124 ANIMAL LIFE AND SOCIAL GROWTH 



more oxygen per individual than do the isolated 

 animals. Under these conditions the outer cat- 

 fishes are continually pushing in toward the center 

 of the group and consequently use up more oxygen 

 in the course of their greater activity. Again it is 

 evident that the type of the response obtained 

 depends in part on the behavior of the different 

 species of fishes under the stimulus furnished by 

 being grouped together. 



Many pond and aquarium fishes grow best in 

 relatively stagnant water. If such fishes are 

 subjected to daily changes of water so that their 

 relatively large aquaria are almost constantly 

 full of relatively uncontaminated water, the fishes 

 do not grow as rapidly as when they are put into 

 water in which freshwater mussels have lived for 

 at least 24 hours. It is known that such mussels 

 give off slime into the water and that this slime 

 has some food value for associated fishes but the 

 fact remains that after such mussel-conditioned 

 water has been filtered to remove the slime it still 

 has greater growth promoting power than has 

 plain well water also allowed to stand in similar 

 vessels for 24 hours and similarly filtered. It is 

 known that the mussels change the chemical 

 content of the water and that they affect the 

 bacteria present. It is also known, however, 

 that fish will not grow in this mussel-conditioned 



