30 INVERTEBRATE PHYSIOLOGY 



Temperature and Phototaxis 



Since the response to a vertical light beam under various conditions of 

 temperature is up or downswimming, the following tests were all con- 

 ducted in a horizontal light beam to eliminate the possibility of a light-in- 

 duced geotaxis (which is discussed above). There are at least four 

 separate and independent effects of temperature on the phototaxic re- 

 sponse of Daphnia magna and Eubranchipus to white light, which are 

 summarized under "Temperature" and "Changes of Temperature" in 

 Fig. 3. Daphnids reared at 15° C and exposed to a horizontal light beam 

 show vigorous positive phototaxis at temperatures of 0° to 5° C. The 

 organized photopositive response is not a prolonged one at very low tem- 

 peratures in the presence of light intensities of greater than 10-foot candles, 

 since it is superseded by a somewhat violent paralytic seizure ending in 

 death. The paralytic seizure may be induced by similar light intensities at 

 room temperature in the presence of 1 :10'^ acetylcholine and eserine. On 

 the other hand the paralytic seizure at low temperatures may be avoided 

 entirely by making the water in which the daphnids swim 1 :10'^ atropine 

 (Baylor, 1954). At the opposite end of the temperature scale for the 

 daphnid, namely 15 degrees above its normal environmental temperature, 

 there is a vigorous negative phototaxis in response to a horizontal beam 

 of white light. In view of the fact that the direction of the intensity vectors 

 of light in the normal environment of such animals is vertical and that ex- 

 tremes of temperature are hazardous, the positive phototaxis in cold water 

 and the negative phototaxis in warm water may be considered to have 

 some adaptive significance. 



Small but continuous decrements of temperature can significantly in- 

 crease the velocity of downswimming in response to any stimulus which 

 will initiate downswimming. Thus a population of daphnids will continue 

 to swim downward through a thermal gradient at high and svistained ve- 

 locities in spite of the fact that the stimulus which initiated the response 

 may have decreased in intensity. The downswimming pattern performed 

 through such a thermal gradient will be maintained until the population 

 leaches 10 to 15 degrees below normal environmental temperature, where 

 the incipient positive phototaxis in response to cold temperature halts 

 the downward migration. 



Small but continuous increments in temperature can significantly in- 

 crease the velocity of upswimming, once such a response is initiated by 

 any of a number of stimuli already discussed. The pattern is now exactly 

 the converse of that described above for decrements of temperature. Once 

 the population of daphnids is started swimming upwards through such a 

 thermal gradient, they will continue to swim upwards till an upper tem- 

 perature is reached which will halt the pattern. 



