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We analyzed cryptophyte nucleomorph 18S rRNA gene sequences retained in natural Myrionecta rubra cells and plastid 16S rRNA gene and psbA sequences retained in natural cells of several Dinophysis species collected from Japanese coastal waters. A total of 715 nucleomorph sequences obtained from 134 M. rubra cells and 564 plastid 16S rRNA gene and 355 psbA sequences from 71 Dinophysis cells were determined. Almost all sequences in M. rubra and Dinophysis spp. were identical to those of Teleaulax amphioxeia, suggesting that M. rubra in Japanese coastal waters preferentially ingest T. amphioxeia. The remaining sequences were closely related to those of Geminigera cryophila and Teleaulax acuta. Interestingly, 37 plastid 16S rRNA gene sequences, which were different from T. amphioxeia and amplified from Dinophysis acuminata and Dinophysis norvegica cells, were identical to the sequence of a D. acuminata cell found in the Greenland Sea, suggesting that a widely distributed and unknown cryptophyte species is also preyed upon by M. rubra and subsequently sequestered by Dinophysis. To confirm the reliability of molecular identification of the cryptophyte Teleaulax species detected from M. rubra and Dinophysis cells, the nucleomorph and plastid genes of Teleaulax species isolated from seawaters were also analyzed. Of 19 isolates, 16 and 3 clonal strains were identified as T. amphioxeia and T. acuta, respectively, and no sequence variation was confirmed within species. T. amphioxeia is probably the primary source of prey for M. rubra in Japanese coastal waters. An unknown cryptophyte may serve as an additional source, depending on localities and seasons.The marine dinoflagellate genus Dinophysis comprises photosynthetic and nonphotosynthetic members and is globally distributed in coastal and oceanic waters (12,29,30). Several members of the genus Dinophysis produce potent polyether toxins that can accumulate in filter-feeding bivalves, leading to a syndrome known as diarrhetic shellfish poisoning (DSP) in humans who consume tainted shellfish. These toxic algal species are important not only for their potential impact on public health but also from an ecological point of view because of their dual role as primary and secondary producers in complex microbial food webs. Despite extensive studies over the last 2 decades, little is known about the ecophysiology, toxicology, and bloom mechanisms of DSP-causing species of Dinophysis, primarily due to an inability to culture them. Since the first successful cultivation of Dinophysis acuminata by Park et al. (42), the understanding of Dinophysis biology and ecology has progressed considerably. Three other species (Dinophysis caudata, Dinophysis fortii, and Dinophysis infundibulus) are now available in culture, and it has become clear that these different Dinophysis species require the presence of both cryptophytes and the marine ciliate Myrionecta rubra to grow and proliferate (36,37,38). When presented with the marine ciliate M. rubra as prey, the four Dinophysis species mentioned...

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