THE SALT CONTENT OF NATURAL WATERS IN 

 RELATION TO RHEOTAXIS IN ASELLUS. 1 



W. C. ALLEE, 

 LAKE FOREST COLLEGE. 



Much of my work on rheo taxis of the isopod Asellus communis 

 Say has been based on the fact that pond and stream mores of 

 the same species give different rheotactic reactions. Among 

 other differences the pond mores have a low, and the stream 

 mores, a high degree of positiveness ('12). I have wished for 

 some time to determine whether there were differences in the 

 salt content of the two classes of habitats that might account for 

 the differences in behavior. Upon my return to the Chicago 

 area after an absence of three years, I hastened to make this 

 survey. The subject is the more interesting since experiments 

 in the meantime have shown that the rheotactic reaction may 

 be controlled by various salts when present in sufficient con- 

 centration. 



Through the kindness of Mariner and Hoskins, to whom I was 

 already indebted for similar analyses, the spring and autumn 

 salt content was determined for County Line Creek, near Brae- 

 side, 111., and for a point near Osborn, Indiana (Slough 93; 

 Shelford, '13). Many of the isopods used in experimental work 

 have been obtained from these habitats. The analyses are 

 from approximately maximum-minimum water level for the 

 year 1916 and are given in detail in Table I. 



Of the elements found, calcium and magnesium are known to 

 decrease the positiveness of rheotaxis in Asellus when present in 

 sufficient concentration. If the salt content is at all responsible 

 for the low positive rheotactic response of pond isopods, its effect 

 should be most marked in the most concentrated water analyzed. 

 This water contained 228.4 m g- f calcium per liter of water, the 



1 I am indebted to the Elizabeth Thompson Fund and to the Bache Fund of the 

 National Academy for money grants and to Mariner and Hoskins for water analyses 

 without charge; without their aid this work could not have been done. 



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