Symbioses involving sulfur-oxidizing bacteria and invertebrate hosts require a source of reduced sulfur, a source of O-2, and transport mechanisms that ensure them a supply of both. We investigated these mechanisms using the symbiosis between the sessile ciliate Zoothamnium niveum (Hemprich and Ehrenberg 1831) and bacteria living on its surface. The stalked colonies of Z niveum grow on peat walls around the openings of centimeter-scale conduits created when mangrove rootlets decompose. Using in situ, time-series measurements with fast-responding amper-ometric microelectrodes, we found that the conduits were charged with H2S by diffusion from the decaying rootlets during periods of low boundary-layer flow speed. During these times, the feeding current of the zooids transported oxygenated seawater from outside the peat wall toward the ectobiotic bacteria. During periods of high flow speed, H2S-rich seawater from the conduits was drawn along the colonies and over the bacteria. We conclude that this symbiosis exploits a combination of two transport mechanisms: (1) venting of H2S-rich seawater due to pulsating boundary-layer current over ciliate groups and (2) the continuous and rapid feeding current generated by the host's cilia. This discovery raises the possibility that other systems in which pockets of decay are exposed to pulsating flow could support similar symbioses.