Metabolic costs during predator‐induced dielvertical migration of Daphnia

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Abstract
Life‐history parameters were determined for a cohort of 60 clonal Daphnia magna grown individually in thermally stratified, 1‐m‐long flow‐through tubes under a constant high‐food regime (2.0 mg C liter−1). The individuals could migrate freely in the tubes and their depth was noted at regular intervals. Half of the tubes received water from a reservoir containing a fish. The animals in these tubes stayed deep during the day and higher at night, thus showing a “normal” diel vertical migration behavior. The other half of the tubes was supplied with the same water, but free of fish. The individuals in this set exhibited no diel migration, but stayed in the warm upper strata day and night. The migrating animals in the fish‐treated set grew at rates (0.21 d−1) only a third those of individuals in the no‐fish treatment (0.57 d−1). Most of the difference between the treatments can be attributed to the lower temperatures experienced by the migrating individuals. However, an experiment with no thermal stratification showed that the chemical presence of fish in the water can significantly retard Daphnia growth.

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