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Predation is a well-known factor that structures rotifer communities. However, the role of protists as predators is relatively understudied. Here, we investigated predatory behavior of Actinosphaerium sp., a freshwater heliozoan, on seven rotifer species. Predators and prey were collected from a local playa; except for Brachionus calyciflorus that served as a naive prey control. Prey included large species (≥ 175 µm mean length: Asplanchna sieboldii , B. calyciflorus , Platyias quadricornis, and Lacinularia flosculosa ) and small species (< 175 µm: Cephalodella gibba, Euchlanis dilatata, and Lepadella patella ). Four experiments were conducted. (1) Single prey items of varying size and motility. Larger prey types were ~ 1.7 to 3.0 times more likely to be ingested than small prey. No L. flosculosa were ingested, contrary to field observations. No correlation was found between swimming speed and predation risk. (2) Preference tests. Asplanchna sieboldii and B. calyciflorus were favored prey. (3) Growth rate of Actinosphaerium on mixed diets, with and without Asplanchna . Highest population growth of Actinosphaerium was observed in presence of A. sieboldii . (4) Prey defenses. Susceptibility of spined versus unspined B. calyciflorus resulted in no significant difference in predation risk. Thus, size and being mobile (compared to sessility) are the primary risk factors influencing rotifer predation vulnerability.

To test the hypotheses that multigenerational predation risk favors clones with longer postero-lateral spines and a larger body, and that fitness consequences of selection depend on both tested fitness variables and selection intensity, we expose replicate Brachionus dorcas populations in low (LK) and high (HK) concentrations of Asplanchna kairomones. Over 60 days (23 asexual generations) of selection, LK-selected populations showed the highest equilibrium population densities, and median mictic ratios and postero-lateral spine lengths, and HK-selected populations showed the lowest equilibrium population densities, the highest mictic ratios, and the longest postero-lateral spines, among all the control and treatment populations. In the absence of Asplanchna kairomones, and compared with populations with the control selection history, populations with an LK selection history showed increased growth rates/intrinsic rates of population increase and net reproductive rates, but decreased mictic ratios, indicating three fitness gains and a fitness cost; and those with an HK selection history showed increased life expectancy at hatching and mictic ratios, but decreased growth rates, indicating two fitness gains and a fitness cost. Asplanchna kairomones might increase the heritability of higher equilibrium densities or mictic ratios, and longer postero-lateral spines, and thus enable the prey to survive predation.

To test the hypothesis that non-consumptive predation pressures favor genotypes that have higher equilibrium densities or mictic ratios, we expose replicate populations of the rotifer Brachionus angularis to low (LK) and high (HK) concentrations of predator Asplanchna kairomones. Over 75 days (25 asexual generations) of selection, LK-treated populations have higher equilibrium densities and lower mictic ratios than the control populations. The opposite occurs for HK-treated populations. These results suggest that compared with the control, LK and HK select for genotypes with higher equilibrium densities and mictic ratios, respectively. Common garden bioassays reveal that in the absence of Asplanchna kairomones, populations with an LK selection history have a higher average intrinsic rate of population increase, and those with an HK selection history have a higher average mictic ratio than those with the control selection history, which suggest that adaptations to LK and HK increase average intrinsic rate of population increase and mictic ratio of prey populations, respectively. Increased intrinsic rate of population increase and mictic ratio can serve as strategies of offsetting mortality from predation and escaping the predator, and thus enable prey to survive low and high predation pressures, respectively.

The planktonic rotifer, Brachionus calyciflorus, displays extensive variation in the length of its anterior and posterior spines. Notably, posterolateral spines may be absent or near body length. Studies of laboratory and natural populations have identified the different factors controlling this variation and have investigated the trade-offs associated with increased spine development. Low temperature and low food availability can induce modest spine elongation that may reduce sinking rate. A kairomone released by the carnivorous rotifer Asplanchna induces pronounced spine elongation, without detectable reproductive cost, that can provide an effective defense against this predator. Endogenous mechanisms also operate: Spine development is inhibited in females hatched from fertilized resting eggs and can be promoted by increasing maternal age. Genetic variation for the length of spines in both noninduced (default) and induced phenotypes occurs among and within populations. Asplanchna in natural communities likely leads to seasonal selection for genotypes that can develop increasingly long spines.

