Daphnia Pulex Research Articles

Comparative demography of two wild cladoceran species, Alona quadrangularis and Scapholeberis mucronata, collected in western Washington state and reared under laboratory conditions

AbstractPopulations of two cladoceran species, Alona quadrangularis and Scapholeberis mucronata, were collected from a pond in Puyallup, Washington, and reared under controlled laboratory conditions. Life tables were developed for each species under the same environmental conditions and feeding regimen to develop basic demographic data. We also compared the major demographic parameters of these two wild species with those of several cladoceran species that are commonly used in laboratory research, Daphnia pulex, D. magna, and Ceriodaphnia dubia. Compared to A. quadrangularis, S. mucronata had a much higher net reproductive rate (R0), a longer generation time (T), but a similar intrinsic rate of increase (rm), similar finite rate of increase, similar doubling time (DT), and a lower birth rate (b). Both wild species had lower R0, shorter T, and higher b than in D. pulex, D. magna, and C. dubia. Results of this study showed that demographic rates differed between A. quadrangularis and S. mucronata, and differed from those reported for the other three laboratory species. These results suggest that data developed for commonly used cladoceran species in the laboratory should not be used to make generalizations about effects on wild species.

What can we infer about mutation calling by using time-series mutation accumulation data and a Bayesian Mutation Finder?

Accurate estimates of mutation rates derived from genome-wide mutation accumulation (MA) data are fundamental to understanding basic evolutionary processes. The rapidly improving high-throughput sequencing technologies provide unprecedented opportunities to identify single nucleotide mutations across genomes. However, such MA derived data are often difficult to analyze and the performance of the available methods of analysis is not well understood. In this study, we used the existing Bayesian Genotype Caller adapted for MA data that we refer to as Bayesian Mutation Finder (BMF) for identifying single nucleotide mutations while considering the characteristics of the data. We compared the performance of BMF with the widely used Genome Analysis Toolkit (GATK) by applying these two methods to time-series MA data as well as simulated data. The time-series data were obtained by propagating Daphnia pulex over an average of 188 generations and performing whole-genome sequencing of 14 MA lines across three time points. The results indicate that BMF enables more accurate identification of single nucleotide mutations than GATK especially when applied to the empirical data. Furthermore, BMF involves the use of fewer parameters and is more computationally efficient than GATK. Both BMF and GATK found surprisingly many candidate mutations that were not confirmed at later time points. We systematically infer causes of the unconfirmed candidate mutations, introduce a framework for estimating mutation rates based on genome-wide candidate mutations confirmed by subsequent sequencing, and provide an improved mutation rate estimate for D. pulex.

Green Packaging Films with Antioxidant Activity Based on Pectin and Camellia sinensis Leaf Extract.

In the packaging materials sector, increasing globalization has created the need for increased efforts to develop consumer protection measures. Consequently, new packaging materials are being sought to replace petroleum-based materials in the future. For this reason, global awareness of the environmental problems associated with the use of synthetic and non-degradable packaging has increased the attention paid to bio-packaging based on natural and biodegradable polymers. The bio-packaging sector is developing innovations to address the sustainability issues facing the food packaging industry. Our research has shown that green matcha extract can be a promising source of antioxidants for the production of bioactive pectin films. This study further confirmed that green matcha extract can be a promising source of antioxidants for the production of bioactive pectin films. The antioxidant activity test showed high activity of films containing matcha extract. The antioxidant activity of films without matcha addition, P, PJ, PC, PJC, was negligible. The addition of matcha to the polymer matrix did not significantly affect the mechanical properties (TS, EB) of the films obtained. The addition of cellulose had the greatest effect on changing the mechanical properties. It caused a twofold increase in the mechanical properties of the obtained packaging films. The addition of matcha significantly improved the barrier properties (for PM films, the WVTR was 3.40 [g/m2d]; for PJM films the WVTR was 1.70 [g/m2d]). The green packaging films showed no toxic effects on the plants (Phacelia tanacetifolia, Salvia hispanica, Brassica napus) and invertebrates (Daphnia pulex, Chaoborus, Chironomus aprilinus) tested. The half-solubility time of the membranes in a solution mimicking gastric acid was also determined. The longest half-dissolution time of the films was about 2 min. Our research has therefore shown that the biodegradable and environmentally safe green packaging films with antioxidant activity that we have developed can be used as edible functional casings in the future, e.g., for sausages and other food products.

