Growth Rate and Variance in the Razor Clam

Abstract

In discussing the growth of the razor clam (Siliqua patula, Dixon) we have pointed out a correlation between the absolute growth rate and the variability as measured by “D”, the interdecile range. 1 In this work we used samples of clams representing the normal population of the respective beds. For this reason the numbers in the older age groups were small, and it was not possible to determine the presence or extent of selective mortality which might greatly affect the relation between growth and variance.

Recently we have selected 2 samples of clams, one from Little River, Humboldt County, California, containing 300 specimens, all 7 years of age, and one from Hallo Bay on the Alaskan Peninsula, containing 76 specimens 13 years old. A complete growth record was obtained for each individual by measuring the consecutive annual rings. The mean length and the variance were calculated for each year for the 2 localities, and the tenth, the fiftieth (median), and the ninetieth percentiles were determined for the distribution of sizes at each age. We have, therefore, by this means obtained data which is obviously unaffected by selective mortality.

The absolute growth rate (annual increments) shows a high positive correlation with the variance. During the period preceding the inflection of the growth curve both the growth rate and the variance is increasing, and the point of maximum absolute growth occurs at the same time as that of the maximum variance. As the growth rate declines the variability also declines. As the animals approach final adult size they tend to reach a uniform length.

During the time the growth rate is increasing the successive annual increments are apparently mutually independent. The variability increases with time, in such a manner that we might consider it to be due to chance.