224 L. B. SLOBODKIN 



The level of food is set initially by the experimenter and as a rule does not 



change during the history of the population. 



The experimenter may act as a predator on the population by removing 

 animals according to some fixed pattern. The most common pattern of 

 predation in these experiments was to remove animals in proportion to the 

 number of births occurring in the population (although the animals removed 

 were not necessarily new-bom). The experimenter might act as a source of 

 immigrants (Armstrong, i960; Armstrong & Slobodkin MS.). In this case 

 animals were added to the populations in proportion to the occurrence of 

 births. 



The precise size and shape of the experimental containers and the kind of 

 food and method of feeding varied from species to species. Daphnia obtusa 

 was fed Chlamydomonas moewusii in early experiments (Slobodkin, 1954). In 

 later Daphnia experiments, C. reinhardi was used as food for D. pulex. The 

 algae were grown separately on sterile agar, washed off into water and the 

 optical density measured with a Klett colorimeter. The Daphnia populations 

 were removed from their old medium, counted, sorted and subjected to 

 either predation or the addition of new immigrants, and were then returned 

 to fresh medium. The appropriate amount of food was added and the 

 animals were returned to their incubator. For some populations food was 

 also added on days when the medium was not changed. 



Hydrids require live animal food. Eggs o£ Artemia salina can be purchased 

 in sealed cans and hatch readily in salt solution. The Artemia nauplii were 

 washed through bolting silk and suspended in graduated centrifuge tubes. 

 When the centrifuge tubes are immersed in ice-water the Artemia are 

 immobilized and form a wet pack at the bottom of the tube. An ahquot of 

 resuspended Artemia of known concentration can be fed to the hydra. Since 

 hydrids seem very sensitive to the presence of various waste products, the 

 hydrids populations are washed one half hour after feeding. Probably several 

 washings during the day would have resulted in increased population size 

 (Loomis, personal communication). 



Flatworms also eat animal food but have difficulty capturing hve Artemia, 

 since Dugesia are repelled by hght while Artemia are attracted. Armstrong 

 (i960) therefore killed the Artemia by dipping the centrifuge tubes in hot 

 water. 



The Daphnia were grown in wide-mouth bottles, the hydrids in Petri plates 

 and the flatworms in fmger-bowls. All of the populations were confmed in 

 relatively small spaces. If there was any reaction which would cause these 

 animals to disperse under starvation conditions, or to adjust their distance 

 from their neighbours in some way in response to environmental conditions 

 it would be impossible to manifest under the conditions of these experiments. 



