Suspension Feeding in Oscillating Flow: The Effect of Colony Morphology and Flow Regime on Plankton Capture by the Hydroid Obelia longissima

¹ Zoology Department, 4079 Life Science Bldg., University of California, Berkeley, California 94720

The effect of flow regime on the ability of the hydroid Obelia longissima to capture plankton was examined in a laboratory flume and a wave tank. Feeding effectiveness—the proportion of the gastrozooids with tentacles successfully capturing food in a fixed amount of time—is ignificantly greater in oscillating flow than in unidirectional flow at the same average velocity and particle flux. Thus quantitative feeding studies in unidirectional flow may seriously underestimate feeding in the field if flow in the natural habitat is unsteady. Increasing colony bushiness (weight/colony length) decreases feeding effectiveness in uni-directional and low frequency oscillating flow, but not in high frequency oscillating flow. Longer colonies (0.083 m) show significantly lower feeding effectiveness than short (0.033 m) colonies. This decrease in feeding effectiveness with increasing colony length and bushiness is offset by a rapid increase in polyp number with increasing colony length.