FRESH-WATER MUSSELS. 1 23 



nite limited directions. During the winter of 1904 a muskrat was observed feeding on mussels along 

 the broad ice crack that extended from the end of Long Point northeastward across the lake. The 

 muskrat was about 50 feet from the shore. It repeatedly dived from the edge of the ice crack and 

 reappeared with a mussel in its mouth. Upon reaching the surface with its catch it sat down on its 

 haimches on the edge of the crack and, holding the mussel in its front feet, pried the valves apart 

 with its teeth and scooped or licked out the contents of the shell. Some of the larger mussels were 

 too strong for it to open, and a part of these were left lying on the ice. The bottom of the lake near 

 Long Point, and also over by Norris's, is well paved by shells that have been killed by muskrats. 

 Muskrats do not seem to relish the gills of gravid mussels; these parts are occasionally found untouched 

 where the animal had been feeding. 



In spite of all these enemies mussels held their own and throve and flourished 

 until the appearance of man upon the scene, when depletion of the mussel beds became 

 noticeable. Man exterminates a good many mussel beds by sewage discharge, by 

 drainage, through which sand is washed down over the beds, by dredging and construc- 

 tion of wing dams for navigation, by pearling, but, most of all, by exhaustive clamming 

 for the shells. 



CONDITIONS UNFAVORABLE FOR MUSSELS. 



Since mussels are animals of generally sedentary habit, with limited powers of loco- 

 motion, they are more helpless to escape from unfavorable conditions of environment 

 thanare fish or other active creatures of the water. This relative helplessness does not 

 characterize the adult mussel alone, but is even exaggerated for the young stages. 

 From the time the larval mussel attaches itself to a fish until it is liberated it is entirely 

 dependent upon the movements of its host for its future home ; it may be dropped in a suit- 

 able environment or in a place wholly unfavorable to its survival. On the other hand, 

 adult mussels of many species can endure unfavorable conditions for a considerable 

 period of time. This is found to be especially true of several species of Quadrula- 



NATURAL CONDITIONS. 



Some natural conditions uiifavorable to mussel life are shifting bottom, turbidity, 

 sedimentation, floods, and droughts. These conditions pertain usually to streams 

 rather than to lakes. They have received some consideration in various paragraphs 

 of this section on "Habitat"; therefore it is only necessary to summarize them in 

 this connection. 



The paucity of mussels in the Missouri River, as well as in the greater part of the 

 Red River and other streams of the plains, is no doubt due to its exceedingly shifting 

 bottom. Similar conditions apply in lesser degree in the lower stretches of many 

 streams. In fact, all rivers, for some distances above their mouths, are 3^ a rule very 

 deficient in mussels as compared with sections farther up where bottom and other con- 

 ditions are more favorable. Shifting bottoms not only prevent mussels from securing a 

 foothold, but may also entirely destroy estabUshed beds. 



Interrelated with shifting bottom are turbidity and sedimentation. All three factors 

 and the extent to which they may be operative are largely dependent upon flood condi- 

 tions. In nearly all large rivers floods commonly plow new channels here and there in 

 the stream bed, cut away banks to a greater or less extent, and build new shoals or 

 change the form and dimensions of old ones. Such changes in navigable streams are 



