200 ANIMAL AGGREGATIONS 



respiration results observed are due to muscular tension acting as 

 a direct result of the previous position of the isopods or whether 

 they are due to the difference in muscular tension resulting from 

 the differential behavior, which is in turn conditioned by their previ- 

 ous position. 



The possibility that the increased carbon-dioxide tension and the 

 decreased oxygen supply within the aggregation may have reduced 

 the oxygen requirements of these animals is negatived by the fact 

 that the rate of oxygen consumption of isopods from the center of 

 the aggregations was slightly, but not significantly, higher than that 

 of those taken from the upper swamp where the oxygen tension and 

 pH were approximately that of the main current above the large 

 clusters. 



The results obtained from the study of these enormous aggrega- 

 tions of isopods in nature demonstrate that such masses can affect 

 environmental conditions markedly, even in flowing water. Obvi- 

 ously, such groupings would have still greater effect in quiet water 

 or on land. As we shall soon see, such aggregations may have survi- 

 val value in the laboratory, due to the modification they produce 

 in their environment. No tests were made of the possible survival 

 value of the isopod aggregations in nature, but the fact that they 

 do alter their environment to a measurable degree extends the possi- 

 bility of the application of the laboratory studies on the survival 

 value of masses of animals. As in the case of the laboratory groups 

 of land isopods and of Ophioderma, these water isopods in nature 

 under adverse conditions tend to use other isopods in place of in- 

 animate elements of their physical environment, when the former are 

 present in considerable numbers and the latter are lacking in their 

 usual ratio of abundance in proportion to the numbers of animals. 



In conclusion we may recall the statement at the beginning of 

 this chapter, which these later studies have served to confirm, that 

 under laboratory conditions the formation of aggregations serves 

 to make these animals more quiet, and in the long run proves to be 

 what is usually called an "adaptive reaction"; and, so far as we have 

 information, the same results come also from aggregations under 

 natural conditions. 