To cope with predation, many prey species have developed inducible defenses in terms of morphology, behavior, and life history. Rotifers were the first model organisms used to evaluate the ecology and evolution of inducible defenses in aquatic ecosystems. Since the middle of last century, only visible morphological defenses, such as spine development, have been found and only in a few rotifer species. Given the development of ultrastructural defenses is taxonomically widespread in aquatic ecosystems, we hypothesize that rotifer prey, particularly small-sized species, can develop such inducible defenses. We evaluated morphological response of two common Brachionus herbivores ( B . calyciflorus and B . angularis ) to predatory rotifer Asplanchna brightwellii . Results confirmed existence of predator-induced ultrastructural defenses, which are expressed as increased lorica thickness and enhanced lorica hardness. Such inducible defenses are more evident and effective in the smaller sized B . angularis , leading to higher fitness of B . angularis in predator-prey interactions. As anticipated, development of defenses has inevitable fitness costs manifested as decreased reproduction or reduced sex investment. Our results not only extend understanding of inducible ultrastructural defense to other planktonic taxa that were previously observed only in cladocerans, but also verify effective mechanical protection of such hidden defensive morphology.

We compared trans‐generational (F0–F12) morphological trait plasticity induced by Asplanchna kairomones between two Brachionus dorcas morphotypes (long‐posterior spines, LS; short‐posterior spines, SS) along with life‐table parameters of the non‐induced morphotypes. Under control conditions, SS rotifers tended to show higher fertility and smaller body size than LS rotifers. Low kairomone concentrations (50 and 200 ind./L) tended to increase body size in SS offspring, while exposure to 50, 200, and 800 ind./L kairomones induced spine elongation in both morphotypes, with posterolateral spine (PS) length increasing with kairomone concentration. Compared to the F0 generation, offspring of both morphotypes in unexposed controls showed generational fluctuations in body size; LS offspring exhibited shortening or no change in anteromedian spine (AMS) and anterolateral spine (ALS) lengths, while SS offspring showed elongation or no change in these spine lengths and PS length. Across all kairomone treatments, significant elongation of AMS and ALS in LS offspring was typically observed only in later generations, whereas SS offspring exhibited significant elongation from F1 through F12; LS offspring showed significant PS elongation from F2 through F12, with maximum lengths in the later generations (F5–F12), while SS offspring showed significant PS elongation from F1 through F12, peaking in early generations (F2–F4). Notably, the multi‐generational mean PS length in SS offspring remained significantly shorter than that in LS offspring under each kairomone treatment. Overall, SS offspring appeared to employ a synergistic defense combining increased body size and spine elongation favoring a “rapid and moderate response,” whereas LS offspring exhibited a “slow and extreme defense” strategy. These divergent strategies may result from evolutionary trade‐offs involving resource allocation, environmental predictability, and genetic constraints. We compared life‐table parameters and trans‐generation (F0–F12) morphological developmental plasticity induced by Asplanchna kairomones between two Brachionus dorcas morphotypes (long‐posterior spines, LS; short‐posterior spines, SS). The results revealed that the SS offspring employed a synergistic defense combining increased body size and spine elongation, and favored a “rapid and moderate response,” while the LS offspring exhibited a “slow and extreme defense” strategy, which likely resulted from evolutionary trade‐offs involving resource allocation, environmental predictability, and genetic constraints. This study provides new insights into the evolutionary strategies of adaptive developmental plasticity and the ecological mechanisms of morphological polymorphism in rotifers, and Oikos is the best platform to publish this article.

