Eukaryotic parasites are believed to play important roles in bloom dynamics of red-tide dinoflagellates; however, little is known about their impact on host behavior. To address this issue, we examined the influence of the parasitic dinoflagellate Amoebophrya sp. on diel vertical migration (DVM), phototaxis/geotaxis, and swimming speed of the bloom-forming dinoflagellate Akashiwo sanguinea. Uninfected A. sanguinea exhibited strong DVM, yielding subsurface aggregations during the day and evenly

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... ispersed populations at night. DVM of parasitized A. sanguinea was comparable to that of uninfected host for much of the infection cycle; however, subsurface accumulations of host cells were dramatically reduced or eliminated as parasites matured. The absence of DVM in the latter part of the infection cycle appeared to be associated with changes in phototaxic response and swimming speed of parasitized cells. While uninfected A. sanguinea exhibited strong positive phototaxis, hosts with late-stage infections exhibited strong negative phototaxis. By contrast, infected and uninfected A. sanguinea did not differ in geotaxis or sinking. Swimming speed of infected A. sanguinea decreased with parasite age, differing significantly from that of uninfected hosts 24 h after initiation of infections. A. sanguinea with late-stage infections exhibited a 37.3% lower swimming speed than uninfected hosts. Results indicate that parasitism by Amoebophrya sp. disrupts 'internal' factors that control DVM in dinoflagellates. Data also suggest that modification of host behavior by parasitism may contribute to vertical variations in phytoplankton species-composition and -abundance, particularly during epidemics when late-stage infections are prevalent. KEY WORDS: Parasitism · Diel vertical migration · Taxis · Swimming speed · Behavior · Red tide · Akashiwo sanguinea Resale or republication not permitted without written consent of the publisher Editorial responsibility: William Li,