Generational and dual-species exposures to invertebrate predators influence relative head size in Daphnia mendotae

Laboratory cyclomorphosis experiments with crustacean zooplankton often fail to produce morphological trait expression as extreme as that observed in field populations. A seven-generation laboratory experiment found that the maximum expression of relative head area in a round-helmet morph of Daphnia mendotae was delayed until the third generation of non-lethal, chemical exposure to Leptodora kindtii. By the third generation and after, relative head area remained high and comparable in degree to field specimens. The induction pattern from parents to offspring was statistically positive into the fourth generation. Published studies on kairomone induction of morphological trait expression extending only one generation may fail to measure transgenerational effects, which could account for mismatch between laboratory and field results. In a second laboratory experiment with a pointed-helmet morph of D. mendotae exposed to L. kindtii and Chaoborus punctipennis, we show that induction levels are not the same for all predators or combinations of predators. The maximum relative head area for pointed morph individuals exposed to both predator cues was still not as extreme in the laboratory as in the field. This is the first demonstration that delayed generational effects of kairomone induction of helmet expression in the Cladocera can reconcile laboratory experiments with the field observations of cyclomorphosis.