Cumulative effects of mutation accumulation on mitochondrial function and fitness

The impact of mutations on the mitochondria deserves specific interest due to the crucial role played by these organelles on numerous cellular functions. This study examines the effects of repeated bottlenecks on mitochondrial function and fitness. Daphnia pulex mutation accumulation lines (MA) lines were maintained for over 120 generations under copper and no copper conditions. Following the MA propagation, Daphnia from MA lines were raised under optimal and high temperatures for two generations before assessing mitochondrial and phenotypic traits. Spontaneous mutation accumulation under copper led to a later age at maturity and lowered fecundity in the MA lines. Mitochondrial respiration was found to be 10% lower in all mutation accumulation (MA) lines as compared to the non-MA control. MtDNA copy number was elevated in MA lines compared to the control under optimal temperature suggesting a compensatory mechanism. Three MA lines propagated under low copper had very low mtDNA copy number and fitness, suggesting mutations might have affected genes involved in mtDNA replication or mitochondrial biogenesis. Overall, our study suggests that mutation accumulation had an impact on life history traits, mtDNA copy number, and mitochondrial respiration. Some phenotypic effects were magnified under high temperatures. MtDNA copy number appears to be an important mitigation factor to allow mitochondria to cope with mutation accumulation up to a certain level beyond which it can no longer compensate.

Innovative method of environmental safety research of starch‐based films with silver nanoparticles

AbstractIn this work, films based on three different starches have been developed and modified by adding silver nanoparticles. We examined the films in terms of their mechanical properties, barrier properties and environmental toxicity to plants and invertebrates. For the toxicity studies, we used the following seeds: Berny rapeseed, Brassica napus; Salvia hispanica—chia, Salvia hispanica; and Karo narrowleaf lupine, Lupinus angustifolius. The invertebrates used for toxicity studies were Daphnia pulex, Tubifex tubifex, Chaoborus sp. (larva), Chironomus aprilinus, and Artemia sp. The tests showed that KD, TD, and ZD films were the most phytotoxic, which was confirmed by the percentages of germination and growth inhibition calculated for all test plants. K2D, D2T, and Z2D films exhibited considerable toxicity to lupine and chia. The conducted biotoxicity tests concerning the modified films and their effect on the selected invertebrates showed that Daphnia pulex and Artemia were highly sensitive to the tested factors. Only Chaoborus sp. (larva) exhibited total resistance to the tested films, which did not cause death of the organism in any case. Biotests on invertebrates and plants developed and used in this research can be successfully used to determine the toxicity of other substances.

Photoperiod-dependent effects of zinc oxide nanoparticles on the growth and reproduction of Daphnia pulex

The discharge of metal nanoparticles into the water inevitably poses a threat to aquatic organisms and the balance of the aquatic ecosystem. Photoperiod is one of the most important ecological factors for the development of cladocerans. In addition, different light conditions can also affect the toxicity of metal nanoparticles. In this study, we studied the effects of four photoperiods (8L/16D, 10L/14D, 14L/10D, and 16L/8D) combined with three concentrations of ZnO NPs (0 mg L−1, 0.05 mg L−1, and 0.10 mg L−1) on the growth and reproduction of Daphnia pulex. With the increase of photoperiod, the maternal body size and growth rate increased first and then decreased; the first time to reproduction was advanced, and broods and the total offspring also increased. Under the influence of ZnO NPs, growth rate and reproductive capacity were inhibited. The photoperiod 8L/16D and 16L/8D interacted with ZnO NPs on the growth of D. pulex, which significantly decreased the growth rate. Besides, the interaction between photoperiod 8L/16D and ZnO NPs decreased the reproduction ability of D. pulex. These results suggest that the effects of zinc oxide nanoparticles on the growth and reproduction of D. pulex is photoperiod dependent, which is useful for assessing the risk of pollutants to cladoceras under different light conditions.