Experiments with two strains of the facultative epibiont, Brachionus rubens, tested the ability of this rotifer to avoid predation by the rotifer Asplanchna, and its propensity for sexual reproduction and consequent diapause at different population densities. Unlike some congeners, B. rubens did not have a morphological response to Asplanchna by developing longer spines or a larger body. However, it responded to this predator, and its kairomone, with a behavioral defense: a higher propensity to transition from free-swimming to attachment, typically in dense aggregations. Attached individuals were less likely to be captured and ingested, so that B. rubens outlived Asplanchna in some mixed-species cultures. Although crowding induces sex in some congeners, it did not do so in B. rubens. Instead, the proportion of sexual (mictic) daughters produced by females cultured in different volumes (0.5–60 ml) was density-independent: ~ 0.2 for the Argentina strain and ~ 0.6 for the Australia strain. Such fixed levels of sex have rarely been detected in rotifers. In B. rubens, they may be a strategy to ensure some sex with diapause at all times, but permit continued population growth via female parthenogenesis at the very high densities that normally occur on hosts and other surfaces.

We quantified the feeding behavior (encounter, attack, capture and ingestion) and demographic parameters (survival and reproduction) of the predatory rotifer Asplanchna sieboldii fed on the prey Plationus patulus previously exposed to microplastics (MPs), Cd or their combination. As compared to controls, capture and ingestion rates of P. patulus by A. sieboldii decreased by 71 and 61%, respectively, with prey previously exposed to mixed MPs and Cd treatment. Life table data showed that the predator died earlier in controls than when fed on prey exposed to both Cd and microplastics. Regardless of the prey treatment, the offspring production by A. sieboldii increased as the available prey numbers increased (from 1 to 4 ind. ml−1). Compared to controls, the fecundity rate of the predator decreased when contaminated prey was offered as food.

In the presence of the predatory rotifer Asplanchna, some Brachionus and Keratella species develop inducible morphological, behavioral, and life history defenses that enable prey to survive predation pressure. However, whether clones belonging to the same rotifer species but with different behaviors (attached and free-swimming) exhibit divergent responses in morphology and life history remains largely unknown. In this study, neonates of an attached and a free-swimming clone of B. dorcas were exposed to media conditioned by A. brightwellii neonates at densities of 0 (control), 40, 80, and 160 ind l−1 for 24 h, and their morphological and life-table demographic parameters were measured and calculated. We showed that the Asplanchna kairomone significantly increased the propensity to attach in the attached clone only, with attached individuals effectively protected from Asplanchna predation. In response to increasing Asplanchna kairomone concentration, the attached clone did not show any morphological changes, but it did exhibit a decreased net reproductive rate and intrinsic rate of population increase. The free-swimming clone developed elongated posterolateral spines and a decreased intrinsic rate of population increase, as well as a decrease in the proportion of sexual offspring. Thus, both clones exhibited reproductive costs.

Spatial distribution and the diet composition of Asplanchna species were studied in 18 water bodies in Matsuyama, Japan. The abundance of Asplanchna and other rotifers, crustaceans, phytoplankton and microbial plankton, together with basic environmental parameters, were determined between October and December 2006, and the distribution and diet composition of Asplanchna species were estimated. Three species of Asplanchna, A. herricki, A. priodonta and A. girodi were found in the present study, but A. herricki was rather less abundant than the other two species. Their diet composition was different among the species, showing that A. herricki consumed only particulate matter while the diet of A. priodonta included mainly phytoplankton, dominated by dinoflagellates. In contrast, A. girodi was rather carnivorous, and included other rotifers in its diet. Their different food habits are not explained by their morphotypes and trophi structures, suggesting this difference might be related to their feeding abilities. For A. girodi, prey selectivity (Chesson's α) for rotifer prey was negative, except for Keratella cochlearis. The amount of rotifers consumed was also low at a mean prey number of less than 3 per A. girodi gut. The result suggests that the predation impact of Asplanchna as a top-down controller of rotifer populations is species-specific and can be apparent only when Asplanchna population reaches high density in these ponds. From the present results, three Asplanchna species were found to belong to basically different feeding groups, A. herricki is detritivore while A. priodonta and A. girodi are omnivores; but A. girodi is more predacious.