Changes in Induced-Antipredation Defense Traits and Transcriptome Regulations of Daphnia magna in Response to 5-HT1A Receptor Antagonist.

The serotonin signaling system plays a crucial role in regulating the ontogeny of crustaceans. Here, we describe the effects of different concentrations of the 5-hydroxytryptamine 1A receptor antagonist (WAY-100635) on the induced antipredation (Rhodeus ocellatus as the predator), morphological, behavioral, and life-history defenses of Daphnia magna and use transcriptomics to analyze the underlying molecular mechanisms. Our results indicate that exposure to WAY-100635 leads to changes in the expression of different defensive traits in D. magna when faced with fish predation risks. Specifically, as the length of exposure to WAY-100635 increases, high concentrations of WAY-100635 inhibit defensive responses associated with morphological and reproductive activities but promote the immediate negative phototactic behavioral defense of D. magna. This change is related to the underlying mechanism through which WAY-100635 interferes with gene expression of G-protein-coupled GABA receptors by affecting GABBR1 but promotes serotonin receptor signaling and ecdysteroid signaling pathways. In addition, we also find for the first time that fish kairomone can significantly activate the HIF-1α signaling pathway, which may lead to an increase in the rate of immediate movement. These results can help assess the potential impacts of serotonin-disrupting psychotropic drugs on zooplankton in aquatic ecosystems.

Species-Specific Effects of Planktonic Bacteria on the Predator-Induced Life-History Defense of Daphnia: Based on Hierarchical Cluster Analysis and Structural Equation Model.

Planktonic bacteria are an important part of aquatic ecosystems and interact with zooplankton. However, it is still unclear whether different planktonic bacteria differentially interfere with the responses of zooplankton to their predators. Here, we investigated the effects of different planktonic bacteria, which were isolated and purified from natural lakes, on the anti-predation (Rhodeus ocellatus as the predator) defense responses of Daphnia magna. Our results showed that the effects of planktonic bacteria on the induced life-history defenses of Daphnia were species-specific. Bacteria which increased (e.g., Escherichia coli, Citrobacter braakii) Daphnia body size also promoted the induced defense of body size, whereas bacteria which decreased (e.g., Pseudomonas spp.) Daphnia body size also inhibited the induced defense of body size. In addition, the same bacteria had different effects on induced defense traits. Some bacteria (e.g., E. coli) promoted the induced defense of body size but reduced the induced defense of offspring number, whereas other bacteria (e.g., Aeromonas hydrophila, Aeromonas veronas) weakened the induced defense of body size but had no significant effect on the induced defense of offspring number. Moreover, the differential effects of planktonic bacteria on Daphnia's induced defenses were not related to the bacterial degradation of kairomone. This study illustrated, for the first time, the species-specific effects of planktonic bacteria on predator-induced responses of Daphnia. IMPORTANCE This study is the first to reveal the differential effects of different species of planktonic bacteria on fish kairomone-induced defense traits and energy redistribution in Daphnia. Our results not only help deepen the understanding of Daphnia's inducible defenses in environments containing a variety of bacteria but also provide insights into the energy reallocation involved in anti-predator defenses.

Enhancing harvest of biodiesel-promising microalgae using Daphnia domesticated by amino acids.

Mass production of microalgal biodiesel is hindered by microalgae harvesting efficiency and costs. In this study, Daphnia domesticated by amino acids were used to harvest microalgae via ingesting. The main factors (density of Daphnia, salinity, pH, light-environment, temperature and algal concentration) that were conducive to Daphnia feeding were optimized. Under the optimal condition, Microalgae-feeding Daphnia were domesticated by adding D-glutamic acid and L-cysteine as stimulating factors. After that, the ingestion rate of domesticated Daphnia increased by 24.93%. The presence of Daphnia as a predator can induce microalgae to mass into clusters. Combining Daphnia feeding and the inductive defense flocculation of microalgae, the harvesting rate of mixed algae (Chlorella pyrenoidosa and Scenedesmus obliquus) reached over 95% after 9 h. Overall, this work suggested that Daphnia feeding process is a green and economical approach for microalgae harvesting.

