Physical Factors involved in the Vertical Migration of Plankton

It is suggested that vertical migration is an inescapable consequence of the use of light by planktonic forms as a means of attaining a level within the euphotic zone; no other sense organ appears capable of producing this result. The mechanism of light response in Daphnia fulfils the necessary requirements for vertical migration, and field observations on other species suggest the presence of similar if not identical physiological features in them. These features are a positive photokinesis which is combined with negative geotaxis at low light intensities; at a critical higher value of light intensity a typical dorsal light reflex cuts off the vertical rise. The dorsal light reflex is probably a response to illumination of the eye within the critical cone of submarine illumination, and is therefore not related to the direction of a particular light source such as the sun.