GROWTH RATES IN LATITUDINALLY AND VERTICALLY SEPARATED POPULATIONS OF MYTILUS CALIFORNIANUS

¹ Department of Zoology, University of California, Los Angeles 24, California

1. Rates of growth have been studied in populations of Mytilus californianus collected from southern California and southeastern Alaska. As nearly as possible natural rates of growth have been determined from populations in the field.

2. Length, width and height of the shell are used as criteria for determining rates of growth in this species. Comparisons of rates of growth between northern and southern populations are made by the use of relative rate (k) values.

3. Mussels were measured at the 1.0 ft. and 3.0 ft. tidal levels in southern California and the 5.5 ft. level in Alaska.

4. Rates of growth of Mytilus are found to he greater in the sample from southern California, comparing the 3.0 ft. level, than in that from Alaska. However, it was found that the southern California population is submerged for a greater period of time per week. The greater period of submergence and the greater weight of soft parts for a given shell size, recently shown by Rao (1953) for the southern population, mean a greater food intake in spite of acclimation of pumping rate. Attempts to correct for these factors suggest that the actual discrepancy in intrinsic growth rate is small or absent. In particular, Rao's findings give reason to believe that the heavier bodied southern mussels lay down no more shell length per unit of body weight than comparable northern ones.

5. Comparison of relative rates of growth in vertically separated populations of this species shows a large positive effect of submergence upon rate of increase of shell dimensions.