Plancton Pulses 305 



its abundance in the plancton is directly related to the 

 abundance of that alga. Each animal may have its 

 food preference. The filaments of Lyngbya are too 

 large for the small and immature crustaceans to handle. 

 Ceratium has too hard a shell; it appears to be eaten 

 only by the rather omnivorous adult Cyclops. For 

 animal planctonts in general Anabaena and its allies 

 and the diatoms and small flagellates appear to be the 

 favorite food. 



Obviously, the amount of food available to any 

 species is in part determined by the numbers of other 

 species present and eating the same things. 



Plaficton pulses — The organisms of the plancton 

 come in waves of development. Now one and now 

 another appears to be the dominant species. In most 

 groups there are a number of forms that are competitors 

 for place and food. The diatoms Asterionella, 

 Fragillaria and Tabelaria may fill the upper waters of 

 a lake together or in succession. A species of Diap- 

 tomus may dominate the waters this May, and 

 species of Cyclops may appear in its stead next May. 

 Yet while species fluctuate, the representation of the 

 groups to which they belong remains fairly con- 

 stant. 



These sudden waves of plancton production are 

 made possible, as every one knows, by the brief life 

 cycle of the planctonts, and by their rapid rate of 

 increase. If a flagellate cell, for example, divide no 

 oftener than every three days, one cell may have more 

 than a thousand descendants, within a month. The 

 rotifer, Hydatina is said to have a length of life of some 

 thirteen days, but during most of this time it is rapidly 

 producing eggs, and the female is mature and ready to 

 begin egg laying in 69 hours from hatching. Some of 

 the larger animals live much longer and grow more 