Differing Escape Responses of the Marine Bacterium Marinobacter adhaerens in the Presence of Planktonic vs. Surface-Associated Protist Grazers.

Protist grazing pressure plays a major role in controlling aquatic bacterial populations, affecting energy flow through the microbial loop and biogeochemical cycles. Predator-escape mechanisms might play a crucial role in energy flow through the microbial loop, but are yet understudied. For example, some bacteria can use planktonic as well as surface-associated habitats, providing a potential escape mechanism to habitat-specific grazers. We investigated the escape response of the marine bacterium Marinobacter adhaerens in the presence of either planktonic (nanoflagellate: Cafeteria roenbergensis) or surface-associated (amoeba: Vannella anglica) protist predators, following population dynamics over time. In the presence of V. anglica, M. adhaerens cell density increased in the water, but decreased on solid surfaces, indicating an escape response towards the planktonic habitat. In contrast, the planktonic predator C. roenbergensis induced bacterial escape to the surface habitat. While C. roenbergensis cell numbers dropped substantially after a sharp initial increase, V. anglica exhibited a slow, but constant growth throughout the entire experiment. In the presence of C. roenbergensis, M. adhaerens rapidly formed cell clumps in the water habitat, which likely prevented consumption of the planktonic M. adhaerens by the flagellate, resulting in a strong decline in the predator population. Our results indicate an active escape of M. adhaerens via phenotypic plasticity (i.e., behavioral and morphological changes) against predator ingestion. This study highlights the potentially important role of behavioral escape mechanisms for community composition and energy flow in pelagic environments, especially with globally rising particle loads in aquatic systems through human activities and extreme weather events.

Induction of Phlorotannins and Gene Expression in the Brown Macroalga Fucus vesiculosus in Response to the Herbivore Littorina littorea.

Mechanisms related to the induction of phlorotannin biosynthesis in marine brown algae remain poorly known. Several studies undertaken on fucoid species have shown that phlorotannins accumulate in the algae for several days or weeks after being exposed to grazing, and this is measured by direct quantification of soluble phenolic compounds. In order to investigate earlier inducible responses involved in phlorotannin metabolism, Fucus vesiculosus was studied between 6 and 72 h of grazing by the sea snail Littorina littorea. In this study, the quantification of soluble phenolic compounds was complemented by a Quantitative real-time PCR (qRT-PCR) approach applied on genes that are potentially involved in either the phlorotannin metabolism or stress responses. Soluble phlorotannin levels remained stable during the kinetics and increased significantly only after 12 h in the presence of grazers, compared to the control, before decreasing to the initial steady state for the rest of the kinetics. Under grazing conditions, the expression of vbpo, cyp450 and ast6 genes was upregulated, respectively, at 6 h, 12 h and 24 h, and cyp450 gene was downregulated after 72 h. Interestingly, the pksIII gene involved in the synthesis of phloroglucinol was overexpressed under grazing conditions after 24 h and 72 h. This study supports the hypothesis that phlorotannins are able to provide an inducible chemical defense under grazing activity, which is regulated at different stages of the stress response.

Ecology of predator-induced morphological defense traits in Daphnia longispina (Cladocera, Arthropoda).

Inducible defenses against predators are widespread among plants and animals. For example, some Daphnia species form neckteeth against predatory larvae of the dipteran genus Chaoborus. Though thoroughly studied in D. pulex, knowledge about neckteeth in other Daphnia species is limited. The occurrence of this trait in the D. longispina species complex is only sporadically reported and the specific shape of neckteeth or the occurrence of other morphological defense traits is scarcely known in this widespread group. Here, we explored neckteeth occurrence in a large number of D. longispina populations across Scandinavia and studied neckteeth formation and other morphological defense traits on three D. longispina clones in the laboratory. In the study region, neckteeth on juvenile D. longispina s. str. were observed frequently in permanent ponds, but only when Chaoborus spp. larvae were present. In the laboratory experiments, all three D. longispina clones developed neckteeth (very similar to D. pulex) in response to Chaoborus kairomone exposure. The D. longispina clones also developed a longer tail spine, wider body, and larger neckteeth pedestal in response to predation threat-likely as a defense against the gape-limited predator. The intensity of neckteeth expression also depended on the clone studied and the concentration of Chaoborus kairomone. Our results demonstrate that neckteeth on D. longispina can be common in nature and that D. longispina can also induce other morphological defenses against predators. The similarity of neckteeth in D. longispina and D. pulex imposes yet unresolved questions on the evolutionary origin in these distantly related Daphnia groups.

