Can crowding stop snails from forming long-term memories?

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Crowding is an environmental stressor. Researchers found that it prevented Lymnaea snails from forming long-term memory (LTM) after operant conditioning of aerial respiratory behaviour.

They compared LTM formation in crowded snails (20 snails per 100 ml of pond water) with snails kept in uncrowded conditions (two snails per 100 ml of pond water). Crowding immediately before or after either of two operant conditioning procedures—the traditional training procedure and the memory augmentation procedure—blocked LTM formation. However, LTM formed if crowding was delayed until more than 1 hour after training or stopped 1 hour before training. If a memory had already formed, crowding did not block its recall.

Pond water from a crowded aquarium, crowding with clean shells from dead snails, or a combination of both was insufficient to block LTM formation. Crowding also did not block intermediate-term memory (ITM) formation. ITM depends on new protein synthesis, whereas LTM depends on both new protein synthesis and altered gene activity. The researchers hypothesized that crowding alters the genomic activity in neurons necessary for LTM formation.