Maternal effects on the induction of defensive morphology are regarded as an adaptive strategy. For instance, a female can transmit environmental conditions related to predation risk to offspring of the same generation or succeeding generations. However, studies have presented limited information regarding such an adaptive maternal effect on monogonont rotifers. In the present study, the maternal effects on the spine development of rotifers were evaluated using Brachionus calyciflorus-Asplanchna models. Asplanchna can release soluble kairomones into the environments, thereby inducing B. calyciflorus to form defensive morphs (e.g., elongated posterolateral spines). Our empirical data supported the hypothesis that an amictic Brachionus-experienced Asplanchna kairomones could produce offspring with longer posterolateral spine in the current or succeeding generation than those of a non-experienced amictic female. On the basis of our data, we speculated that the maternal effect of B. calyciflorus may involve the transfer and dilution of inducers (e.g., hormone) within and between generations. On one hand, our results reinforced the within-generation maternal effects on induction of defensive morphology of rotifers; on the other hand, our results enhanced transgenerational inducible defense to other planktonic taxa that have been only observed in cladocera.

Effects of temperature (18, 24, and 308C), salinity (5-40 ppt, five intervals) and algal foods (Synechococcus sp., Chlorella pyrenoidosa, Isochrysis zhanjiangensis, Dunaliella salina and Tetraselmis cordiformis) on the life table demography of six geographical Brachionus plicatilis sensu stricto clones, which had been identified according to the prevalent taxonomy and biometric analysis of B. plicatilis sensu lato, were studied. The results showed that temperature, salinity and temperature · salinity significantly influenced the life history parameters. Genotype (clone) had no effect on the population growth rate but did influence the net reproductive rate, generation time and lifespan. All rotifer clones showed the expected increase in population growth rate with increasing temperature. B. plicatilis s. s. attained a higher population growth rate at lowmedium salinities (5-20 ppt) than at high salinities (25-40 ppt). The equivalent spherical diameter (ESD) of food algae, salinity and ESD · salinity had significant effects on the life history parameters. In this case, genotype had no effect on population growth rate, net reproductive rate and generation time but did influence lifespan. The population growth rate of B. plicatilis s. s. evaluated against particle retention spectrum of algae at two salinities resulted in bell-shaped curves. Dunaliella salina with an ESD = 7.7 mm was considered to be the best food for B. plicatilis s. s. while Synechococcus appeared to be an inadequate food algae.

Summary Inducible defences reduce the probability of prey being killed by their predators. However, these defences are often accompanied by costs. Most studies associated with aquatic communities have focused on the effects of inducible defences on the growth and reproduction of prey species, while their effects on sex investment remain unclear. Brachionus calyciflorus is a common freshwater rotifer that develops a defensive morphology in response to the kairomones released by the predatory rotifer Asplanchna. Some studies have reported that no detectable cost, such as decreased reproductive output, is associated with the spines in B. calyciflorus. Thus, we studied the sex investment (i.e. production of mictic offspring and resting eggs) of B. calyciflorus populations at three Asplanchna kairomone concentrations (0, 10, and 100 Asplanchna L−1) and two food levels (1 and 12 mg C L−1) to evaluate the sex cost of inducible defences. We also evaluated the crowding threshold required to trigger sex investment (i.e. lowest population density at which mictic female is produced) in B. calyciflorus under different kairomone and food treatments. Results showed that (i) the B. calyciflorus population exposed to kairomones produced offspring with elongated posterolateral spines; (ii) the production of mictic offspring and resting eggs decreased in the B. calyciflorus population exposed to kairomones, suggesting an investment trade‐off exists between inducible defence and sex investment; (iii) the decrease in sex investment was higher at the high kairomone concentration than at the low kairomone concentration; (iv) the decrease of sex investment was higher at the low food level than at the high food level; and (v) the crowding threshold of the B. calyciflorus population increased upon exposure to kairomones. The results of this study suggest that the decreased sex investment in defended B. calyciflorus is a form of defence cost. Our findings add to our understanding of the effect of predation on the ecology and evolution of phenotypic plasticity in rotifers.

Freshwater green algae, Chlorella, have heavy cell walls and their size usually exceeds the lower limits of limb size of herbivorous Daphnia (Cladocera). According to the optimal foraging theory, we speculated that Daphnia would graze more exposed and relatively large Clamydomonas rather than Chlorella, and this process would lead to small-sized Chlorella becoming a superior competitor in the presence of Daphnia. We used Daphnia magna, Clamydomonas sajao and Chlorella pyrenoidosa to test this hypothesis. Our grazing experiments showed that Daphnia preferred C. sajao to C. pyrenoidosa, regardless of the concentration and relative abundance of these two algae. The decrease in relative abundance of high-quality Clamydomonas in Clamydomonas-Chlorella assemblages did not diminish the grazing efficiency of Daphnia on this algal species, but increased selectivity of low-quality Chlorella. However, when the concentration of Clamydomonas was extremely high, the grazing of Daphnia on Clamydomonas decreased. In competition experiments, we observed that the presence of Clamydomonas restrained the growth potential of Chlorella; however, the introduction of herbivorous Daphnia into the competing environment weakened this influence and to some extent enhanced the growth ability of Chlorella. Moreover, we also observed that the intensity of herbivory, imposed by different densities of Daphnia, had an obvious influence on the competition outcome between Clamydomonas and Chlorella. At the highest intensity of herbivory (10 Daphnia), C. sajao was eliminated from the culture medium whereas C. pyrenoidosa could persist, but at low cell density.

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