Bythotrephes longimanus (the spiny waterflea) is a carnivorous zooplankter that invaded the Laurentian Great Lakes in the 1980s and is now expanding its range to inland water bodies in the Great Lakes regions of the United States and Canada. Bythotrephes invasions have been shown to decrease the biodiversity of zooplankton assemblages, and to cause changes in fish diets and energy flow in food webs. While the ecological impacts of Bythotrephes on food web structure have been well studied, relatively little is known about how native prey populations have adapted to this invader. This research investigates the heart rate (HR) response of Daphnia pulicaria (prey for Bythotrephes) from invaded and uninvaded lakes in northeastern Minnesota (USA) to chemical cues (kairomones) exuded by Bythotrephes. An increase in HR after exposure to Bythotrephes kairomones represents a fright response that indicates recognition of the predator as a threat. We predicted that Daphnia clones obtained from uninvaded lakes would be less responsive to Bythotrephes kairomones than those from an invaded lake.  

Results/Conclusions

To determine the exposure time that would maximize our ability to detect a HR response, we exposed Daphnia from Island Lake (invaded by Bythotrephes in 1990) to kairomones for various time periods (1, 2, 4, 8, and 16 min). The kairomone elicited a significant elevation in HR that was maximal after a 2 min exposure, but became less pronounced over time. Using the 2 min exposure method, we compared the HR response of D. pulicaria clones from Island Lake, with those from two uninvaded lakes in northeastern Minnesota. Consistent with our expectation, Daphnia from the invaded lake showed a significant increase in HR after exposure to Bythotrephes kairomones, while those from the uninvaded lakes did not. Lastly, we investigated whether Island Lake Daphnia were responding to a cue from Bythotrephes itself, from alarm cues exuded by injured conspecifics, or to a combination of the two. Compared to controls, Daphnia from Island Lake showed significant increases in HR after exposure to alarm cues (from macerated Daphnia), and after exposure to kairomones from Daphnia-fed Bythotrephes and food-deprived Bythotrephes. Future research will examine the HR response of Daphnia from other invaded lakes (e.g., Lake Superior), and of clones ‘resurrected’ from resting eggs in sediments deposited before and after the invasion of Bythotrephes to Island Lake to further explore if this invasive species has induced physiological changes in a native prey populations.