Taxon‐specific reaction norms to predator cues in a hybridDaphniacomplex

Abstract

Summary1. Previous studies have shown that interspecific hybridisation is common among taxa from theDaphnia galeata/hyalina/cucullataspecies complex. We investigated the influence of predator kairomones on the morphology and life histories of nine clones belonging to three taxa (pureD. galeata, F1hybrids betweenD. galeataandD. hyalina, and backcrossedD. hyalina) of this species complex. Predators exerting positive (fish) and negative (Chaoboruslarvae) size‐selective predation were tested.2. The most responsive traits were size at maturity and size of neonates. Despite large between‐clone variation, discriminant analysis revealed that the three taxa were distinct from each other in key life‐history traits. F1hybrids did not react in an intermediate way compared to the other taxa: the multivariate distances between F1hybrids and either taxon were larger than between pureD. galeataand backcrossedD. hyalina.3. The average plasticity (calculated across all traits) was similar for all three taxa. With regard to the size at maturity and neonate body size, the strength of the response was a function of the intrinsic values of these traits expressed in the control. For example, for size at maturity, smaller individuals showed a significantly stronger reaction toChaoboruskairomones than larger ones.4. Finally, we monitored seasonal changes in body size, egg number and population density of pureD. galeataand F1hybrids in Greifensee (Switzerland). The two taxa experienced similar seasonal changes in body size but, on some sampling dates, they differed in mean egg number. The observed seasonal changes inDaphniabody size were consistent with what would be expected if the predator assemblage shifted from fish toChaoborusover the course of the summer. The fluctuations in the frequencies ofDaphniataxa, however, were not related to seasonal variation inDaphniabody size.5. Experimental data suggest that temporally heterogeneous predation regimes might be an important condition stabilising the co‐occurrence ofDaphniahybrids with parental taxa. Predation regimes, however, cannot solely explain dynamic changes in taxon frequency in Greifensee.