CONSEQUENCES AND PLASTICITY OF THE SPECIALIZED PREDATORY BEHAVIOR OF STREAM-DWELLING STONEFLY LARVAE

The purpose of this study was to determine experimentally the potentia l mechanism(s) favoring specialized foraging behavior of stream-dwelli ng predatory stonefly larvae (Megarcys signata), and whether this spec ialized behavior was fixed or flexible. We measured stonefly growth ra tes after 10 d of conditioning on Baetis bicaudatus (Ephemeroptera) or one of two alternative mayfly prey species in replicated circular ple xiglass flow through chambers powered by water from a stream in wester n Colorado. We then compared attacks per encounter (as an index of pre dator choice) of conditioned stoneflies to those of unconditioned cont rols given equal densities of all three prey types. We conducted addit ional behavioral observations to determine whether predators different ially encountered the alternative prey types on natural substrates, an d to determine whether predator-prey microhabitat overlap was a mechan ism explaining differential prey encounter rates among prey species. M egarcys exhibited a stereotyped attack bias toward swimming mayflies, such as Baetis, following the simple rule, ''attack any prey that swim ,'' which results in Megarcys' selection of prey types that swim in re sponse to stonefly encounters. This behavior remained unmodified after extended experience with alternate but palatable nonswimming mayfly s pecies. A potential fitness advantage (measured as relative growth rat e) of this behavioral specialization occurred for female Megarcys, but not for males. Female Megarcys grew faster on Baetis diets because th ey ate more Baetis, not because Baetis was a more intrinsically profit able prey item (in terms of promoting predator growth). Further, high feeding rates on Baetis were a result of disproportionately high encou nter rates and attacks per encounter with Baetis compared to the other mayfly species. Our data are consistent with the hypothesis that this predatory behavior arose and is maintained in female stoneflies due t o a fitness advantage to individuals specializing on a relatively abun dant, easily recognized prey resource. We speculate that this speciali zed behavior has not been lost in male stoneflies, for which we did no t measure a fitness advantage, because there is no strong selection pr essure against it. Effective pre-and post-contact prey defenses may ex ert enough selection pressure to prevent stoneflies from evolving spec ialization on alternative mayfly types.