RATE OF WATER PROPULSION IN MYTILUS CALIFORNIANUS AS A FUNCTION OF LATITUDE

¹ Department of Zoology, University of California, Los Angeles, California

1. Shell weight relative to the weight of soft parts is a constant in any given population of Mytilus californianus, but it increases with increasing latitude.

2. The absolute rate as well as weight-specific rate of pumping is greater at any temperature in mussels from higher latitudes than in those of the same weight from lower latitudes.

3. The rate of decline, in the absolute as well as weight-specific rate of pumping, with increasing size, is shown to be slower in higher latitudes than in the lower. This perhaps affords a physiological basis for the larger size of the more northern forms.

4. The Q₁₀ of the rate of pumping, between 10 and 20° C., is higher in larger individuals of any given population. But the Q₁₀ of individuals of the same weight at different latitudes, shows a decrease with increasing latitude.

5. It is suggested that the center of dispersal of species like Mytilus californianus is in the lower latitudes. This suggestion stems from the fact that the difference in Q₁₀ values between smaller individuals of lower latitudes and larger individuals of higher latitudes is negligibly small, while the opposite comparison results in a large difference, requiring a great physiological adjustment.

6. A comparison of relative degrees of compensation to temperature at different latitudes is made possible by a proposed temperature compensation coefficient. If the phenomenon under study is a phenotypic change in the lifetime of individuals, this becomes a measure of the degree of acclimatization (Roberts, 1952). This coefficient shows a greater degree of compensation for temperature in more northern samples and in smaller individuals.

7. Such a comparison, and several other instances from earlier work, seem to suggest that the relative compensation to change increases with increasing change.