DIURNAL MIGRATION OF PLANKTON 33 



No destruction of the mechanism results. Hardy and Bainbridge (1951) 

 noted low-pressure responses in marine plankton. 



pH and Phototaxis 

 The influence of pH, or carbon-dioxide-induced changes in pH, on 

 daphnid behavior has been observed often since Jacques Loeb ( 1904) re- 

 ported reversal of phototaxis when he profligately poured beer into his 

 experimental aquaria of daphnids. In general we have confirmed these ob- 

 servations ; high pH values induce positive phototaxis while low pH values 

 induce negative phototaxis. The crossover point from positive to negative 

 phototaxis appears to depend to some extent on the pH at which the or- 

 ganisms were reared. Daphnids reared at pH 8.0 become vigorously posi- 

 tive at pH 8.5 and vigorously negative at pH 7.0. The adults are apt to be 

 normally photopositive to white light at pH 8.0 when fed on green algae. 

 Conditions of culturing may change the sign of taxis ; these will be dis- 

 cussed under redox potentials. Immature forms often exhibit the reverse 

 photo- and geotaxis of the adults. This fact has often been observed and is 

 believed by Skadowsky ( 1939) to be a function of metabolic rate. 



pH and Geotaxis 



Geotaxes in response to pH changes occur in the dark (vertical distri- 

 bution of a population of daphnids is easily measured by a flash photo- 

 graph). Results of such studies show that Daphnia magna swims upward 

 in the dark at pH 9.0 and swims downward at a pH of 7.0 or below. Such 

 geotaxis may occur in addition to the phototaxis in response to pH since 

 it takes longer for a population to reach equilibrium position in the dark 

 than in the light. 



Probably the most startling induction of geotaxis by pH is to be observed 

 in the Inland Waterway of Florida, where small changes of pH are of pri- 

 mary importance in controlling the vertical migration of the zooplankton. 

 Special care must be taken in the collection of the plankton sample to avoid 

 a change of more than 2° C or 0.7 pH units. Either of these will de- 

 stroy the response to pH or temperature. Vertical-movement tests in re- 

 sponse to pH were run in the laboratory on three representative species : 

 a hydrozoan medusa, a pontellid copepod, and blue crab megalops and 

 zoea larvae. Some efi^ort was made to obtain relatively pure populations 

 of the three species tested for pH responses ; this was, however, imprac- 

 tical so that other unidentified organisms were often included in the tests. 

 The responses of these unidentified organisms were identical with those 

 of the carefully isolated species studied. When the water temperature was 

 maintained at 31° C and the pH was varied through the range of 8.0 to 

 8.1, there was a uniform response of the species tested ; a pH of 8.0 caused 

 persistent upswimming while changing the pH to 8.1 induced persistent 



