Hydrodynamic forces on larvae affect their settlement on coral reefs in turbulent, wave-driven flow

This study investigates a key aspect of how ambient flow affects the recruitment of water-dispersed marine larvae onto benthic substrata: the sweeping away of larvae that have landed on surfaces. Through a combination of field and laboratory measurements, we studied how waves interact with a complex substratum to affect hydrodynamic forces encountered by microscopic larvae sitting at different positions on the bottom terrain. We used larvae of the nudibranch Phestilla sibogae settling onto coral reefs as the system to address this question. Laser Doppler anemometry was utilized within a laboratory flume to measure water velocities encountered 200 mm from coral surfaces by microscopic larvae sitting at different locations within a reef of the branching coral Porites compressa. Comparing wave-driven flow, based on conditions measured over P. compressa reefs in Hawaii, with unidirectional flow of the same mean velocity, we found that peak shear stresses along coral surfaces were similar to 15 times greater and hydrodynamic forces on larvae were similar to 10 times higher in wave conditions. Peak forces on larvae sitting on the reef top were 3-10 times greater than on larvae 5-10 cm below the reef top. Intermittent bursts of high velocity, which occurred more often at the reef top than within it, determined the settlement probability for larvae in different reef microhabitats. The faster and more strongly a larva sticks to a surface, the higher the probability of successful settlement. Using values for P. sibogae adhesive strength, we predict they can settle only on sheltered surfaces within reefs.