Chemical cues from predators induce morphological defenses such as helmets, neck teeth, and elongated tail spines in many Daphnia species. These induced morphologies are defensive agents against predators, halting or reducing predation. The goal of this study was to compare the morphology of native Daphnia species when exposed to cue/no cue from the invasive predator Bythotrephes longimanus. A resurrection ecology approach was used to recover and hatch Daphnia resting eggs from lake sediments that were deposited before (pre-invasion) and after (post-invasion) B. longimanus invasion. Established Daphnia genotypes (i.e., clones) were used to examine whether a plastic (in response to B. longimanus cue) and/or evolutionary morphology of pre- and post-invasion daphniids differs. To address this question, clones of three Daphnia species (D. pulicaria, D. mendotae, D. ambigua) representing pre-/post-invasion time periods, were raised for four weeks in B. longimanus cue and no cue media, then were preserved and key morphological traits (i.e., eye diameter, head length, body length, and tail-spine length) were measured. This was done to elucidate predator cue-induced morphological change in pre- and post-invasion clones of the three Daphnia species. The results of this study will help address how native Daphnia are responding to the invasion of B. longimanus.
Results/Conclusions:
The morphology of all three Daphnia species was significantly different between cue and no cue treatments (MANCOVA; p<0.001 for D. pulicaria, p=0.017 for D. mendotae, and p<0.001 for D. ambigua), indicating an induced morphological response to B. longimanus chemical cue. The morphology of pre- and post- invasion Daphnia was not the same, thus indicating evolutionary as well as plastic responses in morphology (MANCOVA; p=0.037 for D. pulicaria, p<0.001 for D. ambigua, only post-invasion D. mendotae were tested). In addition, there was a significant interaction effect of treatment (cue/no cue) and history of coexistence (pre-/post-invasion), demonstrating that change in morphology in response to the presence of cue was dependent on whether the individual was from pre- or post-invasion times (MANCOVA; p=0.044 for D. pulicaria, p<0.001 for D. ambigua, only post-invasion D. mendotae were tested). Results from our study indicate that native Daphnia species are responding to the invasion of B. longimanus with both plastic and evolutionary changes in morphology. We will discuss the implications of these findings in relation to the ecology and evolution of predator-prey interactions in invaded lake systems.