We isolated 13 polymorphic microsatellites from the red-tide causing dinoflagellate Akashiwo sanguinea. These loci were highly variable, with between 2 and 10 alleles per locus, and estimated gene diversity ranging from 0.08 to 0.82. These loci have the potential to reveal genetic structure and estimate gene flow among A. sanguinea populations.

To determine the process of population expansion and ascertain the origin of the Sea of Japan population, in a noxious red tide forming dinoflagellate Cochlodinium polykrikoides, 13 samples, isolated from 11 different localities in Japanese and Korean coasts, were analysed using 10 polymorphic microsatellites. Analyses by nonmetric multidimensional scaling plots of pairwise F-ST, global amova, and genetic admixture analysis identified three clusters - the Sea of Japan populations, Yatsushiro Sea (Kumamoto Pref.) populations, and other populations - indicating genetic structuring of the 13 samples into three distinct populations. In the proportion of shared alleles by pairwise individuals (P-SAxy) analyses between the Sea of Japan and the other samples, P-SAxy was extremely low compared with that among the Sea of Japan or among other samples, indicating that a large genetic barrier has occurred between the populations. No significant relationship of isolation-by-distance patterns and almost no genetic distance were detected between pairwise samples of the Sea of Japan, although there is a maximal distance of > 600 km between samples. In addition, P-SAxy data among the samples were extremely high compared with those among other samples, clearly showing that a large-scale transfer from west to east has occurred via the Tushima Warm Current. In the P-SAxy data of the Seto Inland Sea and Pacific samples, individuals showing relatively high P-SAxy were concentrated in the three areas of Nagasaki, Harima, and Mie, suggesting that frequent transfer may have occurred by human-assisted dispersal, although Nagasaki and Mie are separated by a distance of approximately 700 km.

We isolated 12 polymorphic microsatellites from the noxious red-tide-causing alga Chattonella ovata. These loci provide a class of highly variable genetic markers, as the number of alleles ranged from four to 12, and the observed and expected heterozygosities ranged from 0.238 to 0.850 and from 0.310 to 0.889, respectively. These loci are useful for revealing the genetic structure of and gene flow among C. ovata populations.

We isolated 15 polymorphic microsatellites from one of the most noxious red tide-causing dinoflagellate species, Heterocapsa circularisquama. These loci provide one class of highly variable genetic markers, as the number of alleles ranged from two to six, and the estimate of gene diversity was from 0.205 to 0.684 across the 15 microsatellites. These loci have the potential to reveal genetic structure and gene flow among H. circularisquama populations.

We isolated 15 polymorphic microsatellites from Cochlodinium polykrikoides. These loci provide a class of highly variable genetic markers, as the number of alleles ranged from two to 15, and the estimate of gene diversity was from 0.083 to 0.880 across the 15 microsatellites. We consider that these loci have a potential to reveal the genetic structure and gene flow among C. polykrikoides populations.

Fluctuations of the genus Dinophysis, which contained several toxic species of diarrhetic shellfish poisoning (DSP), were investigated during blooms in Hiroshima Bay, Mutsu Bay and Ise Bay, Japan. The co-occurrences of small phytoplanktons (cryptophytes, other nanophytoplanktons, cyanobacteria and eukaryotic picophytoplanktons) were investigated to search for relationships with mixotrophic Dinophysis. Cryptophytes were divided into three size-groups based on length of their chloroplasts (>10, 5–10 and <5μm) during counting by epifluorescence microscopy. Clear relationships were not found between the occurrences of Dinophysis spp. and nanophytoplanktons, cyanobacteria and eukaryotic picophytoplanktons. However, the fluctuations of small-sized cryptophytes (<5μm) showed a close relationship with that of D. acuminata in Hiroshima Bay. In Mutsu Bay, small-sized cryptophytes also accompanied the first occurrence peak of Dinophysis spp. In Ise Bay, peaks of the occurrences of middle- and small-sized cryptophytes were observed 2–3 weeks before the peak of D. acuminata. These cryptophytes decreased rapidly with increase in D. acuminata. These results suggest the possibility that small-sized cryptophytes may be food organisms for mixotrophic Dinophysis, with the abundance of Dinophysis dependent on these cryptophytes.

Monitorings were conducted on DSP toxins in mid-gut gland of scallop (mouse assay), cell numbers of toxic dinoflagellate species of Dinophysis, and diarrhetic shellfish poisoning (DSP) toxins in small-sized (0.7–5 μm) plankton fraction of seawater collected from surface (0 m) and 20 m depth at a station in Mutsu Bay, Aomori Prefecture, Japan, in 2000. A specific enzyme-linked immunosorbent assay (ELISA) was employed for the analysis of DSP toxins in small-sized plankton fraction using a mouse monoclonal anti-okadaic acid antibody which recognizes okadaic acid, dinophysistoxin-1, and dinophysistoxin-3. DSP toxins were detected twice in the mid-gut gland of scallops at 1.1–2.3 MU (mouse units) g−1 on 26 June and at 0.6–1.2 MU g−1 on 3 July, respectively. Relatively high cell densities of D. fortii were observed on 26 June and 11 September, and may only contribute to the bivalve toxicity during late June to early July. D. acuminata did not appear to be responsible for the toxicity of scallops in Mutsu Bay in 2000. ELISA monitoring of small-sized plankton fraction in seawater could detect DSP toxins two weeks before the detection of the toxin in scallops, and could do so two weeks after the loss of the bivalve toxicity by mouse assay. On 17 July, toxic D. fortii was detected at only small number, <10 cells l−1, but DSP toxins were detected by the ELISA assay, suggesting a presence of other toxic small-sized plankton in seawater. For the purpose of reducing negative impacts of DSP occurrences, monitorings have been carried out hitherto on DSP toxins of bivalve tissues by mouse assay and on cell densities of “toxic” species of Dinophysis. Here we propose a usefulness of ELISA monitoring of plankton toxicity, especially in small-sized fraction, which are possible foods of mixotrophic Dinophysis, as a practical tool for detecting and predicting DSPs in coastal areas of fisheries grounds of bivalve aquaculture.