DIURNAL MIGRATION OF PLANKTON 31 



While the adaptive significance is not so obvious here, we hypothesize 

 that the value may lie in carrying out the pattern of vertical migration (for 

 whatever value it may have) in spite of the fact of accommodation to the 

 light stimulus and in spite of the fact that appropriate intensity and wave 

 lengths decrease rapidly with depth. Conversely, it allows a rapid approach 

 to the surface waters in spite of rapidly increasing intensities of light. 



In the zooplankters of the Inland Waterway of Florida vertical responses 

 to temperature were also observed. However, the range of useful tempera- 

 tures was considerably limited. At pH 8.05 a shift of minus 0.5 degrees 

 from 31° C would induce upswimming, while a shift to 3L5° C would in- 

 duce downswimming. A shift of as much as 5 degrees would irreversibly 

 destroy the response to either pH or temperature. 



Temperature and Geotaxis 



In the dark, positive geotaxis results from immersing Daphnia magna 

 in water 10 to 15° C above the temperature at which it was reared. This 

 behavior pattern is a parallel of the phototaxis described in response to 

 temperature. Small increments or decrements of temperature will cause 

 the sustained high velocities of locomotion described for phototaxis in re- 

 sponse to these stimuli for Cladocera only under white light. 



Hydrostatic Pressure and Vertical Movement 



In general, the effects of pressures on certain marine and freshwater 

 zooplankters appear to be identical and to fall into three categories. 



First, there is the efifect of pressures from 750 to 6,000 psi. Pressures of 

 these magnitudes cause paralysis followed by death within minutes, par- 

 ticularly at low temperatures. In Daphnia magna the efifect appears to be 

 on the choline esterase system, since it can be alleviated by the use of atro- 

 pine in physiological concentrations. It is not known whether or not the 

 same protection is afforded marine organisms. 



Second, there is the effect of pressure ranging from 30 to 500 psi. 

 Throughout this range, pressure can cause upswimming depending on the 

 temperature or pH of the water within the pressure bomb. For example, 

 the megalops larvae of portunid crabs swim downwards at all tempera- 

 tures above 30° C when the pH is 8.1, but can be made to swim upwards 

 by application of pressure. The higher the temperature the more pressure 

 is required to induce upswimming. At 40° C the response is poor and 

 heat death ensues in minutes. Death is not particularly delayed by high 

 pressure. 



The third category is small pressure changes in a low pressure range, 

 from partial vacua to 10 psi. For the Florida Inland Waterway zooplank- 

 ters such small changes in hydrostatic pressure were found to produce 