Investigation of pharmaceutical bioaccumulation in Daphnia sp. living in a wastewater treatment plant

Non-steroidal anti-inflammatory drugs (NSAIDs) are among the most widely used pharmaceuticals. Their presence in natural waters is due to the low removal efficiency in conventional wastewater treatment plants (WWTPs). Interestingly, certain zooplankton species can survive the mixture of pollution and abnormal water conditions in WWTPs. In our study, for the first time, we tested the in-situ bioaccumulation of NSAIDs and their metabolites in Daphnia pulex, which were obtained in high numbers in one WWTP during the summer. It was found that diclofenac (DCF) and 4-hydroxy DCF were present in the studied clarifiers and ponds. Among these chemicals, only DCF was detected in daphnia. The bioaccumulation factor of DCF in daphnia was below 36 L kg−1ww and was lower than those obtained under experimental conditions for Daphnia magna. The tested daphnia adapted to chronic exposure to mixtures of drugs in μg L−1 level and could be implemented in biobased WWTPs. According to our data, there is a need to supplement the risk assessment of anthropogenic pollutants with in-situ cases to demonstrate the adaptation possibilities of wild-living organisms.

Proteome-wide neuropeptide identification using NeuroPeptide-HMMer (NP-HMMer)

Neuropeptides are essential neuronal signaling molecules that orchestrate animal behavior and physiology via actions within the nervous system and on peripheral tissues. Due to the small size of biologically active mature peptides, their identification on a proteome-wide scale poses a significant challenge using existing bioinformatics tools like BLAST. To address this, we have developed NeuroPeptide-HMMer (NP-HMMer), a hidden Markov model (HMM)-based tool to facilitate neuropeptide discovery, especially in underexplored invertebrates. NP-HMMer utilizes manually curated HMMs for 46 neuropeptide families, enabling rapid and accurate identification of neuropeptides. Validation of NP-HMMer on Drosophila melanogaster, Daphnia pulex, Tribolium castaneum and Tenebrio molitor demonstrated its effectiveness in identifying known neuropeptides across diverse arthropods. Additionally, we showcase the utility of NP-HMMer by discovering novel neuropeptides in Priapulida and Rotifera, identifying 22 and 19 new peptides, respectively. This tool represents a significant advancement in neuropeptide research, offering a robust method for annotating neuropeptides across diverse proteomes and providing insights into the evolutionary conservation of neuropeptide signaling pathways.

Effects of environmental concentrations of 6PPD and its quinone metabolite on the growth and reproduction of freshwater cladoceran

The widespread occurrence and accumulation of N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine (6PPD) and its quinone metabolite, 6PPD quinone (6PPD-Q), have been globally recognized as a critical environmental issue. However, knowledge on the adverse effects of 6PPD and 6PPD-Q on freshwater invertebrates is limited. This study investigated the effects of 6PPD and its oxidative byproduct, 6PPD-Q, on the growth and reproduction of Daphnia pulex. Through 21-day exposure experiments, we measured the uptake of 0.1, 1, and 10 μg/L 6PPD and 6PPD-Q by D. pulex and assessed the effects on growth and fecundity of D. pulex. While 6PPD and 6PPD-Q did not affect the mortality rate of D. pulex, 6PPD-Q exposure inhibited the growth of D. pulex, indicating potential ecological risks. In particular, the reproductive capacity of D. pulex remained unaffected across the tested concentrations of 6PPD and 6PPD-Q, suggesting specific toxicological pathways that warrant further investigation. This study underscored the importance of evaluating the sublethal effects of emerging contaminants such as 6PPD and 6PPD-Q on aquatic invertebrates, and highlighted the need for comprehensive risk assessments to better understand their environmental impacts.

