Abstract
The present investigation was aimed at gaining more insight into the physiological mechanism underlying the diurnal vertical migration of planktonic animals. The experiments were carried out on Daphnia magna. Only reactions to decreases in light intensity were studied.
Kinetical aspects.
Experiments with instantaneous decreases in light intensity revealed that the reaction to
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this stimulus is a positively phototactic swimming, the speed of which increases with increasing stimulus intensity. For threshold stimuli the light intensity reached when the reaction stops is equal to the initial intensity. The swimming distance depends on the turbidity of the water. There is experimental evidence that a kind of feed-back is involved.
Series of sub-threshold decreases in light intensity may lead to a phototactical swimming reaction, which demonstrates the additional effect of these sub-threshold decreases.
Continuous decreases in light intensity lead to a series of separate responses that are identical with the positively phototactic reactions caused by instantaneous decreases. With increasing rate of the relative decrease in light intensity these separate responses merge into a continuous upward swimming. The period of time after which the first reaction is initiated depends on the rate of the continuous decrease in illumination; a relation is found between the logarithm of the stimulus strength and the duration of the stimulus needed to initiate a reaction, comparable to such a relation valid in muscle stimulation. The swimming speed is constant as long as the reactions are separated, but as soon as they flow together the swimming speed starts to increase linearly with the logarithm of the stimulus intensity.
At a small rate of the relative decrease in light intensity the reacting animals lag behind the initial light intensity. With increasing rate this lagging behind disappears gradually, but at a certain rate the lagging behind increases again. A theory is built up, which tries to account for several observed phenomena.
An observed migration of Daphnia magna in nature demonstrated the applicability of the experimental results to the natural phenomenon. Also the interpretation of the migration of three scattering layers in terms of the theoretical concepts proved to be promising.
Directional aspects.
It is argued that spatial differences in light intensity, such as contrasts and gradients, might be proper orientational beacons for Daphnia magna. In the angular light distribution of the aquatic habitat relatively large changes in light vectors (contrasts or gradients) are present between 30° and 60° with the vertical. Therefore, the orientation responses of Daphnia magna to contrasts were studied experimentally. Daphnia magna proved to be unable to swim normally when the light from every direction is of equal intensity, but increasing the light intensity from one direction enables the animal to reassume a normal body position. Below a certain light intensity normal swimming is possible, independently of the angular distribution.
When a Daphnia magna, transversally pinned through the broodpouch and lying on its side, is illuminated in the medial plane by light of equal intensity from every direction in this plane, forward or backward rotations are observed. Screening off the light for 270° stops these rotations, the formerly forward rotating animals direct the body axis toward darkness, the formerly backward rotating animals direct the body axis toward the lighted area. It was proved that the animals are orientated in these two cases to different edges of the screen (beacon contrasts). In adjusting the compound eye and the body axis to displacements of the beacon contrasts the animal lags behind.
A decrease in total light intensity of an angular light distribution with light of equal intensity coming from every direction evokes a dorsal eye rotation in Daphnia magna. This fact demonstrates that no optical beacons are needed to induce eye rotations.
It is concluded that the directing stimulus for the phototactic reaction is a contrast or a gradient present in the angular light distribution. Previous hypotheses on the orientation mechanism in Daphnia are compared with the one developed in this paper.
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