The avoidance reaction ofCalanus finmarchicuscopepodids was observed in the presence of quantifiable fluid mechanical signals. The experimental apparatus was a rotating cylinder of sea water with fixed obstacles (diameters 6 mm and 10 mm) which disturbed the flow and stimulated avoidance. The experimental conditions prevented unambiguous discrimination of signals; the amount and rate of fluid deformation appear to be the effective stimuli. Avoidance reactions began at a mean distance of 7 mm from the small obstacle and 6 mm from the large obstacle. Mean values for fluid deformation for the small and large obstacles were 0.12 and 0.24 mm/mm; fluid deformation rate values were 0.80 and 1.0 sec−1. There was less scatter for deformation values. The copepods preferentially avoided obstacles at angles of about 60°with respect to the approach streamline. Avoidance speeds averaged 16 cm.sec−1; mean distance of avoidance flights was 1
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