EFFECT OF CROWDING ON SURVIVAL 191 



differential length of life under experimental conditions is shown in 

 the earlier and greater reduction in oxygen consumption in final 

 stages of the isolated rather than of the bunched individuals. 



The possibility of consuming a similar amount of oxygen in the 

 same time was the same for the lot of isolated as for the bunched 

 Ophioderma, had they been given equal treatment. The conclusion 

 is obvious that when similar Ophioderma are isolated without food 

 in clean glass receptacles of approximately equal volume per number 

 of individuals present, their rate of oxygen consumption, following 

 a period of initial stimulation, is significantly depressed, and their 

 expectation of remaining intact, or of living, is less than if they are 

 allowed to aggregate. 



FACTORS CONTRIBUTING TO GROUP PROTECTION 



The question immediately arises as to the type of beneficial in- 

 fluence exerted by the grouped individuals upon each other. There 

 has not been opportunity to investigate this aspect of the problem 

 thoroughly. Certain data have been collected that are of decided 

 interest, and these will be presented not as a final answer but as a 

 possible guide to the ultimate solution of the problem. 



The earlier students of the effects of crowding, who uniformly 

 found only deleterious effects, supposed that there is some harmful 

 secretion given off by the crowded animals. Similarly, workers who 

 have reported beneficial effects of increased numbers of individuals 

 in relation to volume of the containing water have postulated some 

 beneficial secretion which is preservative in action. Drzewina and 

 Bohn, in their series of studies upon this subject, in which they used 

 a wide range of aquatic organisms, suggested that the observed 

 beneficial effects are due to a hypothetical chemical which is secreted 

 by the mass of animals in sufficient amounts to have an autopro- 

 tective effect. When the crowded conditions produce harmful 

 results, they postulated another chemical substance which is auto- 

 destructive. 



Inasmuch as these Ophioderma live naturally among eelgrass 

 where they crawl over the blades, and since the formation of dense 

 aggregations is greatly retarded if not entirely prevented by the 

 presence of eelgrass in the stock aquaria, it seemed possible that 



