EFFECTS OF FISH PRESENCE AND SIMULATED MOONLIGHT GRADIENTS ON NIGHTTIME HORIZONTAL MOVEMENTS OF A PREDATORY ZOOPLANKTER, CHAOBORUS-PUNCTIPENNIS

Some zooplankton, including Chaoborus punctipennis, have recently been reported to undergo diel horizontal migrations in addition to their w idely known vertical migrations. In a series of laboratory and field e xperiments, we tested the influences of gradients in light intensity a nd fish presence on horizontal migration in C.punctipennis. In a small chamber, C.punctipennis showed no response to simulated moonlight gra dients. They did, however, show significant movement away from fish he ld at one end of the chamber under uniform moonlight. A field experime nt in a large chamber indicated that the response to fish was induced by chemical cues. When tested in the laboratory chamber with both moon light gradients and fish present, C.punctipennis became positively pho totactic. This response increased the movement away from fish when the fish were at the darker end of the gradient, or reduced it when the f ish were at the brighter end. In combination, these experiments demons trate that C.punctipennis show directed horizontal movements in respon se to fish presence and moonlight gradients, suggesting a potential fo r these stimuli to influence horizontal migrations observed in the fie ld.