Heart Rate of Mussels as a Function of Latitude, Intertidal Height, and Acclimation Temperature

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Abstract
Heart rates in field and laboratory acclimated populations of 2 species of mussels (Mytilus californianus and M. edulis) were measured during several months of the year. In summer, rates in natural populations acclimated to colder habitats are higher than those in warm water populations measured at the same temperature, so that apparent temperature compensation exists. However, differences or changes in rate do not appear in patterns which would support the thesis that mussels are always compensating for temperature, especially in the fall, winter, and spring or after reciprocal transplantation and experimental acclimation in the laboratory. Furthermore, there are differences in rate when habitat temperatures are the same, and identical rates when habitat temperatures differ. Although temperature compensation in the heart rate of mussels is not ruled out, it is suggested that a more likely explanation for these rate changes and differences is that they reflect adjustments to food conditions. Differences in rate between populations could be eliminated within 6 weeks by reciprocal transplantation of mussels to each other''s habitat. However, one group of mussels in which rates could not be modified significantly was the Alaskan population of M. edulis. whose limits of activity and survival limits were also different from those of a southern California population.