Changes in cytokinins are sufficient to alter developmental patterns of defense metabolites in Nicotiana attenuata.

Plant defense metabolites are well known to be regulated developmentally. The optimal defense (OD) theory posits that a tssue's fitness values and probability of attack should determine defense metabolite allocations. Young leaves are expected to provide a larger fitness value to the plant, and therefore their defense allocations should be higher when compared with older leaves. The mechanisms that coordinate development with defense remain unknown and frequently confound tests of the OD theory predictions. Here we demonstrate that cytokinins (CKs) modulate ontogeny-dependent defenses in Nicotiana attenuata. We found that leaf CK levels highly correlate with inducible defense expressions with high levels in young and low levels in older leaves. We genetically manipulated the developmental patterns of two different CK classes by using senescence- and chemically inducible expression of CK biosynthesis genes. Genetically modifying the levels of different CKs in leaves was sufficient to alter ontogenic patterns of defense metabolites. We conclude that the developmental regulation of growth hormones that include CKs plays central roles in connecting development with defense and therefore in establishing optimal patterns of defense allocation in plants.

Geographic variation in the sensitivity of an herbivore-induced seaweed defense.

Intraspecific variation in primary producer induced defenses may affect how these defenses contribute to population and community regulation. For instance, inducible defenses may play larger roles in primary producer populations that are sensitive to low levels of grazing than populations that only respond to high levels of grazing. Although the incidence of herbivore-induced defenses is well documented, and there are examples of geographic variation in these defenses, we have limited knowledge of the factors that affect sensitivity to grazing (i.e., the minimum grazer density needed to elicit these responses) within and among populations. Filling this gap is necessary to understand the importance of these defenses under different conditions. To address whether the sensitivity of seaweed induced defenses varies geographically, we exposed Northern and Southern California populations of the seaweed Silvetia compressa to five densities of the snail Tegula funebralis, under ambient, regional environmental conditions. Southern seaweeds required high levels of grazing to induce defenses, while all levels of herbivory decreased Northern seaweed palatability. To better understand these different responses to low levels of grazing, we conducted common garden experiments to directly test the roles of experimental environment and herbivore source. Consistent with initial experiments, Northern Silvetia responded to low levels of grazing, regardless of environmental conditions, while Southern Silvetia never responded to low levels of grazing, even after being acclimated to Northern environmental conditions for 24 d. Additionally, Southern Silvetia did not respond to grazing by Northern snails, suggesting that herbivore source did not explain this geographic pattern in inducible seaweed defenses. Together, these observations suggest that the seaweed source explains this geographic pattern in sensitivity. Trade-offs with constitutive defenses did not help explain this pattern, as Southern Silvetia had weaker constitutive defenses than Northern seaweeds. This may be due to a combination of low grazing pressure and low predictability of attack in the South. Thus, population variation in sensitivity may be due to long-term differences in environmental histories, resulting in local adaptation or legacy effects from exposure to local conditions. Overall, our results indicate that these herbivore-induced responses may be more important in Northern Silvetia populations than Southern ones.

Good neighbors make good defenses: associational refuges reduce defense investment in African savanna plants.

