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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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]