34 INVERTEBRATE PHYSIOLOGY 



downswimming. Light has Httle effect on this reaction. The same responses 

 could be obtained with more marked shifts of pH, although departures of 

 more than 0.5 units from the normal environmental pH quickly destroyed 

 the mechanism. The daytime pH of the Inland Waterway is 8.1, the noc- 

 turnal pH 8.0. There is sufficient turbulence to prevent a gradient. The 

 pH values were obtained with a Beckman Model B pH meter equipped 

 with titrating electrodes. Each reading was preceded by a buffer and 

 temperature check. 



Redox Potentials and Phototaxis 



Oxidizing substances to which daphnids are permeable cause positive 

 phototaxis while reducing substances to which daphnids are permeable 

 cause negative phototaxis. The redox poising compounds utilized for 

 such tests are largely those intravitam stains which readily penetrate 

 daphnids, some of which appear preferentially concentrated in the cells 

 of the nauplius eye. Additionally, catechol and cysteine were used. Catechol 

 (10"^M) having an E'o of + 0.33 volts produces strong upswimming 

 while (10""M) cysteine having an E'o of —0.14 volts produces strong 

 downswimming (Smith 1954). The mid-point of the range where no 

 effect occurs for a population of DapJinia reared on green algae appears to 

 to about -f- 0.045 volts E'o, while the mid-point for a culture reared on bac- 

 teria appears to be lower. The important point here is that the majority 

 of organisms reared on algae appear to be photopositive, while those reared 

 on bacteria having a low redox potential are photonegative. We feel that 

 these results will largely explain the photopositivity reported for daphnids 

 by Clarke (1930) and the photonegativity later reported for the same 

 species by the same author (1932). A summary of our observations on 

 phototaxis and redox poising compounds is given in Fig. 3. 



REFERENCES 



Baylor, E. R., 1954. The interaction of light and drugs in the cold narcosis of Daphnia. 



Proc. Fed. Soc. for Exp. Biol. 13, 543. 

 Baylor, E. R., and F. E. Smith, 1953. The orientation of Cladocera to polarized light. 



Am..Nat.B7,97-\0\. 

 Clarke, G. L., 1930. Change of phototropic and geotropic signs in Daphnia induced by 



changes of light intensity. /. Exp. Biol. 7, 109-131. 

 Clarke, G. L., 1932. Quantitative aspects of the change of phototropic sign in Daphnia. 



J. Exp. Biol. 9, 180-211. 

 Fraenkel, G. S., and D. L. Gunn, 1940. The Orientation of Animals. Clarendon Press, 



Oxford. 

 Grosser, B. I., E. R. Baylor, and F. E. Smith, 1953. Analysis of geotactic responses 



in Daphnia magna. Ecology 34, 804-805. 

 Hardy, A. C, and R. Bainbridge, 1951. Vertical migration of plankton animals. Na- 

 ture 168, 327-328. 

 Loeb, J., 1904. The control of heliotropic reactions in fresh-water crustaceans by 



chemicals, especially CO,. Univ. of Calif. Pub. in Physiol. 2, 1-2. 



