DIURNAL MIGRATION OF PLANKTON 29 



zontal phototaxis show light-induced geotaxis ; however, some species 

 (medusae, Mnemiopsis, and a pteropod, Creseis) which have no lateral 

 phototaxis show light-induced geotaxis. Certain marine organisms may- 

 show a reversal of behavior outlined above. For example, Callinectid 

 megalops larvae, usually being geopositive to increased intensity, may 

 sometimes show the reverse pattern and swim upward in response to an 

 increased intensity. In general, if inshore plankters are geopositive in the 

 dark ( in response to factors other than light ) , they will upswim to an in- 

 crease in light intensity, while, if they are geonegative or show neither 

 tendency in the dark, they will downswim to an increase of light intensity. 

 In the presence of long wave lengths of light, the addition of short wave 

 lengths either has no effect at all, or else the organisms downswim with 

 the addition or upswim with the subtraction of short wave lengths. 



Light and Geotaxis 



There are two effects of light on geotaxis. One eff'ect is associated with 

 wave length while the other eft'ect is associated with intensity. We have 

 not determined the action spectrum of the daphnid eye accurately in the 

 violet region and hence these effects may be identical. In our tests geotaxis 

 is distinguished from phototaxis by the use of a horizontal light beam. 

 Thus phototaxes will be directed toward or from the light source while 

 geotaxes are directed up or down. The behavior pattern here is readily dis- 

 tinguished from blue-dancing in a horizontal beam where movement is also 

 at right angles to the beam. 



The effect of an increase of intensity is downswimming (see Fig. 3). 

 Provided the animal is accommodated to the intensity of the horizontal 

 beam, the etiect is almost a pure downswimming in the near ultraviolet 

 with a very small negative phototaxis. The eft'ect of shortening the wave 

 length of a horizontal beam of violet light from a monochromator is also 

 downswimming. If one assumes that the peak of the violet action spectrum 

 for Daphnia magna is 3,000 A or below, then the response to either an in- 

 crease in intensity with the wave length held constant or a decrease in 

 wave length is indistinguishable operationally. Both may appear as an 

 increase of intensity in the experimental animal. Conversely, either 

 dimming the intensity or increasing the wave length will produce up- 

 swimming, provided the animal is accommodated to the initial intensity 

 and wave length. Thus, geotaxis in response to light stimuli appears asso- 

 ciated with phototaxis in the integrated behavior of Daphna magna. In 

 the natural environment the two behavior patterns of phototaxis and geo- 

 taxis operate together and in the same direction in response to the same 

 stmulus although one is oriented to light and the other to gravitv (see 

 Fig. 3). 



