Flow regime determines whether larval settlement hydrodynamics are governed by swimming speed or shape

Abstract

Larval dispersal and settlement govern the replenishment of sessile marine populations and the assembly of benthic communities, yet how larval traits interact with near-surface flow structures to direct settlement remains unclear. We used coupled hydrodynamic and larval-scale simulations to resolve how swimming speed and body shape, from circular to elongated, influence settlement in oscillatory flow over ridged substrates. Settlement success differed by up to an order of magnitude across trait combinations. Near-surface flow acted as a trait filter by differentially regulating transport to, retention near, and encounters with the substrate. The trait that mattered most depended on the local flow structure. Where rapid flow restricted access to settlement surfaces, swimming speed was most important. Where flow recirculated between substrate ridges, shape-dependent alignment controlled transport and retention. These results link larval traits to flow-mediated settlement mechanisms and provide a physical framework for understanding how recruitment varies across complex benthic habitats.