Millimeter-scale topography enables coral larval settlement in wave-driven oscillatory flow

TL;DR

This study demonstrates that millimeter-scale topography on substrates significantly enhances coral larval settlement in wave-like oscillatory flows by creating flow recirculation zones that increase settlement likelihood.

Contribution

The paper introduces the use of millimeter-scale benthic topography to improve coral larval settlement, combining experimental, computational, and simulation approaches.

Findings

Millimeter-scale ridges increase settlement by an order of magnitude.

Flow recirculation zones redirect larvae toward the substrate.

Enhanced settlement observed across experiments and models.

Abstract

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.