Abstract
We observed the individual swimming behavior of a clone of Daphnia hyalina swiniming freely inside a mesocosm-scale plankton tower. Changes in light intensity and the presence or absence of fish smell induced vertical migration through ∼4 m. The results of analysis of video records of individual swimming behavior include the following when lights were turned on, Daphnia moved down by fast downward swimming, not by sinking or moderate swimming; when lights were turned off, the Daphnia rose by fast upward swimming, not by upward moderate swimming (with hops). Moderate swimming was nearly horizontal and fast swimming was nearly vertical. Fish smell increased the proportion of the population swimming fast in response to a light stimulus, but inhibited the expression of sinking behavior. These results, interpreted in the light of the predator-avoidance hypothesis of diel vertical migration, suggest that vertical migration through fast swimming is less dangerous than vertical migration via either sinking or vertical moderate swimming.
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