Transient eddies over ridged substrates enhance trapping and settlement of coral larvae

The low settlement success of planktonic larvae is an important problem that can inhibit the recovery of reefs from environmental damage. Tiny coral larvae ($<$ 1mm) must navigate the water column to find a suitable surface for permanent settlement, a process influenced by diverse chemical, biological, and physical mechanisms acting over multiple length scales. Using a custom-built oscillatory flume tank, we investigate coral larval settlement on substrates of different roughness scales to understand the effect of hydromechanical forces on larvae due to boundary layer flows. Dynamic flow fields, characterized by particle image velocimetry, contain regions of recirculation near millimeter-scale substrate features that correlate strongly with larvae settlement positions, despite the mean flow speed in the flume tank being an order of magnitude higher than the larval swimming speed. Using simulations, we explore how the recirculation regions influence the settlement of active swimmers over substrates of varying surface topography.