Multiscale flow between the branches and polyps of gorgonians

Gorgonians, or sea fans, are soft corals that are well known for their elaborate branching structure and they way that they sway in the ocean. This branching structure can create feeding flows that are optimized for a particular range of velocities, and presumably for a particular size of prey. As water moves through the elaborate branches, it is slowed, and recirculation zones can form downstream of the fan. At the smaller scale, individual polyps that emerge from the branches expand their tentacles, further slowing the flow. At the smallest scale, the tentacles are covered in tiny bristles where prey capture and exchange occur. In this paper, we quantify the gap to diameter ratios for a variety of sea fans at the scale of the branches and the bristles on the polyp tentacles. We then use computational fluid dynamics to determine the flow patterns at both levels of branching. We quantify the leakiness between the branches and bristles over the biologically relevant range of Reynolds numbers and gap to diameter ratios. We find that the branches and bristles can either act as a leaky rake or solid plate depending on these dimensionless parameters. The results of the study have implications for how sea fans can generate efficient feeding flows while also reducing drag during storms.