ANNUAL GROWTH OF FRESH-WATER MUSSELS 
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provoke a major interruption of growth has been observed by various writers (Hazay, 
1881; Isley, 1914). Weinland (1918) working with the European fresli-water mussel, 
Anodonta cygnea, found that the daily oxygen consumption fell from 5.3 milligrams 
in November to 1.7 milligrams during December, January, and February; rising 
again to 5.3 milligrams in March, 9.6 milligrams in April and May, and 17.0 milli- 
grams in June. This fall in oxygen consumption during the winter months to a level 
only one-tenth of that maintained by the same animal during the month of June, 
implies metabolic changes which might readily provide a physiological basis for the 
winter growth interruption and account for the extent of this interruption. 
Eliminating the few problematic individuals which are always found in any 
large series of animals, this major annual ring was rather easily differentiated from 
the other narrower interruption rings. 
During the analysis of the data, the length was plotted against age on semiloga- 
rithmic paper to ascertain whether the growth in length in fresh-water mussels repre- 
sents a logarithmic function comparable to many other biological corollaries of 
growth. (See Brody, 1927; Brody, Comfort, and Matthews, 1928.) In the main, 
up to the Vl-year class, the length values from mussel shells were reducible to a 
straight line by this treatment. After passing the Yl-year class, the gains in length 
were less than the logarithmic values required to maintain a straight line, indicating 
a more abrupt decline in growth rate than that represented by a simple logarithmic 
progression; that is, there was a distinct slowing of linear growth, suggesting a senile 
stage. (See Minot, 1908.) 
Since the commercial value of the fresh-water shells decreases very rapidly 
beyond the Vl-year class, this period of senile decline is not discussed in detail here. 
As several factors, such as increase in the weight of the shell and changes in the 
metabolism of the older animals, may be correlated with this change in the linear 
growth rate of the older mussels, additional data are being collected on this phase of 
the problem. 
SUMMARY 
By means of the annual ring method, the year classes of over 1,100 fresh-water 
mussel shells have been determined. Studies of length, weight, and thickness were 
made on the shells of these groups. 
Four commercial species, the yellow sand shell, Lampsilis anodontoides; the 
Lake Pepin mucket, Lampsilis siliquoidea pepinensis; the buckhorn, Tritogonia 
verrucosa; and Pope’s purple, Unio popei, were used. Growth curves were developed 
for each of these species. 
Yellow sand shells from three localities — the Mississippi River at Fail-port, 
Iowa; the White River at Newport, Ark.; and the Rio Grande Valley, near Mercedes, 
Tex. — were studied. These localities represented the northern, middle, and southern 
portions of the range of this species. 
The growth curves for length of the yellow sand shell show that the Mississippi 
shells made their greatest gain in length during the second year; that the White 
River shells made about equal gains during the first and second years; and that the 
Rio Grande shells made their greatest gain during the first year. After passing the 
maximum rate gain, the rate of gain in length declines very rapidly to the Vl-year 
class, beyond which the gains in length are small. 
By comparing the weights of the yellow sand shells from the three localities, it 
may be seen that the Texas shells were conspicuously lighter in proportion to their 
