234 Proceedings of Indiana Academy of Science 
rate should be higher. Then, with higher metabolism, more food, and 
more oxygen, lake mussels should be the larger, if these were the de- 
termining factors. In the upper reaches of lake-fed rivers the mussels 
may profit to a certain extent by the water flowing from the lakes above. 
It is likely that the Najades originally populated the fresh waters 
through the rivers rather than originating in the lakes. The lakes are 
younger, more transient, less extensive, at greater altitudes, and at the 
extremes of the drainage systems, and mussels have had less time in 
which to grow adjusted to them than to rivers. 
Feeding Conditions upon Stream Deltas. Northwesterly winds 
have diverted the mouth of Pocahontas Creek southward into a shallow 
bay. The bivalve population of this bay were observed at times to 
have an almost complete change of diet. Ordinarily the food is lake 
plankton. After heavy rainfall the increased volume of creek water 
usually spreads out in a sheet of a few inches depth over the entire 
bottom of the bay. On such occasions the food of the mussels is greatly 
altered. The same phenomenon was sometimes observed to take place 
when there had been no rainfall, and at first it was puzzling to explain 
the sudden changes of diet from lake to stream plankton. The explana- 
tion turned out to be simple, when it was found to correspond to the 
diurnal or cyclonic temperature changes. The creek is shallow and its 
temperature changes more rapidly than that of the lake. After a cold 
period, its cooler water sinks into the water of the lake and spreads out 
in a thin layer at the bottom of the embayment. Its planktonts become 
the food of the mussels there, and they are excluded from their normal 
food supply. When the creek water is turbid and cold at the same time, 
it may easily be seen to underlie the warmer clearer lake water. It 
follows the bottom closely until the edge of the terrace is reached, where 
it spreads out horizontally in the region of the thermocline, in water of 
virtually equal temperature. 
This alternation in temperature and food does not show evidence of 
inciting movement. But, during freshets, when the lake level is greatly 
changed upon the littoral, movements shoreward begin, due to pressure 
change. 
In streams one may often see the siphonal regions of living shells 
used as holdfasts by such filamentous stream algae as Cladophora. 
This does not ordinarily occur in the lake. Yet it is a common observa- 
tion in the above-mentioned bay where the water of the creek lies next 
to the substratum, even well out from the mouth of the creek. 
Evermann and Clark acknowledge the greater food content of lakes. 
They suggest that fertilization is favored in the current of rivers and 
take no cognizance of the movements of lake water which accomplish the 
same purpose. Their explanation of the distribution of mussels upon 
rifles and other parts of a river bed lays emphasis upon the current as 
the distributional factor. Since lake species tend also to seek out grav- 
elly or sandy beds and few choose soft bottom, the correspondence to 
river forms is exact. In lakes it is certainly the character of the bottom 
which is of most importance, and this factor can as readily explain dis- 
tribution on river bottom. The current has of course produced the form 
of the bottom, and is thus an indirect factor. 
