PREDATOR-AVOIDANCE RESPONSES IN FRESH-WATER DECAPOD-GASTROPOD INTERACTIONS MEDIATED BY CHEMICAL STIMULI

Previous studies on single species of snails reached conflicting conclusions regarding the importance of chemical cues in predator-avoidance behavior. We performed laboratory experiments to determine if chemically mediated, predator-avoidance behavior, such as crawling out of the water by freshwater gastropods, occurred among prey of different species from several geographic sources: Oklahoma, Oregon, and Wisconsin. We compared responses of eight gastropod populations when they were exposed to predation by three species of crayfish. We observed no crawl-out from two prosobranch species. Individuals from four of six populations of pulmonates responded to crayfish predators; physid and lymnaeid populations consistently responded whereas some planorbid populations did not respond. We also examined individual variation in response to predators and the proximate costs and benefits to prey associated with crawling out of the water. Trade-offs between risks of immediate, direct mortality from aquatic, amphibious, or terrestrial predation relative to delayed, indirect mortality associated with desiccation, are influenced by shell size, thickness, and morphology, as well as physiological adaptations among various pulmonate populations. If water depth is sufficient to provide spatial refuge from crayfish, smaller snails can reduce their losses to predation by moving to the surface of the water or by crawling out. In previous studies, Physella virgata and Planorbella trivolvis frequently used predator-avoidance behavior when young, but as shell size and strength increased with age, the larger individuals spent less time crawling out of the water. In this study none of the planorbids (Helisoma anceps or Gyraulus parvus) crawled out, even when young. These differences are apparently due to differences in selection pressures caused by differences in use of various microhabitats where risk of predation varies over time.