The genome-wide signature of short-term temporal selection

Despite evolutionary biology's obsession with natural selection, few studies have evaluated multigenerational series of patterns of selection on a genome-wide scale in natural populations. Here, we report on a 10-y population-genomic survey of the microcrustacean Daphnia pulex. The genome sequences of [Formula: see text]800 isolates provide insights into patterns of selection that cannot be obtained from long-term molecular-evolution studies, including the following: the pervasiveness of near quasi-neutrality across the genome (mean net selection coefficients near zero, but with significant temporal variance about the mean, and little evidence of positive covariance of selection across time intervals); the preponderance of weak positive selection operating on minor alleles; and a genome-wide distribution of numerous small linkage islands of observable selection influencing levels of nucleotide diversity. These results suggest that interannual fluctuating selection is a major determinant of standing levels of variation in natural populations, challenge the conventional paradigm for interpreting patterns of nucleotide diversity and divergence, and motivate the need for the further development of theoretical expressions for the interpretation of population-genomic data.

Toxicity Evaluation of Water and Pore Water from a Pilot-Scale Pit Lake in the Alberta Oil Sands Region to Daphnia Species.

Significant amounts of tailings and oil sands process-affected water (OSPW) are generated by bitumen extraction in the Alberta Oil Sands region. These by-products are potentially toxic to aquatic organisms and require remediation. The study site was Lake Miwasin, a pilot-scale pit lake integrated into broader reclamation efforts. It consists of treated tailings overlaid with blended OSPW and freshwater, exhibiting meromictic conditions and harboring aquatic communities. This study assessed the potential toxicity of Lake Miwasin surface water (LMW) and pore water (LMP) using saline-acclimated Cladocera, including lab strains of Daphnia magna and Daphnia pulex and native Daphnia species collected in brackish Humboldt Lake (HL) and Lake Miwasin (LM). The pore water evaluation was used to represent a worst-case water quality scenario during pond stratification. Additionally, the inclusion of native organisms incorporated site-specific adaptations and regional sensitivity into the toxicity evaluation. Our results showed that LMW did not display acute or chronic toxicity to lab species and native Daphnia sp. (HL). Conversely, LMP was acutely toxic to both lab species and native D. pulex (LM). In chronic tests (12days exposure), LMP negatively affected reproduction in D. pulex (lab), with reductions in the number of offspring. Limited ability to acclimated organisms to the high salinity levels of LMP resulted in a shortened exposure duration for the chronic toxicity test. In addition to salinity being identified as a stressor in LMP, toxicity identification evaluation (TIE) phase I findings demonstrated that the observed toxicity for D. magna (lab) and D. pulex (LM, native) might be attributed to ammonia and metals in LMP. Further investigations are required to confirm the contributions of these stressors to LMP toxicity.

Selenium bioaccumulation in Daphnia pulexvia aqueous and dietary exposure.

Pit lakes are currently being investigated as a way to store and reclaim waste materials in the Alberta Oil Sands (AOS) region, Canada. Lake Miwasin (LM) is a pilot-scale pit lake consisting of treated fine tailings overlayed with oil sands process-affected water (OSPW) blended with fresh surface water. In October 2021, the surface water contained a mean concentration of 1.33 ± 0.04µg/L dissolved selenium (Se), slightly above the Canadian Council of Ministers of Environment water quality guideline for long-term protection of aquatic life (1µg Se/L). This study assessed the bioaccumulation of Se by the cladoceran Daphnia pulex under laboratory conditions through both aqueous and dietary exposure routes for comparison to field-collected specimens. In 12-day semi-static tests, lab-cultured D.pulex were exposed to water, and algae grown in media spiked with selenate. Results showed that Se bioaccumulation by lab-cultured D. pulex increased in all exposure treatments from days 5 to 12, with maximum Se concentrations of 3.08-3.47µg/g dry weight (dw) observed within the exposure range tested. Interestingly, lower Se bioaccumulation concentrations (1.26-1.58µg/g dw) were observed in the highest dissolved Se and dietary Se treatments, suggesting potential internal regulatory mechanisms. In addition, native D. pulex (LM) collected from Lake Miwasin and cultured in-house were exposed in 8-day semi-static tests to Lake Miwasin surface water and algae cultured in Lake Miwasin surface water. Selenium bioaccumulation in native D. pulex (LM) ranged from 2.00 to 2.04µg/g dw at day 8 and was not significantly different (p > 0.05) compared to Se concentrations in D. pulex collected from Lake Miwasin (2.15 ± 0.28µg/g) in summer 2022.

