Bythotrephes longimanus is a non-native invasive zooplanktivore found in many north-temperate lakes of North America. A preferred prey of B. longimanus is the herbivorous zooplankter, Daphnia, which plays an important role both as a grazer of algae/bacteria and as a vital food source for recreationally and commercially important fish species. Thus, predation of B. longimanus on native Daphnia species has the potential to disrupt the functioning of many aquatic ecosystems. It has been documented that the vertical distribution of zooplankton in invaded lakes is changing. A resurrection ecology approach was used to recover and hatch Daphnia resting eggs from lake sediments that were deposited before and after the B. longimanus invasion. Established Daphnia genotypes (i.e., clones) were used to examine whether anti-predator responses of pre- and post-invasion daphniids differed in response to B. longimanus chemical cue. To address this question, 15-minute phototactic assays were performed to elucidate predator cue-induced depth selection behavior in pre- and post-invasion clones of three Daphnia species. The results of this study will help address why the vertical distribution of Daphnia in invaded lakes is changing (i.e., are they changing migration patterns or are they simply being eliminated from certain depth strata).
Results/Conclusions
We found that Daphnia vertical distribution was significantly affected by clonal identity and the interaction between predator chemical cue and clonal identity (two-way ANOVA; p= 0.008, p= 0.017), indicating that the response to the predator chemical cue was clone-specific. The relationship between clone and average depth depends on the treatment condition (the presence or absence of predator chemical cue). Naïve Daphnia clones displayed a wide variety of phototactic tendencies, but none of the clones qualified as either strongly positively or negatively phototactic. Results of this experiment indicate that Daphnia show genetic variation in response to B. longimanus chemical cue and that native Daphnia species are responding to chemical cues from an invasive predator (B. longimanus) despite the lack of a shared evolutionary history. We will discuss the implications of these findings in relation to the ecology and evolution of predator-prey interactions in these invaded lake systems.