitats. And, since there was not a full recovery after three generations in clean media, an indication of epigenetic changes for both species may also be considered.

Vertical change of cladoceran ephippial densities in the sediments of Lake Nanyi and its correlation with the nutrients

We examined the vertical variations of cladoceran ephippia in the sediments (1-28 cm) of Lake Nanyi. Nine cladoceran ephippia (Daphnia similoides sinensis, Daphnia galeata, Daphnia pulex, Ceriodaphnia sp., Moina micrura, Bosmina sp., Scapholeberis sp., Leydigia leydigii and Chydorus sp.) were identified. The ephippial densities of Daphnia species and small cladocerans ranged from 0 to 2.0 ind&middot;g-1 DM and from 0 to 10 ind&middot;g-1 DM, respectively. D. similoides sinensis and Ceriodaphnia sp. were respectively dominant species of large and small cladocerans in the sediments of Lake Nanyi. The ephippial densities of D. similoides sinensis had a significant negative correlation with total phosphorus content of sediments in the 1# sampling site of Lake Nanyi and had positive correlations with total nitrogen and total phosphorus contents in the 3# sampling site. In three sampling sites (except for total nitrogen contents in the 2# sampling site), there were significant positive relationships between the ephippial densities of Ceriodaphnia sp. and both total nitrogen and total phosphorus contents. Our results suggested that eutrophication and fish predation affected the historical succession of cladoceran community structure in Lake Nanyi.

The current state of knowledge for toxicity of corexit EC9500A dispersant: a review

Corexit EC9500A is an oil dispersant used prominently in the Deepwater Horizon spill both below water and on the ocean surface. Aquatic tests show that it has low toxicity to many crustaceans, corals and fish but is more toxic to plankton, daphnia and many species early life stages. Human health concerns found low acute toxicities but potentials for chronic effects with large exposures. Dispersant use should be based on many factors, of which toxicity is just one which should be considered. However, freshwater system toxicity requires more study before widespread use is recommended.

Mitogenome of Daphnia laevis (Cladocera, Daphniidae) from Brazil

This study presents the first mitogenome of Daphnia laevis, a species that plays an important ecological role in Brazilian aquatic environments. The mitochondrion is 15,246 bp in size, with 13 protein-coding genes, 22 tRNAs genes, two rRNAs and d-loop region. Fourteen overlaps were found, a total of 98 bp. The percentages of A, T, C and G were 35.0%, 33.6%, 18.0%, and 13.4%, respectively, with A + T 68.6%. A cluster analysis confirmed that Daphnia laevis is close to other Daphnia species.

Bioaccumulation and morphological traits in a multi-generation test with two Daphnia species exposed to lead

Anthropic pressure negatively affects natural environments. Lead (Pb) is a non-essential highly toxic metal that is present in aquatic ecosystems. Two daphnid species from two different latitudes, the temperate Daphnia magna and the tropical Daphnia similis were used as test-organisms to evaluate a long-term Pb exposure. Both species were exposed for nine generations to a chronic concentration of Pb (50 μg/L) and the effects were explored, considering some endpoints not commonly used in toxicity tests: body burden of Pb and presence of granules in the dorsal region of neonates, hemoglobin contents, carapace deformation and morphology, production of males and ephippia (or dormant haploid egg), changes in the eggs' colour and eggs abortion. This multi-generation test was conducted under two food regimes, the usual (3 × 105 cells/mL) and the restricted (1.5 × 105 cells/mL) regime. On generation F6, Pb acclimated neonates were changed to a clean media for three generations, to evaluate exposure retrieval (recovery period). Negative and adverse effects occurred through generations, but no disparity was shown between D. magna and D. similis. The D. magna Pb accumulation showed different patterns regarding food regime. Bioaccumulation was faster under usual food, rapidly reaching a saturation point, whereas a gradual increase occurred under food restriction. A successful retrieval happened regarding Pb in D. magna, since no difference between control and recovering organisms was evidenced regarding their Pb body burdens. Generational Pb exposure led to carapace malformations, Pb aggregation in neonates' dorsal region, reddish extremities, production of males, ephippia (or dormant haploid egg), and aborted eggs, and changes in the eggs’ colour (green and white). Food restriction also induced the production of males. Reddish extremities disappeared in recovering organisms and ephippia (or dormant haploid egg) did not occurred during the recovery period. Existent males revealed a shorter lifespan than females (under stress). D. magna and D. similis presented similar responses, for the endpoints analysed; however, it does not mean that this lack of sensitivity difference will be observed when other endpoints (e.g. survival, reproduction) are considered. Bioaccumulation of Pb and adverse effects occurred at the tested concentration of 50μg/L, although higher Pb levels are allowed in the environment as safe concentrations, as reported by the Brazilian legislation and the literature where effects are evidences above 400 μg/L of Pb. Pb effects on reproduction, respiration, malformation, and other adverse effects suggest that a chronic generational exposure can be harmful to both D. magna and D. similis, and that such chronic contaminated environments should not be disregarded when it comes to environmental monitoring.

