INFECTION OF THE BLOOM-FORMING THECATE DINOFLAGELLATES ALEXANDRIUM AFFINE AND GONYAULAX SPINIFERA BY TWO STRAINS OF AMOEBOPHRYA (DINOPHYTA)1

In autumn 2002, parasitic dinoflagellates of the genus Amoebophrya Koeppen infected populations of the bloom-forming thecate dinoflagellates Alexandrium affine (Inoue and Fukuyo) Balech and Gonyaulax spinifera (Claparede and Lachmann) Diesing in coastal waters of Korea. Here we present the first documentation of Amoebophrya infections in A. affine and G. spinifera and use host–parasite cultures to provide information on parasite development and total generation times. Parasites of the two dinoflagellate hosts differed in their site of infection, developing in the cytoplasm of A. affine but in the nucleus of G. spinifera. Developmental stages of the parasite strains were similar to the previous descriptions of Amoebophrya spp. infecting other dinoflagellates. A prominent feature of Amoebophrya infection in A. affine from natural field samples was the presence of abnormal “giant cells” in the long chains formed by this host species. The characteristic green autofluorescence of Amoebophrya infections was not evident under blue light excitation until very late in the infection cycle of the A. affine–Amoebophrya sp. system but was detectable throughout the infection cycle in the G. spinifera–Amoebophrya sp. system. Despite the relatively long duration (2–10 min) of the emergence process of Amoebophrya spp. from these thecate hosts, total parasite generation times were shorter (53–55 h) than those previously reported for athecate host–parasite systems. These observations provide the basis for better assessing the impact of parasitism by Amoebophrya sp. as a significant loss factor for thecate dinoflagellate populations in Korean waters.