Existence of PUmkton 41 



In such a situation plankton and detritus accumulate and form a 

 "false bottom" that provides a potentially rich feeding zone. 



Important changes in viscosity also occur as temperature is al- 

 tered. At 25°C the viscosity of water is only half that at 0°C. Since 

 at higher temperatures the effect of the decreased viscosity is added 

 to that of the lower density, the rate of sinking tends to be greater. 

 Probably as an adaptation to this difference through natural selection 

 many planktonic organisms in warmer waters are more profusely pro- 

 vided with bristles and other feathery structures tending to retard 

 sinking (Fig. 2.8). Certain planktonic diatoms and dinoflagellates 

 also show a graded difference in the length of spines in tropical and 



Aug. Sept. Oct. Nov. Jan. Apr. May June June July 



Redrawn from Wesenberg-Lund, 1910 



Fig. 2.9. Cyclomorphosis in Daphnia cuctillata (upper) and Bosmiiw coregoni 

 (lower), showing seasonal changes in body form. 



polar areas. Whether such geographical variants should be divided 

 into separate species may not be settled until controlled culturing ex- 

 periments are undertaken. 



In some plankters the body form changes with the season of the 

 year. This phenomenon, known as cyclomorphosis, is strikingly ex- 

 hibited by the increased size of the "helmet" and length of spines 

 in certain Cladocera during warm months ( Fig. 2.9 ) . These changes 

 produce increase in body surface and were originally believed to be 

 entirely an adaptation to the lower density and viscosity of warmer 

 water, but subsequent work has shown that factors other than tem- 

 perature also influence body form (Brooks, 1946). In all such re- 

 lationships the ecologist must distinguish between actual causal fac- 

 tors and possible beneficial results. For a further discussion of 



