Studies on temperature acclimation in the freshwater pulmonate mollusc Lymnaea stagnalis (L.)

Abstract

This work is concerned with processes of thermal acclimation in the freshwater pulmonate, Lymnaea stagnalis. Three physiological functions were studied: heart rate, rate of oxygen consumption and assimilation efficiency, Seasonal changes in rate-temperature curves of the first two processes were investigated and compared with alterations induced by exposure to constant temperatures in the laboratory. Simple comparisons were made to determine whether season affected assimilation efficiency. The aims of the investigation were to show whether the measured physiological functions exhibited acclimatory responses to determine the precise nature and interrelationship of any such adaptations, and to suggest possible mechanisms responsible for the changes. It was found that changes in the heart rate-temperature relation were induced both by season and by laboratory acclimation. Results of the seasonal study showed apparent capacity adaptations, so that winter animals had a higher heart rate than summer animals at temperatures between 15 and 25 degrees C, and also resistance adaptations, which gave summer animals increased resistance to heat and winter animals greater tolerance of cold. Laboratory acclimation induced resistance adaptations at both temperature extremes but capacity adaptation was absent. Observed bimodality in heart rate-temperature curves of both studies indicated that control of heart rate is complex. These results are discussed further with reference to changes in physiological mechanisms. Seasonal changes occurred in the size-rate regression for oxygen consumption and in the general shape of the rate temporature curves. There was evidence for a 'reverse acclimation' in response to seasonal changes in temperature. These seasonal responses were not produced, however, by exposure to constant temperature in the laboratory. It is proposed that the observed changes resulted essentially from reproductive activity and seasonal changes in dietary conditions. Hormonal influences are thought to be most important in mediating these changes. No significant differences were found in the assimilation efficiencies of winter and summer snails. Results of this and other studies suggest that the assimilation function does not show acclimatory changes in response to either temperature or season. The results are discussed in relation to the known biology of Lymnaea stagnalis and with reference to fundamental temperature acclimation.