88 FRESHWATER ANIMALS 



pium, she may, if nutritive conditions continue to be poor, have 

 no eggs in the brood chamber (no oocytes having matured 

 while she was carrying the ephippium). If conditions should 

 have improved, however, it is possible for her to resume quickly 

 the production of parthenogenetic eggs. 



The males necessary for the fertilization of the haploid ephip- 

 pial eggs develop from diploid parthenogenetic eggs. Males usu- 

 ally appear both in nature and in laboratory culture shortly after 

 a population maximum. The exact nature of the environmental 

 stimulus for the development of males instead of females is not 

 clear, nor is the mechanism by which this is accomplished (see 

 Banta, 1939, for a summary of investigations into this problem). 

 Shortly before the time that a population maximum is reached, 

 the amount of food per individual usually falls rapidly. The ap- 

 parent environmental determinant of ephippial egg formation 

 (diminution of food supply) thus initiates the slower process of 

 ephippial egg formation just before males begin to develop (at 

 the time of population maxima ) . 



There can be little doubt of the selective value of uniparental 

 reproduction to Cladocera, and especially to Daphnia in which 

 it is especially prominent. When food is adequate, every feeding 

 member of the population is an egg-producer — making a most 

 efficient transformation of food into eggs. This enormous repro- 

 ductive potential, however, carries with it the danger of over- 

 shooting the population size and density that the food supply 

 can support. Possibly the chief significance of the episodes of 

 sexuality from the point of view of population dynamics is that 

 they quickly reduce both the reproductive rate and the feeding 

 rate of the population. The diversion of some diploid partheno- 

 genetic eggs into males reduces the number of egg producers 

 developing from oocytes already formed. The oocytes beginning 

 to form in other females follow a path of development which, if 

 fertilization ensues, produces an embryo with a dormant period 

 of varying duration. Thus a part of the population will exist for 

 a time as nonfeeding, nonreproducing individuals. These mech- 

 anisms permit a temporary reduction in the size of the feeding 



