EFFECT OF CROWDING ON SURVIVAL 195 



were located in a stream 15-18 cm. deep where the surface water 

 was moving at the rate of 25 cm. a second. 



Field tests were run to determine the effect of aggregation upon 

 the rate of oxygen consumption of the aggregated isopods. For this 

 purpose, a respiration chamber was made by firmly attaching a 

 short piece of snugly fitting rubber tubing to a small wide-mouthed 

 bottle of about 14 cc. capacity. The rubber tubing was long enough 

 to extend beyond the bottle mouth about the length of the bottle 

 neck. A second similar bottle inserted into this rubber collar until 

 the mouths of the two met tightly made a respiration chamber of 

 27.3-30 cc. capacity. The rubber tubing served as a collar to hold 

 the two bottles firmly bound together, and came in contact with 

 the water only to a very slight extent. At the end of the respiration 

 period the contained isopods were all shaken down into the first 

 bottle. The other was removed, closed with its glass stopper, and 

 used as an isopod-free collection sample. The agitation necessary 

 to shake the isopods all into one bottle must have mixed the con- 

 tained water fairly thoroughly. 



A sample collected in this manner was treated according to the 

 usual Winkler's method, except that only one-fourth of the usual 

 amount of the reagents was added. Titration was carried on in the 

 field, using a standardized sodium-thiosulphate solution of about 

 0.002 normality. Blanks were run with each set of five respiration 

 tests. These tests were always run in parallel series — one composed 

 of animals taken from the aggregations; the other containing the 

 same number of isopods of similar size that w^ere scattered about 

 singly when collected, and which had not been aggregated recently 

 if at all. Ten male isopods, or 20 of the smaller females, were the 

 standard number used at one time in each respiration chamber. 

 The respiration tests of lots from the aggregation and those that 

 before the tests had been scattered singly were, with one exception, 

 run simultaneously. In this exceptional case, the results obtained 

 may be affected by the greater amount of oxygen present in the 

 water when the isolated animals were tested. 



Care was taken to collect the water for the tests from the same 

 depth at the same point in the stream, and in as nearly the same 