Differential Toxicity of Arsenic in Daphnia pulex Under Phosphorus and Food Limitation.

The on-going anthropogenic degradation of freshwater habitats has drastically altered the environmental supply of both nutrients and common pollutants. Most organisms living in these altered habitats experience interactive effects of various stressors that can initiate adjustments at multiple levels impacting their fitness. Hence, studies measuring response to a single environmental parameter fail to capture the complexities of the status quo. We tested both the individual and the interactive effect of arsenic (As) exposure, food quantity, and dietary phosphorus (P)-supply on six life-history traits (Juvenile Growth Rate; Adult Growth Rate; Age and Size at Maturity, Lifespan, and Fecundity) as surrogates for organismal fitness in the keystone aquatic grazer Daphnia pulex. We also tested the effect of food quantity and P-supply on somatic As accumulation in Daphnia. Our results indicated an influence of P-supply on neonatal growth and an influence of As and food quantity on growth and maintenance later in life. Maturation was strongly influenced by all three variables, with no reproduction observed in the presence of two or more environmental stressors. We found a strong interaction between As and dietary P, with increased P-supply intensifing the toxicity effect of As. No such effects were seen between As and food quantity, indicating a differential role of quantity versus quality on As toxicity. We found a nominal effect of diet on somatic As accumulation. The results from the present study emphasize the importance of considering such interactions between co-occurring environmental stressors and the dietary status of organisms, to better predict and manage impacts and risks associated with common environmental toxicants in highly vulnerable ecosystems. Environ Toxicol Chem 2024;43:1807-1819. © 2024 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Morphological and life‐history trait plasticity of two Daphnia species induced by fish kairomones

AbstractDaphnia can avoid predation by sensing fish kairomones and producing inducible defenses by altering the phenotype. In this study, the results showed that the morphological and life‐history strategies of two Daphnia species (Daphnia pulex and Daphnia sinensis) exposed to Aristichthys nobilis kairomones. In the presence of fish kairomones, the two Daphnia species exhibited significantly smaller body length at maturity, smaller body length of offspring at the 10th instar, and longer relative tail spine of offspring. Nevertheless, other morphological and life‐history traits of the two Daphnia species differed. D. pulex showed a significantly longer relative tail spine length and earlier age at maturity after exposure to fish kairomones. The total offspring number of D. sinensis exposed to fish kairomones was significantly higher than that of the control group, whereas that of D. pulex was significantly lower. These results suggest that the two Daphnia species have different inducible defense strategies (e.g., morphological and life‐history traits) during prolonged exposure to A. nobilis kairomones, and their offspring also develop morphological defenses to avoid predation. It will provide reference for further exploring the adaptive evolution of Daphnia morphology and life‐history traits in the presence of planktivorous fish.

Copy number and sequence variation in rDNA of Daphnia pulex from natural populations: insights from whole-genome sequencing.

Ribosomal DNA (rDNA) has a vital role in ribosome biogenesis as it contains the genes that encode ribosomal RNA (rRNA) separated by intergenic spacers (IGSs). The rRNA genes occur in hundreds to tens of thousands of copies per haploid genome in eukaryotes and are generally highly conserved with low variation within species. Due to the repetitive nature and large size of rDNA arrays, detecting intraindividual variation can be difficult. In this study, we use whole-genome sequences of 169 Daphnia pulex individuals from 10 natural populations to measure the copy number and sequence variation in rDNA. This revealed that variation in rDNA copy number between individuals spans an order of magnitude. We further observed a substantial level of sequence variation within individual genomes. As expected, single-nucleotide polymorphisms occurred in regions of lower functional constraint such as the IGS and expansion segments of the rRNA genes. The presence of strong linkage disequilibrium among variants facilitated identification of haplotypes within each population. Although there was evidence of recombination among haplotypes from different populations, it is insufficient to eliminate linkage disequilibrium within populations. Estimating copy number and haplotype diversity within individuals revealed that the level of intraindividual sequence variation is not strongly correlated with copy number. The observed patterns of variation highlight a complex evolutionary history of rDNA in D. pulex. Future research should explore the functional implications of rDNA copy number and sequence variation on organismal phenotypes.

