FRESH-WATER MUSSELS. 125 



ensuing changes cause new sand bars to form and to bury previously existing beds. 

 Wing dams constructed for improvement of the Mississippi River, built of rock and 

 brush and projecting from the shore to the channel, have far-reaching effects upon the 

 course of the current, upon sedimentation, and upon the formation of sand bars. The 

 area between the dams may fill up with sand, so that eventually willows are growing 

 where a mussel bed once flourished. Such changes have been obser^^ed in the Mississippi 

 River near Fairport, Iowa, and at Homer, Minn. 



The effect of the construction of dams directly across the channel of a river, as for 

 water-power development, has been discussed on page 97. 



Greater irregularity of stream flow resulting from the clearing of forests greatly 

 influences the life of mussels. The drying up of ponds inhabited by mussels and the 

 extreme low stages of water which allow clammers to obtain the mussels by wading, form 

 disastrous conditions to which mussel beds are occasionally exposed. Extreme low 

 stages of lakes and streams in summer may lead to mortality of mussels resulting from 

 high temperature of the water and diminished oxygen supply. (See Strode, 1891; 

 Sterki, 1892; Farrar, 1892.) 



GROWTH AND FORMATION OF SHELL. 



MEASUREMENTS OF GROWTH. 



Methods of propagation, estimate of results, and measures for protection all depend 

 in a considerable degree upon knowledge of the rate of growth of mussels. It is impor- 

 tant to know how many years elapse before a mussel may attain a market size, as well 

 as at what age it may be expected to begin breeding. Furthermore, these questions 

 require answers for more than 40 economic species, even if consideration were not g^ven 

 to the more than 50a additional American species of fresh-water mussel. The rate of 

 growth is not, however, easily ascertainable for most species. 



Mussels of any species may be left under observation for a considerable period in 

 tanks or troughs, but experiments indicate that normal growth does not occur under 

 the conditions of. life in tanks. Even large ponds do not offer the conditions required 

 by many species. The data to be offered on this subject are derived principally from 

 experiments conducted at the Fairport station. Further data on growth of mussels 

 will be found in Isley's paper (191 4). 



Pocketbooks, Lampsilis ventricosa, reared in one of the ponds at the Fairport 

 station attained a length of 41 to 47 mm. (1.6 to 1.85 inches) in two growing seasons, 

 and about 65 mm. (2.56 inches) by August of the third season. Examples 45 to 47 

 mm. long (1.76 to 1.85 inches), and these evidently in the second year of free life, were 

 measured and planted in the Mississippi River by Lefevre and Curtis in June, 1908, 

 and recovered by the senior author of this paper in November, 1910, at the close of the 

 fourth year of growth (Lefevre and Curtis, 1912, p. 180 ff). They had attained lengths 

 of 81 to 85 mm. (3.18 to 3.35 inches). (See fig. 6, p. 133.) 



It is evident, then, that pocketbook mussels under only ordinarily favorable con- 

 ditions may attain a marketable size by the end of the fourth season of independent 

 life (at 3K years of age from date of infection). The observations reported in the 

 following table (10) show that a nearly equal rate of growth applies to the Lake Pepin 

 mucket, Lampsilis luieola. 

 75412°— 22 9 



