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A variety of photic stimuli were presented to the hydromedusan Polyorchis penicillatus under controlled conditions to characterize the photic responses of various sizes of P. penicillatus and to determine the role of these responses in its behavior. "Treadmill" experiments showed that for all but very small hydromedusae, swimming frequencies at different constant light intensities did not differ. Swimming frequency of P. penicillatus was, however, directly proportional to rates of decrease in light intensity. Slowly increasing light intensity caused an inhibition of swimming and "crumpling." Rapid, 100% shadows of various absolute magnitudes usually caused only a single swimming contraction. The maximal response to rapid shadows of monochromatic light occurred around 450-550 nm. These results suggest that the shadow response of P. penicillatus does not function in predator avoidance, but more likely contributes to nighttime upward movement in the water column. The inhibition of swimming during increasing light intensity may initiate dawn sinking. Most of the photic responses of P. penicillatus show size-(age) related differences which may result in ontogenic changes in distribution and feeding behavior.

THE SHADOW RESPONSE OF A. HYDROMEDUSAN (POLYORCHIS PENICILLATUS): BEHAVIORAL MECHANISMS CONTROLLING DIEL AND ONTOGENIC VERTICAL MIGRATION