Invertebrates from naturally brackish areas are less impacted by road salt and alternative deicers

AbstractRoad salt (NaCl) is the most common deicer used to melt snow and ice from roadways in cold climates, but its use results in runoff of sodium and chloride ions into sewers and into soil and freshwater ecosystems. Road salt causes harmful effects on wetland habitats, which host an abundance of macroinvertebrate species. Alternatives to road salt, such as beet juice, are commercially available, but their impact on invertebrates is less well studied. We examined the impact of road salt alternatives on four invertebrate species (Daphnia pulex, Eisenia fetida, Heterocypris sp., and Culicoides sp. larvae), some of which come from a naturally brackish area. Our expectation was that road salt alternatives (beet salt, molasses, pickle juice, and sand) would be less toxic to the animals than traditional road salt. This study comprised two experiments: the first exposed individuals of the four invertebrate species to road salt alternatives, and the second exposed the invertebrates to a range of road salt concentrations. Only individuals of D. pulex were negatively impacted by road salt. The other invertebrates were not significantly affected by any salt alternative, demonstrating interspecific differences in response to salt. Species found in naturally salty environments were more tolerant of both road salt and alternatives. This research shows species‐ and habitat‐specific responses to road salt alternatives, which has implications for municipalities looking to salt roads without negatively impacting freshwater ecosystems.

A Chronic Aquatic Hazard Assessment for the Perfume Raw Material Octahydro-tetramethyl-naphthalenyl-ethanone.

Octahydro-tetramethyl-naphthalenyl-ethanone (OTNE) is a high-production volume fragrance material used in various down-the-drain consumer products. To assess aquatic risk, the Research Institute for Fragrance Materials (RIFM) uses a tiered data-driven framework to determine a risk characterization ratio, where the ratio of the predicted-environmental concentration to the predicted-no-effect concentration (PNEC) of <1 indicates an acceptable level of risk. Owing to its high production volume and the conservative nature of the RIFM framework, RIFM identified the need to utilize a species sensitivity distribution (SSD) approach to reduce the PNEC uncertainty for OTNE. Adding to the existing Daphnia magna, Danio rerio, and Desmodesmus subspicatus chronic studies, eight new chronic toxicity studies were conducted on the following species: Navicula pelliculosa, Chironomus riparius, Lemna gibba, Ceriodaphnia dubia, Hyalella azteca, Pimephales promelas, Anabaena flos-aquae, and Daphnia pulex. All toxicity data were summarized as chronic 10% effect concentration estimates using the most sensitive biological response. Daphnia magna was the most sensitive (0.032 mg/L), and D. subspicatus was the least sensitive (>2.6 mg/L, the OTNE solubility limit). The 5th percentile hazardous concentration (HC5) derived from the cumulative probability distribution of the chronic toxicity values for the 11 species was determined to be 0.0498 mg/L (95% confidence interval 0.0097-0.1159 mg/L). A series of "leave-one-out" and "add-one-in" simulations indicated the SSD was stable and robust. Add-one-in simulations determined that the probability of finding a species sensitive enough to lower the HC5 two- or threefold was 1/504 and 1/15,300, respectively. Given the high statistical confidence in this robust SSD, an additional application factor protection is likely not necessary. Nevertheless, to further ensure the protection of the environment, an application factor of 2 to the HC5, resulting in a PNEC of 0.0249 mg/L, is recommended. When combined with environmental exposure information, the overall hazard assessment is suitable for a probabilistic environmental risk assessment. Environ Toxicol Chem 2024;43:1378-1389. © 2024 SETAC.

