FRESH- WATER MUSSEI^S. Ill 



and Wilson (191 2) describe the Maumee River as rather muddy most of the time, and it 

 is interesting to find that they report that two-thirds of the spikes, Unio gibbosus, in that 

 river were white-nacred and that the black sand-shells were usually white-nacred. 



The reputed migration of certain mussels toward shore in time of flood may be an 

 accommodation to light conditions associated with turbidity of water under such con- 

 ditions. We have virtually no data on the distribution of mussels with respect to 

 permanently shaded areas or with regard to the reactions to daily changes in light. 



CURRENT. 



The luxuriant development of certain mussels in streams where the current is 

 strong, in contrast with their growth in sluggish portions of rivers and lakes, bears 

 witness to the significance of current as a favorable factor of environment for fresh- 

 water mussels. Current is a characteristic feature of streams, and the rate of flow is 

 largely determined by the gradient of the channel. Currents producing a circulation 

 of water occur also in lakes, where they are caused chiefly by wind and to a less extent 

 by changes of temperature. In some lakes the circulation extends from top to bottom, 

 but in small deep lakes only a partial surface circulation commonly prevails (Birge and 

 Juday, 191 1). Undertow currents are also developed where there is wave action, and 

 under some conditions convection currents must exist in natural bodies of water, but 

 we have little data on this. 



Shelford (191 3) emphasizes the relation of water animals to current as follows: 



The distribution of dissolved salts and gases is dependent upon the circulation of the water, as 

 their diffusion is too slow to keep them evenly distributed. The water of streams has been found to 

 be supersaturated with oxygen [citing Birge and Juday, 1911]. Oxygen is taken up by water near the 

 surface. Nitrogen and carbon dioxide are produced especially near the bottom, and if the water did 

 not circulate they would be too abundant in some places and deficient in others for animals to live 

 (p. 60). * * * 



The current in streams differs from that in lakes in that it. is for the most part in one direction 

 while the lake currents often alternate. There are backward flows and eddies at various points in 

 streams in front of and behind every object encountered in the current. As we pass across a stream 

 we find the current swiftest near the surface in the middle and least swift at the bottom near the sides 

 (p. 61). * * * 



The factors of greatest importance in governing the distribution of animals in streams are current 

 and kind of bottom. They influence carbon dioxide, light, oxygen content, vegetation, etc. (p. 66). 



Since mussels are bottom dwellers and largely stationary in habit, one can appreciate 

 how dependent they must be upon circulation of the water to bring renewed supplies 

 of organic food, mineral matter in solution, and oxygen, and to remove the poisonous 

 products of metabolism that are produced in their own bodies and in those of other 

 organisms living about. Mussels, of course, cause by their respirative currents cir- 

 culation of the water immediately about them, but this is not sufficient to prevent 

 an early exhaustion of food supply unless broader currents prevail. 



It must be emphasized, too, that flowing water carries more matter in suspension 

 than still water. It has been seen (p. 91) that the food of mussels consists to a con- 

 siderable extent of the finely divided solid matter; but such materials, however abun- 

 dant on the bottom, are not available to the mussel until they are taken up in the water 

 and carried to the mussel. The efi'ects of the current, then, both in lifting soHd matter 

 from the bottom and in holding it in suspension play a foremost part in its relation 



