Not-so-naïve: A native prey recognizes and responds specifically to a novel predator in a freshwater system

Background/Question/Methods

Native prey may be expected to lack the ability to detect and behaviorally respond to non-native predators due to a lack of shared evolutionary history, particularly in isolated systems like lakes. Nevertheless, seemingly adaptive responses by native prey to invasive predators have been observed. Four mechanisms can explain these observations. Native prey could perceive and respond to non-native predators: (1) through cue similarity between the non-native predator and pre-existing predators in the native system; (2) from cues from conspecifics eaten by the non-native predator; (3) through learning based on recent experience with the non-native predator; and (4) through constitutive cues produced specifically by the non-native predator. In the Great Lakes, the native cladoceran Daphnia mendotae responds in a seemingly adaptive manner to the invasive planktivore Bythotrephes longimanus; migrating to deeper, darker water in the presence of Bythotrephes, which helps it avoid detection and capture by this visual predator. We conducted a series of laboratory experiments designed to determine which of the above four mechanisms was responsible for Daphnia’s adaptive response to Bythotrephes.

Results/Conclusions

We found that cue similarity between Bythotrephes and with pre-existing predators does not explain Daphnia’s response to Bythotrephes. Specifically, whereas Daphnia responded adaptively to water-borne chemical cues from Bythotrephes by migrating downward, it did not respond to cues from taxonomically-similar predatory cladocerans, and responded adaptively to cues from two taxonomically-distant predators: to native fish by migrating downward and native shrimp by migrating upward. Conspecific cues associated with Bythotrephes predation were also not responsible for Daphnia’s response to Bythotrephes, as these cues actually reduced the response of Daphnia to Bythotrephes. Finally, associative learning cannot explain Daphnia’s response to Bythotrephes as Bythotrephes-naïve neonates and F3 generation individuals (i.e., individuals with no previous experience or exposure to Bythotrephes) respond adaptively to Bythotrephes cues. Taken together, our experiments rule out three of the four mechanisms that could explain a native prey’s response to non-native predators. We therefore conclude that Daphnia responded specifically to constitutive cues produced directly from Bythotrephes. Such a finely tuned response may be retained from an ancestral Daphnia stock that coevolved with Bythotrephes in its native range, or may have rapidly evolved in present-day populations due to strong selection by the invasive predator.