Predictability of the impact of multiple stressors on the keystone species Daphnia

Eutrophication and climate change are two of the most pressing environmental issues affecting up to 50% of aquatic ecosystems worldwide. Mitigation strategies to reduce the impact of environmental change are complicated by inherent difficulties of predicting the long-term impact of multiple stressors on natural populations. Here, we investigated the impact of temperature, food levels and carbamate insecticides, in isolation and in combination, on current and historical populations of the freshwater grazer Daphnia. We used common garden and competition experiments on historical and modern populations of D. magna ‘resurrected’ from a lake with known history of anthropogenic eutrophication and documented increase in ambient temperature over time. We found that these populations response dramatically differed between single and multiple stressors. Whereas warming alone induced similar responses among populations, warming combined with insecticides or food limitation resulted in significantly lower fitness in the population historically exposed to pesticides. These results suggest that the negative effect of historical pesticide exposure is magnified in the presence of warming, supporting the hypothesis of synergism between chemical pollution and other stressors.

Divergent developmental patterns of induced morphological defenses in rotifers and Daphnia: Ecological and evolutionary context

AbstractInduced morphological defenses commonly develop in some loricate rotifers as greatly increased spine development and in many species of the cladoceran genus Daphnia as an alteration in the shape and size of its head, producing helmets, crests, and neck spines. This restructuring of the shape of these animals during development, which reduces their vulnerability to a variety of planktonic predators, involves a series of developmental challenges and potential evolutionary constraints for each specific type of defense. In this review, we examine these challenges and the divergent developmental patterns that evolved in rotifers and Daphnia in response to the particular forms of predation pressure they face in their lake and pond environments. These patterns involve differences in the sensitivity of various developmental stages to predator kairomones, the timing of when the defense is present during postembryonic development, the ability to reverse the induced defense if the predator disappears from the community, and the effect of water temperature on the induction process. The type of developmental pattern associated with each induced defense appears to have been shaped by natural selection to effectively protect the animal, at minimal cost, from the specific kind of predation pressure to which it is commonly subjected.

Pattern of DNA Methylation in Daphnia: Evolutionary Perspective.

DNA methylation is an evolutionary ancient epigenetic modification that is phylogenetically widespread. Comparative studies of the methylome across a diverse range of non-conventional and conventional model organisms is expected to help reveal how the landscape of DNA methylation and its functions have evolved. Here, we explore the DNA methylation profile of two species of the crustacean Daphnia using whole genome bisulfite sequencing. We then compare our data with the methylomes of two insects and two mammals to achieve a better understanding of the function of DNA methylation in Daphnia. Using RNA-sequencing data for all six species, we investigate the correlation between DNA methylation and gene expression. DNA methylation in Daphnia is mainly enriched within the coding regions of genes, with the highest methylation levels observed at exons 2–4. In contrast, vertebrate genomes are globally methylated, and increase towards the highest methylation levels observed at exon 2, and maintained across the rest of the gene body. Although DNA methylation patterns differ among all species, their methylation profiles share a bimodal distribution across the genomes. Genes with low levels of CpG methylation and gene expression are mainly enriched for species specific genes. In contrast, genes associated with high methylated CpG sites are highly transcribed and evolutionary conserved across all species. Finally, the positive correlation between internal exons and gene expression potentially points to an evolutionary conserved mechanism, whereas the negative regulation of gene expression via methylation of promoters and exon 1 is potentially a secondary mechanism that has been evolved in vertebrates.

The first complete mitochondrial genome of Limnadia lenticularis (Branchiopoda, Spinicaudata), with new insights on its phylogeography and on the taxonomy of the genus

We present the complete mitochondrial (mtDNA) genome of the branchiopod Limnadia lenticularis (Linnaeus, 1761). The circular genome is 15,151 bp in size and includes the standard complement of 37 genes (13 protein-coding genes, 22 tRNA genes and two rRNA genes) plus one non-coding region (control region). Genes were encoded on one or the other of both strands of the DNA, with a nucleotide composition of 35% GC. We perform the first phylogenetic reconstruction (Bayesian analysis) with eleven complete mtDNAs of branchiopods indicating that L. lenticularis is close to Daphnia species. This phylogeny and gene order showed two distinct groups: Anostraca (Artemia, Phallocryptus and Streptocephalus) and a second group formed by L. lenticularis, Daphnia and Triops. These results were also confirmed through a comparison of nucleotide identity for the 13 protein-coding genes and the two ribosomal RNA genes between these eleven mtDNAs. In addition to presenting the complete L. lenticularis mtDNA, we investigated the genetic diversity and relationships of different specimens from remote localities of this species using two partial markers: one nuclear (28S rRNA) and one mitochondrial (COX1). Our results suggest that Limnadia lenticularis is composed of three cryptic species (with allopatric distribution in Europe, East Asia and America).