Intraspecific variation in plant defense phenotype is common and has wide-ranging ecological consequences. Yet prevailing theories of plant defense allocation, which primarily account for interspecific differences in defense phenotype, often fail to predict intraspecific patterns. Furthermore, although individual variation in defense phenotype is often attributed to ecological interactions, few general mechanisms have been proposed to explain the ubiquity of variable defense phenotype within species. Here, we show experimentally that associational refuges and induced resistance interact to create predictable intraspecific variation in defense phenotype in African savanna plants. Physically defended species from four families (Acanthaceae, Asparagaceae, Cactaceae, and Solanaceae) growing in close association with spinescent Acacia trees had 39-78% fewer spines and thorns than did isolated conspecifics. For a subset of these species, we used a series of manipulative experiments to show that this variability is maintained primarily by a reduction in induced responses among individuals that seldom experience mammalian herbivory, whether due to association with Acacia trees or to experimental herbivore exclusion. Unassociated plants incurred 4- to 16-fold more browsing damage than did associated individuals and increased spine density by 16-38% within one month following simulated browsing. In contrast, experimental clipping induced no net change in spine density among plants growing beneath Acacia canopies or inside long-term herbivore exclosures. Associated and unassociated individuals produced similar numbers of flowers and seeds, but seedling recruitment and survival were vastly greater in refuge habitats, suggesting a net fitness benefit of association. We conclude that plant-plant associations consistently decrease defense investment in this system by reducing both the frequency of herbivory and the intensity of induced responses, and that inducible responses enable plants to capitalize on such associations in heterogeneous environments. Given the prevalence of associational and induced defenses in plant communities worldwide, our results suggest a potentially general mechanism by which biotic interactions might predictably shape intraspecific variation in plant defense phenotype.

Maternal Effects Via Resting eggs in Predator Defense of the Rotifer Brachionus calyciflorus.

Maternal effects play important evolutionary and ecological roles. Amictic female mothers of monogonont rotifer Brachionus calyciflorus can transmit predatory information of Asplanchna brightwellii in their environment to their offspring by changing the offspring's defensive morphology to increase their fitness. However, it remains unclear whether such maternal effects also exist during sexual reproduction of a mictic mother. This study explored the maternal effect in mictic mothers using the B. calyciflorus and A. brightwellii as a prey-predator model. We collected resting eggs from two groups of mictic mothers that previously experienced environments with (P environment) or without (NP environment) Asplanchna kairomones. Stem females from the resting eggs of each maternal group were also hatched and reared in P and NP environments. The population growth rate of offspring who experienced the same environment as their mictic mothers was significantly higher than those that experienced a different environment. When exposed to a gradient of predator kairomone levels, the posterolateral spine of the offspring elongated with increasing kairomone concentration. Offspring from the P mictic mother showed significantly shorter posterolateral spines than those from the NP mictic mother at each predator kairomone level. Offspring originating from NP mictic mothers clearly elongated their posterolateral spines at low concentrations of predator kairomones, while those from P mothers elongated their posterolateral spines only at the highest levels of predator kairomone. Our findings highlight the existence of anticipatory maternal effects during the sexual process via resting eggs of B. calyciflorus in response to predator kairomone.

Predator lipids induce paralytic shellfish toxins in bloom-forming algae.

Interactions among microscopic planktonic organisms underpin the functioning of open ocean ecosystems. With few exceptions, these organisms lack advanced eyes and thus rely largely on chemical sensing to perceive their surroundings. However, few of the signaling molecules involved in interactions among marine plankton have been identified. We report a group of eight small molecules released by copepods, the most abundant zooplankton in the sea, which play a central role in food webs and biogeochemical cycles. The compounds, named copepodamides, are polar lipids connecting taurine via an amide to isoprenoid fatty acid conjugate of varying composition. The bloom-forming dinoflagellate Alexandrium minutum responds to pico- to nanomolar concentrations of copepodamides with up to a 20-fold increase in production of paralytic shellfish toxins. Different copepod species exude distinct copepodamide blends that contribute to the species-specific defensive responses observed in phytoplankton. The signaling system described here has far reaching implications for marine ecosystems by redirecting grazing pressure and facilitating the formation of large scale harmful algal blooms.

Predator ontogeny affects expression of inducible defense morphology in rotifers.

