Early warning lowers risk of stonefly predation for a vulnerable mayfly

Abstract

Behavior of Baetis bicaudatus (Ephemeroptera; Baetidae) was video taped in a western Colorado, USA stream to identify the specific stimuli eliciting noncontact responses to predatory stoneflies (Kogotus modestus; Plecoptera; Perlodidae). In situ behavioral and feeding trial experiments were also conducted to assess the effect of removal of Baetis cerci on these responses to predators, on measured reactive distances, and on the vulnerability of Baetis to stonefly predation. Predatory stimuli tested included live stoneflies (tethered and untethered), stonefly models (plasticized), excised fresh stonefly antennae, and wires. Intact Baetis primarily turned their cerci (tail curl behavior) with similar frequencies toward live or model stoneflies that were between 1 and 2 cm downstream, and primarily moved when stoneflies were within 1 cm even before contact. Removal of Baetis' cerci reduced the frequency of tail curl responses and the reactive distance, but increased the frequency of movement responses to noncontact with Kogotus. Neither intact nor cerciless Baetis showed much response to Kogotus antennae and wires. These results suggest that stimuli associated with the whole stonefly, such as visual or more probably hydrodynamic cues, cause Baetis tail curl behavior. However, movement behavior was uniformly high after contact with all predator treatments, and the pattern of movement (swim, drift or crawl) varied with predator stimulus, suggesting that Baetis can discriminate live or model Kogotus, Kogotus antennae, and wires using chemotactile or tactile cues. Removal of Baetis cerci resulted in increased encounter rates (contact) with live, untethered Kogotus, higher capture success, and greater predation rates than on intact Baetis. When touched by all predator treatments, Baetis usually swam, a behavior that stimulates stonefly attacks, but after noncontact tail curl responses, Baetis most often remained stationary. Thus, cerci were apparently effective as an early warning system, enabling Baetis to detect Kogotus before contact, thereby reducing the incidence of dangerous swimming escape behavior, and the susceptibility to predation.