Induction of egg-laying in the pond snailLymnaea stagnalisby environmental stimulation of the release of ovulation hormone from the Caudo—Dorsal Cells

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Abstract
Ovipository activity decreases when specimens of the pond snail, Lymnaea stagnalis, are kept in closed jars that are not cleaned regularly. When the snails are transferred from dirty to clean water, egg-laying occurs within 3 h in over 90% of the animals. The number of eggs per egg mass laid after water change is directly related to the length of the period between the induced and the previous oviposition. The relationship is similar to that found when snails are injected with a very high dose (20-times threshold) of egg-laying hormone. The latency of oviposition, which depends on the dose of ovulation hormone, is not affected by the oviposition interval. The size of the egg capsule depends on the number of ripe eggs in the ovotestis and the packaging capacity of the female accessory sex organs. The role of a number of component factors of the water-change stimulus was studied. Elevated O2-content of the water, a clear jar and clean water all have a positive effect on egg-laying. Clean water is the most effective factor. High O2-content is slightly less effective, but in dirty water a sudden elevation of O2-content still induces oviposition in about 80% of the snails. It is concluded that the neuroendocrine Caudo-Dorsal Cells producing the ovulation hormone of Lymnaea respond to water change with the release of an ample dose of egg-laying hormone. The effectiveness as well as the character of the water-change stimulus suggest that the change in quality of the water may be a natural stimulus for egg-laying.