Predation is an important selective pressure shaping phenotypic plasticity in aquatic organisms. As the key mediator between environmental changes and host physiology, gut microbiota and their metabolism play crucial roles in regulating host fitness. Although a few studies have extended the effects of predation risk on gut microbial composition, relatively little is known about whether and how the gut metabolite profiles are reshaped and linked to host defensive responses. This study integrated phenotypic assays, 16S rDNA sequencing and metabolomic analysis, systematically revealing the coordinated shifts in gut microbes and metabolites of Daphnia magna under fish kairomone exposure, which may be associated with D. magna's morphological and reproductive defences. Particularly, the enrichment of the indicator taxa Selenomonadaceae and Sporichthyaceae was negatively correlated with dAMP and adenine in the purine metabolism pathway, suggesting restricted nucleotide synthesis and ATP production. The resulting energy deficit may activate AMPK while inhibiting mTOR signalling, reallocating energy from somatic growth to reproductive investment. Moreover, Selenomonadaceae enrichment was linked to reduced PGD2 in the neuroactive ligand-receptor interaction pathway, potentially weakening Gs-cAMP-PKA signalling, suppressing cell proliferation and leading to a smaller body size of D. magna. These coordinated associations suggest a potential mediating role for gut microbe-metabolite interactions in the growth-survival trade-off of Daphnia under predation risk, which requires further experimental validation. These findings expand our understanding of host ecological adaptation from a gut microbial functional perspective.

The concomitant prevalence of toxic cyanobacteria blooms and plastic pollution in aquatic ecosystems is emerging as a pressing global water pollution dilemma. While toxic cyanobacteria and microplastics (MPs) can each independently exert significant impacts on aquatic biota, the magnitude and trajectory of the combined interactions remains rudimentary. In this study, we evaluated how MPs influences cyanobacterial stress on keystone grazer Daphnia, focusing on population, individual, biochemical and toxicogenomic signatures. We found that toxic Microcystis (TM) adversely affected the fitness of Daphnia populations (intrinsic rate of population increase), and these adverse effects were amplified in the presence of MPs. Through detailed observation, it was ascertained that MPs promoted the ingestion of TM, culminating in enhanced microcystin bioaccumulation. Using the Eco-Evo model, we found that there was potential absence of correlation between the MPs toxicity and the effect size of MPs on the TM. Utilizing gene set enrichment analysis (GSEA), we further identified a marked suppression of molecular pathways and entities crucial to individual growth and development in the TM-MPs consortium compared to exposure to TM alone. The present study provides important insights about the influence of MPs on cyanobacteria toxicity and the prediction the risk of harmful algal blooms in aquatic ecosystems.

The increasing frequency of cyanobacterial blooms poses escalating ecological stress on freshwater ecosystems worldwide. These disturbances primarily act through impacts on basal trophic groups—most notably zooplankton. Apoptosis is a vital mechanism for preserving cellular homeostasis and managing stress at the organismal level, yet the role of its conserved regulators, such as caspase-2, remains poorly understood in aquatic grazers. We characterized a pivotal caspase-2 homolog in the freshwater water flea Moina macrocopa, an important model in aquatic ecology, and designated it Mm-cas2. Mm-cas2 possesses the characteristic caspase CASs domain, and phylogenetic analysis confirmed its classification within the caspase-2 family. Exposure to toxic Microcystis aeruginosa induced a dose-dependent increase in Mm-cas2 expression, confirmed at both mRNA (qRT-PCR) and protein (Western blot) levels. Spatial mapping via Whole-mount in situ hybridization (WISH) further indicated heterogeneous Mm-cas2 expression within the M. macrocopa organism. Critically, RNAi-mediated silencing of Mm-cas2 significantly heightened developmental retardation and reproductive obstruction in cyanobacteria-exposed M. macrocopa, confirming its essential role in governing sensitivity to M. aeruginosa. Our findings collectively reveal how the apoptotic regulator caspase-2 governs the sensitivity of M. macrocopa to M. aeruginosa, establishing Mm-cas2 as a candidate biomarker to evaluate ecological risks in freshwater ecosystems prone to cyanobacterial blooms outbreaks.

