Predator mediated coexistence of hybrid and parental Daphnia taxa

We used Daphnia as a model to study the effect of predators on clonal and taxon coexistence within the same habitat. Different Daphnia species and their hybrids coexist in many lakes worldwide. We studied the potential influence of planktivorous fish on the maintenance of these species assemblages using 20 Daphnia clones, belonging to five hybrid and parental taxa of the Daphnia galeata-hyalina-cucullata complex originating from the Plussee (Northern Germany). With these clones, two competition experiments were conducted in two large-scale indoor mesocosms, the Plankton Towers in Plon, Germany. We varied the presence of fish and kairomones. Using allozyme electrophoresis we observed Diel Vertical Migration (DVM) of indi- vidual clones. Without fish, two clones belonging to the largest taxa (D. galeata and D. galeata × hyalina) became numerically dominant after three weeks. The mere presence of fish kairomones led to a significant decrease in the niche overlap between the five taxa. With fish present in the towers relative densities of smaller D. cucullata × ga- leata clones were highest. We observed that under different predation intensities differ- ent taxa showed the highest instantaneous rate of increase. Without fish and fish kairo- mones this rate was the highest for D. galeata, when fish kairomones were present D. galeata × hyalina had the highest relative r. With increasing fish predation D. cucul- lata and D. cucullata × galeata had the highest relative r, respectively, showing that changing predation pressure facilitates the co-occurrence of taxa in this species com- plex.