Moving with purpose: kelp zoospore motility governs encounters, settlement, and early recruitment

Kelp forests depend on the successful settlement of microscopic zoospores, the motile dispersal stage of the kelp life cycle. Settlement begins with arrival at a surface and culminates in stable attachment through adhesive contact and curing. This is a critical transition between zoospore dispersal and gametophyte development, during which individual swimming behavior and zoospore traits may shape settlement success. Despite its importance, settlement metrics are rarely considered in kelp ecology, restoration, and ecotoxicology frameworks. Here, we explore and quantify how zoospore swimming behavior relates to settlement in three habitat-forming kelp species, Laminaria hyperborea, L. digitata, and Saccharina latissima (Laminariales, Phaeophyceae). Zoospores were cultured under controlled conditions and analyzed using high-resolution video microscopy at both individual zoospore and population levels. Across species, motile fraction declined with culture age, and cultures with higher proportions of motile zoospores exhibited substantially greater settlement. Motile fraction exerted a strong multiplicative effect on settlement, independent of species, season, and site effects. Species also differed significantly in zoospore size, aspect ratio, and surface ornamentation, revealing a previously under-quantified axis of early-life functional differentiation. Integrating swimming speed and directional persistence of individual zoospores into an encounter metric revealed a clear behavioral hierarchy, with highly progressive trajectories exhibiting the greatest encounter potential and immobile zoospores the least. These results provide a mechanistic link between zoospore swimming behavior, encounter rates, and settlement success, demonstrating that a simple motile/non-motile classification captures the likelihood of settlement, while kinematic metrics explain how sublethal impairment of motility translates into reduced recruitment. Therefore, zoospore motility and morphology are sensitive, mechanistically-grounded functional metrics for kelp ecology, restoration, and ecotoxicology.