Some aspects of the chemical ecology of the crayfish Orconectes virilis and Orconectes rusticus.

Abstract

When multiple stimuli are encountered by an animal, tradeoffs may be elicited. The animal could fully respond to one stimulus and ignore the second stimulus or exhibit a response that is intermediate between the responses elicited by either stimulus alone. I present here tests of the effects of environmental variation on behavioral tradeoffs in response to chemical stimuli in the virile crayfish, <italic>Orconectes virilis</italic>, and the rusty crayfish, <italic>O. rusticus</italic>. I first tested the effects of recent history with predation risk on behavioral tradeoffs. In response to food and predation risk cues, <italic>O. virilis </italic> was affected by previous risk exposure, whereas <italic>O. rusticus </italic> was not. Reactions to starvation and sensory acclimation are discussed as possible explanations for this difference. In response to a combination of female sex pheromone and predation risk cues, male <italic>O. virilis</italic> exhibited greater antipredator behavior than when the stimuli were encountered without previous exposure to risk. To complement the work on the reproduction-risk conflict, a field test of the seasonal responses by male <italic>O. virilis </italic> to female sex pheromone was conducted, with limited results. In the waters sampled, <italic>O. virilis</italic> is native, and <italic> O. rusticus</italic> is introduced. Invasive crayfish are known to displace natives, and reproductive interference is a factor contributing to displacement. The persistence of natives is thus favored if reproductive isolation is present between the native and introduced species. In response to pairs of sex pheromones, male <italic>O. virilis</italic> sympatric with <italic>O. rusticus</italic> preferred conspecific females over congeneric females. Male <italic>O. virilis </italic> from an allopatric population did not prefer conspecific females over <italic>O. rusticus</italic> females, but they did prefer same-population, conspecific females over conspecific females from the invaded population. The results indicate isolation via pheromones and are consistent with a pattern of character displacement. Finally, I tested the effects of hydrodynamics on behavioral tradeoffs. The responses by <italic>O. virilis</italic> to a combination of food and predation risk cues were not affected by habitat type. Habitat type also did not affect behavioral tradeoffs in <italic>O. virilis</italic> in response to sex pheromone and predation risk cues. These results are discussed in terms of the natural history of <italic>O. virilis</italic>.