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Reproduction is influenced by many external factors. For egg laying of pond snails, one important trigger is the transfer from dirty, oxygen-poor water to clean, oxygen-rich water. This response is due to the combined effects of elevated oxygen level, chemical water composition and clean substrate. Whether this clean-water stimulus (CWS) resembles the natural egg-laying process has remained untested. Given that the response relies heavily on a pretreatment that suppresses egg laying, the animal's internal state is clearly important. Egg laying is known to be influenced by day length, hence external factors signifying time of day or season may be involved. We here study the effect of light on the CWS in the freshwater pulmonate Lymnaea stagnalis. Clean water was more effective in inducing oviposition in the light than during darkness, irrespective of the presence of eyes. Thus, light has a profound influence on egg laying, which is most likely mediated by nonocular photoreceptors. We show that more eggs are laid during the day than during the night in wild-caught animals kept outside, which indicates that the effect of light on CWS-induced egg laying is relevant for the induction of egg laying under natural conditions. [ABSTRACT FROM AUTHOR]