Identification of General Patterns of Sex-Biased Expression in Daphnia, a Genus with Environmental Sex Determination.

Daphnia reproduce by cyclic-parthenogenesis, where phases of asexual reproduction are intermitted by sexual production of diapause stages. This life cycle, together with environmental sex determination, allow the comparison of gene expression between genetically identical males and females. We investigated gene expression differences between males and females in four genotypes of Daphnia magna and compared the results with published data on sex-biased gene expression in two other Daphnia species, each representing one of the major phylogenetic clades within the genus. We found that 42% of all annotated genes showed sex-biased expression in D. magna. This proportion is similar both to estimates from other Daphnia species as well as from species with genetic sex determination, suggesting that sex-biased expression is not reduced under environmental sex determination. Among 7453 single copy, one-to-one orthologs in the three Daphnia species, 707 consistently showed sex-biased expression and 675 were biased in the same direction in all three species. Hence these genes represent a core-set of genes with consistent sex-differential expression in the genus. A functional analysis identified that several of them are involved in known sex determination pathways. Moreover, 75% were overexpressed in females rather than males, a pattern that appears to be a general feature of sex-biased gene expression in Daphnia.

Daphnia diversity on the Tibetan Plateau measured by DNA taxonomy.

Daphnia on the Tibetan Plateau has been little studied, and information on species diversity and biogeography is lacking. Here, we conducted a 4‐year survey using the barcoding fragment of the mitochondrial COI gene to determine the distribution and diversity of Daphnia species found across the Plateau. Our results show that species richness is higher than previously thought, with total described and provisional species number doubling from 5 to 10. Six of the taxonomic units recovered by DNA taxonomy agreed well with morphology, but DNA barcoding distinguished three clades each for the D. longispina (D. galeata, D. dentifera, and D. longispina) and D. pulex (D. pulex, D. cf. tenebrosa, and D. pulicaria) complexes. The sequence divergence between congeneric species varied within a large range, from 9.25% to 30.71%. The endemic D. tibetana was the most common and widespread species, occurring in 12 hyposaline to mesosaline lakes. The lineage of D. longispina is the first confirmed occurrence in west Tibet.

Seasonal effects of food quality and temperature on body stoichiometry, biochemistry, and biomass production in Daphnia populations

AbstractFood quality and temperature can affect zooplankton production in lakes by altering organismal metabolism. However, the influence of these factors on consumer nutritional physiology and population biomass remains relatively understudied in natural populations. Here, we examined seasonal changes in body stoichiometry, biochemistry, and population biomass in two Daphnia species collected from two separate lakes differing in dietary phosphorus (P) supply. Food quality, measured as seston carbon:P (C:P) ratios, varied throughout the study in each lake, and water temperatures generally increased across the growing season. Daphnid elemental composition was correlated with food quality in both populations, but relationships between daphnid body stoichiometry and temperature were consistently stronger as Daphnia body C:P ratios and content of major biochemical pools declined simultaneously throughout the summer, which largely coincided with increased water temperatures. Warmer temperatures were associated with relaxed %P‐RNA coupling as daphnid body RNA content declined and P content remained relatively high. These responses combined with temperature related decreases in Daphnia body %lipids and %C appeared to explain declines in daphnid body C:P ratios in both lakes over the growing season. Seasonal changes in population biomass were related to both food quality and water temperature in the lower nutrient lake. Biomass production under more eutrophic conditions however was unrelated to food quality and was instead associated with seasonal temperature changes in the higher nutrient lake. Overall, our study shows that seasonal changes in temperature and resource quality may differentially affect consumer stoichiometry and biomass production in lake ecosystems by altering consumer elemental metabolism.

Degree of high phenotypic plasticity in wild populations of Daphnia in early spring

Inducible defenses have been empirically known as defensive phenotypes that are triggered by predator kairomones. We hypothesized that morphological defense of wild Daphnia varies not only with predator density, but also with the predator regime in the field. We observed how the morphological defenses of two Daphnia species (D. ambigua and D. pulex) changed according to the population density in the water column at daytime of predatory insect larvae Chaoborus flavicans in Lake Fukami-ike, Japan, from February to July. In both Daphnia species, the inducible defense morphology was highly expressed in March and April. Its degree of expression decreased, and did not change with increases in predator density from May to July. These seasons are generally considered as when the number of the larvae and fish increases, and predation becomes more active due to their growth and breeding. We suggest that the degree of inducible defense of Daphnia is higher at the end of the overwintering season, when Chaoborus larvae began to inhabit the water column during the daytime and is constant in other seasons regardless of predators regime changes. Field observation of wild populations is important for understanding the seasonal changes in the morphology, and to provide more realistic explanations of phenomena in inducible defense.