Mayfly cerci as defense against stonefly predation: deflection and detection

Abstract

In situ behavioral experiments were conducted in flow-through observation boxes in a Colorado and a New York stream to compare and contrast the cercal responses of Ephemerellidae and Baetidae mayflies to predaceous stoneflies. Ephemerella infrequens (Colorado) exhibited primarily a posture when touched by stoneflies. This posture was stimulated by the touch of any body part of an intact stonefly (antenna, mouthpart, leg, cercus), with some variation in frequency depending on the stonefly, and occurred more often when a mayfly was touched dorsally than on anterior, posterior or lateral body aspects. Scorpion posturing sometimes occurred without physical contact between predators and prey, usually when predators approached Ephemerella from upstream. With increasing size, both E. infrequens (Colorado) and E. invaria (New York) increased the frequency of scorpion posture responses to stoneflies. Very few Ephemerella were consumed by stoneflies during the experiments. Alternatively, the baetid mayfly, Baetis bicaudatus (Colorado) exhibited a posture during which cerci and posterior abdominal segments were flexed laterally in the direction of actively foraging predators. This response always occurred before touch by the stoneflies and usually when predators passed beside the mayflies rather than approached directly from upstream or downstream. Tail curl responses were followed by active swimming, drifting, or crawling by Baetis -behavior that prevented touch encounters between predators or prey. Thus, no Baetis were consumed after a tail curl response. I speculate that while the use of cerci by Ephemerella deflects stoneflies, cerci are used by B. bicaudatus to detect water wave disturbances produced by foraging stoneflies, enabling the prey to avoid predator encounters and thereby reduce the risk of predation.