PREDATION AND EFFICIENCY IN LABORATORY POPULATIONS 231 



The relation between yield per unit food and F is also essentially the same 

 as that found in Daplmia (Fig. 7.). 



Experiments on the interaction between predation and competition in 

 H. littoralis and Chlorohydra viridissima are still in progress. Some prehminary 

 results will be reported here. 



The green hydrid, C. viridissima, eats Artemia but also contains presumably 

 symbiotic algae. Populations of green hydrid are four times more numerous 

 when grown under a fluorescent lamp than when grown in partial darkness. 

 Complete darkness was impossible since the animals had to be counted under 

 a light microscope. They do not blanch completely in the dark but their 



•25 



■75 



I'O 



Fig. 6. — Relation between Pp per unit food and F in thirty-two populations of Hydra oligactis. 

 The mean values of Pp at each F are indicated by >. The line is drawn from equation (i). 



green colour becomes much paler, tight is of no apparent significance in the 

 population growth of H. littoralis. 



To the degree that present data permit conclusions, the outcome of 

 competition between the species depends on light. Of the six mixed species 

 populations in the light, two were not subjected to predation and in both of 

 these the green hydrid were the only survivors. Two populations were 

 subjected to predation of F = 25 per cent for each species separately. The 

 brown hydrid was eliminated in both of these populations. One mixed 



