NATURAL HISTORY AND PROPAGATION OF FRESH-WATER 



MUSSELS. 



By 



R. E. COKER, Assistant in Charge Scientific Inquiry, 



A. F. SHIRA, Lately Director Fisheries Biological Station, Fairport, Iowa, 



H. WALTON CLARK, Scientific Assistant, and 



A. D. HOWARD, Scientific Assistant. 



INTRODUCTION. 



Adult fresh-water mussels are free-living but sedentary in habit. Though attached 

 to nothing they remain for indefinite periods nearly as still as if their position were irre- 

 vocably fixed. They have powers of locomotion but only occasionally use them. A 

 snail is expected to be in travel, however slow, in the search for food; but when a mussel 

 is found in motion the obser\'er is inclined to look for a special cause of this behavior. 



If a living animal remains generally in one place without going after its food, it 

 must have some effective mechanism for bringing food to itself, and it must also depend 

 in part upon outside agencies to convey its food within reach. In the fresh-water mus- 

 sel, the mechanism employed for food gathering consists of hundreds of thousands of 

 active microscopic paddles covering the flaps that hang from the side of its body. These 

 paddles are exceedingly minute and all within the sheU; each is weak and ineffective 

 alone, but the effect of their concurrent action is to keep a strong current of water passing 

 into the mussel and out again. The water is filtered in passing, and the food, of course, 

 consists of the fine materials suspended and perhaps in part dissolved in the water. 

 The food in the water that lies within the influence of its currents is thus available to 

 the mussels; the natural circulation of the outside water must do the rest. 



A single animal that finds food brought within its reach might live the full period 

 of its life in one spot, but all animals of a species can not live in the same spot. It is 

 inevitable, therefore, that at some stage in the life history of such an animal as the 

 fresh-water mussel, there must be a period of movement or of distribution by outside 

 agencies. Through one of nature's nice adaptations, such a period of migration or dis- 

 tribution occurs in the life history of the mussel at the stage of infancy. Even then the 

 mussel shifts the burden of its distribution upon fish, as will be more fully told in the 

 section on life history. As inactive in youth as in old age, the fresh-water mussel, hav- 

 ing taken passage upon a fish, may then travel extensively to find a new home far removed 

 from the scene of its birth. Its living conveyance dispensed with, the mussel settles 

 down to a relatively immobile existence. 



Peculiarly victims of circumstance at all stages of existence, the fresh-water mus- 

 sels under natural conditions yet throve abundantly and broadly in streams and lakes 



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