Exploring the role of stress sensitivity in memory formation: why do some animals learn while others do not? Lessons from <i>Lymnaea stagnalis</i>

The ability to learn and form memory is critical for survival, yet even genetically similar individuals can vary considerably in their cognitive performance. Using the pond snail Lymnaea stagnalis, we investigated how individual sensitivity to stress influences configural learning-a higher-order form of associative learning in which the simultaneous exposure to two contrasting stimuli, such as a predatory odour and an appetitive taste, results in the appetitive stimulus becoming associated with risk and evoking anti-predator behaviours. We used freshly collected, predator-naive snails from Margo Lake, Canada. While group-level data suggested the Margo strain failed to learn, individual-level analysis revealed that some snails successfully formed configural memories, while others did not. We hypothesized that this divergence reflects differences in individual (predator-related) stress responsiveness, which may modulate the engagement of memory-related molecular pathways. To test this, we measured expression levels of selected genes in the central ring ganglia. Snails that formed configural memories showed significantly higher expression of stress-responsive genes, components of the serotonin pathway and markers of neuroplasticity, along with increased endocannabinoid turnover. These findings suggest that individual variation in stress reactivity can drive adaptive differences in cognitive performance, offering new insights into the molecular and behavioural mechanisms underlying learning and memory.