94 BtrivLETlN OF THE BUREAU OF FISHERIES. 



HABITAT. 



The pearly mussels, as inhabitants of fresh water, are found in diverse habitats, in 

 lakes and in rivers, in shallow and in deeper waters, in cold and in warm waters, in mud, 

 in sand, and among rocks. Yet they do not occur in all lakes and rivers, nor in all parts 

 of the lakes and rivers in which they do live ; and the several species of mussels, when 

 living together, are not always found in the same relative abundance. It may, therefore, 

 be supposed that fresh-water mussels, like other animals, are adapted rather definitely 

 to particular conditions of environment; that some find congenial environment in still 

 or sluggish water, while others thrive best in strong currents ; that a mud bottom supports 

 certain species, while a firmer soil is required by others. 



Adult mussels in some cases thrive, or continue to live at least, in environments 

 where the young would perish, for delicately balanced conditions are required by very 

 young mussels of many species, and only where these conditions exist can a mussel bed 

 originate or perpetuate itself. On the degree of stability of the conditions favorable to 

 the growth of the young the permanency of the bed must depend, since, when replenish- 

 ment fails, the bed can continue only as long as the life of the adult mussels it contains. 

 As any mussel has rather limited powers of independent locomotion, the place where it 

 lives (or prematurely dies) is probably, as a general rule, near where it falls when it 

 drops from its fish host; yet the early juvenile can be carried by the current, and doubtless 

 this means of transportation may sometimes aid the young mussel in finding a suitable 

 habitat. An adult niggerhead mussel lived in apparently healthy condition in a balanced 

 aquarium at the Fairport station for nearly nine months; yet in nature this species is 

 found only in strong currents, the favored environment of its fish host, the river herring. 



The relationship of fresh-water mussels to the environment may be treated with 

 reference to body of water, bottom, depth, light, current, water content, vegetation, 

 and animal associates. 



BODY OF WATER. 



The various geographic types of fresh water in which mussels occur are rivers, lakes, 

 ponds, sloughs, swamps, marshes, and canals. In so far as distinctive conditions char- 

 acterize these various types of waters, each may have its characteristic mussel fauna. 

 It may be said in general, that wherever conditions suitable for a particular species of 

 animal prevail, that species will be found, except as it may have been naturally excluded 

 through features of geologic history or other factors governing the distribution of animals; 

 in the case of fresh-water mussels, however, emphasis must be placed upon a qualifica- 

 tion of this general statement. Though all conditions in a body of water may be other- 

 wise suitable, mussels can not naturally occur where conditions do not permit the entry 

 and survival of the species of fish which serve as hosts. 



STREAMS. 



Mussels have undoubtedly reached their greatest development, as to numbers, both 

 of species and of individuals, in flowing water. From the commercial standpoint, also, 

 the quality of shells from streams is almost invariably superior. In general, where other 

 conditions are favorable to mussels, larger bodies of flowing water are more productive 

 than the smaller. Brooks do not usually contain mussels. Morphologically, mussels 

 adapted to life in strong currents are differentiated from those adapted to still water by 



