ABSTRACT: Parasitic dinoflagellates of the genus Amoebophrya commonly infect free-living dinoflagellates, some of which cause toxic or otherwise harmful red tides. These parasites prevent reproduction of their hosts and kill infected cells on a time scale of days. Thus, epidemic outbreaks of Amoebophrya spp. are thought to facilitate the decline of red tides by causing mass mortality of host taxa. However, little is known about biotic and abiotic factors that regulate epidemic occurrence of Amoebophrya spp. in nature. We addressed the hypothesis that grazing by ciliate microzooplankton on the infective stage of Amoebophrya sp. can regulate parasite prevalence in the bloom-forming dinoflagellate Akashiwo sanguinea. In culture, the choreotrich ciliate Strobilidium sp. rapidly ingested and digested infective dinospores of Amoebophrya sp. ex A. sanguinea. Laboratory experiments also showed that grazing by Strobilidium sp. could decrease infection of A. sanguinea by 70 to 80% relative to controls. Field experiments using plankton assemblages from Chesapeake Bay, USA, indicated that grazing by natural populations of ciliates may contribute to the regulation of parasitism in A. sanguinea. Thus, grazing by ciliates and other microzooplankton may indirectly influence the occurrence of red tides by limiting the spread of parasites like Amoebophrya sp.

KEY WORDS: Parasitic dinoflagellate · Harmful algal bloom · Ciliate grazing · Amoebophrya sp. · Akashiwo sanguinea · Strobilidium sp. · Chesapeake Bay

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Published in AME Vol. 28, No. 1 (2002) on May 16
Print ISSN: 0948-3055; Online ISSN: 1616-1564. Copyright © Inter-Research, Oldendorf/Luhe, 2002