Diel Vertical Migration and Feeding Behavior of a Demersal Hydromedusan (Polyorchis penicillatus)

The hydromedusan Polyorchis penicillatus makes a small amplitude diel vertical migration. During the day, over 85% of the individuals were observed within 1 m of the bottom, while at night, medusae diffused several metres up into the water column. This movement occurred concomitantly with the emergence of many of the major taxa of demersal Zooplankton whose activity was quantified by emergence traps. Food boli contents collected from Polyorchis showed some selectivity for large, fast-moving demersal plankters and low capture and utilization of invertebrate larvae. Diel shifts in feeding behavior and water column position enable Polyorchis to feed efficiently on high densities of demersal plankters at all times. Medusae may be exempt from migration constraints imposed by visual predators and are thus able to move into areas of high prey density to supply energetic requirements. Hydromedusae, such as Polyorchis, are shown to utilize a variety of factors, such as tentacle position and swimming speed, in an "ambush" strategy to optimize prey encounters.