ANNUAL GROWTH OF FRESH-WATER MUSSELS 
723 
Grande Valley, growth in length is very much the greatest for any year class. Fol- 
lowing as second in growth rate are the shells from the mid range of the species — the 
White River, Ark. — while those from the Mississippi in Iowa, made the slowest gains. 
This is true not only of the length gains but also to a large extent of the gains in 
weight and thickness. 
A study of the extent of growth in length for the different years brings out some 
interesting characteristics of the different localities. In both the Mississippi and 
White River shells, the amount of increase in length the second year is as great, or 
even greater, than that shown in the first year. A decline begins the third year, 
which is apparently associated with sexual maturity. In the case of the Rio Grande 
shells on the other hand, not only is there very much greater growth the first year 
than in other years, but the decline in rate begins the second year. It is suggested 
that these Texas mussels mature a year earlier than the northern forms, but no 
definite data are on hand regarding this point. It is common knowledge that the 
growth rate of many organisms declines on reaching sexual maturity. Since the 
Texas mussels may possibly reach maturity a year earlier than the northern forms, 
the slump in growth rate begins that much sooner; but apparently the advantages 
gained by the greater juvenile growth, in part at least, are maintained throughout 
life. Figure 9 shows the relative annual growth for the species in the three locali- 
ties; not only for each year, but also for periods of the year during which growth 
takes place in each locality. The point at which decline in growth rate begins is 
clearly brought out. The growing season increases progressively in length from the 
northern to the southern limits of the range. To this fact must be added the state- 
ment that the first season of growth for fresh-water mussels is also their shortest. 
In Iowa and northward the close of the parasitic period — that is, the beginning of 
free existence when actual growth starts — may come as late as July for the yellow 
sand shell. The cooling of the water with the approach of winter, may stop shell 
growth in September. In other words, an estimated average period of growth for 
this species in this locality during the first year is only 3 months. In Arkansas, the 
corresponding period may be 5 months; and in southern Texas, possibly 7 % months. 
The greater length of the growing season during the first year gives the shells from 
the more southern localities a pronounced gain in linear growth; but the earlier ap- 
proach of maturity, which is also apparently correlated with the southern habitat, 
may, however, partially discount this advantage. 
The variations in rate of growth in length between the sexes for the species in 
the different localities may be compared. In all three localities, males and females 
remain at about the same corresponding length until after the formation of the third 
ring. At this time, the shells are beginning to take on pronounced secondary sexual 
characteristics, which are brought out by the measurements. It is noticed that at the 
time of the formation of the third ring, the females of all three localities are slightly 
longer. This excess in length of the females also holds in the first and second years in 
the case of the shells from the White River and the Rio Grande. In the case of the 
Mississippi shells, the males exceed the females during these first two years. After 
the third year, and apparently associated with the species period of maximum sexual 
activity, pronounced and characteristic differences in growth between the sexes 
appear. With the approach of old age and lessened sex activity, these differences 
in increase in length tend to diminish, and a tendency in both sexes toward an equal 
length for a given age seems to be discernable. 
