Millimeter-scale ridges increase the duration of larval settling windows.

(a) Visualization of the relative fluid speed over flat (left) and ridged (right) substrates during peak (top) and turning point (bottom) flow. The fluid velocity (U) is normalized by the average larval swimming velocity (uℓ). Dotted black boxes represent regions ≤1.5 mm above the substrate surface that were used to calculate duration of settlement windows. Black arrows near the bottoms of the ridges indicate regions where the velocity remains low even at the turning points. (b) The average relative flow speed within the dotted black regions plotted over an average period for flat (top) and ridged (bottom) substrates. The yellow regions highlight the settling windows during which the local flow speed drops below uℓ (black line) plus one standard deviation (grey band).</p