112 BULLETIN OF THE BUREAU OF FISHERIES. 



to the welfare of mussels. The power of water to move solid matter on the bottom 

 increases very rapidly with the rate of flow. 



The capacity of water to move solid matter from a condition of rest on the bottom of a stream 

 varies with the sixth power of the velocity of the stream. If the velocity is doubled, the increase 

 in the force which is capable of putting the particle in motion is multiplied 64 times. (New York 

 report of Metropolitan Sewerage Commission, 1912, p. 41.) 



Fish frequent areas near the current but maintain themselves in eddies or in places 

 where the current is relatively slack, as at the bottom and near the shores (Shelford, 191 3). 

 In view of the essential part that fish play in the distribution of mussels, the habits 

 of the fish may be a very significant factor in the distribution of mussels with reference 

 to current. It has been suggested by Evermann and Clark (191 8, p. 252) that currents 

 may promote the reproduction of mussels by making fertilization of the egg more 

 certain and by decreasing the chance for inbreeding through the conveyance of sperm 

 from mussels farther upstream. In still waters the chance for fertilization of eggs 

 may be less favorable. 



The relations of mussels to temperature have not been fully investigated, but it 

 seems certain that flowing water must protect mussels from excessively high tempera- 

 tures and thus permit many species to live in much shallower water in streams than 

 in ponds or lakes. 



The tendency of mussels to locate apart from the main channel and nearer the 

 banks of the streams has previously been mentioned (p. 97). While this distribution 

 may be partly due to the fact that there the full force of the current is avoided while 

 many of its benefits are received, nevertheless it must not be overlooked that many 

 species of mussels thrive in rapid shallow streams and that such regions of swift water 

 in the Mississippi River, as the former "rapids" at Keokuk or the existing "rapids" 

 above Davenport, have been among the most prolific mussel grounds of the entire river. 

 In these circumstances, however, the rocky nature of the bottom affords the mussels 

 protection against some effects of the current. Evidently the barrenness of the main 

 channel in most cases is due rather to the nature of the bottom combined with the force 

 of flow than to the strength of current alone. 



On page 99 there have been listed the species of mussels which are characteristic 

 of lakes and ponds, regions of comparatively still water. The more common mussels 

 of rivers may be classified according to apparent adaptation to sluggish water, strong 

 current, and rapids (Table 8). These general comments should be made: In a firm 

 bottom, such as furnishes good anchorage, a mussel may dwell in a current swifter 

 than is characteristic of its common habitats; where rocks furnish shelter, mussels 

 below them may be in rather slow water despite the current around them; deep water 

 may be fairly sluggish under a swift surface current. 



EXPLANATION OF TABLE 8. 



The symbols are those used in Table 6, C representing Clark; D, Howard; F, Shira; and G, Coker. 

 The large capital denotes preference in the opinion of the observer, for a particular condition of current. 

 The small capital denotes that the condition is favorable but not, so far as is known, preferred to other 

 conditions. When no large capital occurs on a line, no preference is indicated; and when a particular 

 letter appears in small capital throughout a line, the observer denoted by the letter has no evidence 

 upon which to base an opinion of discrimination on the part of the particular mussel between the 

 different conditions of current regarded as favorable. 



