8o 



A BIOLOGY OF CRUSTACEA 



have already mentioned, the young which are produced by the 

 resting eggs are all females, which lay eggs not requiring fertilisa- 

 tion and give rise to more females. The population increases rapidly. 

 A mature female can produce over a hundred live young ever) 

 three days, and each of these young matures within a fortnight, or 

 even less than a week when the water is warm enough. When a 

 certain density has been reached the effects of crowding make 

 themselves felt, and some of the eggs develop into males. Some of the 

 females then produce resting eggs. The population often diminishes 

 after this due to deaths from overcrowding and food shortage. 

 Towards the end of the summer the small algae in the pond begin 

 to increase again so that more food becomes available to the water 

 flea. The population again increases rapidly until crowding effects 

 intervene once more; males and resting eggs appear again, and the 

 population dies down or disappears, leaving only resting eggs to 

 overwinter. 



This cycle has been described in terms of the population, no one 

 individual survives for the whole year. It has been found that the 

 length of life of Daphnia magna varies with temperature, it can 

 live for 108 days at 8° C, but only 29 days at 28 C. Some of the 

 females which overwinter may live for about six months. It has also 

 been found experimentally that females grow to a larger size at low 

 temperatures, although they grow more slowly. This fits in with 

 what is found in nature; the largest specimens are found at the 

 end of the winter, and when these individuals die there is a marked 



Fig. 38. An idealised diagram of the yearly cycle of Daphnia magna in tem- 

 perate Europe. The precise location of the peaks is not important — they vary 

 from year to year according to the weather. Variations from pond to pond 

 also occur. Sometimes the population does not diminish in summer, some- 

 times it may disappear. Notice the coincidence of the onset of sexual 

 reproduction with periods of maximum population. 



