Phenotypic plasticity in defensive traits is an appropriate mechanism to cope with the variable hazard of a frequently changing predator spectrum. In the animal kingdom these so-called inducible defences cover the entire taxonomic range from protozoans to vertebrates. The inducible defensive traits range from behaviour, morphology, and life-history adaptations to the activation of specific immune systems in vertebrates. Inducible defences in prey species play important roles in the dynamics and functioning of food webs. Freshwater zooplankton show the most prominent examples of inducible defences triggered by chemical cues, so-called kairomones, released by predatory invertebrates and fish. The objective of this review is to highlight recent progress in research on inducible defences in freshwater zooplankton concerning behaviour, morphology, and life-history, as well as difficulties of studies conducted in a multipredator set up. Furthermore, we outline costs associated with the defences and discuss difficulties as well as the progress made in characterizing defence-inducing cues. Finally, we aim to indicate further possible routes in this field of research and provide a comprehensive table of inducible defences with respect to both prey and predator species.

A holistic understanding of plant strategies to acquire soil resources is pivotal in achieving sustainable food security. However, we lack knowledge about variety‐specific root and rhizosphere traits for resource acquisition, their plasticity and adaptation to drought. We conducted a greenhouse experiment to phenotype root and rhizosphere traits (mean root diameter [Root D], specific root length [SRL], root tissue density, root nitrogen content, specific rhizosheath mass [SRM], arbuscular mycorrhizal fungi [AMF] colonization) of 16 landraces and 22 modern cultivars of temperate maize (Zea mays L.). Our results demonstrate that landraces and modern cultivars diverge in their root and rhizosphere traits. Although landraces follow a ‘do‐it‐yourself’ strategy with high SRLs, modern cultivars exhibit an ‘outsourcing’ strategy with increased mean Root Ds and a tendency towards increased root colonization by AMF. We further identified that SRM indicates an ‘outsourcing’ strategy. Additionally, landraces were more drought‐responsive compared to modern cultivars based on multitrait response indices. We suggest that breeding leads to distinct resource acquisition strategies between temperate maize varieties. Future breeding efforts should increasingly target root and rhizosphere economics, with SRM serving as a valuable proxy for identifying varieties employing an outsourcing resource acquisition strategy.

The long-chain polyunsaturated fatty acids (PUFA) eicosapentaenoic acid (EPA, ω-3, or n-3) and arachidonic acid (ARA, ω-6 or n-6) are known to have distinct physiological functions, yet can both support growth and reproduction of consumers, raising the question of whether EPA and ARA are ecologically substitutable dietary resources. We explored the relative importance of EPA and ARA for the growth and reproduction of the freshwater keystone herbivore Daphnia in a life-history experiment. Both PUFA were supplemented in a concentration-dependent manner to a PUFA-free diet, separately and in combination (50% EPA: 50% ARA mixture). The growth-response curves obtained with EPA, ARA, and the mixture were virtually congruent and the thresholds for PUFA limitation did not differ, indicating that EPA (n-3) and ARA (n-6) were substitutable dietary resources under the applied experimental conditions. The actual requirements for EPA and ARA might change with growth conditions, e.g., under the influence of parasites or pathogens. The higher retention of ARA in Daphnia suggests that EPA and ARA are subject to different turnover rates, which also implies different physiological functions. Studies on the ARA requirements of Daphnia could provide valuable information on the presumably underestimated ecological importance of ARA in freshwater food webs.

Various species of freshwater zooplankton feature defensive spines, spikes or spinules across a comparatively wide size range. Among those is Daphnia magna, which has previously been found to express an enhanced spinescence in response to the predatory tadpole shrimp Triops cancriformis. This involves an elongated tail-spine, further protruding fornices and elongated, as well as further spread spinules along its carapace margins. Of these, the predator-induced trait changes concerning the spinules of D. magna at the age of first reproduction are known. However, their morphological development throughout the juvenile instars, in comparison to the changes of the prominent tail spine during the same periods, has thus far been unclear. Our study shows that D. magna expresses spinules across all investigated juvenile instars. Furthermore, during ontogeny, the development of D. magna’s rather small spinescence traits, i.e. the dorsal and ventral mean spinule length, as well as the dorsal spinules bearing area, in response to predation pressure, differs distinctly from that of the prominent tail-spine. Thus, our study provides further insight into the connectedness of traits contributing to the overall defensively enhanced spinescence of D. magna, with possible implications for the functional benefit of the traits at certain developmental periods.

Zooplankton displays different inducible defenses against invertebrate and vertebrate predators. The response pattern to gape‐limited invertebrate predators involves increased somatic growth and offspring body size but delayed maturity and reduced offspring numbers. In contrast to this general pattern, the freshwater model organism Daphnia magna has been reported to exhibit a different response when encountering the gape‐limited tadpole shrimp Triops cancriformis. Under laboratory conditions, D. magna showed increased somatic growth, earlier maturation, and an increase in both offspring number and size. We propose here that the discrepancy between the previously observed and the theory‐based response patterns against invertebrate predators is due to differences in food availability in the applied laboratory settings and assessed whether the defensive response of D. magna against T. cancriformis is modulated differently by food quantity and quality. We found a strong impact of food quantity and quality on the defense response of D. magna to T. cancriformis kairomones. The prey seem to be able to overcome trade‐offs between morphological defense traits and reproductive traits, but distinctly between high food quantity and high food quality. Thereby, reproductive traits were preferred over morphological defenses. Furthermore, the removal of particles from the T. cancriformis‐conditioned water caused a defense pattern in D. magna that was consistent with the general response pattern known from other invertebrate predators, thus explaining the described discrepancy to previous studies with T. cancriformis. Our study highlights the importance of assessing food‐related effects on predator–prey interactions to understand trophic relationships and food web processes.

Microplastics (MP) derived from commodity plastics are considered a threat to ecosystems. Bio-based and biodegradable (BB-) plastics are proposed as eco-friendly alternatives to petroleum-based (PB-) plastics. However, studies on the effects of MP on the model organism Daphnia magna often report a wide range of effects for both PB- and BB-plastics. While different physical and chemical MP properties may contribute, other factors, such as differences in clonal sensitivities, are rarely considered. Additionally, only a few studies included a particle control to differentiate between effects caused by particles themselves and those specifically by MP. In this study, these knowledge gaps are addressed by (1) comparing the effects of PB-MP (PET) and two BB-MP (PBS, PLA) on the life-history and morphology of D. magna, and (2) examining the responses of two D. magna clones to chronic MP exposure (all MP < 20 µm), always including cellulose as a particle control, in a setup with reduced food availability to mimic real environmental scenarios. When comparing PB- to BB-MP, the latter caused similar adverse effects on survival and sublethal life-history traits, while cellulose had no effect or positive effects. We observed a similar concentration-dependent increase in mortality for both clones while data on sublethal parameters were significantly different between the clones. We conclude that particle controls and genetic variability are crucial parameters that should be considered in D. magna experiments and that MP effects need to be investigated under environmentally relevant conditions.