Aquatic snails kept in crowded conditions become so stressed that they are unable to form long-term memories.
Scientists kept Lymnaea, a genus of snails commonly kept in ponds and aquariums, in small tanks under crowded and uncrowded conditions and then trained the snails to perform a task.
Crowded snails, which were stocked at a rate of 20 per 100ml of water, were so stressed by crowding that they long-term memory (LTM) formation was blocked.
The uncrowded snails, which were kept at the lower density of two per 100ml of water, remembered what they'd learnt during training.
De Caigny and Lukowiak of the University of Calgari, who reported their findings in the Journal of Experimental Biology said: "Crowding either immediately before or after two different operant conditioning procedures - the traditional training procedure and the memory augmentation procedure - blocked LTM formation.
"However, if crowding is delayed by more than 1h following training or if crowding stops 1h before training, LTM results. If memory is already formed, crowding does not block memory recall."
The scientists also used water from a crowded aquarium, as well as clean shells from dead snails, to place in the aquariums to see whether these too had an effect on long-term memory formation; they didn't.
For more information see the paper: De Caigny P, Lukowiak K (2008) - Crowding, an environmental stressor, blocks long-term memory formation in Lymnaea. Journal of Experimental Biology 2008 Aug;211(Pt 16):2678-88.