  Larval settlement in wave-dominated, nearshore environments is the most
critical life stage for a vast array of marine invertebrates, yet it is poorly
understood and virtually impossible to observe in situ. Using a custom-built
flume tank that mimics the oscillatory fluid flow over a shallow coral reef, we
show that millimeter-scale benthic topography increases the settlement of
slow-swimming coral larvae by an order of magnitude relative to flat
substrates. Particle tracking velocimetry of flow fields revealed that
millimeter-scale ridges introduced regions of flow recirculation that
redirected larvae toward the substrate surface and decreased the local fluid
speed, effectively increasing the window of time for larvae to settle. In
agreement with experiments, computational fluid dynamics modeling and
agent-based larval simulations also showed significantly higher settlement on
ridged substrates. These findings highlight how physics-based substrate design
can create new opportunities to increase larval recruitment for ecosystem
restoration.
