FRESH-WATER MUSSELS. 83 



in a vertical direction, the mussel going down deeper into the bottom; rarely does it go 

 completely beneath the surface of the bottom; more frequently the mussel moves 

 horizontally, leaving a distinct path behind it, which reveals the direction and dis- 

 tance of travel. Locomotion is accomplished by thrusting the muscular foot forward 

 into the bottom, expanding the outer end, and then contracting special muscles so as 

 to draw the shell and body nearer to the end of the foot. Mussels that are most likely 

 to travel in this way are the yellow sand-shell, black sand-shell, and slough sand-shell, 

 species that are relatively long and narrow. Rotund forms, like most of the species of 

 Quadnila, are less likely to migrate, but, of the Quadrulas, perhaps the most vagrant 

 fonn is the very elongate rabbit's-foot, Quadrula cylindrica.'^ The pocketbook, Lamp- 

 silis ventricosa, and the pink lieel-splitter, Lampsilis alaia, are also fairly active. Juvenile 

 mussels are more active than adults. (PI. V, fig. 3.) 



The causes of movements from one location to another are not known and the 

 subject offers. an interesting field of study. Change of pressure (depth), temperature, 

 or more probably light may be the governing factor. Yellow sand-shells move up on 

 the shoals or toward shallow water in times of flood, and return toward deeper water as 

 the stage of water recedes. It is a matter of common report that after high-flood stages 

 these mussels are sometimes found stranded in the swamps at some distance from the 

 ordinary channel of a river, but the authenticity of such reports is not established. 

 Headlee and Simonton (1904, p. 175) observed that fat muckets moved away from shore 

 during periods of high-wave action. 



Isley (1914) tagged and planted large numbers of mussels in comparatively shallow 

 natural waters and after several months recovered a considerable percentage of them, 

 finding very little evidence of migration. The Quadrulas placed in water over 3 feet 

 deep remained approximately where planted ; those placed in water as shallow as i foot 

 moved to deeper water, which was easily reached. The species of Lampsilis used in 

 the experiments showed more activity, but none were discovered which had moved more 

 than a few yards. He concluded from his experiments and field obser\^ations that 

 mussels, especially the Quadrulas (heavy-shelled mussels) and related species, were 

 unable to help themselves if conditions became unfavorable, but that, on the other hand, 

 their power to endure unfavorable conditions was remarkable. 



From observations in Lake Maxinkuckee, Ind., Evermann and Clark (191 8, p. 256) 

 say: 



The mussels in shallow waters near the shore move into greater depths at the approach of cold 

 weather in late autumn or early winter and btnry themselves more deeply in the sand. This movement 

 is rather irregular and was not observed every year. It was strikingly manifest in the late autumn of 

 1913, when at one of the piers off Long Point a large number of fmrows was observed heading straight 

 into deep water, with a mussel at the outer end of each. The return of the mussels to shore dviring 

 spring and summer was not observed. [These were mostly Lampsilis luteola, the fat mucket.] 



It is evident from the available data that the locomotion of fresh-water mussels 

 can play little part in their distribution. Distribution is, in fact, effected principally 

 during the period of parasitism on fish, when it is governed by the migrations of the 

 hosts. When dropping from the fish, the little mussels are naturally subject to the 

 force of the current, and some that fall in unfavorable environments may be carried to 

 a more suitable place, while others falling upon good ground may drift into a less favor- 



« Wilson and Clark (1914, pp. 35 and 59) have noted a particularly vagrant habit for Quadruta cytindrica. 