Many prey organisms show induced morphological responses to predators including changes in protective spine length, such as in rotifers, although previous studies have mainly focused on how prey become larger than the predator gape-size optimum. Here we show that a large-sized predator makes prey rotifers escape below the gape-size optimum of the predator by reducing spine length. In experiments and field studies we show that during part of their ontogeny fish larvae feed intensively on the common rotifer Keratella cochlearis, and that larval fish predation reduces rotifer spine length both through induction of shorter spines and selective predation on long-spined individuals. We also describe a global scale pattern in spine length of K. cochlearis, showing an increasing variance in spine length with latitude. This pattern may be explained by differences in fish reproduction from once per year at high latitudes to several times per year at lower latitudes. That spine length is adaptively adjusted to the ontogeny of a dominant predator taxa provides a novel view on our understanding of factors affecting temporal and spatial variations in prey defense morphology.

Interplay between microbial trait dynamics and population dynamics revealed by the combination of laboratory experiment and computational approaches.

Filament formation is a common bacterial defense mechanism and possibly has a broad impact on microbial community dynamics. In order to examine the impact of filament formation on population dynamics, we developed an experimental system with a filamentous bacterium Flectobacillus sp. MWH38 and a ciliate predator Tetrahymena pyriformis. In this system, the effective defense of Flectobacillus resulted in the extinction of Tetrahymena by allowing almost no population growth. The result of a kairomone experiment suggested the existence of chemical signals for filament formation. To examine the mechanism further, we developed a quantitative mechanistic model and optimized the model for the experimental result using the simulated annealing method. We also performed a global parameter sensitivity analysis using an approximated Bayesian computation based on the sequential Monte Carlo method to reveal parameters to which the model behavior is sensitive to. Our model reproduced the population dynamics, as well as the cell size dynamics of Flectobacillus. The model behavior is sensitive to the nutrient uptake of Flectobacillus and the propensity of filament formation. It robustly predicts the extinction of Tetrahymena at the condition used in the experiment and predicts the transition from equilibrium to population cycle at higher nutrient conditions. Contrary to the previous study that disproved the presence of chemical signals for filament formation, our result suggested the importance of chemical signals at low predator density, suggesting the variety in bacterial resistance mechanisms that act at different stages of predator-prey interactions.

Naive tadpoles do not recognize recent invasive predatory fishes as dangerous.

Invasive alien predators (IAP) are spreading on a global scale-often with devastating ecological effects. One reason for their success may be that prey species fail to recognize them due to a lack of co-evolutionary history. We performed a comprehensive test of this "prey naiveté" hypothesis using a novel approach: we tested whether predator-naive tadpoles of the agile frog (Rana dalmatina) display antipredator behavior upon encountering chemical cues produced by native, invasive (established or recent) or allopatric fishes (four perciforms, four siluriforms, and two cypriniforms). We studied the influence of population origin on predator-detection ability by presenting chemical cues to predator-naive tadpoles that originated from fishless hill-ponds or fish-infested floodplain populations. Before trials, we fed fishes with tadpoles or an alternative food to test whether direct chemical cues from the predator's diet influences the tadpoles' recognition of potential predators. Tadpoles reduced their activity upon exposure to cues from native and long-established invasive perciforms, but not in response to recent invaders, allopatric predators, or to any siluriforms. Also, predators that were previously fed with tadpoles did not universally induce behavioral defensedefenses upon first encounter. Finally, tadpoles originating from isolated hill-ponds exhibited higher baseline activity and responded in weaker fashion than their conspecifics from floodplain populations, which co-exist with predatory fishes. Our results indicate that tadpoles may be vulnerable to invading predatory fishes due to their inability to recognize them as dangerous, though their ability to recognize invasive IAP may evolve rapidly, in fewer than 30 generations.

Inhibitory effects of toxic Dolichospermum flos-aquae and anatoxin-a on inducible defenses of Daphnia magna.

