The effect of photoperiod on female reproductive activity and growth of the freshwater pulmonate snailLymnaea stagnaliskept under laboratory breeding conditions

Abstract

At a Long Day photoperiod (LD = 16 h light, 8 h dark) egg laying of snails kept under laboratory breeding conditions started 2 weeks earlier than in animals kept at Medium Day (MD = 12 h light, 12 h dark) or Short Day (SD = 8 h light, 16 h dark) photoperiods. At the end of the experimental period of 24 weeks the total number of egg masses produced by the LD animals had been 6 and 10 times higher than that of the MD and the SD animals, respectively, while the mean size (number of eggs) of the egg masses of the LD snails was only 30 and 10% larger. The high ovipository activity of the LD animals was reflected in the proportional wet weights of the female accessory sex organs, which tended to be larger. The energy metabolism of the LD animals had been clearly affected as was apparent from the metabolic drain to the female reproductive activity and a change in calcium metabolism, which reflects changes in energy metabolism. Growth of the LD snails, measured as the increase in body wet weight and in shell height, was slower than that of the MD and SD animals. As the differences between MD and SD animals were relatively small, whereas these were very different from LD animals, the threshold for the stimulatory action of the photoperiod on female reproductive activity will be between 12 and 16 h light per day. Most probably the effects of the photoperiod are mediated by the female gonadotropic centres: the lateral lobes, the dorsal bodies and the caudo-dorsal cells. The role of the photoperiodic response of the pond snail in its reproductive strategy is discussed.