Presenter: Dominic Eastburn (University of Washington)

Description:
Many hydromedusae exhibit distinct behavioral responses to changing photic stimuli utilizing light detecting organs called ocelli. These behavioral responses directly affect swimming behaviors which include vertical migration, pulse frequency, and fishing posture. With this in mind, we studied the two hydromedusae species, Polyorchis penicillatus and Halimedusa typus, to test how swimming behaviors shifted with light and dark conditions. Each species was tethered and observed in response to short periods (15 mins.) of light and dark to quantify pulse frequency. Both P. penicillatus and larger H. typus (>1 cm) had a higher pulse frequency in the dark than in light. During qualitative observations over a 24-hr period in a larger tank, H. typus exhibited a sinking behavior not previously described in the literature. H. typus also sank at a faster rate in the dark (0.20±0.095 cm/s; ± SD) than light (0.076±0.016 cm/s). Finally, to assess whether the vertical distribution of both species varied between day and night in the natural environment, we utilized plankton nets to sample at different depths within the water column. We observed P. penicillatus at the surface in greater numbers at night, while H. typus was mostly found at the surface during the day and reduced numbers during the night. It remains unclear if the reduced number of H. typus at the surface during the night is related to a reverse diel migration to deeper water. Variation in swimming behavior may indicate different feeding efficiency and prey interactions, and help us determine the trophic positions of these hydromedusae.