Cyanobacterial blooms, resulting from serious eutrophication, can produce various cyanotoxins and severely disrupt aquatic ecosystems. Inducible defenses are adaptive traits developed by prey in response to predation risks. However, the effects of the increasing proportion of cyanobacteria and cyanotoxins produced during cyanobacterial blooms on the inducible defenses of cladocerans, particularly in terms of behavioral defenses, remain unclear. In this study, we selected Daphnia magna and investigated the defensive traits against predation risks by the predator Rhodeus ocellatus under different ratios of cyanobacteria (Dolichospermum flos-aquae) and green algae (Scenedesmus obliquus), as well as varying concentrations of anatoxin-a (ATX), a cyanotoxin. We recorded the inducible defensive traits involving to morphology, behavior, and offspring production of D. magna. Results showed that the body length of D. magna at sexual maturity and the number of offspring in the first brood were significantly reduced by the presence of D. flos-aquae. Moreover, when the proportion of D. flos-aquae reached 75% and 100%, D. magna did not develop to sexual maturity. Furthermore, D. flos-aquae inhibited the formation of inducible behavioral defense of D. magna, with a stronger inhibitory effect as the proportion of D. flos-aquae increased. In this experiment, the effects of ATX on the morphological traits at sexual maturity and offspring production of D. magna were minor, but ATX still had the potential to inhibit the formation of inducible behavioral defense. We confirmed that changes in the proportion of cyanobacteria and green algae as well as the production of ATX by cyanobacteria during cyanobacterial blooms can affect the growth, development, and inducible defensive traits of cladocerans, potentially altering their population dynamics during such events.

Differential interference effects of thermal pollution on the induced defense of different body-sized cladocerans.

Climate warming influences the biological activities of aquatic organisms, including feeding, growth, and reproduction, thereby affecting predator-prey interactions. This study explored the variation in thermal sensitivity of anti-predator responses in two cladoceran species with varying body sizes, Daphnia pulex and Ceriodaphnia cornuta. These species were cultured with or without the fish (Rhodeus ocellatus) kairomone at temperatures of 15, 20, 25, and 30 °C for 15 days. Results revealed that cladocerans of different body sizes exhibited varying responses to fish kairomones in aspects such as individual size, first-brood neonate size, total offspring number, average brood size, growth rate, and reproductive effort. Notably, low temperature differently affected defense responses in cladocerans of different body sizes. Both high and low temperatures moderated the intensity of the kairomone-induced response on body size at maturity. Additionally, low temperature reversed the reducing effect of fish kairomone on the total offspring number, average brood size, and reproductive effort in D. pulex. Conversely, it enhanced the increasing effect of fish kairomone on these parameters in C. cornuta. These results suggest that inducible anti-predator responses in cladocerans are modifiable by temperature. The differential effects of fish kairomones on various cladocerans under temperature influence offer crucial insights for predicting changes in predator-prey interactions within freshwater ecosystems under future climate conditions.

Resource supply and intraspecific variation in inducible defense determine predator-prey interactions in an intraguild predation food web.

This study investigated the dynamics of reciprocal phenotypic plasticity entailing inducible defense and offense in freshwater ciliate communities in response to altered resource supply and the extent of intraspecific trait variation. Communities consisted of Euplotes octocarinatus (intraguild prey) capable of inducible defense to escape predation, Stylonychia mytilus (intraguild predator) capable of inducible offense to expand its prey spectrum, and Cryptomonas sp. (algal resource). The extent of inducible defense was tested in ten different Euplotes strains in response to freeze-killed Stylonychia concentrate, revealing significant differences in their width and length development. In a subsequent 30-day experiment, four strains were incubated in monoculture and mixture with Stylonychia under continuous and pulsed microalgae supply. The polyclonal Euplotes population outperformed the monoclonal populations, except one, which developed the most pronounced inducible defense and retained the highest biovolume. Stylonychia fluctuated in size, but dominated all communities irrespective of clonal composition. Pulsed resource supply promoted biovolume production of both species. However, periods of resource depletion resulted in more Stylonychia resting cysts, allowing Euplotes to resume growth. Our study provides new insights into interactions of induced defense and intraguild predation under variable environmental conditions, emphasizing the relevance of intraspecific trait variation for predator-prey interactions and community dynamics.

Polystyrene microplastics weaken the predator-induced defenses of Daphnia magna: Evidences from the changes in morphology and behavior.

