MICROGEOGRAPHIC VARIATION AS THERMAL ACCLIMATION IN AN INTERTIDAL MOLLUSC

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Abstract
1. Highest and lowest members of a eurytopic intertidal species, A. limatula, have been compared in the following: heart rate, gonad size, and spawning behavior. 2. Within the weight range of 0.4 to 1.0 gm., the heart rate varies inversely with increasing weight. The regression coefficients fall between – 0.042 and – 0.172; thus no single expression is available to describe fully the effect of weight on heart rate in this species. 3. Sex and size of gonad (as divorced from turgidity) have not been found to contribute to the variation in heart rate between samples. 4. Comparing equal weight animals, it is found that low intertidal individuals have faster heart rates than high intertidal individuals at any temperature from 4° to 29° C. 5. From data on field temperatures it is suggested that the significant parameter of the intertidal difference is temperature. High-level animals are subjected to considerable periods of warmer as well as to some periods of cooler temperatures than are low-level animals. 6. An attempt was made to characterize the difference in heart rate by: transplanting the animals in the field, following the seasonal changes, and maintaining samples of both populations in the laboratory at a cool temperature (14° C.) and without food. 7. When low-level animals are transplanted to high-level their heart rates slow so that within 29 days it is equal to that of the high-level animals when measured at any given temperature. The half-acclimation time was about 6 days. In the field, acclimation to cold was also shown to be complete within 29 days. 8. Comparisons of the heart rate during winter and summer showed that both high and low intertidal animals have faster rates in winter at any given temperature from 9° to 29° C. Acclimation to cold was also shown in the laboratory. 9. The above results lead to the interpretation that the microgeographic difference in heart rate is a phenotypic expression of a compensatory phenomenon operating to maintain approximately equal heart activity in spite of the habitat temperature differences. Latitudinal differences in physiological rate activities which are clearly correlated with habitat temperature are sufficiently similar to the intertidal differences reported here to warrant the suggestion that the same phenomenon is involved. 10. Low-level and winter animals show a heart rate that is less dependent on temperature changes in the range from 9° to 19° C. This same response is not observed consistently above 19° C. While both high-and low-level animals appear to be approaching cold depression below 9° C., the low-level animals are cold depressed at a higher temperature than the relatively warm-adapted high-level animals; the low-level animals possess a higher Q10 in this range. It is suggested that the physiological temperature range of the warm-acclimated group extends both higher and lower than that of the cold-acclimated group. In the field, the Q10 of the heart rate changes within two days after the animals are transplanted. 11. The size of the gonad also varies with intertidal height. Low-level animals maintain a larger gonad during winter and spring than do high-level animals. Transplantation also reveals this difference to be reversible. 12. Analysis of spawning behavior (using turgidity as the criterion of pre-spawning readiness) presents the possibility that either (1) the two groups spawn a number of months out of phase with each other, or (2) high-level individuals do not contribute to the breeding population.