SENSORY ORGANS AND RECEPTION 



325 



There is general agreement that vertical migration depends on changing 

 light penetration during the day. Migratory planktonic animals aggregate 

 in a band or region of optimal light intensity. At levels below the optimum, 

 locomotory movements are initiated or increased; and at intensities above 

 the optimum, movements slow down. Experimental evidence for several 

 species reveals that there is a linear relationship between velocity of loco- 

 motory movements and log 10 light intensity. 



Under laboratory conditions Daphnia, a freshwater cladoceran, can be 

 made to execute a complete cycle of vertical migration by varying cyclically 



Time 



1400- 1300 



2200-0200 



0600-1000 



WOO -2200 



0200-0600 



woo- noo 



First calyptopis 



a so- o 



f 



* 100- 50 



3 250- 100 



ti 

 ^ 500-250 



o 

 vj* 750-500 



aS 1000- 750 



Second calyptopis 

 fc 50- 

 | 100- so 



3 250- 100 

 C^ 500- 250 



<*> 750- 500 

 C? 

 <3 7 000- 150 



Third calyptopis 

 *£ SO- 



100- so 



-i 250- 100 



I 500^ ISO 



"o 750-500 

 <*> 



£ WOO- ISO 



+"'H 



Fig. 8.15. Diagram Showing Vertical Migration in Calyptopis Stages 1-3 

 of Euphausia superba in the Falkland Islands Sector of the Antarctic 



(from Fraser, 1936.) 



the light-intensity. At low intensities, the movement of Daphnia is inde- 

 pendent of the direction of the light and is determined solely by photo- 

 kinesis; the dawn rise is a manifestation of this factor. The photokinetic 

 response continues even in blinded animals. Superimposed on the photo- 

 kinetic response at high light intensities is a phototactic response, in which 

 the animal moves towards the light at reduced light intensities, and away 

 from it when the light intensity is increased. In this way, Daphnia is able 

 to follow a zone of optimum light intensity. One or both of these mechan- 



