. The biology of marine animals. Marine animals; Physiology, Comparative. 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 log10 light intensity. Under la
. The biology of marine animals. Marine animals; Physiology, Comparative. 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 log10 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- 0 | 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. 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-. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustratio
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