Abstract
Stress alters long-term memory formation sometimes enhancing its formation whilst at other times blocking it. It is unclear what the causal mechanisms are that allow stress to either enhance or suppress memory. We have made use of a relatively simple invertebrate model system to attempt to explore the causal mechanisms of how stress alters memory. Here we explore the consequences of presenting to the organism two different ecologically relevant stressors: detection of a predator and crowding. We find that the suppressive effect on memory formation elicited by crowding is more powerful than is the enhancing effect on predator-detection. That is, when the two stressors are experienced by the snail, long- term memory formation is suppressed.