A central goal in ecotoxicology and bio-based strategies for controlling water pollution is to uncover the molecular mechanisms by which organisms detect and respond to environmental challenges. Cyanobacterial blooms, especially when dominated by toxic Microcystis, present considerable ecological threats to freshwater by disrupting zooplankton communities, which are essential to the structure and function of aquatic food webs. Investigating the molecular mechanisms that govern zooplankton responses to cyanobacterial toxicity is essential for understanding their vulnerability and adaptive capacity. The objective of this study was to elucidate how the endoplasmic reticulum stress sensor IRE1 mediates the responses of the freshwater zooplankton Moina macrocopa to toxic Microcystis exposure. To address this objective, we characterized an endoplasmic reticulum stress-associated gene (designated Mm-IRE1) from M. macrocopa. Structural analysis showed that Mm-IRE1 contains conserved IRE1 domains-a luminal sensor, serine/threonine kinase, and RNase-suggesting its role in the unfolded protein response. Expression profiling showed that Mm-IRE1 was significantly upregulated after exposure to Microcystis, indicating its potential involvement in stress responses. Importantly, RNA interference (RNAi)-mediated knockdown of Mm-IRE1 resulted in lowered survivorship and body length, especially exposed to cyanobacteria, highlighting the gene's potential protective function. These results reveal a key role of Mm-IRE1 in regulating zooplankton sensitivity to cyanobacterial blooms and highlight its potential as a stress-responsive molecular indicator for reflecting cyanobacterial stress in freshwater zooplankton.

Aquatic organisms are normally exposed to waters where multiple pollutants coexist. Although the concentration of each single pollutant in natural waters is extremely low and may not have harmful effects, the combined effects of multiple low concentration pollutants may cause substantial harm to Daphnia. Therefore, we selected 11 kinds of pollutants including microplastics, antibiotics, heavy metals, agricultural and industrial pollutants, and then exposed Daphnia magna to the combination of these pollutants at the environmental concentrations (ng L−1-μg L−1 range) to evaluate the possible negative effects. Results showed the combination of multiple pollutants significantly decreased heart rate, body size, survival, and fecundity of D. magna and delayed maturation. In the filial generation constantly exposed to the pollutant combination, the growth, survival, and reproduction further decreased. The diversity of the gut microbiota decreased, but the abundance of bacteria with functions related to xenobiotics degradation increased under the pollutant combination. The expressions of genes related to antioxidant, xenobiotics catabolism, and energy absorption were upregulated by the pollutant combination, with downregulating expressions of the genes related to cell division and nitrogen metabolism, which reveals the underlying mechanism of the harmful effects of multiple pollutants on life history traits of D. magna. This study demonstrated the ecological risks of multiple pollutants at environmentally relevant concentrations to D. magna, providing a new perspective for evaluating the consequences of low environmental pollution in natural waters.

As cyanobacterial blooms and herbicide pollution, which are often detected in eutrophic waters, can separately jeopardise zooplankton populations, there is an urgent and on‐going need to understand the strength and direction of their interactive effects. This is a crucial step toward realistic risk‐evaluation of agricultural pollution in eutrophic waterbodies. In this study, we evaluated how the herbicide, atrazine (ATZ), alters the effects of cyanobacterial food on the zooplankter, Daphnia magna . We found survival time of Daphnia decreased with increasing amounts of Microcystis in their diets, and the magnitude of this cyanobacterial effect was independent of ATZ. In contrast, ATZ exposure triggered faster growth and larger body size in Daphnia fed diets containing Microcystis compared to those fed the good‐food diet. Although toxic Microcystis reduced the overall reproductive output of Daphnia , the presence of ATZ, regardless of the type of food treatment, exhibited a masking effect by further significantly decreasing Daphnia 's overall reproductive output, resulting in a low level of reproduction. Finally, we found an expression trade‐off at the molecular scale between growth and reproduction genes on one side, and antioxidation gene on the other, which could account in part for ATZ's influence on Microcystis toxicity to Daphnia at maturation stage. These results demonstrated that ATZ can reshape Daphnia 's responses to Microcystis , predominantly with effects on growth and reproductive traits. These trait‐dependent responses were found to be closely linked to the regulation of key metabolic pathways. Collectively, our study enhances current knowledge regarding the potential interaction between fundamental trophic levels in cyanobacteria‐dominated lakes around farmlands, and is helpful to achieve more realistic environmental risk management of agricultural pollution in eutrophic waterbodies.