On the Hydrofauna of the First Batabat Water Reservoir

For the first time generalized data on hydrofauna of the First Batabat reservoir are given. In the fauna 6 species of zooplanktonic organisms are revealed, with absolute predominance of large predatory copepod Hemidiaptomus rylovi Charin, 1928. The mean body length of fertilized egg-bearing female crustaceans was 3.6 mm to the furca and 5.1 mm to the tip of the telson. In the ecosystem, Daphnia pulex Leydig, 1860 typically produces its last parthenogenetic generation in September. In the macrozoobenthos of the reservoir, 40 species of macrobenthic organisms were found, with the Heteroptera (9 species) and the Chironomidae family (9 species) predominating in terms of the number of species. According to all its ecological indicators Gammarus lacustris (Sars, 1863) is the dominant bottom organism of the ecosystem. The maximum number of population individuals — 346 spec./m2 and biomass — 2804 g/m2 was calculated in the last decade of June 2017. Being a detritophage with a wide food spectrum, individuals of the higher crayfish have an abundant food base in the reservoir. The first Batabat reservoir located in the high mountainous region of the autonomous republic and having favorable natural and geographical conditions is characterized by high biological productivity.

Daphnia uses its circadian clock for short-day recognition in environmental sex determination

Some organisms have developed a mechanism called environmental sex determination (ESD), which allows environmental cues, rather than sex chromosomes or genes, to determine offspring sex.1,2,3,4 ESD is advantageous to optimize sex ratios according to environmental conditions, enhancing reproductive success.5,6 However, the process by which organisms perceive and translate diverse environmental signals into offspring sex remains unclear. Here, we analyzed the environmental perception mechanism in the crustacean, Daphnia pulex, a seasonal (photoperiodic) ESD arthropod, capable of producing females under long days and males under short days.7,8,9,10 Through breeding experiments, we found that their circadian clock likely contributes to perception of day length. To explore this further, we created a genetically modified daphnid by knocking out the clock gene, period, using genome editing. Knockout disrupted the daphnid's ability to sustain diel vertical migration (DVM) under constant darkness, driven by the circadian clock, and leading them to produce females regardless of day length. Additionally, when exposed to an analog of juvenile hormone (JH), an endocrine factor synthesized in mothers during male production, or subjected to unfavorable conditions of high density and low food availability, these knockout daphnids produced males regardless of day length, like wild-type daphnids. Based on these findings, we propose that recognizing short days via the circadian clock is the initial step in sex determination. This recognition subsequently triggers male production by signaling the endocrine system, specifically via the JH signal. Establishment of a connection between these two processes may be the crucial element in evolution of ESD in Daphnia.

Warming temperatures exacerbate effects of microplastics in a widespread zooplankton species.

The emergence of microplastics as a global contaminant of concern has coincided with climate change induced temperature warming in aquatic ecosystems. Warmer temperatures have been previously demonstrated to increase the toxicity of certain contaminants, but it is currently unclear if microplastics are similarly affected by temperature. As aquatic organisms simultaneously face microplastic pollution and both increasing and variable temperatures, understanding how temperature affects microplastic toxicity is pertinent in this era of human-induced global change. In this study, we investigate the effects of environmentally relevant microplastic exposure to Daphnia pulex survival, reproduction, and growth at three different temperatures. To simulate an environmentally relevant exposure scenario, we created microplastics with physicochemical characteristics often detected in nature, and exposed organisms to concentrations close to values reported in inland waters and 1-2 orders of magnitude higher. The three temperatures tested in this experiment included 12 °C, 20 °C, and 24 °C, to simulate cool/springtime, current, and warming scenarios. We found the highest concentration of microplastics significantly impacted survival and total offspring compared to the control at 20 °C and 24 °C, but not at 12 °C. The adverse effect of high microplastic concentrations on total offspring at warmer temperatures was driven by the high mortality of the juveniles. We observed no effect of microplastics on time to first reproduction or average growth rate at any temperature. Warmer temperatures exacerbated microplastic toxicity, although only for concentrations of microplastics not currently observed in nature, but these concentrations are possible in pollution hotspots, through pulses pollution events or future worsening environmental contamination. The results of our study illustrate the continued need to further investigate climate change related co-stressors such as warming temperatures in microplastic and pollution ecology, through environmentally realistic exposure scenarios.