On the different role of alarm substances and fish kairomones in diapause induction in a freshwater planktonic crustacean

Many aquatic organisms anticipate predation risk via infochemical detection. In a laboratory experiment, we investigated the expression of life-history responses in planktonic Daphnia magna under long-lasting exposure to various concentrations of fish kairomones (FK) and alarm substances of Daphnia (AS). Daphnia were exposed to one of six concentrations of AS (0, 0.0005, 0.005, 0.05, 0.5, 5 homogenized D. magna/L) mixed with the highest concentration of FK, or to one of six concentrations of FK (diluted fish feces of 0, 0.001, 0.002, 0.01, 0.02, 0.1 fish/L) accompanied by the highest concentration of AS. FK alone at the highest concentration were sufficient to induce diapause in 21% of Daphnia, while AS alone at the highest concentration did not trigger diapause. Mixed at the highest concentrations, both cues induced diapause in 94% of Daphnia, whereas in the control treatment free of infochemicals no ephippial individuals occurred. No significant size or fecundity changes accompanied the diapause response. The graded type of diapause response observed across a wide concentration AS concentrations suggests that Daphnia use AS concentration as a proxy for the level of non-specific predation risk. In contrast, the abrupt change of diapause response across a narrow concentration of FK suggests that they were more critical to identify predator origin than level of risk.