As a key form of post-transcriptional regulation, microRNAs (miRNAs) regulate gene expression by binding to target mRNAs, leading to mRNA decay or translational repression. Recently, the role of miRNAs in the response of aquatic organisms to environmental stressors has emerged. Daphnia, widely distributed cladocerans, play a crucial role in aquatic ecosystems. Cyanobacterial blooms often cause Daphnia populations to decrease, thereby disrupting ecosystem functionality and water quality. However, the post-transcriptional mechanisms behind Daphnia's response to toxic cyanobacteria are insufficiently understood. This study investigated the role of miR-210, a multifunctional miRNA involved in stress response and toxicity pathways, and its target genes (MLH3, CDHR5, and HYOU1) in two Daphnia magna clones exposed to toxic Microcystis aeruginosa. Results showed that M. aeruginosa inhibited somatic growth rates, led to microcystin accumulation, caused abnormal ultrastructural alterations in the digestive tract, and induced DNA damage in both clones. Notably, exposure significantly increased miR-210 expression and decreased the expression of its target genes compared with the controls. We identified miR-210s regulation on clonal-tolerance variations in D. magna to M. aeruginosa, emphasizing miRNAs' contribution to adaptive responses. Our work uncovered a novel post-transcriptional mechanism of cyanobacterial impact on zooplankton and provided essential insights for assessing cyanobacterial toxicity risks.

Cyanobacterial blooms, which are becoming more frequent in aquatic ecosystems across the globe, pose a significant health threat to the aquatic keystone species, Daphnia magna. Given that D. magna solely rely on innate immunity centered around tumor necrosis factor receptor-associated factor 4 (TRAF4), the aim of this study is to analyze how the TRAF4 gene in D. magna (Dm-TRAF4) participates in the response to cyanobacterial stress. First, TRAF4 sequence was identified bioinformatically in the D. magna genome. Then, Dm-TRAF4 expression levels were measured at different developmental stages of D. magna. Furthermore, the effects of exposure to the toxic cyanobacteria (Microcystis aeruginosa) on Dm-TRAF4 expression was investigated. Structural analysis revealed that Dm-TRAF4 contained several conserved functional domains, including three canonical zinc finger motifs and a MATH domain, indicating its potential role in immune signaling. Moreover, Dm-TRAF4 was evolutionarily more related to the insect sequences than to those of copepods. The expression results showed a significant progressive increase in Dm-TRAF4 expression levels in D. magna from embryonic development to aging stages. Furthermore, when exposed to Microcystis, Dm-TRAF4 expression was markedly downregulated compared to the control. In addition, miR-4443 showed a negative correlation with Dm-TRAF4 expression under the threat of M. aeruginosa in D. magna, indicating that post-transcriptional modification of the TRAF4 gene was involved in D. magna response to M. aeruginosa stress. In conclusion, the current findings provide novel insights into the TRAF4-mediated innate immune response to cyanobacteria pollution in zooplankton.

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Harmful cyanobacterial blooms, particularly those caused by Microcystis species, pose significant ecological threats to freshwater environments by negatively impacting zooplankton populations, essential components of aquatic food webs. Understanding the molecular mechanisms underlying zooplankton responses to these toxic blooms is crucial for assessing and mitigating these impacts. The mitogen-activated protein kinase (MAPK) pathway, known for its critical role in stress response signaling, offers a promising area of study to elucidate these mechanisms. However, the specific involvement of MAPK in zooplankton responses to cyanobacterial stress remains unclear. In this study, we identify and characterize the p38 MAPK gene (MmMAPK) from the zooplankton Moina macrocopa. The gene contains conserved structural elements typical of MAPKs, including a Thr-Gly-Tyr (TGY) motif and a substrate-binding site, Ala-Thr-Arg-Trp (ATRW), indicating its potential functional relevance in stress signaling pathways. Expression analysis reveals a significant upregulation of MmMAPK in M. macrocopa upon exposure to toxic Microcystis, suggesting its role in mediating the organism's stress response. Furthermore, RNA interference (RNAi) experiments demonstrate that knockdown of MmMAPK results in reduced survival and decreased body size, particularly under cyanobacterial stress, underscoring its importance in maintaining stress sensitivity. These findings provide new insights into the molecular mechanisms by which M. macrocopa responds to harmful algal blooms and highlight the potential of MmMAPK as a biomarker for ecological risk assessment and management of cyanobacterial pollution in freshwater ecosystems.