Polystyrene microplastics are ubiquitous in freshwater ecosystems and have significant impacts on freshwater organisms. Stable interspecific relationships, including the predation and defense relationships between predator and prey, play an extremely important role in maintaining the health of aquatic ecosystems. However, it still remains unknown whether polystyrene microplastics can interfere with predator-induced defenses of prey, especially in behavior change. Therefore, we studied the effects of different particle sizes and concentrations of polystyrene microplastics on the induced defenses related to morphology, reproduction, and behavior of Daphnia magna exposed to the predation risks from a species of zooplanktivorous fish Rhodeus ocellatus. Results showed that polystyrene microplastics weakened the predator-induced defenses of D. magna, including morphology, reproduction, and behavior. Polystyrene microplastics did not affect the beat rate of the second antennae (swimming activity) and thoracic appendage (filter-feeding activity) of D. magna, but R. ocellatus kairomone reduced Daphnia swimming activity. Moreover, in the absence of R. ocellatus predation risks, polystyrene microplastics did not alter the vertical distribution of D. magna in the water column, whereas D. magna exposed to R. ocellatus kairomone stayed deeper in the water; however, when both polystyrene microplastics and fish kairomone were present, D. magna inhabited in shallower water. Furthermore, small-sized microplastics interfered with the inducible behavioral defense of D. magna more strongly than large-sized polystyrene microplastics. Such findings suggested that polystyrene microplastics can weaken the predator-induced defenses of Daphnia, which may increase their risk of predation and alter the population dynamics of zooplankton.

Delicate plasticity: Maladaptive responses to fish predation risk in Daphnia magna caused by sertraline pollution.

An emerging environmental pollutant may have a greater impact on phenotypic plasticity than its direct toxicity, causing maladaptive responses of organisms to their current environment. To better understand such ecological risks, we proposed a delicate plasticity hypothesis: if an emerging stressor acts on the fundamental processes underlying a specific adaptive plastic response, it is more likely to pose high risks to the phenotypic plasticity. Endocrine regulation is one of the critical processes of plasticity and is becoming a target for emerging pollutants. To test this hypothesis, we measured individual traits and the expression of endocrine-related genes in Daphnia magna in response to fish predation risk under exponentially increasing concentrations of the antidepressant sertraline, a selective serotonin reuptake inhibitor. The results showed that sertraline impaired most of the defense responses of D. magna at concentrations lower than the effective concentrations of its direct toxicity. The high risks of sertraline on inducible defenses were also visually reflected in the relationships between toxicity and plasticity strength, that is, most of the defense responses exponentially decayed with an increase in sertraline toxicity. In addition, the expression of genes involved in serotonin synthesis was significantly correlated with the expression of other endocrine-related genes and with changes in morphological traits. These results revealed that environmental sertraline pollution could disturb endocrine regulation and cause high risks to inducible defenses of D. magna, providing evidence supporting the delicate plasticity hypothesis.

Chemical cues in the interaction of herbivory-prey induce consumer-specific morphological and chemical defenses in Phaeocystis globosa.

Bloom-forming algae Phaeocystis globosa is one of the most successful blooming algae in the oceans due to its capacity to sense grazer-associated chemical cues and respond adaptively to these grazer-specific cues with opposing shifts in phenotype. P. globosa produces toxic and deterrent compounds as chemical defenses. However, the origin of the signals and underlying mechanisms that triggered the morphological and chemical defenses remain enigmatic. Rotifer was chosen to establish an herbivore-phytoplankton interaction with P. globosa. The influences of rotifer kairomone and conspecific-grazed cue on morphological and chemical defenses in P. globosa were investigated. As a result, rotifer kairomones elicited morphological defenses and broad-spectrum chemical defenses, whereas algae-grazed cues elicited morphological defenses and consumer-specific chemical defenses. According to multi-omics findings, the difference in hemolytic toxicity caused by different stimuli may be related to the upregulation of lipid metabolism pathways and increased lipid metabolite content, while the inhibition of colonial formation and development of P. globosa may be caused by the downscaled production and secretion of glycosaminoglycans. The study demonstrated that zooplankton consumption cues were recognized by intraspecific prey and elicited consumer-specific chemical defenses, highlighting the chemical ecology of herbivore-phytoplankton interactions in the marine ecosystem.