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Abstract

Nereocystis luetkeana is a large canopy-forming seaweed that supports nearshore ecosystems throughout the Pacific Northwest. Despite its importance to canopy formation and primary productivity, the mechanisms underlying thallus development remain poorly understood. Unlike most seaweeds, the blades that constitute canopies originate from repeated divisions of a single blade, producing complex habitat that is well suited to a variety of environmental conditions. This thesis clarifies the process of blade division in Nereocystis and identifies the developmental and mechanical factors that regulate blade number. Using controlled growth experiments across a range of flow speeds, I demonstrated that blade division increases under faster flow, resulting in higher blade numbers. Comparisons among thalli originating from different populations but grown in a common garden confirmed that blade division is a plastic response to hydrodynamic environment rather than a fixed, population-level trait. Measurements of drag before and after blade division revealed that blade division mitigates drag, indicating that higher blade numbers confer a mechanical advantage in high-flow habitats. Blade division in kelps is often assumed to result from tissue degradation or programmed cell death. In contrast, histological analyses showed that blade division in Nereocystis is driven by mechanical tearing, followed by rapid and extensive healing of the blade margin. I further demonstrated that lines of dehiscence (LOD)—linear features that frequently precede blade divisions—correspond to regions of reduced tissue thickness and distinct chemical properties associated with wound healing. Finally, I examined the effect of LOD on blade division and the capacity of water flow to provide the energy for blade separation. Flow experiments showed that splits readily extend under moderate flow and that split propagation becomes increasingly likely as tears lengthen. LOD facilitate longitudinal blade division by reducing tearing strength and increasing the probability of tearing in flow, particularly in juvenile blades, which otherwise tear isotropically and risk tissue loss. These findings demonstrate that blade division in Nereocystis luetkeana is a mechanically mediated developmental process that channels hydrodynamic stress to regulate blade number and thallus form, enabling persistence and canopy formation across the broad range of flow conditions characteristic of the Pacific Northwest.