Tetravalent metal (e.g., Zr4+, Hf4+) phosphonate frameworks featuring remarkable chemical and radiolytic stabilities have been newly utilized as high-performing adsorbents for actinide in harsh solutions. Nevertheless, the practical applications have been impeded by the as-synthesized powder form that is not compatible with continuous actinide recovery or removal. Herein, we incorporate hafnium phosphonate (HfP) fine powder into polyacrylonitrile (PAN) via a simple and economical electrospinning technique, engendering a stable and hy-drophilic nanofibrous membrane with the first-rank permeate flux for the potential treatment of a large volume of actinide-containing wastewater. This composite membrane can capture more than 90% Th(IV) at ppm level and 95% Pu(IV) and 90% Np(V) at tracer amount level in strong acidic solutions, which retains the excellent adsorption efficacy of HfP powder. Besides, it has a breakthrough volume larger than 880 mL for Th(IV) and 760 mL for U(VI) at the ppb level under a high permeate flux of 785 +/- 11.2 L center dot m-2 center dot h-1, representing one of the top nanofibrous membranes for the dynamic removal of actinides. This work will pave an avenue for fabricating highly efficient and stable adsorptive membranes, which are promising candidates for capturing actinides from large-volume of acidic nuclear wastewater.

The salinization of the global freshwater system caused by various human activities and climate change has become a common problem threatening freshwater biodiversity and resources, which may affect a variety of species of cladocerans at individual and population levels. In order to comprehensively evaluate the impact of salinization on different-sized cladocerans at individual and population levels, we exposed two species of cladocerans with obvious body size difference, Daphnia magna and Moina macrocopa, to seven salinities (0, 0.02, 0.04, 0.06, 0.08, 0.10, 0.12 M), recorded individual life history traits and population growth dynamics, and used multiple mechanistic models to fit the data. At the individual level, the median effect concentration of survival time, total offspring per female, and number of broods of D. magna were significantly higher than those of M. macrocopa. At the population level, the decrease in carrying capacity of D. magna with increasing salinity was significantly less than that of M. macrocopa. At the same salinity treatment, the integrated biomarker response indexes value of M. macrocopa is higher than that of D. magna. Therefore, it was further inferred that the sensitivity of small-sized species M. macrocopa to salinity stress is significantly higher than that of big-sized species D. magna. Thus, freshwater salinization may result in the replacement of smaller salt-intolerant cladocerans with larger salt-tolerant cladocerans, which may have dramatic effects on freshwater communities and ecosystems. Additionally, the increase of salinity had a greater impact on the population level of D. magna and M. macrocopa than on the individual level, indicating that population level of cladocerans was more susceptible to salinity stress. Experiments only based on individuals may underestimate the ecologically related changes in populations and communities, thus understanding the impact of salinization on freshwater systems needs to consider multiple ecological levels.

Shallow lakes and ponds, providing essential ecological and environmental services, are simultaneously disrupted by various pollutants of emerging concern (PECs). As a group of PECs, microplastics (MPs) ubiquitously found in freshwater are toxic to a huge variety of organisms. However, the consequence of secondary factors such as food quantity determining MPs toxicity, and the corresponding water safety risks await assessment is still poorly understood. Accordingly, we investigated how MPs across three particle sizes (10, 1 and 0.07 µm) interacted with food abundance to affect survival, reproduction and population performance in the waterflea Daphnia magna. Across multiple population traits, we found that MPs toxicity on Daphnia population performance was attenuated by higher food quantity, but this attenuation size was strongly dependent on MPs size. Path analysis results showed population growth rate was mainly constrained by reduced survival rather than fecundity. Furthermore, the additive null model revealed that the interactive effects of food abundance and MPs were predominately recognized as synergism and trait dependency. The present findings underscore the importance of considering the complexity of interactions that can occur in the wild, when assessing the effects of plastics pollution on population dynamics of the basic trophic level in lakes and ponds.

