324 



Aquatic Societies 



The animal population of the shores is likewise dis- 

 tributed largely in relation to water movement, or to 

 conditions resulting therefrom. There is a zonal 

 arrangement of animal life along shores that is only a 

 little less definite than that of plants. It is much less 

 obvious, for plants are fixed in position and come out 

 more into the open and into view. Nevertheless, even 

 the most free-roving animals, the fishes, as we have 

 already seen (p. 233), keep in the main to certain shore- 

 ward limits. 



Distribution in relation 

 to depth and to character 

 of bottom comes out 

 clearly in Headlee's stud- 

 ies of the mussels of Win- 

 ona Lake. In that lake 

 the play of the waves on 

 shore yields a clean beach 

 line of sand and gravel, 

 and sifts the finer mater- 

 ials into deeper water. 

 The succession is gravel 

 and sand, marly [sand, 

 sandy marl, coarse white 

 marl, marly mud and very 

 soft black mud. The last 

 named, beginning at a 

 depth of some 20 feet, 

 covers a very large central portion of the lake bottom. 

 Mussels cannot live in it for they sink too deeply 

 and the fine sediment clogs their gills. Hence the 

 mussels are restricted to the strip along shore. With- 

 in this strip they are arranged according to hard- 

 ness of bottom and exposure to waves. The accom- 

 panying diagram illustrates the distribution of four of 

 the common species. The two Anodontas, having 



FIG. 191. Diagram of distribution of 

 mussels in Winona Lake, Indiana 



A, outline of lake with the mussel zone 

 stippled and marked out by two ten-foot 

 contours. 



B, shows the relation of four of the common- 

 est species to depth and character of bottom: 



1. Anodonta edentula. 3. Unio rubigniosa. 



2. Anodonta grandis. 4. Lampsilis luteolus. 



