Pelagic-benthic transition of the harmful alga, Heterosigma akashiwo: Changes in swimming and implications for benthic cell distributions

► Heterosigma exhibits distinct swimming modes during pelagic-benthic transition. ► Our video-based motion assay can identify state-specific movement behaviors.► Heterosigma does not use active down-swimming to influence benthic distributions. ► Distributions of Heterosigma resting cells mirror distributions of passive particles.