Most aquatic ecosystems are at risk of being polluted by new environmental pollutant nanoparticles. As the main food source of zooplankton, the biomass of algae always fluctuates. Cladocerans, an important part of zooplankton, are usually be simultaneously exposed to different abundance of algae and nanoparticles in aquatic environment. To evaluate the combined effects of food abundance and ZnO nanoparticles concentration on the development and early reproductive performance of cladocerans, we exposed Daphnia magna, a common and representative model organism in cladocerans, to the combinations of different abundances of Chlorella pyrenoidosa and different concentrations of ZnO nanoparticles, recorded the key life-history traits, and used multiple models to fit the data. Results showed that high level of ZnO nanoparticles and low abundance Chlorella had an interactively negative effect on the life history of D. magna. When D. magna was exposed to ZnO nanoparticles, some life history traits, such as survival time, body length at maturation, and offspring per female, increased exponentially with the increase of food abundance, and then reached a theoretical maximum value, whereas some other life history traits, such as time to maturation and time to first brood, showed an opposite trend. However, higher Chlorella abundance reduced the negative effect of ZnO nanoparticles on D. magna, but the negative effect could not be eliminated with the increase of food abundance. Below Chlorella 0.30 mg C L-1, food plays a decisive role, while at or above this threshold, ZnO nanoparticles play a decisive role. Therefore, the effect of different ZnO nanoparticles concentrations can be fully reflected only when food is sufficient, and the negative effects of food shortages may mask the toxic effects of ZnO nanoparticles on D. magna. The findings indicated that the effects of food abundance should be considered in evaluating the realistic impact of pollutants on zooplankton.

In aquatic ecosystems, cladocerans, an important part of zooplankton, are often exposed to new pollutant nanoparticles and poor quality food simultaneously. To evaluate the combined effects of poor quality food and nanoparticles on the development and early reproductive performance of cladocerans, we exposed Daphnia magna , a representative cladoceran, to different concentrations of ZnO nanoparticles under the food conditions containing different proportions of non-toxic Microcystis , recorded some key indicators of the early life history, and analyzed the possible differential dose effects. The results showed that non-toxic Microcystis and ZnO nanoparticles significantly delayed the times to maturation and reproduction and also significantly reduced the survival time, the body length at maturation, the number of offspring in the first brood, and the total offspring per female of D. magna . When the food contained non-toxic Microcystis , D. magna did not reach sexual maturity at 0.25 mg L −1 ZnO nanoparticles and did not develop eggs and reproduce offspring at 0.20 mg L −1 ZnO nanoparticles, especially non-toxic Microcystis and ZnO nanoparticles had a significant or nearly significant synergistic effect on the time to maturation, time to first brood, and the total offspring per female of D. magna , whereas for other life history indicators, non-toxic Microcystis and ZnO nanoparticles showed an additive effect, based on comparing the slopes of linear fitting of the relevant indicators with nanoparticle concentration under different food quality conditions. Such findings are helpful to realistically evaluate the comprehensive stress effect on zooplankton under the complex conditions of poor food quality and coexistence of pollutants.

Nitrite and ammonia are two of the most common toxic nitrogenous pollutants in aquatic ecosystem, which can pose a serious threat to the health of aquatic organisms. Cladocerans, as an important part of freshwater ecosystem, will inevitably be harmed by these pollutants. To evaluate the combined toxic effects of these nitrogenous pollutants on cladocerans, we simulated 15 combinations of five nitrite concentrations (0, 0.5, 2, 4, 8 mg L−1) and three ammonia concentrations (0, 0.1, 1 mg L−1) to study the changes of life history traits of Daphnia pulex exposed to these combinations for 21 days. Results showed that under the combined stress of nitrite and ammonia, the survival time of D. pulex was shortened, the time to first batch of eggs and time to first brood were delayed, the body size and tail spine length at maturation were reduced, and the total offspring and the number of broods were decreased. There were some synergistically negative effects between the two nitrogenous pollutants. The presence of ammonia reduced the tolerance of D. pulex to nitrite, and vice versa. These findings provided new insights into the combined toxic effects of nitrite and ammonia on the life history traits of cladocerans, and were of great significance for understanding the population dynamics of specific species in cladocerans community under the condition of compound nitrogenous pollutants.

Microplastics are an emerging and increasingly serious pollutant in freshwater environment, which have become a threat to freshwater organisms. However, whether microplastics interfere with the responses of organisms to their predators is still unclear. In this study, we investigated the effects of microplastics with tiny different particle size (diameter: 0.7 and 1 µm) on the anti-predation (Rhodeus ocellatus as the predator) defense responses of different body-sized cladocerans, Daphnia pulex and Moina macrocopa. Results showed that microplastics had a size-based inhibitory effect on the induced defense of both D. pulex and M. macrocopa. Specifically, 0.7 µm microplastics had stronger effects on reduced survival time, delayed maturation time, and decreased offspring numbers. In addition, the effects of microplastics also varied with different body-sized cladocerans, i.e. medium-sized cladoceran (D. pulex) were more sensitive than the small-sized one (M. macrocopa) regarding the maturation time. This study illustrated for the first time that the effect of microplastics on induced defense was related to cladoceran species and microplastics size, and further revealed the extensive negative effects of microplastics from the perspective of interspecific relationship.

