  While many organisms filter feed and exchange heat or nutrients in flow, few
benthic organisms also actively pulse to enhance feeding and exchange. One
example is the pulsing soft coral (Heteroxenia fuscescens). Pulsing corals live
in colonies, where each polyp actively pulses through contraction and
relaxation of their tentacles. The pulses are typically out of phase and
without a clear pattern. These corals live in lagoons and bays found in the Red
Sea and Indian Ocean where they at times experience strong ambient flows. In
this paper, $3D$ fluid-structure interaction simulations are used to quantify
the effects of ambient flow on the exchange currents produced by the active
contraction of pulsing corals. We find a complex interaction between the flows
produced by the coral and the background flow. The dynamics can either enhance
or reduce the upward jet generated in a quiescent medium. The pulsing behavior
also slows the average horizontal flow near the polyp when there is a strong
background flow. The dynamics of these flows have implications for particle
capture and nutrient exchange.
