Effects of fish presence and simulated moonlight gradients on night-time horizontal movements of a predatory zooplankter, Chaoborus punctipennis

Abstract

Some zooplankton, including Chaoborus punctipennis, have recently been reported to undergo diel horizontal migrations in addition to their widely known vertical migrations. In a series of laboratory and field experiments, we tested the influences of gradients in light intensity and fish presence on horizontal migration in C. punctipennis. In a small chamber, C. punctipennis showed no response to simulated moonlight gradients. They did, however, show significant movement away from fish held at one end of the chamber under uniform moonlight. A field experiment in a large chamber indicated that the response to fish was induced by chemical cues. When tested in the laboratory chamber with both moonlight gradient and fish present, C. punctipennis became positively phototactic. This response increased the movement away from fish when the fish were at the darker end of the gradient, or reduced it when the fish were at the brighter end. In combination, these experiments demonstrate that C. punctipennis show directed horizontal movements in response to fish presence and moonlight gradients, suggesting a potential for these stimuli to influence horizontal migrations observed in the field.