Both predation risk and crowding can induce sexual reproduction in Daphnia, and predator kairomones can also cause population crowding. It is, however, unclear as to whether sexual reproduction is caused by kairomone per se or indirectly by kairomone-induced crowding. In this study, we cultured Daphnia pulex in the presence of fish kairomone, with newborn offspring being retained (increasing population density) or removed (constant population density). The relationships among fish kairomone, population density, and indicators of sexual reproduction were assessed using structural equation models. The results showed that when offspring were retained in the population, the population density and sexual reproduction by Daphnia simultaneously increased in response to the presence of the fish kairomone, whereas sexual reproduction was not directly affected in the presence of fish kairomones alone. Conversely, when maintaining a constant population density, the number of ephippia and ratio of male to female of Daphnia increased with increasing population density, but the presence of fish kairomone did not promote sexual reproduction. Structural equation model analysis indicated that population density, rather than the fish kairomone, was the significant factor promoting sexual reproduction. The findings of this study indicate that population density directly triggers sexual reproduction in Daphnia populations. Our observations provide new insights into environmentally induced sexual reproduction, and on this basis, we suggest that a more thorough consideration of the interactions among environmental factors in determining sexual reproduction is necessary.

Microbes perform a variety of vital functions that are essential for healthy ecosystems, ranging from nutrient recycling, antibiotic production and waste decomposition. In many animals, microbes become an integral part by establishing diverse communities collectively termed as “microbiome/s”. Microbiomes defend their hosts against pathogens and provide essential nutrients necessary for their growth and reproduction. The microbiome is a polygenic trait that is dependent on host genotype and environmental variables. However, the alteration of microbiomes by stressful condition and their recovery is still poorly understood. Despite rapid growth in host-associated microbiome studies, very little is known about how they can shape ecological processes. Here, we review current knowledge on the microbiome of Daphnia, its role in fitness, alteration by different stressors, and the ecological and evolutionary aspects of host microbiome interactions. We further discuss how variation in Daphnia physiology, life history traits, and microbiome interactive responses to biotic and abiotic factors could impact patterns of microbial diversity in the total environment, which drives ecosystem function in many freshwater environments. Our literature review provides evidence that microbiome is essential for Daphnia growth, reproduction and tolerance against stressors. Though the core and flexible microbiome of Daphnia is still debatable, it is clear that the Daphnia microbiome is highly dependent on interactions among host genotype, diet and the environment. Different environmental factors alter the microbiome composition and diversity of Daphnia and reduce their fitness. These interactions could have important implications in shaping microbial patterns and their recycling as Daphnia are keystone species in freshwater ecosystem. This review provides a framework for studying these complex relationships to gain a better understanding of the ecological and evolutionary roles of the microbiome.

Phenotypic plasticity is common, and the induction and reversibility of plastic traits is closely related to the fitness of organisms in fluctuating environments. However, understanding the expression and reversion patterns of inducible traits in environments with fluctuating predation intensity is still a challenge. The present study focused on the inducible horns of Ceriodaphnia cornuta and its interaction with predation by Chaoborus larvae. We described the expression and reversion patterns of horns under increasing predation risks, which included response time, rate, and intensity. Additionally, we used predation trials to measure the effect of inducible horns on the survival rate and resistance to Chaoborus larvae predation by juvenile and adult C. cornuta. The results revealed that the size and expression patterns of horns at different body parts contributed to individual horn induction, which ultimately led to adaptive expression and reversion patterns under increasing predation risks. Specifically, the expression time and intensity increased linearly under increasing predation risks. Correspondingly, the reversion time also increased linearly, reversibility decreased linearly, and induction and reversion rates increased logarithmically. Corresponding with the plasticity pattern, predation trials showed that adult individuals with horns were remarkably less vulnerable to predation by Chaoborus larvae than undefended individuals. This finding provides a direct evidence of the benefit of inducible horns in C. cornuta. By combining benefits and response patterns, the present study provided a basic understanding of the ontogenetic plasticity of horns in C. cornuta. Furthermore, our study indicates that C. cornuta could be an efficient organism for understanding the induction and reversibility of inducible defensive traits and related interspecific relationships under fluctuating conditions in aquatic environments.