A Quantitative Study of Environmental Factors Influencing the Seasonal Onset ofReproductive Behaviour in the South American Apple-snail Pomacea canaliculata(Gastropoda: Ampullariidae)

Abstract

Two laboratory experiments were carried out on sets of winter (inactive) pairs of Pomacea canaliculata (Lamarck), one to study the influence of water temperature (either 18C or 25C, corresponding to late winter and summer temperatures in Mendoza, r e s p e c t i v e l y ) and day length (either 10 h light/day or 14 h light/day, corresponding to the shortest and the longest day of the year in Mendoza, respectively), and the other to determine the effects of food availability (ad libitum feeding vs. restriction to 25% of the ad libitum requirements) on the frequency of copulation and spawning, on spawn measurements (number and volume of eggs, and spawn volume), and on an index of the reproductive effort. Animals in all experimental groups were observed during 4 weeks, after seven days of acclimation to the experimental aquaria (under low water temperature and short photoperiod). Low water temperatures induced a lesser and later activation of copulation and spawning than that induced by warm temperatures. Day length was without effect on both behaviours. Both the frequency of spawning and the reproductive effort index decreased under restricted feeding, without altering the spawn measurements. The latency periods for both copulatory and spawning behaviours were similar in both ad libitum and restrictedly fed groups. We conclude that water temperature may be the critical factor that causes the seasonal onset of copulatory and spawning behaviour in these applesnails. However, food availability may also be a determ i n i n g factor, mainly to trigger spawning activity. These results suggest that the factors that control the seasonal onset of the sexual activities in Pomacea canaliculata are similar to those that control the seasonal arrest, thereby establishing the annual pattern of reproduction in this species.