Anti-Predator Responses of Mayfly Larvae to Conspecific and Predator Stimuli

Abstract

Hydrodynamic and water-borne chemical stimuli are considered important cues that initiate anti-predator behaviours of mayfly larvae. We tested the hypotheses that chemical stimuli from conspecifics and the predacious fish Rhinichthys cataractae and hydrodynamic stimuli from a Rhinichthys model would initiate anti-predator responses by mayfly larvae of Ephemerella aurivillii, Paraleptophlebia heteronea, and Baetis tricaudatus. A second set of experiments tested the hypotheses that chemical stimuli from conspecifics and the predacious stonefly Claassenia sabulosa and hydrodynamic stimuli from a Claassenia model would initiate responses by Baetis. These hypotheses were tested in laboratory streams where mayfly larvae received either chemical stimuli, hydrodynamic stimuli, or the combination of hydrodynamic and chemical stimuli. Responses by larvae to these stimulus types, in terms of use of substratum surfaces and drift rates, were compared with those by larvae in control streams without conspecific and predator stimulus types. Responses to chemical stimuli were variable depending upon mayfly species and the chemical stimulus type. For instance, while Ephemerella and Paraleptophlebia responded to Rhinichthys odours, Baetis did not respond to either Rhinichthys or Claassenia odours. Paraleptophlebia responded to conspecific odours whereas Ephemerella and Baetis did not. In contrast, all three mayfly species responded to hydrodynamic stimuli either alone or when combined with chemical stimuli. Responses by Baetis were more complex when chemical stimuli were combined with hydrodynamic stimuli. For instance, although Baetis did not respond to conspecific odours alone, drift was significantly higher in streams receiving the three stimuli of conspecific odours, predator odours from Rhinichthys, and hydrodynamic stimuli from a Rhinichthys model when compared with streams that received only the conspecific odours combined with the fish model. Similarly, an enhanced response was observed when Baetis odours were combined with Claassenia odours and the Claassenia model. Thus, the lack of a response to a chemical stimulus did not preclude an enhanced response when combined with other stimulus types. These observations suggest that although some mayflies are capable of detecting the presence of some chemical stimuli, a response is not elicited unless the stimulus is accompanied by additional stimuli.