The influence of fish kairomones on the induction and vertical distribution of sexual individuals of the Daphnia galeata species complex

To assess the potential production of hybrids and backcrosses in a semi-nat ural environment, we studied the combined effect of fish kairomone, and foo d level on the production of males and ephippial females in different clone s of five Daphnia taxa from the D. galeata species complex. We also studied the diel vertical migration (DVM) of these sexual daphnids under the same varying conditions. This was done to test the hypothesis that males and eph ippial females have different migrating strategies, which would increase th eir mating probability. The study was carried out in two large-scale indoor mesocosms, the so-called 'plankton towers' in the Max-Planck Institute in Plon, Germany. Although all of the Daphnia taxa produced ephippial females in the course o f the experiment, only D. galeata produced a significant number of males. F ish kairomones had a significant negative influence on the production of ep hippial females. We found no DVM in the D. galeata males. They stayed at a depth between 5 and 6 m both day and night, 1 or 2 m above the thermocline. The ephippial females of D. cucullata x hyalina migrated, whereas ephippia l females of the other taxa showed no DVM but came significantly closer to the surface in the presence of fish kairomones. We conclude that males and sexual females co-occur in this species complex both in time and space. The refore, a regular production of hybrids and backcrosses in this species com plex seems likely. Fish kairomones do not seem to significantly influence t his process.