As the first step to study relationships between development and learning in the molluscan central nervous system, we examined developmental changes in acquisition and retention of a conditioned taste aversion (CTA) in the pond snail, Lymnaea stagnalis. We found that snails developed ability of a CTA as a long-term memory through three critical stages. Embryos in veliconcha started to respond to appetitive sucrose at the first critical stage. This response was in good agreement with morphological observations that embryos at this developmental stage seemed to be physically ready to eat. However, they could not associate this appetitive stimulus (conditioned stimulus: CS) with an aversive stimulus of KCI (unconditioned stimulus: UCS). At the second critical stage, embryos just before hatching acquired the CTA, but the conditioned response did not persist. Through this stage, they may acquire learning ability to safely seek out food in an external environment. At the third critical stage, immature snails with a 10 mm shell could use a long-term memory to maintain the conditioned response. This memory persisted for at least a month, showing that now they are able to maintain a long-term memory so that they can safely eat a variety of food when they cover wide territory to search for a mate. The present findings indicate that the development of learning ability in snails, which secures acquisition of better survival ability, is coincident with the major changes in their life cycle.
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