Simple SummaryWhether reproductive behaviour is affected by physical activity (exercise) remains largely unknown. We addressed this question using a simple model organism, the pond snailLymnaea stagnalis. In this animal, one can investigate events at the molecular and cellular levels. We found that intense crawling in shallow water (exercise) for two hours resulted in an increased number of egg clutches and the total number of eggs laid in the following 24 h. This effect was stronger from January to May, in contrast to September–December. Synthesis of RNA transcripts of the egg-laying hormone was higher in the central nervous system of snails that exercised. Additionally, the CDC neurons, which produce the egg-laying hormone and play a key role in egg-lying behaviour, showed a stronger response to electrical stimulation in exercised snails. Our data suggest that exercise enhances reproduction inL. stagnalis, in spite of its obvious energetic costs. The finding agrees with the hypothesis that behavioural effects of exercise were formed early in evolution and have an adaptive significance: to prepare an organism to enter a new environment.AbstractIntense species-specific locomotion changes the behavioural and cognitive states of various vertebrates and invertebrates. However, whether and how reproductive behaviour is affected by previous increased motor activity remains largely unknown. We addressed this question using a model organism, the pond snailLymnaea stagnalis. Intense crawling in shallow water for two hours had previously been shown to affect orienting behaviour in a new environment as well as the state of the serotonergic system inL. stagnalis. We found that the same behaviour resulted in an increased number of egg clutches and the total number of eggs laid in the following 24 h. However, the number of eggs per clutch was not affected. This effect was significantly stronger from January to May, in contrast to the September–December period. Transcripts of the egg-laying prohormone gene and the tryptophan hydroxylase gene, which codes for the rate-limiting enzyme in serotonin synthesis, were significantly higher in the central nervous system of snails that rested in clean water for two hours after intense crawling. Additionally, the neurons of the left (but not the right) caudo-dorsal cluster (CDC), which produce the ovulation hormone and play a key role in oviposition, responded to stimulation with a higher number of spikes, although there were no differences in their resting membrane potentials. We speculate that the left–right asymmetry of the response was due to the asymmetric (right) location of the male reproductive neurons having an antagonistic influence on the female hormonal system in the hermaphrodite mollusc. Serotonin, which is known to enhance oviposition inL. stagnalis, had no direct effect on the membrane potential or electrical activity of CDC neurons. Our data suggest that (i) two-hour crawling in shallow water enhances oviposition inL. stagnalis, (ii) the effect depends on the season, and (iii) the underlying mechanisms may include increased excitability of the CDC neurons and increased expression of the egg-laying prohormone gene.