J. L. BROOKS 85 



of the distinctive characters of ten of the thirteen species by 

 which the subgenus Daphnia is represented in North America. 



Our main task is to discover the reasons why the characteriza- 

 tion and identification of these common freshwater animals has 

 presented so much difficulty. But before proceeding to a con- 

 sideration of these sources of difficulty, we should consider some 

 of the basic biological peculiarities of the animals being studied. 



The average life span of a Daphnia is between a few weeks 

 and several months, depending on the temperature. At the tem- 

 perature (about 20° C.) common in ponds and the upper waters 

 of lakes in summer, the female of most species lays her first brood 

 of eggs about a week after she is born. Subsequent broods of 

 eggs are laid at two- or three-day intervals. Although it is diffi- 

 cult to assess the life spans under natural conditions, probably 

 two or three weeks would be a reasonable estimate of the most 

 frequent life span at these temperatures (Brooks, 1946). These 

 same species grow much more slowly at the low temperatures 

 (4° C. or less) characteristic of temperate lakes (dimictic lakes 

 in the terminology of Hutchinson and Loffler, 1956) during 

 winter. A few of the individuals born in the fall survive until the 

 waters warm in the spring. Then they produce a few broods of 

 eggs and die. Although it is clear that the few females which do 

 survive have a life span of three or four months, it is difficult 

 to assess the average life span at these temperature under nat- 

 ural conditions. 



A Daphnia population living in a large permanent body of 

 water may have as many as ten or fifteen generations per year 

 (see below for the basis of this estimate). At the other extreme 

 are the temporary-pond populations, which may have only one 

 or two generations per year. The species maintains itself for 

 the remainder of the year in the form of dormant early embryos 

 (so-called resting eggs) within a resistant case, the ephippium. 



The populations of Daphnia consist almost entirely of females 

 (the exceptional presence of males is discussed later) that are 

 able to maintain themselves because the eggs produced under 

 propitious environmental conditions are diploid, and they begin 

 cleavage soon after they leave the oviduct. The diploidy of this 



