82 BULLETIN OF THE BUREAU OF FISHERIES. 



finely divided nonliving organic materials. The rate of growth of mussels generally is 

 much higher and the size attained is greater in rivers than in lakes. Other factors 

 than currents, of course, enter into consideration, and these will be discussed in the 

 appropriate places. 



A chief condition of rich growth of mussels is a plentiful food supply, and not all 

 rivers are alike in this regard. There are relatively fertile and relatively sterile rivers 

 and lakes, and the fertility of streams is likely to correspond in a rather general way 

 with the fertility of the lands from which the drainage is derived. Whatever materials 

 suitable for the construction of plant tissues are brought into the waters are likely ulti- 

 mately to be converted in great part into plant or animal life, and no little part of the 

 plant life that is formed is likely to be converted ultimately into animal life. 



A primary condition for the formation of thick shells of good quality is the presence 

 in the water of suitable minerals, principally calcium, and all of the important mussel- 

 bearing streams are those whose tributaries flow from regions of limestone or other cal- 

 careous deposits. Consequently it is the Mississippi Basin which largely supports the 

 pearl-button industry, though shells of commercial value are also found in the Great 

 Lakes and Gulf of Mexico drainages, and some in the Red River of the North. The 

 streams of the Atlantic and Pacific slopes are almost or entirely barren of valuable 

 shells. 



Many factors, indeed, enter into the suitability of waters for mussels, and of the 

 various species of mussels — more than 500 in the United States — each has its special 

 requirements; some will thrive where others will not. Much remains to be learned 

 concerning the relation of mussels to their environment, and the subject is particularly 

 complex because of the great number of species involved; but it will be attempted to 

 place the several phases of that subject in general review in a later section on habitat 

 (p. 94). It is the purpose of this section to give such a general account of the habit of life 

 and the conditions of existence as is necessary to establish the peculiar dependence of 

 fresh-water mussels upon the immediate environment. 



LOCOMOTION. 



As regards their place of abode, fresh-water mussels are very largely creatures of 

 circumstances. Since they are not frequently seen in motion it is probable that most 

 of them spend their lives after the period of infancy very near to the place where they 

 first settle down. Nevertheless they can and do move, and certain species, principally 

 the more elongate forms, manifest a condition of restlessness at times. 



All mussels are sensitive to some stimuli; a splash of the water near them, a touch 

 on the edges of the mantle especially at the siphons, or the passing of a shadow over 

 them, will cause the siphons to be withdrawn and the shell to be tightly closed. There 

 are evidences to indicate that when the disturbance is severe, as when the mussel is 

 taken entirely out of the water, or is exposed to the sun by an unusually low stage of 

 water, or is affected by extreme cold, the withdrawal of the mantle is so extreme as to 

 break the living connection with the edge of the shell, and thus to cause, when growth 

 is resumed, an interruption line or plane in the shell which is present ever afterwards. 

 (See p. 132.) 



The reaction to evident stimuli consists merely in closing up; there are times, 

 however, when a mussel is impelled to change its position. The movement may then be 



